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Control Circuits

TWO-WIRE CIRCUITS

Control circuits are divided into two basic types: the two-wire and the three-wire. Figure 19–1 shows a simple two-wire control circuit. In this circuit, a simple switch is used to control the power applied to the coil of a motor starter. If the switch is open, there is no complete path for current flow, and the motor will not operate. If the switch is closed, power is supplied to the motor starter and it closes the M contacts, connecting the three- phase motor to the power line.

THREE-WIRE CIRCUITS

Three-wire control circuits are used because they are more flexible than two-wire circuits. Three-wire circuits are characterized by the fact that they are operated by a magnetic relay or motor starter. These circuits are generally controlled by one or more pilot devices.

ELECTRICAL SYMBOLS FOR PILOT/CONTROL DEVICES

When people first begin to learn to read, they learn a set of symbols that are used to represent different sounds. This set of symbols is generally referred to as the alphabet. When learning to read electrical diagrams, it is necessary to learn the symbols used to rep- resent different devices and components. The symbols that follow are commonly used on control schematics and wiring diagrams. These are not all the symbols used. Unfortunately, there is no real set standard for the use of electrical symbols. Most of these symbols are approved by the National Electrical Manufacturers Association (NEMA). These symbols are as follows:

The contact symbols shown are standard and relatively simple to understand. There can be instances, however, in which symbols can be used to show something that is not at first apparent. For example, the symbol for a normally closed pressure switch is shown in Figure 19–2. Notice that this symbol shows not only the movable arm making contact with the stationary contact but also the movable arm drawn above the stationary contact. Figure 19–3 shows a normally closed held open pressure switch. This symbol is not a normally open contact symbol even though the contact arm is shown not making connection with the stationary contact because pressure is used to keep the contact open. If pressure decreases to a certain point, the switch contact will close.

Figure 19–4 shows a normally open pressure switch. Notice that the contact arm is drawn below the stationary contact. Figure 19–5 shows the same symbol except that the movable arm is making connection with the stationary contact. This symbol represents a normally open held closed pressure switch, that is, a pressure switch that is wired normally open but has its contact held closed by pressure. If the pressure decreases to a certain level, the switch will open and break connection to the rest of the circuit.

SCHEMATIC AND WIRING DIAGRAMS

Schematic diagrams show components in their electrical sequence without respect to physical location. Schematic diagrams are used to troubleshoot and install control circuits. Schematics are generally easier to read and understand than are wiring diagrams.

Wiring diagrams show components mounted in their general location with connect- ing wires. A wiring diagram is used to represent how the circuit generally appears. To help illustrate the differences between wiring diagrams and schematics, a basic control circuit will first be explained as a schematic and then shown as a wiring diagram.

Reading Schematic Diagrams

To read a schematic diagram, a few rules must first be learned. Commit the following rules to memory:

1. Reading a schematic diagram is similar to reading a book in English. It is read from left to right and from top to bottom.

2. Electrical symbols are always shown in their off or deenergized position.

3. Relay contact symbols are shown with the same numbers or letters that are used to designate the relay coil. All contact symbols that have the same number or letter as a coil are controlled by that coil regardless of where in the circuit they are located.

4. When a relay is energized, or turned on, all of its contacts change position. If a contact is shown as normally open, it will close when the coil is energized. If the contact is shown normally closed, it will open when the coil is turned on.

5. There must be a complete circuit before current can flow through a component.

6. Components used to provide a function of stop are generally wired normally closed and connected in series. Figure 19–6 illustrates this concept. Both switches A and B are normally closed and connected in series. If either switch is opened, connection to the lamp will be broken and current will stop flowing in the circuit.

7. Components used to provide the function of start are generally wired normally open and connected in parallel. In Figure 19–7, switches A and B are normally open and connected in parallel with each other. If either switch is closed, a current path will be provided for the lamp and it will turn on.

The circuit to be discussed is a basic control circuit used throughout industry. Figure 19–8 shows a start–stop push button circuit. This schematic shows both the control circuit and the motor circuit. Schematic diagrams do not always show both control and motor connections. Many schematic diagrams show only the control circuit.

Notice in this schematic that there is no complete circuit to M motor starter coil because of the open-start push button and open M auxiliary contacts. There is also no connection to

the motor because of the open M load contacts. The open M contacts connected in parallel with the start button are small contacts intended to be used as part of the control circuit. This set of contacts is generally referred to as the holding, sealing, or maintaining contacts. These contacts are used to provide a continued circuit to M coil when the start button is released.

The second set of M contacts is connected in series with the overload heater element and the motor, and the contacts are known as load contacts. These contacts are large and designed to carry the current needed to operate the load. Notice that these contacts are normally open and there is no current path to the motor.

When the start button is pushed, a path for current flow is provided to M motor starter coil. When M coil energizes, both M contacts close (Figure 19–9). The small auxiliary contact provides a continued current path to the motor starter coil when the start button is released and returns to its open position. The large M load contact closes and provides a complete circuit to the motor and the motor begins to run. The motor will continue to operate in this manner as long as M coil remains energized.

If the stop button is pushed (Figure 19–10), the current path to M coil is broken and the coil deenergized. This causes both M contacts to return to their normally open position. When M holding contacts open, there is no longer a complete circuit provided to the coil when the stop button is returned to its normal position. The circuit remains in the off position until the start button is again pushed.

Notice that the overload contact is connected in series with the motor starter coil. If the overload contact opens, it has the same effect as pressing the stop button. The fuse is connected in series with both the control circuit and the motor. If the fuse should open, it has the effect of disconnecting power from the line.

A wiring diagram for the start–stop push button circuit is shown in Figure 19–11. Although this diagram looks completely different, it is the same electrically as the schematic diagram. Notice that the push button symbols indicate double-acting push buttons. The stop

button, however, uses only the normally closed section, and the start button uses only the normally open section. The motor starter shows three load contacts and two auxiliary contacts. One auxiliary contact is open and one is closed, but only the open contact is used.

The overload relay shows two different sections. One section contains the thermal heater element connected in series with the motor. The other section contains the normally closed contact connected in series with the coil of M starter.

