Synchronous Motors

Question 1

Synchronous Motors require a magnetic field in the rotor to rotate. The field can be supplied by:

Answer 1

A brushless excitation source
A brushed dc excitation source
A permanent magnet

Question 2

Name the two starting characteristcs of a Synchronous Motor:

Answer 2

Low starting current per horsepower
Low starting torque

Question 3

What are the two names for a Synchronous Motor operating at no load for PFC?

Answer 3

Synchronous Condenser
Rotary Capacitor

Question 4

The rotor of a synchronous motor is constructed like the rotor of an induction motor. T or F?

Answer 4

False

Question 5

Synchronous Motors are doubly excited machines. T or F?

Answer 5

True. It means they have two electrical inputs, the stator and the rotor.

Question 6

The rotational frequency or speed of the stator’s rotating magnetic field is directly proportional to what?

Answer 6

To the frequency of the supply voltage

Question 7

Name the three common methods to start a Synchronous Motor:

Answer 7

• Use a shaft-mounted induction motor to start the synchronous motor rotor
• Use the load from the synchronous motor as a starting motor
• Use an amortisseur winding on the rotor

Question 8

Egen can also be called:

Answer 8

The CEMF

Question 9

A field-discharge resistor is connected across the field utlizing a synchronizing switch. What type of switch is used?

Answer 9

A make before break

Question 10

The CEMF opposes the supply voltage applied to the stator armature windings by how much degrees?

Answer 10

180

Question 11

Egen can be varied by changing the field excitation. T or F?

Answer 11

True

Question 12

The angle between the centre of a stator pole and the centre of a corresponding rotor pole is called:

Answer 12

The torque angle

Question 13

Describe Pull-out Torque:

Answer 13

The value of the load counter-torque required to RIP the rotor out of synchronism.

Question 14

The overload condition that usually happens when the torque angle exceeds what amount of degrees for SAILENT pole rotors?

Answer 14

60-70 degrees

Question 15

Rated Load on a Synchronous Motors equals what power factor?

Answer 15

Unity
or PF of 1

Question 16

What is synchronous reactance?

Answer 16

Is the combination of the inductive reactance of the armature and armature reaction.

Question 17

Changing mechanical load on the shaft of the motor has the greatest effect on ________ ________

Answer 17

Torque Angle

Question 18

The motor is over excited when the amount of excitation results in a:

Answer 18

Leading power factor

Question 19

A Bell Curve represents Field Current vs Power Factor. T or F?

Answer 19

True

Question 20

I of the Armature Leads the VResultant angle by 90. T or F?

Answer 20

False. It lags VResultant by 90

Question 21

The Torque Angle of a Synchronous Motor can never be at 0° due to what 2 things?

Answer 21

• Rotational Drag
• Mechanical Load