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