Control Circuits

TWO-WIRE CIRCUITS

Control circuits are divided into two basic types: the two-wire and the three-wire. Figure 19–1 shows a simple two-wire control circuit. In this circuit, a simple switch is used to control the power applied to the coil of a motor starter. If the switch is open, there is no complete path for current flow, and the motor will not operate. If the switch is closed, power is supplied to the motor starter and it closes the M contacts, connecting the three- phase motor to the power line.

THREE-WIRE CIRCUITS

Three-wire control circuits are used because they are more flexible than two-wire circuits. Three-wire circuits are characterized by the fact that they are operated by a magnetic relay or motor starter. These circuits are generally controlled by one or more pilot devices.

ELECTRICAL SYMBOLS FOR PILOT/CONTROL DEVICES

When people first begin to learn to read, they learn a set of symbols that are used to represent different sounds. This set of symbols is generally referred to as the alphabet. When learning to read electrical diagrams, it is necessary to learn the symbols used to rep- resent different devices and components. The symbols that follow are commonly used on control schematics and wiring diagrams. These are not all the symbols used. Unfortunately, there is no real set standard for the use of electrical symbols. Most of these symbols are approved by the National Electrical Manufacturers Association (NEMA). These symbols are as follows:

The contact symbols shown are standard and relatively simple to understand. There can be instances, however, in which symbols can be used to show something that is not at first apparent. For example, the symbol for a normally closed pressure switch is shown in Figure 19–2. Notice that this symbol shows not only the movable arm making contact with the stationary contact but also the movable arm drawn above the stationary contact. Figure 19–3 shows a normally closed held open pressure switch. This symbol is not a normally open contact symbol even though the contact arm is shown not making connection with the stationary contact because pressure is used to keep the contact open. If pressure decreases to a certain point, the switch contact will close.

Figure 19–4 shows a normally open pressure switch. Notice that the contact arm is drawn below the stationary contact. Figure 19–5 shows the same symbol except that the movable arm is making connection with the stationary contact. This symbol represents a normally open held closed pressure switch, that is, a pressure switch that is wired normally open but has its contact held closed by pressure. If the pressure decreases to a certain level, the switch will open and break connection to the rest of the circuit.

SCHEMATIC AND WIRING DIAGRAMS

Schematic diagrams show components in their electrical sequence without respect to physical location. Schematic diagrams are used to troubleshoot and install control circuits. Schematics are generally easier to read and understand than are wiring diagrams.

Wiring diagrams show components mounted in their general location with connect- ing wires. A wiring diagram is used to represent how the circuit generally appears. To help illustrate the differences between wiring diagrams and schematics, a basic control circuit will first be explained as a schematic and then shown as a wiring diagram.

Reading Schematic Diagrams

To read a schematic diagram, a few rules must first be learned. Commit the following rules to memory:

1. Reading a schematic diagram is similar to reading a book in English. It is read from left to right and from top to bottom.

2. Electrical symbols are always shown in their off or deenergized position.

3. Relay contact symbols are shown with the same numbers or letters that are used to designate the relay coil. All contact symbols that have the same number or letter as a coil are controlled by that coil regardless of where in the circuit they are located.

4. When a relay is energized, or turned on, all of its contacts change position. If a contact is shown as normally open, it will close when the coil is energized. If the contact is shown normally closed, it will open when the coil is turned on.

5. There must be a complete circuit before current can flow through a component.

6. Components used to provide a function of stop are generally wired normally closed and connected in series. Figure 19–6 illustrates this concept. Both switches A and B are normally closed and connected in series. If either switch is opened, connection to the lamp will be broken and current will stop flowing in the circuit.

7. Components used to provide the function of start are generally wired normally open and connected in parallel. In Figure 19–7, switches A and B are normally open and connected in parallel with each other. If either switch is closed, a current path will be provided for the lamp and it will turn on.

The circuit to be discussed is a basic control circuit used throughout industry. Figure 19–8 shows a start–stop push button circuit. This schematic shows both the control circuit and the motor circuit. Schematic diagrams do not always show both control and motor connections. Many schematic diagrams show only the control circuit.

Notice in this schematic that there is no complete circuit to M motor starter coil because of the open-start push button and open M auxiliary contacts. There is also no connection to

the motor because of the open M load contacts. The open M contacts connected in parallel with the start button are small contacts intended to be used as part of the control circuit. This set of contacts is generally referred to as the holding, sealing, or maintaining contacts. These contacts are used to provide a continued circuit to M coil when the start button is released.

The second set of M contacts is connected in series with the overload heater element and the motor, and the contacts are known as load contacts. These contacts are large and designed to carry the current needed to operate the load. Notice that these contacts are normally open and there is no current path to the motor.

When the start button is pushed, a path for current flow is provided to M motor starter coil. When M coil energizes, both M contacts close (Figure 19–9). The small auxiliary contact provides a continued current path to the motor starter coil when the start button is released and returns to its open position. The large M load contact closes and provides a complete circuit to the motor and the motor begins to run. The motor will continue to operate in this manner as long as M coil remains energized.

If the stop button is pushed (Figure 19–10), the current path to M coil is broken and the coil deenergized. This causes both M contacts to return to their normally open position. When M holding contacts open, there is no longer a complete circuit provided to the coil when the stop button is returned to its normal position. The circuit remains in the off position until the start button is again pushed.

Notice that the overload contact is connected in series with the motor starter coil. If the overload contact opens, it has the same effect as pressing the stop button. The fuse is connected in series with both the control circuit and the motor. If the fuse should open, it has the effect of disconnecting power from the line.

A wiring diagram for the start–stop push button circuit is shown in Figure 19–11. Although this diagram looks completely different, it is the same electrically as the schematic diagram. Notice that the push button symbols indicate double-acting push buttons. The stop

button, however, uses only the normally closed section, and the start button uses only the normally open section. The motor starter shows three load contacts and two auxiliary contacts. One auxiliary contact is open and one is closed, but only the open contact is used.

