Module 5 - Introduction to Generators and Motors
Navy Electricity and Electronics Training Series (NEETS)
Chapter 2:  Pages 2-11 through 2-16

The interpole coil in a motor is connected to carry the armature current the same as in a generator. As the load varies, the interpole flux varies, and commutation is automatically corrected as the load changes. It is not necessary to shift the brushes when there is an increase or decrease in load. The brushes are located on the no-load neutral plane. They remain in that position for all conditions of load.

Q15. What current flows in the interpole windings?

The DC motor is reversed by reversing the direction of the current in the armature. When the armature current is reversed, the current through the interpole is also reversed. Therefore, the interpole still has the proper polarity to provide automatic commutation.

MANUAL and AUTOMATIC STARTERS

Because the DC resistance of most motor armatures is low (0.05 to 0.5 ohm), and because the counter EMF does not exist until the armature begins to turn, it is necessary to use an external starting resistance in series with the armature of a DC motor to keep the initial armature current to a safe value. As the armature begins to turn, counter EMF increases; and, since the counter EMF opposes the applied voltage, the armature current is reduced. The external resistance in series with the armature is decreased or eliminated as the motor comes up to normal speed and full voltage is applied across the armature.

Controlling the starting resistance in a DC motor is accomplished either manually, by an operator, or by any of several automatic devices. The automatic devices are usually just switches controlled by motor speed sensors. Automatic starters are not covered in detail in this module.

Q16.   What is the purpose of starting resistors?

Summary

This chapter presented the operating principles and characteristics of direct-current motors. The following information provides a summary of the main subjects for review.

The main PRINCIPLE of a DC MOTOR is that current flow through the armature coil causes the armature to act as a magnet. The armature poles are attracted to field poles of opposite polarity, causing the armature to rotate.

The CONSTRUCTION of a DC motor is almost identical to that of a DC generator, both physically and electrically. In fact, most DC generators can be made to act as DC motors, and vice versa.

COMMUTATION IN a DC MOTOR is the process of reversing armature current at the moment when unlike poles of the armature and field are facing each other, thereby reversing the polarity of the armature field. Like poles of the armature and field then repel each other, causing armature rotation to continue.

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COUNTER-ELECTROMOTIVE forCE is generated in a DC motor as armature coils cut the field flux. This EMF opposes the applied voltage, and limits the flow of armature current.

In Series MOTORS, the field windings are connected in series with the armature coil. The field strength varies with changes in armature current. When its speed is reduced by a load, the series motor develops greater torque. Its starting torque is greater than other types of DC motors. Its speed varies widely between full-load and no-load. Unloaded operation of large machines is dangerous.

In Shunt MOTORS, the field windings are connected in parallel (shunt) across the armature coil. The field strength is independent of the armature current. Shunt-motor speed varies only slightly with changes in load, and the starting torque is less than that of other types of DC motors.

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In COMPOUND MOTORS, one set of field windings is connected in series with the armature, and one set is connected in parallel. The speed and torque characteristics are a combination of the desirable characteristics of both series and shunt motors.

Load on a motor is the physical object to be moved by the motor.

DC MOTOR ARMATURES are of two types. They are the Gramme-ring and the drum-wound types.

The GRAMME-RING ARMATURE is inefficient since part of each armature coil is prevented from cutting flux lines. Gramme-ring wound armatures are seldom used for this reason.

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The DRUM-WOUND ARMATURE consists of coils actually wound around the armature core so that all coil surfaces are exposed to the magnetic field. Nearly all DC motors have drum-wound armatures.

MOTOR REVERSAL in a DC motor can be accomplished by reversing the field connections or by reversing the armature connections. If both are reversed, rotation will continue in the original direction.

SPEED CONTROL IN a DC MOTOR is maintained by varying the resistance either in series with the field coil or in series with the armature coil. Increasing shunt-field circuit resistance increases motor speed. Increasing the armature circuit resistance decreases motor speed.

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ARMATURE REACTION is the distortion of the main field in a motor by the armature field. This causes the neutral plane to be shifted in the direction opposite to that of armature rotation. Interpoles and compensating windings are used to reduce the effect of armature reaction on motor operation.

STARTING ResistorS are necessary since the DC resistance of a motor armature is very low. Excessive current will flow when DC voltage is first applied unless current is limited in some way. Adding resistance in series with the armature windings reduces initial current. It may then be removed after counter EMF has been built up.

Answers to Questions Q1. Through Q16.

A1.     Direction of armature current, and direction of magnetic flux in field.

A2.     Direction of conductor movement (rotation), direction of flux, and the direction of current flow.

A3.     There are no differences.

A4.     Generator action.

A5.     Speed.

A6.     The device to be driven by the motor.

A7.     It must have a load connected to avoid damage from excess speed.

A8.     High torque (turning force) at low speed.

A9.     It maintains a constant speed under varying loads.

 A10.  Only outside of coils cut flux (inefficient).

A11.    By winding the armature in a way that places the entire coil where it is exposed to maximum flux.

A12.    By reversing either field or armature connections.

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A13.    Motor will slow down.

A14.    Opposite the rotation. A15. Armature current.

A16.    To limit armature current until counter EMF builds up.

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