Development of Control Circuits part2

6 ·5 DEVELOPMENT OF CIRCUIT 4

The specifications for this circuit are as follows. It must give limit-switch control for forward and reverse running of the motor by the use of momentary-contact limit switches. It must also provide low-voltage protection. The initial start and stop for the control system will be by momentary-contact START and STOP push buttons.

The requirement that START and STOP push buttons be used to initiate a control of the circuit by limit switches would indicate the use of a control relay. The wiring for this is shown in Fig. 6 ·Sa. Contact CR 1 is used to maintain the circuit to the control relay during the running operation of the circuit. Contact CR2 is used to make and break the line circuit to the forward and reverse control circuit, thus satisfying the provision . that the START and STOP buttons initiate and terminate the automatic control of the motor by limit switches. The use of the control relay and its START and STOP buttons also provides low-voltage protection.

The specifications call for the use of momentary-contact limit switches, which would require a three-wire control circuit for forward and reverse. These limit switches necessarily have two sets of contacts, one normally open and the other normally closed. When wired as shown in Fig. 6 ·Sa, the normally closed contact of limit switch 2 would act as the stop for the forward controller, and the normally open contact of limit switch 1 would act as the start contact for the forward controller. The auxiliary contact of the forward starter must be connected in parallel with the normally open contact of limit switch 1 in order to maintain the circuit during the running of the motor in the for­ ward direction.

Figure 6 ·5b shows the additional wiring required for the re­ verse starter. The normally closed contact of limit switch 1 is wired as a stop contact for the reverse starter, and the normally open contact of limit switch 2 is wired as a start contact for the reverse starter. The auxiliary contact on the reverse starter is wired in parallel with the normally open contacts of limit switch 2 to maintain the circuit while the motor is running in reverse.

This circuit satisfies all the requirements of the specifications with the exception of electrical interlock, which is shown in Fig. 6 ·5c. This electrical interlock is accomplished by the addi­ tion of a normally closed contact in series with each starter and operated by the starter for the opposite direction of rotation of the motor.

PI ugging reversal is provided in this circuit by the action of the limit switches themselves. When limit switch 1 is moved from its normal position, the normally open contact closes ener­ gizing coil F and the normally closed contact opens and drops out coil R. The reverse action is performed by limit switch 2, thus providing plugging in either direction.

The circuit of Fig. 6 ·5c would work perfectly and satisfy all the conditions of operation if it were always to stop in a position that would leave either normally open limit switch contact closed. This is not very likely to be the case, however, and there­ fore we must provide some means of starting the motor in either forward or reverse in order that the limit switches can take over automatic control. The circuit additions necessary to accomplish this are shown in Fig. 6 ·5d. Here we have added a push button in parallel with the other start components in the forward and reverse circuits. The function of these push buttons is to start the action of the motor in the desired direction so that it can run until the first limit switch is actuated and then will continue to operate automatically until the STOP button is pushed.

Circuits similar to this are used frequently for the control of milling machines and other machine tools which require a repeated forward and reverse action in their operation.

6 ·6 DEVELOPMENT OF CIRCUIT 5

The requirements of this circuit are to add jogging control in both forward and reverse to the circuit of Fig. 6 ·4d. In order to jog a motor, the push button must connect the line to the starter coil while it is held down, causing the motor to run. It must also prevent the auxiliary contact of the starter from maintaining the circuit when the JOG button is released.

The circuit of Fig. 6 ·6 shows two JOG buttons identified as JF and JR. If you follow tqis circuit, you will see that the JOG-FORWARD button has a normally open contact which is con­ nected from the STOP button to the coil side of the auxiliary contact for the forward starter. The normally closed contact

of this push button is connected between the STOP button and all the other control devices. When the JOG-FORWARD button is pressed, the circuit is made from line 1 through its normally open contacts to the coil of the forward starter through the nor­ mally closed electrical interlock. At the same time, the normally closed contact of this JOG button breaks the circuit between the STOP button and all the other push buttons and contacts in the circuit, thus preventing the motor starter from sealing in when the JOG button is released.

The installation and wiring of the JOG-REVERSE button are identical with those of the JOG-FORWARD button, except that it is connected to the rever e starter; its action electrically and mechanically is the same as that for the JOG-FORWARD button.

This circuit incorporates many of the functions of control. It has start and stop in both forward and reverse, manual plug­ ging duty, jog service in forward and reverse, electrical interlock, low-voltage protection, and overload protection .

6 ·7 DEVELOPMENT OF CIRCUIT 6

The assignment here is to add automatic plugging reversal to the circuit of Fig. 6 ·4c. The easiest method of obtaining auto­ matic plugging reversal is by the use of a plugging switch. The connection for this switch is shown in Fig. 6 ·7. The action of this switch is such that when the motor is running in the forward direction, the movable arm of the switch is held in the direction shown by the arrow marked with the letter F. When the STOP button is pressed, the circuit is broken to the forward starter, allowing it to drop its contacts and thus closing the interlock contact marked F2. At this instant, the circuit is made from line 1 through the plugging switch through the normally closed interlock contact F2 to the coil R on the reverse starter. This will plug the motor in the reverse direction. The closing of the maintaining contact R 1 will energize coil R which assures the running of the motor in the reverse direction. Should the STOP button be held down, the flow of current through R 1 to coil R would not be possible and the result is plugging stop for the motor. The action of the plugging switch when the motor is running in reverse is such that its arm is in the position marked R. When the STOP button is pressed, the circuit functions in exactly the same way that it does when the motor is running in forward, except that now the motor is plugged by the energiz­ ing of the forward starter.