The overload relay shows two different sections. One section contains the thermal heater element connected in series with the motor. The other section contains the normally closed contact connected in series with the coil of M starter.

WOUND-ROTOR MOTOR CONTROL

An automatic time-operated speed control for a wound-rotor motor is shown in Figure 19–20. In this circuit, the stator of the motor is connected to the line when M load contacts close. Resistors are connected to the rotor of the motor during starting. When the motor is first started, all resistors are connected in series with the rotor. It will be assumed that all timers have been set for a delay of 10 seconds.

When the start button is pushed, a circuit is completed through M coil and TR1 coil. When M contacts close, the stator of the motor is connected to the line, and M auxiliary contact is used as a holding contact. Ten seconds after TR1 coil is energized, TR1 contact closes. This energizes S1 coil and TR2 coil. When S1 coil energizes, S1 contacts close and short out the first set of resistors. This permits the motor to increase to the second step of speed.

Ten seconds after coil TR2 is energized, contact TR2 closes and energizes coils S2 and TR3 . When S2 energizes, S2 contacts close and short out the second set of resistors. This permits the motor to increase to the third step of speed.

Ten seconds after coil TR3 energizes, contact TR3 closes and energizes coil S3 . This causes S3 contacts to close and short out the remaining resistance and the motor operates at its full speed.

SUMMARY

• The two basic types of control circuits are two-wire and three-wire.

• A schematic diagram shows components in their electrical sequence without respect to physical location.

• A wiring diagram shows components in their physical location with connecting wires.

• Control diagrams are read like a book, from top to bottom and from left to right.

• Control components are always shown in their normal deenergized position.

• Interlocking is used to keep some function or action from happening until some other function or action has taken place.

• Jogging is used to position a machine in some position by starting it with short jabs of power.

• On-delay relays delay changing their contacts when the coil is energized, but change back immediately when the coil is deenergized.

• Off-delay relays change their contacts immediately when they are energized but delay changing them back when they are deenergized.

Achievement Review

1. What are the two basic types of motor control?

2. Define a schematic diagram.

3. Define a wiring diagram.

4. Determine how components for the function of stop are generally wired and connected with the stop button.

5. Determine how components used for the function of start are generally wired and connected with the start button.

6. When reading a schematic diagram, are the components shown in their energized or deenergized position?

7. What does a dashed line drawn between components represent?

8. What is an auxiliary contact?

9. Explain the operation of an on-delay timer.

Explain the operation of an off-delay time

WOUND-ROTOR MOTOR CONTROL

An automatic time-operated speed control for a wound-rotor motor is shown in Figure 19–20. In this circuit, the stator of the motor is connected to the line when M load contacts close. Resistors are connected to the rotor of the motor during starting. When the motor is first started, all resistors are connected in series with the rotor. It will be assumed that all timers have been set for a delay of 10 seconds.

When the start button is pushed, a circuit is completed through M coil and TR1 coil. When M contacts close, the stator of the motor is connected to the line, and M auxiliary contact is used as a holding contact. Ten seconds after TR1 coil is energized, TR1 contact closes. This energizes S1 coil and TR2 coil. When S1 coil energizes, S1 contacts close and short out the first set of resistors. This permits the motor to increase to the second step of speed.

Ten seconds after coil TR2 is energized, contact TR2 closes and energizes coils S2 and TR3 . When S2 energizes, S2 contacts close and short out the second set of resistors. This permits the motor to increase to the third step of speed.

Ten seconds after coil TR3 energizes, contact TR3 closes and energizes coil S3 . This causes S3 contacts to close and short out the remaining resistance and the motor operates at its full speed.

SUMMARY

• The two basic types of control circuits are two-wire and three-wire.

• A schematic diagram shows components in their electrical sequence without respect to physical location.

• A wiring diagram shows components in their physical location with connecting wires.

• Control diagrams are read like a book, from top to bottom and from left to right.

• Control components are always shown in their normal deenergized position.

• Interlocking is used to keep some function or action from happening until some other function or action has taken place.

• Jogging is used to position a machine in some position by starting it with short jabs of power.

• On-delay relays delay changing their contacts when the coil is energized, but change back immediately when the coil is deenergized.

• Off-delay relays change their contacts immediately when they are energized but delay changing them back when they are deenergized.

Achievement Review

1. What are the two basic types of motor control?

2. Define a schematic diagram.

3. Define a wiring diagram.

4. Determine how components for the function of stop are generally wired and connected with the stop button.

5. Determine how components used for the function of start are generally wired and connected with the start button.

6. When reading a schematic diagram, are the components shown in their energized or deenergized position?

7. What does a dashed line drawn between components represent?

8. What is an auxiliary contact?

9. Explain the operation of an on-delay timer.

Explain the operation of an off-delay time

INTERLOCKING

Interlocking is used to prevent some function from happening until some other function or action has occurred. A good example of interlocking is shown in the circuit in Figure 19–12. This circuit is a forward–reverse control. The two normally closed contacts labeled F and R are used to interlock the system. In this circuit, the motor is reversed by changing two of the input lines to the stator. If both motor starters F and R are energized

at the same time, there would be a direct short circuit across two of the phases. Forward– reverse starters are generally interlocked mechanically as well as electrically, but this example shows only electrical interlock.

It will first be assumed that the motor is to be operated in the forward direction. When the forward push button is pressed, a circuit is completed to F coil. This causes all F con- tacts to change position. The normally open F load contacts close and connect the motor to the line. The normally open auxiliary contact closes to complete the circuit around the normally open F push button when it is released, and the normally closed F contact opens. When the normally open F contact opens, it breaks the circuit to R coil. If R push button were to be pressed, R coil could not energize because of the now open contact connected in series with it.

Before the motor can be reversed, the stop button must be pushed and F coil de- energized. This permits all F contacts to return to their normal position. If the reverse push button is now pressed, current can flow through the normally closed F contact and energize R coil. When R coil energizes, all R contacts change position. The R load contacts connect the motor to the line, the normally open R contact closes to complete a circuit around R push button, and the normally closed R contact connected in series with F coil opens. This prevents F coil from being energized if F push button should be pressed.

