I4 Learning Task 5 thru 10

Question 1

How does the rotor of a wound-rotor induction motor differ form that of a squirrel-cage induction motor?

Answer 1

It contains a three-phase winding rather than a squirrel-cage design.

Question 2

What is the purpose of the three slip rings and their corresponding brushes on the wound rotor motor?

Answer 2

the slip rings and brushes provide an electrical connection between the moving rotor and the external rotor leads

Question 3

When is max torque produced in the wound-rotor induction motor?

Answer 3

when the resistance of the rotor cct is approximately equal to the rotor reactance.

Question 4

The starting torque of a wound-rotor motor is improved by

Answer 4

inserting resistance across the secondary terminals M1, M2, and M3

Question 5

To improve the torque output and speed regulation of the wound-rotor motor

Answer 5

the resistance in the rotor cct is decreased as the speed of the motor increases.

Question 6

why is the efficiency of a wound-rotor motor low when resistance is left connected in the secondary cct?

Answer 6

a great deal of energy is being converted into heat at the secondary resistors.

Question 7

The wound-rotor motor is intended to be a

Answer 7

variable torque motor

Question 8

List the advantages of a wound-rotor motor

Answer 8

high starting torque with low current
smoother acceleration under heavy load
no abnormal heating during starting
good running characteristics after the starting resistance is removed
adjustable speed

Question 9

List the disadvantages of the wound-rotor motor

Answer 9

physical size
higher initial and maintenance costs
more complicated control
poor speed regulation when the secondary resistance is left in the cct.

Question 10

Applications of the wound-rotor motor

Answer 10

overhead cranes
ball mills
driving loaded conveyors

Question 11

why is it that increasing the resistance of the rotor of the wound-rotor motor cct increases the torque output of the motor?

Answer 11

Increasing the resistance improves the power factor of the rotor thus bringing the rotor flux more in phase with the stator flux

Question 12

with the secondary terminals M1, M2, M3, short-circuited, what will the starting torque of the wound-rotor motor be? Low or High?

Answer 12

low

Question 13

to decrease the speed of a wound-rotor motor, resistance

Answer 13

added to the rotor circuit

Question 14

why is it not practical to control the speed of a wound-rotor motor below 50% slip

Answer 14

speed becomes unstable

Question 15

Describe the rotor of a synchronous motor

Answer 15

essentially a DC electromagnet with the same number of field poles as in the stator
poles remain constant and do not change polarity

Question 16

Describe the poles in a synchronous motor rotor

Answer 16

the poles are of the salient type i.e. they project out from the rotor

Question 17

the synchronous motor direct-current field pole windings are connected to two

Answer 17

slip-rings mounted on the rotor shaft

Question 18

the synchronous motor direct-current field pole windings are marked

Answer 18

F1 and F2

Question 19

List the three ways of supplying a synchronous motor with DC field current

Answer 19

external converters
built-in exciter
brushless exciter

Question 20

Describe an external converter for supplying the DC current to the field in a synchronous motor

Answer 20

usually a simple rectifier

Question 21

Describe a built-in exciter for supplying the DC current to the field in a synchronous motor

Answer 21

a small DC generator that is mounted on the same shaft as the synchronous motor
field excitation is done by placing a rheostat in series with the shunt field of dc exciter motor

Question 22

Describe a brushless exciter for supplying the DC current to the field in a synchronous motor

Answer 22

a three-phase alternator and rectifier mounted on the rotor of the machine.
the alternator, rectifier and field of the synchronous motor all rotate together so no need for brushes.

Question 23

Why are synchronous motor not self-starting?

Answer 23

when trying to start it produces torque in one direction for half the time and torque back in the other direction the rest of the time. Net 0 torque

Question 24

amortisseur windings is used to

Answer 24

accelerate the synchronous motor close to synchronous speed or in other words used to start the synchronous motor and dampen any speed fluctuations

Question 25

what happens to the amoritsseur windings if they carry the load too long?

Answer 25

can overheat

Question 26

Define Pull-in torque for a synchronous motor

Answer 26

max constant under which the motor will pull it connected inertia load into synchronism when its field excitation is applied.

Question 27

Define Pull-out torque for a synchronous motor

Answer 27

the max sustained torque that the motor can develop at synchronous speed. If it is exceed it will pull the motor out of synchronism

Question 28

In a synchronous motor the amount of cemf produced is the stator
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Answer 28

is the phasor sum of the voltage applied to the stator and the emf induced into the stator by the rotating field

Question 29

In a synchronous motor the two voltage vectors, Es and Ec, add together to yield the resultant voltage: Er. This voltage divided by the impedance of the stator windings (Zs), determines the stator current: Is. therfore

Answer 29

anytime Er increases, Is also increases

Question 30

A synchronous motor with an under-excited rotor field will run with a

Answer 30

lagging power factor and the stator must supply the magnetizing current that the rotor is not supplying

Question 31

A synchronous motor with an over-excited rotor field will run with a

Answer 31

leading power factor and the stator must supply a demagnetizing current to compensate for the over-magnetization caused by the rotor

Question 32

synchronous motors with out an output shaft are called

Answer 32

synchronous capacitors and are used solely for power factor correction.

Question 33

For a synchronous motor, at what point in the starting process is the DC excitation applied to the field?

Answer 33

When the rotor is moving at 95 to 97% of the synchronous speed.

Question 34

Why is no current induced in the amortisseur winding when the rotor is rotating at synchronous speed

Answer 34

the rotor is locked with the rotating field and not cutting any lines of flux