MOTOR CONTROL

Although the function of motor control is fully covered later in this book, a brief outline of its essentials will be of aid in further study of the various types of motors and their associated control circuits.

The elementary functions of control are starting, stopping, and reversing of the motor. These, however, are only a few of the many contributions which the control renders to efficient operation of indus trial motors.

The most common control functions of industrial motors are:

1. To limit torque on the motor and machine,

2. To limit motor starting current,

3. To protect the motor from overheating,

4. To stop the motor quickly,

5. To regulate speed,

6. Miscellaneous functions.

Limiting Torque

One example of the need for limiting torque is that of a belt-driven motor-operated machine throwing the belt when the motor is started.

The pulleys may be correctly lined up and the belt tension may be correct: yet the belt is thrown off in starting. This is the result of applying torque too quickly at standstill, and can be avoided by limiting the torque on the motor in starting. As another example, the blades on centrifugal fans can be sheared off if too much torque is applied to the fan in starting.

Limiting Starting Current

It is a common sight to see de motors flash over at the commutator when too much current is applied to the motor in bringing it up to speed. Also, it is common to see lights blink when a motor on the same power circuit is started. True, this blinking of lights can be reduced by selecting a motor with the right characteristics, but usually the real sol uti on is the selection of a control that limits the starting current, either by inserting resistance in the circuit or by using a reduced voltage source of power.

Protection from Overheating

Motors are designed to produce full-load torque for a definite period without overheating. While the motor is capable of exceeding its normal output for limited periods, there is nothing inherent in the motor to keep its temperature within safe limits. It is therefore the function of the control to prevent the motor from overheating excessively without shutting it down unnecessarily.

Quick Stopping

Where a driven machine has high inertia, it will continue to run for a considerable time after the power has been disconnected. There are several types of controls, such as electric brakes, to stop a motor quickly. The one most generally used on ac motors is the plugging switch. To plug a motor, it is necessary only to disconnect it from the line, and then reconnect it so that the power applied to the motor tends to drive it in the opposite direction. This brakes the motor rapidly to a standstill, at which time the plugging switch cuts off the reverse power.

Speed Regulation

Fans are sometimes run at various speeds, depending on the ventila tion requirements. For some applications, it is advisable to use a two-speed motor, but where a greater variety of speeds is required, a motor with variable speed control may be the best solution to the problem.

Miscellaneous Control Functions

Adequate control equipment covers various other protective functions which are not as common as those enumerated previously. Among these are rel’erse-phase protection, which prevents a motor from running in the wrong direction if a phase is inadvertently reversed; open-phase protection, which prevents the motor from running on single phase in case a fuse blows, and undervoltage protection, which prevents a motor from starting after a power failure unless started by the operator.

SUMMARY

If the south-seeking (S) pole of a magnet is brought near the S pole of a suspended magnet, the poles repel each other. Likewise, if the two north-seeking (N) poles are brought together, they repel each other. However if a N pole is brought near the S pole or if a S pole is brought near the N pole, the two unlike poles attract each other. In other words, like poles repel each other, and unlike poles attract each other. Experiments have shown these attracting or repelling forces between magnetic poles to vary inversely as the square of the distance between the poles.

Oersted discovered the relation between magnetism and electricity in the early 18th Centruy. He observed that when a wire connecting the poles of a battery was held over a compass needle, the N pole of the needle was deflected toward the west when the current flowed from south to north, and a wire placed under the compass needle caused the N pole of the needle to be deflected toward the east. The compass needle indicates the direction of the magnetic lines of force, and an electric current sets up a magnetic field at right angles to the conductor. The so-called left-hand rule is a convenient method for determining the direction of the magnetic flux around a straight wire carrying a cur rent-if the wire is held in the left hand, with the thumb pointed in the direction of the current, the fingers will point in the direction of the 1nagnetic field. Conversely, if the direction of the magnetic field around a conductor is known, the direction of the current in the conductor can be determined by applying the rule.

An electromagnet is a soft-iron core surrounded by a coil of wire. The magnetic strength of an electromagnet can be changed by changing the strength of its applied current. When the current is interrupted, the iron core returns to its natural state. This loss of magnetism is not complete, however, because a small amount of magnetism, or residual 1nagnetism, remains. The electromagnet is used in many electrical devices, including electric bells, telephones, motors, and generators. The polarity of an electromagnet can be determined by means of the left-hand rule, as follows: Grasp the coil with the left hand, with the fingers pointing in the direction of the current in the coil; the thumb will point to the north pole of the coil.

If a coil of wire having many turns is moved up and down over one pole of a horseshoe magnet, a momentary electric current without an apparent electrical source is produced. This current produced by mov ing the coil of wire in a magnetic field is called an induced current. Lenz ‘s law states that an induced current has such a direction that its magnetic action tends to resist the motion by which it is produced. The generator and the motor are examples of useful applications of induced currents.

A generator converts mechanical energy into electrical energy. Its essential parts are a magnetic field, usually produced by permanent magnets, and a moving coil or coils called the armature.

A motor converts electrical energy into mechanical energy. The motor, like the generator, consists of an electromagnet, an armature, and a commutator with its brushes.

The two principal classes of polyphase induction motors are the squirrel-cage motor and the wound-rotor motor. By definition, an induction motor is one in which the magnetic field in the rotor is induced by currents flowing in the stator. The rotor has no connections whatever to the supply line.

Single-phase motors can be divided into two principal classes as follows:

1. Split-phase

a. Resistance-start

b. Split-capacitor

c. Capacitor -start

d. Repulsion -start

2. Commutator

a. Series

b. Repulsion