JOGGING

Jogging in this context is moving a machine with short jabs of power. It is used to bring a machine into some particular position, generally for loading or unloading. Another term, inching, is very similar to jogging. The difference is that jogging applies full volt- age to the motor and inching applies reduced voltage to the motor. Regardless of which method is used, the basic control requirement for either is for the motor starter to energize when the jog button is pushed and deenergize when the jog button is released. This control function can be accomplished by several methods. One method is shown in Figure 19–13. In this circuit, a double-acting push button is used as the jog button. When the jog button is pressed, the normally open section of the button closes to complete a circuit to M coil. The normally closed section opens to prevent the normally open M contact from holding the coil in after the button is released. This permits M coil to deenergize and disconnect the motor from the line. If the start button is pressed, M coil will energize and close all M contacts. The normally open auxiliary contact can now provide a current path through the closed push button to keep the coil energized.

TIMERS

There are two basic types of time-delay relays: the on delay and the off delay. The on-delay timer is often referred to on a schematic as DOE, which stands for delay on energize. Off-delay timers are often referred to as DODE, which stands for delay on de- energize. Timers may contain only time-operated contacts or a combination of both time- operated and instantaneous contacts. Time-operated contacts are operated by the timer mechanism, and instantaneous contacts operate like normal relay contacts.

On-Delay Timer

The operation of an on-delay timer is as follows. When the coil is energized, the contacts delay changing from their normal position. When the coil is deenergized, they change back to their normal position immediately.

The circuit shown in Figure 19–14 illustrates the operation of an on-delay timer. It is assumed that the timer has been set for a delay of 10 seconds. When the normally open push button is pressed, a circuit is completed to TR coil. The normally open contacts connected around the start push button are instantaneous contacts. These contacts close immediately and act as holding contacts to keep the relay energized. A normally open time-operated contact is connected in series with a pilot light. This contact will close 10 seconds after TR coil has been energized. When the stop button is pressed, the circuit to TR coil is opened and the relay deenergizes. All TR contacts open immediately. This disconnects the pilot light from the line.

Off-Delay Timer

The off-delay timer functions as follows. When the relay coil is energized, the contacts change position immediately. When the coil is deenergized, the contacts delay changing back to their normal position.

The circuit in Figure 19–15 illustrates the operation of an off-delay timer. A nor- mally open instantaneous contact is used as the holding contact around the start button. A normally open time-operated contact is connected in series with a pilot light. When the start button is pressed, TR coil energizes and both TR contacts close immediately. When the stop button is pressed, TR coil is deenergized, but the normally open time-operated contact remains closed for 10 seconds before it reopens.

Types of Timers

There are several methods that can be used to achieve a time delay. One type of timer is known as a pneumatic timer because it uses air to provide a time delay. Figure 19–16 illustrates this type of timer. When rod A pushes against the bellows, air is forced out the check valve. Retraction of the bellows causes TR contact to close. When rod A is pulled back, the spring pushes outward on the bellows. Before the bellows can expand, however, air must enter through the air inlet. The needle valve controls the rate at which air can

enter the bellows and therefore the time required to expand the bellows and reopen the TR contact.

Dashpot timers use a piston moving through oil to create a time delay. Some timers use electric clocks to provide time delays. Electronic timers have become very popular because they are inexpensive and have good reliability and repeat accuracy.

The circuit shown in Figure 19–17 illustrates an electronic circuit that can be used as an on-delay timer. When switch S1 closes, capacitor C1 begins to charge through resistor RT. When C1 reaches about 10 V, the unijunction transistor turns on. This provides gate current

to the silicon controlled rectifier (SCR). When the SCR turns on, relay coil K1 energizes. The SCR remains turned on until switch S1 is again opened. Two types of electronic timers are shown in Figures 19–18 and 19–19. One is adjustable and the other is not.

INTERLOCKING

Interlocking is used to prevent some function from happening until some other function or action has occurred. A good example of interlocking is shown in the circuit in Figure 19–12. This circuit is a forward–reverse control. The two normally closed contacts labeled F and R are used to interlock the system. In this circuit, the motor is reversed by changing two of the input lines to the stator. If both motor starters F and R are energized

at the same time, there would be a direct short circuit across two of the phases. Forward– reverse starters are generally interlocked mechanically as well as electrically, but this example shows only electrical interlock.

It will first be assumed that the motor is to be operated in the forward direction. When the forward push button is pressed, a circuit is completed to F coil. This causes all F con- tacts to change position. The normally open F load contacts close and connect the motor to the line. The normally open auxiliary contact closes to complete the circuit around the normally open F push button when it is released, and the normally closed F contact opens. When the normally open F contact opens, it breaks the circuit to R coil. If R push button were to be pressed, R coil could not energize because of the now open contact connected in series with it.

Before the motor can be reversed, the stop button must be pushed and F coil de- energized. This permits all F contacts to return to their normal position. If the reverse push button is now pressed, current can flow through the normally closed F contact and energize R coil. When R coil energizes, all R contacts change position. The R load contacts connect the motor to the line, the normally open R contact closes to complete a circuit around R push button, and the normally closed R contact connected in series with F coil opens. This prevents F coil from being energized if F push button should be pressed.

JOGGING

Jogging in this context is moving a machine with short jabs of power. It is used to bring a machine into some particular position, generally for loading or unloading. Another term, inching, is very similar to jogging. The difference is that jogging applies full volt- age to the motor and inching applies reduced voltage to the motor. Regardless of which method is used, the basic control requirement for either is for the motor starter to energize when the jog button is pushed and deenergize when the jog button is released. This control function can be accomplished by several methods. One method is shown in Figure 19–13. In this circuit, a double-acting push button is used as the jog button. When the jog button is pressed, the normally open section of the button closes to complete a circuit to M coil. The normally closed section opens to prevent the normally open M contact from holding the coil in after the button is released. This permits M coil to deenergize and disconnect the motor from the line. If the start button is pressed, M coil will energize and close all M contacts. The normally open auxiliary contact can now provide a current path through the closed push button to keep the coil energized.