6 · 8 DEVELOPMENT OF CIRCUIT 7

This circuit is to control a three-speed motor, and the require­ ment is that it provide selective speed control (Sec. 3 ·9). To satisfy the requirement that the circuit give selective speed con­ trol would indicate the use of three simple start circuits, one circuit for each speed, so that the operator can start the motor in any desired speed. To increase speed, he need only to press · the button for the desired higher speed. Such a circuit is shown in Fig. 6 ·8a.

This circuit, however, ignores any form of interlock which will prevent two speeds from being energized at the same time unless such interlock is provided mechanically in the starter. The necessary electrical interlock has been added in Fig . 6 ·8b. A careful study of this circuit will reveal that it is possible to increase speed by merely pushing the button for the next speed. For instance, if the motor is running in its first speed and it

is desired to increase to the second speed, the normally closed interlock contact identified as M3 will be closed and the coil M2 will be energized. This will break the normally closed con­ tact identified as M2, thus dropping out coil M 1 and deenergiz­ ing the contactor for speed 1.

If this circuit is to function properly, then contact M2 must be built so that it will break before the line contacts of contactor M2 are made. If this is not done, then the starter will energize two speeds at one time, causing damage to the motor and the wiring. The action of the circuit for speed 3 is similar, in that by energizing coil M3 the normally closed contact M3 is broken before the line contacts for speed 3 are made, thus dropping out either speed 1 or speed 2, whichever is energized at the time.

In order to reduce speed, the STOP button must be pressed first. Analyzing the circuit, we see that if we try to go from speed 3 to speed 2, the pressing of the START button for this speed will result in current flowing only as far as contact M3. The same is true if we try to reduce from speed 3 to speed 1. If we try to reduce speed from speed 2 to speed 1, current can flow only as far as contact M2. The pressing of the STOP button drops out any coil that is energized, thus returning all contacts to their normally closed position and allowing the cir­ cuit to be energized in any desired speed.

Chiefly because it is difficult to obtain contact arrangements on starters which will provide the break-before-make action neces­ sary in this circuit for interlocking, starters of this type generally · employ control relays for each speed, which in turn energize the proper coils. The circuit for use of control relays is shown in Fig. 6 ·8c. It should be noted that the circuit for the three control relays is identical with the circuit of Fig. 6 ·8b, with only the addition of a contact on the relay for each contactor coil. Thus, it gives a three-wire control to the control relays and essentially a two-wire control to the contactor coils.

While this circuit has been developed for speed control of a single motor, it is equally applicable to sequence control of three motors. If coils Ml, M2, and M3 were coils of indi­ vidual starters for individual motors, they would start in selective sequence. This means that the operator could start any motor he desired and could progress upward in the sequence of motors at will. To go backward in the sequence, however, he must first stop whatever motor is running and then select the motor de­ sired. This would be selective sequence control of three motors and could be expanded to any number of motors desired.

6 · 9 DEVELOPMENT OF CIRCUIT 8

This circuit will be a modification of circuit 7 to give sequence speed control (Sec. 3 ·9). The requirement of sequence speed control is that the motor be accelerated by pressing the START button for each successive speed in order until the desired speed is reached. Figure 6 ·9 is a circuit to accomplish sequence speed control of a three-speed motor using control relays. The contact arrangement on these relays in this type of service is critical, and it must be pointed out that normally closed contact CR2b must break before normally open contact CR2c makes. Also. contact CR3b must break before contact CR3a makes.

This circuit will not be developed step by step, because a similar circuit for speed control was developed as circuit 7. Rather we shall analyze the operation of this revised circuit. Suppose that the operator wishes to run the machine in its third speed. Then he must first press the button for speed 1, which will energize coil CR 1. Energizing the coil causes contacts CR 1a and CR1b to close. The closing of contact CRlb energizes the contactor for speed 1, and the motor starts and accelerates to this speed. The closing of contact CR 1a sets up the start circuit for speed 2, and when this button is pressed, the circuit is com­ plete to coil CR2, thus energizing this coil and closing contacts CR2a and CR2c. Also, it opens contact CR2b. The opening of contact CR2b drops out the contactor for speed I.

Immediately thereafter, contact CR2c closes, energizing the contactor for speed 2 and allowing the motor to accelerate and run at the second speed. The closing of contact CR2a sets up the start circuit for the third speed. When the start button for the third speed is pressed, the circuit is complete to coil CR3, which in turn first opens contact CR3b, which drops out the contactor for speed 2. Immediately thereafter, contact CR3a closes, thus energizing the contactor for speed 3, which will allow the motor to accelerate and run in speed 3.

It should be noted that the control relays remain energized until the stop button is pressed and that the only way to reduce speed is to press the STOP button and then progressively acceler­ ate the motor, starting with speed 1 and increasing speed as desired. While this circuit is designed for only three speeds, it could be extended to include as many speeds as desired for the motor in question.

This circuit is not presented as the only or most desirable method of providing sequence speed control. There are many factors involved in a design of a control circuit for a given motor and controller. The control man will find many variations of circuits to accomplish the same purpose and should try to develop an overall understanding of the operation of the compo­ nents and circuits which might be used to accomplish an end. Attempting to memorize a circuit to perform any particular func­ tion is a detriment to the student of control.