Unit 31: DC Motors

Some of the losses of a DC motor are

1. I2R loss in the armature windings 2. Windage loss 3. Bearing friction 4. Brush friction

Permanent magnet motors

1. Servomotors 2. Servodisc motor

What are the three factors that determine the amount of CEMF produced by a motor?

1. The strength of the magnetic field of the pole pieces 2. The number of turns of wire in the armature 3. The speed of the armature

Two factors that determine the torque produced by a DC motor are

1. The strength of the magnetic field of the poles 2. The strength of the magnetic field of the armature

The stator windings for a brushless motor are generally ____ phase but can be ___ or ___ phase

3 / 4 or 2

1 horsepower is equal to __ watts

746

What function does the commutator perform in a DC motor?

A rotary switch and maintains the correct direction of current flow through the armature windings

When full voltage is applied to the armature and reduced voltage is applied to the shunt field, the motor operates ____ speed

Above base

Disc servomotors can be ___

Accelerated very rapidly

Because there is not ____ in the armature if a disc servomotors, it doesn't have a problem with _____

Any iron / cogging

DC motors operate on the principle of ____

Attraction and repulsion of magnetism

When full voltage is applied to both the armature and shunt field, the motor operates at ____ speed

Base

When full voltage is applied to the field and reduced voltage is applied to the armature, the motor operates at ____ speed

Below base

The lower the armature resistance the ___the speed regulation

Better

What limits current flow through the armature?

CEMF and armature resistance

What is cogging?

Caused by the reaction of the iron armature with the field of permanent magnets. This causes jumps because the armature is slotted and stronger attached to the closer pole pieces

How is the speed of the disc servomotors varied by?

Changing the amount of voltage supplied to armature. ( can use pulse width modulation )

In PM motors the field flux is _____. This means the speed is controlled by ____

Constant / the amount of voltage applied to the armature

Shunt motors are sometimes known as ___

Constant speed motors

Brushless DC motors require a ___ to change the __ into ___

Converter / DC into AC

Series motors can develope _____

Extremely high starting torque

Inside out motor is designed in a way to act like a ____ which gives the motor a large amount of ____

Flywheel / inertia ( superior speed regulation)

Servomotors can operate at ____ and ___ or ____ quickly

High speed / stopped / reversed

Series motors are known for their ____

High torque

PM motors are ____ than conventional DC motors

More efficient

___ motors use permanent magnets as the pole prices and do not require___ or ___ windings

PM/ series / shunt

In a brushless DC motors, the armature or rotor contains ____

Permanent magnets

What's another name for a servodisc motor

Printed circuit motor

The speed regulation of a DC motor is ____ to the resistance of the armature

Proportional

Torque is ____ to the armature current if the field excitation current is ____

Proportional / constant

In order for a motor to develop a turning force, the magnetic field polarity is the armature must ____ in relation to the polarity of the pole pieces

Remain constant

Why are brushless motors better than other DC motors?

Require less maintenance and have less downtime

The direction of rotation of a DC motor can be changed by _____ the connection of either the ___ or ___ leads

Reversing / armature / field

Right hand rule is used to determine the ____ when the magnetic field polarity of the pole pieces and direction of current through the armature are known

Rotation of the armature

Compound motors contain both ____ and ____ field windjngs

Series and shunt

What are the three basic types of DC motors?

Series, shunt, and compound

____ have lightweight armatures with low inertia

Servomotors

Disc servomotors contain an armature that is made of ____

Several layers of copper conductors

The operation characteristic Of a PM motor are similar to those of a ____

Shunt motor with external excitation

The amount by which the speed decreases as load is added is called ____

Speed regulation

Servodisc motors have a ____ to the pole pieces

Tangent force

What is the purpose of a shunt field relay ( field loss relay)

To disconnect power to the armature if the if the shunt field current drops below a certain level

What is torque?

Torque is the pushing or turning force created by the magnetic field of the pole pieces and the magnetic field of the loop or armature

Series motors must never operate ____

Under no load condition

DC motors are used in applications that require ___

Variable speed (or large amount of torque at low speed)

Brushless DC motors do NOT contain what three components?

Wound armature, commutator, or brushes