TIMERS

There are two basic types of time-delay relays: the on delay and the off delay. The on-delay timer is often referred to on a schematic as DOE, which stands for delay on energize. Off-delay timers are often referred to as DODE, which stands for delay on de- energize. Timers may contain only time-operated contacts or a combination of both time- operated and instantaneous contacts. Time-operated contacts are operated by the timer mechanism, and instantaneous contacts operate like normal relay contacts.

On-Delay Timer

The operation of an on-delay timer is as follows. When the coil is energized, the contacts delay changing from their normal position. When the coil is deenergized, they change back to their normal position immediately.

The circuit shown in Figure 19–14 illustrates the operation of an on-delay timer. It is assumed that the timer has been set for a delay of 10 seconds. When the normally open push button is pressed, a circuit is completed to TR coil. The normally open contacts connected around the start push button are instantaneous contacts. These contacts close immediately and act as holding contacts to keep the relay energized. A normally open time-operated contact is connected in series with a pilot light. This contact will close 10 seconds after TR coil has been energized. When the stop button is pressed, the circuit to TR coil is opened and the relay deenergizes. All TR contacts open immediately. This disconnects the pilot light from the line.

Off-Delay Timer

The off-delay timer functions as follows. When the relay coil is energized, the contacts change position immediately. When the coil is deenergized, the contacts delay changing back to their normal position.

The circuit in Figure 19–15 illustrates the operation of an off-delay timer. A nor- mally open instantaneous contact is used as the holding contact around the start button. A normally open time-operated contact is connected in series with a pilot light. When the start button is pressed, TR coil energizes and both TR contacts close immediately. When the stop button is pressed, TR coil is deenergized, but the normally open time-operated contact remains closed for 10 seconds before it reopens.

Types of Timers

There are several methods that can be used to achieve a time delay. One type of timer is known as a pneumatic timer because it uses air to provide a time delay. Figure 19–16 illustrates this type of timer. When rod A pushes against the bellows, air is forced out the check valve. Retraction of the bellows causes TR contact to close. When rod A is pulled back, the spring pushes outward on the bellows. Before the bellows can expand, however, air must enter through the air inlet. The needle valve controls the rate at which air can

enter the bellows and therefore the time required to expand the bellows and reopen the TR contact.

Dashpot timers use a piston moving through oil to create a time delay. Some timers use electric clocks to provide time delays. Electronic timers have become very popular because they are inexpensive and have good reliability and repeat accuracy.

The circuit shown in Figure 19–17 illustrates an electronic circuit that can be used as an on-delay timer. When switch S1 closes, capacitor C1 begins to charge through resistor RT. When C1 reaches about 10 V, the unijunction transistor turns on. This provides gate current

to the silicon controlled rectifier (SCR). When the SCR turns on, relay coil K1 energizes. The SCR remains turned on until switch S1 is again opened. Two types of electronic timers are shown in Figures 19–18 and 19–19. One is adjustable and the other is not.

STEPPING MOTORS

Stepping motors are devices that convert electrical impulses into mechanical movement. Stepping motors differ from other types of dc or ac motors in that their output shaft moves through a specific angular rotation each time the motor receives a pulse. Each time a pulse is received, the motor shaft moves by a precise amount. The stepping motor allows a load to be controlled with regard to speed, distance, or position. These motors are very accurate in their control performance. Generally, less than 5% error per angle of rotation exists, and this error is not cumulative regardless of the number of rotations. Stepping motors are operated on dc power but can be used as a two-phase synchronous motor when connected to ac power.

Theory of Operation

Stepping motors operate on the theory that like magnetic poles repel and unlike magnetic poles attract. Consider the circuit shown in Figure 18–39. In this illustration, the rotor is a permanent magnet and the stator winding consists of two electromagnets. If cur- rent flows through the winding of stator pole A in such a direction that it creates a north

magnetic pole, and through B in such a direction that it creates a south magnetic pole, it would be impossible to determine the direction of rotation. In this condition, the rotor could turn in either direction.

Now consider the circuit shown in Figure 18–40. In this circuit, the motor contains four stator poles instead of two. The direction of current flow through stator pole A is still in such a direction as to produce a north magnetic field; the current flow through pole B

produces a south magnetic field. The current flow through stator pole C, however, produces a south magnetic field, and the current flow through pole D produces a north magnetic field. As illustrated, there is no doubt regarding the direction or angle of rotation. In this example, the rotor shaft will turn 90° in a counterclockwise direction.

Figure 18–41 shows yet another condition. In this example, the current flow through poles A and C is in such a direction as to form a north magnetic pole, and the direction of current flow through poles B and D forms south magnetic poles. In this illustration, the permanent-magnet rotor has rotated to a position between the actual pole pieces.

To allow for better stepping resolution, most stepping motors have eight stator poles, and the pole pieces and rotor have teeth machined into them as shown in Figure 18–42. In practice, the number of teeth machined in the stator and rotor determines the angular rotation achieved each time the motor is stepped. The stator–rotor tooth configuration shown in Figure 18–42 produces an angular rotation of 1.8° per step.

Winding

There are different methods for winding stepper motors. A standard three-lead motor is shown in Figure 18–43. The common terminal of the two windings is con-nected to ground of an aboveground–belowground power supply. Terminal 1 is connected to the common of a single-pole, double-throw switch (switch 1), and terminal 3 is connected to the common of another single-pole, double-throw switch (switch 2). One of the stationary contacts of each switch is connected to the positive or aboveground voltage,

and the other stationary contact is connected to the negative or belowground voltage. The polarity of each winding is determined by the position setting of its control switch.

Stepping motors can also be wound bifilar, as shown in Figure 18–44. The term bifilar means that there are two windings wound together. This is similar to a transformer winding with a center-tap lead. A bifilar stepping motor has twice as many windings as does the three-lead type, which makes it necessary to use smaller wire in the windings. This results in higher wire resistance in the winding, producing a better inductive–resistive (L/R) time constant for the bifilar wound motor. The increased L/R time constant results in better motor performance. The use of a bifilar stepper motor also simplifies the drive circuitry requirements. Notice that the bifilar motor does not require an aboveground–belowground power supply. As a general rule, the power supply voltage should be about five times greater than the motor voltage. A current-limiting resistance is used in the common lead of the motor. This current-limiting resistor also helps improve the L/R time constant.

