Synchronous Motors

Question 1

What are the disadvantages of using a synchronous motor over an induction motor?

Answer 1

1. A synchronous motor requires DC excitation from a separate supply source in order to run
2. A synchronous motor may fall out of synchronism if it is overloaded
3. The starting torque of a synchronous motor is quite small compared to an induction motor, and synchronous motors require assistance to get to their operating speed
4. Synchronous motors do not work well in applications that require frequent starting or direction changes

Question 2

What are the advantages of using a synchronous motor over a squirrel cage induction motor?

Answer 2

1. A synchronous motor can be made to operate at a leading power factor to improve the power factor of an entire building
2. A synchronous motor gives a constant speed from no-load to full-load
3. A synchronous motor operates at higher efficiency, especially when operating at a unity power factor

Question 3

What is synchronous speed?

Answer 3

• When the rotor of a motor turns at the same speed as the rotating field of the stator
• Synchronous motors operate at synchronous speed

Question 4

When a synchronous motor is used in power factor corretion purposes, what are they called?

Answer 4

• They are called synchronous condenser’s

Question 5

When a synchronous condenser is used, what needs to be adjusted to cause a change in the level of power factor correction?

Answer 5

• The amount of excitation current

Question 6

Where are the armature windings of a synchronous motor?

Answer 6

• The armature windings are on the stator, which are connected to the supply voltage
• When energized, the armature windings support the full-load current and produce a rotating magnetic field

Question 7

The rotor of a synchronous motor supports the field windings

true or false?

Answer 7

True

Question 8

In addition to supporting the field windings of a synchronous motor, what else differs from the rotor of a squirrel cage induction and the rotor of a synchronous motor?

Answer 8

1. The rotor of a synchronous induction motor also supports an amortisseur winding, which is very similiar to the rotor bars of a squirrel cage induction motor
2. Synchronous motor rotors are manufactured with the same number of poles (and field windings) as there are poles (and armature windings) on the stator

Question 9

Do most synchronous motors use cylindrical rotors or salient pole rotors?

Answer 9

Salient Pole Rotors

Question 10

What holds the rotor in place and maintains the air gap between the rotor and the stator?

Answer 10

• The rotor is held in place by the bearings mounted in the end bells at both ends of the stator

Question 11

In a synchronous motor, how are the rotating field poles magnetized?

Answer 11

• By the direct current flowing through the field windings wound on the pole cores
• You can control the intensity of the rotating magnetic field by varying the field excitation

Question 12

Synchronous motors require a magnetic field in the rotor to rotate, what are the three ways the field of the synchronous motor can be supplied?

Answer 12

1. A brushed DC excitation source - DC voltage is supplied to the spinning rotor through slip rings and brushes
2. A brushless excitation source - a small rotating armature alternator is attached onto the end of the synchronous motor shaft which causes the motor to drive the alternator, the output of the alternator is rectified DC which is connected directly to the synchronous motor field winding
3. A permanent magnet

Question 13

What is the name of the angle between the centre of a stator pole and the centre of a corresponding rotor pole?

Answer 13

The torque angle

Question 14

What happens to the torque angle of a synchronous motor as load is increased?

Answer 14

• The torque angle increases as the load increases

Question 15

What is the torque angle of a motor?

Answer 15

• The angle between the stator and the rotor poles

Question 16

What is Pull-In Torque?

Answer 16

• The torque that the rotor must develop in order to pull into synchronism once the DC windings have been energized
• The Pull-In torque must be greater than the counter-torque of the load

Question 17

What is Pull-Out Torque?

Answer 17

• Pull-out torque is the value of counter-torque required to rip the rotor out of synchronism
• The point where the load is sufficient enough to break the magnetic attraction between the two fields