Four-Step Switching (Full Stepping)

The switching arrangement shown in Figure 18–44 can be used for a four-step sequence. Each time one of the switches changes position, the rotor will advance one-fourth of a tooth. After four steps, the rotor has turned the angular rotation of one “full” tooth. If the rotor and stator have 50 teeth, it will require 200 steps for the motor to rotate one full revolution. This corresponds to an angular rotation of 1.8° per step (360°/200 steps 1.8° per step). Figure 18–45 illustrates the switch positions for each step.

Eight-Step Switching (Half-Stepping)

Figure 18–46 illustrates the connections for an eight-stepping sequence. In this arrangement, the center-tap lead for phases A and B are connected through their own separate current-limiting resistors back to the negative of the power supply. This circuit contains four separate single-pole switches instead of two switches. The advantage of this arrangement is that each step causes the motor to rotate one-eighth of a tooth instead of one-fourth of a tooth. The motor now requires 400 steps to produce one revolution, which

produces an angular rotation of 0.9° per step. This results in better stepping resolution and greater speed capability. The chart in Figure 18–47 illustrates the switch position for each step. Figure 18–48 depicts a solid-state switching circuit for an eight-step switching arrangement. A stepping motor is shown in Figure 18–49.

AC Operation

Stepping motors can be operated on ac voltage. In this mode of operation, they become two-phase ac synchronous constant-speed motors and are classified as permanent-magnet induction motors. Refer to the exploded diagram of a stepping motor in Figure 18–50. Notice that this motor has no brushes, slip rings, commutator, gears, or belts. Bearings maintain a constant airgap between the permanent-magnet rotor and the stator windings. A typical eight-stator pole-stepping motor will have a synchronous speed of 72 r/min when connected to a 60-Hz two-phase ac power line.

A resistive–capacitive network can be used to provide the 90° phase shift needed to change single-phase ac into two-phase ac. A simple forward–off–reverse switch can be added to provide directional control. A sample circuit of this type is shown in Figure 18–51. The correct values of resistance and capacitance are necessary for proper opera- tion. Incorrect values can result in random direction of rotation when the motor is started, change of direction when the load is varied, and erratic and unstable operation, as well as failure of the motor to start. The correct values of resistance and capacitance will be different with different stepping motors. The manufacturer’s recommendations should be followed for the particular type of stepping motor used.

Motor Characteristics

When stepping motors are used as two-phase synchronous motors, they have the ability to virtually start, stop, or reverse direction of rotation instantly. The motor will start within about 11⁄2 cycles of the applied voltage and stop within 5 to 25 ms. The motor can maintain a stalled condition without harm to it. Because the rotor is a permanent magnet, no induced current is in the rotor, and no high inrush of current occurs when the motor is started. The starting and running currents are the same. This simplifies the power requirements of the circuit used to supply the motor. Because of the permanent- magnet structure of the rotor, the motor does provide holding torque when turned off. If more holding torque is needed, dc voltage can be applied to one or both windings when the motor is turned off. An example circuit of this type is shown in Figure 18–52. If dc voltage is applied to one winding, the holding torque will be approximately 20% greater than the rated torque of the motor. If dc voltage is applied to both windings, the holding torque will be about 11⁄2 times greater than the rated torque.

SUMMARY

• Types of single-phase motors (split-phase)

1. Single-phase induction motor

2. Repulsion motor

3. Repulsion-induction motor

4. Series motor (universal motor)

5. Shaded-pole motor

• Three-phase motors generally perform better than do single-phase motors. In many instances, however, only single-phase service is available.

• Most single-phase motors have fractional horsepower ratings. In general, their use is limited to commercial and residential applications.

• Resistance-start, induction-run motor

1. A resistance-start, induction-run motor has the following basic parts:

a. A stator (stationary part).

b. A rotor (revolving part).

c. A centrifugal switch, located inside the motor.

d. Two end shields bolted to the steel frame; these shields house the rotor shaft bearings.

e. A cast steel frame; the stator core is pressed into this frame.

2. The stator contains two windings, which are placed 90 electrical degrees apart.

a. The main or running winding

b. The starting or auxiliary winding

3. At start-up, both windings are connected in parallel to the single-phase line. Once the motor accelerates to two-thirds or three-quarters of the rated speed, a centrifugal switch disconnects the starting winding automatically.

4. The rotor of the resistance-start, induction-run motor is the same as that of a three- phase, squirrel-cage induction motor.

a. The rotor contains no windings, brushes, slip rings, or commutator where faults may develop.

b. Fans are provided with the rotor to keep the temperature of the windings at a reasonable level.

5. The centrifugal switch is mounted inside the motor. It disconnects the starting winding after the rotor reaches approximately 66% to 75% of its rated speed.

6. The running winding has a low resistance and a high inductive reactance. Thus, the current of the running winding lags behind the voltage.

7. The current in the starting winding is more nearly in phase with the voltage because the winding has a high resistance and a low inductive reactance.

8. When a current passes through each of these windings, the resulting pulsating field effect gives rise to a rotating field around the inside of the stator core.

9. While traveling at the synchronous speed, S 120 f /P, the rotating field cuts the copper bars of the squirrel-cage winding (rotor). Voltages are induced in the winding and cause currents in the rotor bars.

10. A rotor field is created. This field reacts with the stator field to develop the torque that causes the rotor to turn.

11. As the rotor nears the rated speed, the centrifugal switch disconnects the start- ing winding from the line. The motor then continues to operate, using only the running winding and the current induced in the rotor. The alternating voltages produced generate a torque that keeps the rotor turning.

12. The motor resembles a two-phase induction motor because the currents of the windings are approximately 90 electrical degrees out of phase with each other. However, a single-phase source supplies the motor. Therefore, the motor is called a split-phase motor.

13. If the motor does not start, but a low humming sound is present, then one wind- ing is open. The centrifugal switch should be checked to determine whether its mechanism is faulty or whether the switch contacts are pitted.

14. If the motor operates on its starting winding for more than 60 seconds, the winding may char or burn out.

15. To reverse the rotation of the motor, interchange the leads of either the starting winding or the running winding (preferably the starting winding). Interchanging these leads reverses the direction of rotation of the field.

16. Dual-voltage motors

a. Some single-phase motors have dual-voltage ratings such as 115/230 V. The running winding consists of two sections, each of which is rated at 115 V.

b. The windings are marked “T1 and T2” and “T3 and T4 .”

c. For the higher voltage, the two windings are connected in series across 230 V.

d. For the lower voltage, the windings are connected in parallel across 115 V.

e. The starting winding consists of one winding rated at 115 V.

f. It is connected in parallel across 115 V with the two running windings.

g. For the higher voltage, the starting winding is connected in parallel across one of the series running windings.

17. Speed regulation

a. A resistance-start, induction-run motor has very good speed regulation from no load to full load.

b. The regulation compares to that of a three-phase, squirrel-cage induction motor.

c. The percent slip ranges from 4% to 6%.

18. Starting torque

a. The starting torque of the motor is poor.

b. A phase angle of 30° to 50° is large enough to provide a weak rotating mag- netic field and the starting torque is small.

• Capacitor-start, induction-run motor

1. This type of motor is similar to the resistance-start, induction-run motor. A capacitor is connected in series with the starting winding. The capacitor is mounted in or on the motor.

2. The starting torque is higher than that of a resistance-start motor.

3. The capacitor limits the starting surge of current to a lower value as compared to the resistance-start motor.

4. This motor is used in refrigeration units, compressors, oil burners, small-machine equipment, and any application where split-phase induction motors are used.

5. The basic operation of this motor is nearly the same as that of the resistance-start, induction-run motor. The capacitor is energized for two or three seconds only at start-up. Thus, it cannot improve the power factor.

6. Defective capacitors often cause problems in the motors using them.

a. The capacitor may short-circuit and blow the fuse on the branch motor circuit.

b. If the fuse rating is high, and the fuse does not interrupt the power supply to the motor, the starting winding may burn out.

• Multispeed motors

1. Two basic types of motors are used for multispeed applications:

a. The consequent pole motor.

b. The capacitor-start, capacitor-run motor.

2. The consequent pole motor changes speed by changing the number of stator poles. This causes the synchronous speed of the rotating magnetic field to change.

3. The capacitor-start, capacitor-run (CSCR) type of multispeed motor changes speed by inserting impedance in series with the run winding.

4. Multispeed motors commonly have from two to five ranges of speed.

5. Capacitor-start, capacitor-run motors designed for use as multispeed motors have higher impedance windings than other CSCR motors.

• Repulsion-type motors

1. There are three types of repulsion motors:

a. The repulsion motor

b. The repulsion-start, induction-run motor

c. The repulsion-induction motor

2. These three types of motors differ in their construction, operating characteristics, and industrial applications.

a. Repulsion motor

(1) The stator is usually wound with four, six, or eight poles.

(2) The rotor is a slotted core containing a winding. The rotor is similar to the armature of a dc motor. The coils are connected to the commutator through segments, or bars, parallel to the armature shaft.

(3) Carbon brushes contact the commutator surface. The brush holders are movable. They are connected together by heavy copper jumpers. The brushes are movable so that they can make contact at different points to obtain the correct rotation and maximum torque output.

(4) To reverse the motor, the brushes are shifted 15 electrical degrees to the other side of the stator field pole centers.

(5) This motor has excellent starting torque.

(6) The speed of the motor can be changed by changing the value of the supply voltage.

(7) The motor has very unstable speed characteristics. It may race to a very high speed if there is no mechanical load.

b. Repulsion-start, induction-run motor

(1) This motor is available in two basic types: the brush-lifting motor and the brush-riding motor.

(2) For the brush-lifting motor, a centrifugal device lifts the brushes from the commutator surface at 75% of the rated speed. A short-circuiting necklace shorts out the commutator. The brushes are lifted from the commutator, thus saving wear and tear on all parts.

(3) The brush-riding type of motor has the same short-circuiting neck- lace, but the brushes ride on the commutator at all times.

(4) This motor starts like a repulsion motor. After it reaches 75% of the rated speed, it runs as an induction motor.

(5) To reverse the direction of rotation, follow the steps outlined for a repul- sion motor.

(6) These motors may be designed for dual-voltage connections.

c. Repulsion-induction motor

(1) This type of motor has the same characteristics as a repulsion-start, induction-run motor. It does not have a centrifugal mechanism.

(2) It does have a squirrel-cage winding beneath the slots of the armature.

The squirrel-cage winding improves the speed regulation from no load to full load.

(3) The speed for this type of motor is similar to that of the dc compound motor.

(4) The motor may be designed for dual-voltage windings.

• The same schematic diagram symbol is used for the repulsion-induction motor, the repulsion motor, and the repulsion-start, induction-run motor.

• The dc series or shunt motor cannot be operated from an ac supply.

• Universal motor

1. The field poles of a universal motor are laminated to reduce eddy currents.

2. The voltage loss across the field poles is reduced by using a small number of field turns on a low-reluctance core.

3. A universal motor can be operated on an ac or dc supply.

4. Universal motors are available with fractional horsepower ratings and are commonly used in household appliances.

5. The field windings and the armature of a universal motor are connected in series.

6. There are two versions of the universal distributed field motor: the two-field motor and the single-field compensated motor. The two-field motor has a main winding and a compensating winding to reduce the reactance voltage developed in the armature by the alternating flux.

7. The speed of a universal motor can be controlled by varying the inductance through taps on one of the field poles. The motor may be operated at an excessive speed with no load. Universal motors are permanently connected to the devices being driven.

8. To reverse the direction of rotation, the current flow is reversed through the field circuit or the armature circuit.

• Conductive and inductive compensation

1. Conductive compensation is achieved by placing special windings in slots cut in the pole faces to overcome the armature reaction under load.

a. The strength of this field increases with an increase in the load current.

b. The compensating winding is connected in series with the series field winding and the armature.

c. A motor with conductive compensation has high starting torque and poor speed regulation.

2. An inductively coupled winding is also used to overcome armature reaction in ac series motors.

a. This winding acts like a short-circuited secondary winding in a transformer.

b. The operating characteristics of an inductively compensated motor are similar to those of the conductively compensated motor.

• Shaded-pole induction motor

1. Small fractional horsepower induction motors may be started by shading coils mounted on each of the stator poles.

2. A standard squirrel-cage rotor is used.

3. The motor does not require a starter mechanism such as a centrifugal switch.

4. The shading coil is a low-resistance copper loop.

5. The shading poles distort the main flux pattern, and therefore set up a rotating magnetic field.

6. The torque produced is small.

7. Shaded poles are used only on motors rated at 1⁄10 hp or smaller.

8. A shaded-pole motor is used in applications where a strong torque is not needed, such as for fans and blowers.

• Variable-speed motors

1. Two types of motors are used for variable-speed control:

a. The capacitor-start, capacitor-run motor

b. The shaded-pole induction motor

2. These motors are generally used because they do not require the use of a centrifugal switch.

3. Motors used for variable-speed control have higher impedance windings than do other types of motors not designed for use as variable-speed motors.

4. There are two general methods used to control the speed of variable-speed motors:

a. Variable-voltage control

b. Inserting impedance in series with the motor winding

5. Two common methods of obtaining variable voltage are

a. A solid-state controller using a triac

b. An autotransformer

6. Only triac controllers designed for use as variable-speed motor controllers should be used.

• Stepping motors

1. Stepping motors generally operate on direct current and are used to produce angular movements in steps.

2. Stepping motors are generally used for position control.

3. Stepping motors can be used as single-phase synchronous motors when connected to two-phase ac.

4. Most stepping motors operate at a synchronous speed of 72 r/min when connected to 60-Hz ac.

5. Stepping motors can produce a holding torque by applying a continuous direct current to their stator windings.

6. Full stepping produces an angular rotation of 1.8° per step for a total of 200 steps per revolution.

7. Half stepping produces an angular rotation of 0.9° per step for a total of 400 steps per revolution.

Achievement Review

1. Explain how a motor is started using the split-phase method.

2. Explain why the starting torque of a capacitor-start, induction-run motor is better than that of a resistance-start, induction-run motor.

3. How is the direction of rotation reversed for each of the following?

a. Resistance-start, induction run motor

b. Capacitor-start, induction-run motor

4. The centrifugal switch contacts fail to close when a resistance-start, induction-run motor is deenergized. Explain what happens when the motor is energized.

5. A resistance-start, induction-run motor is started with a heavy overload. The motor fails to accelerate to a speed high enough to open the centrifugal switch contacts. What will happen?

6. A resistance-start, induction-run motor has a dual-voltage rating of 120/240 V. This motor has two running windings, each rated at 120 V, and one starting wind- ing rated at 120 V. Draw a schematic connection diagram for this motor connected for 240 V.

7. Show the connections for a 120-V, capacitor-start, induction-run motor.

8. a. Compare the operating characteristics of a resistance-start, induction-run motor with those of a capacitor-start, induction-run motor.

b. List three applications for

(1) a resistance-start, induction-run motor.

(2) a capacitor-start, induction run motor.

9. Explain the differences in construction and operation between a capacitor-start, induction-run motor and a capacitor-start, capacitor-run motor.

10. Explain how a repulsion motor operates.

11. a. Explain why torque is not developed by a repulsion motor when the brushes are placed directly in line with the stator pole centers.

b. The brushes are shifted 90 electrical degrees from the position described in part a of this question. Is a torque developed by the motor in this case?

c. At what position should the brushes be placed relative to the stator pole centers to obtain the maximum torque?

12. Explain how a repulsion-start, induction-run motor operates.

13. Explain the difference between a repulsion-start, induction-run motor (brush-lift- ing type) and a repulsion-start, induction-run motor (brush-riding type).

14. How are the following motors reversed?

a. A repulsion motor

b. A repulsion-start, induction-run motor

15. Compare the operating characteristics of a repulsion motor with those of a repulsion- start, induction-run motor.

16. List three practical applications for

a. a repulsion motor.

b. a repulsion-start, induction-run motor.

17. Explain the difference between a repulsion-start, induction-run motor and a repulsion- induction motor.

18. Explain why shunt motors cannot be operated satisfactorily on ac service.

19. What is the difference between an ac series motor and a dc series motor?

20. Why are ac series motors with small fractional horsepower ratings called

universal motors?

21. What is the difference between a series motor with conductive compensation and a series motor with inductive compensation?

22. List four practical applications for ac series motors.

23. Describe the construction and operation of a shaded-pole motor.

24. Where is the shaded-pole motor used?

25. A 120-V, two-pole, 60-Hz, capacitor-start, induction-run motor has a full-load speed of 3450 r/min. Determine

a. the synchronous speed.

b. the percent slip.

26. Name two methods of changing the speed of the rotating magnetic field.

27. What type of multispeed motor changes speed by changing the synchronous speed of the rotating magnetic field?

28. Name a major difference between capacitor-start, capacitor-run motors designed for multispeed use and CSCR motors that are not.

29. What two types of motors are used as variable-speed motors?

30. Why are these types of motors used?

31. Name two methods of controlling the speed of variable-speed motors.

32. What solid-state component is used to control the voltage applied to a variable- speed motor?

33. Explain the difference in operation between a stepping motor and a common dc motor.

34. What is the principle of operation of a stepping motor?

35. Why do stepping motors have teeth machined into the stator poles and rotor?

36. When a stepping motor is connected to ac power, how many phases must be applied to the motor?

37. How many degrees out of phase are the voltages of a two-phase system?

38. What is the synchronous speed of an eight-pole stepping motor when connected to a two-phase, 60-Hz ac line?

39. How can the holding torque of a stepping motor be increased?