Operating Principles of Three-Phase Synchronous Motors

Question 1

Why is the synchronous motor not self starting?

Answer 1

The rotating stator field is travelling too fast for the rotor to accelerate and lock in step. The field moving past the rotor produces constant torque reversals. This results in a net starting torque of zero.

Question 2

At what point in the starting process is the DC excitation applied to the field?

Answer 2

When the rotor is close to synchronous speed. The percentage of slip varies with the mechanical load on the motor

Question 3

During acceleration and deceleration of the rotor, why is the field short-circuited or connected across a discharge resistor

Answer 3

To prevent a high voltage from being induced in it

Question 4

Can the direction of rotation of a synchronous motor be reversed by reversing the polarity of the DC field supply?

Answer 4

No. The direction of rotation is reversed by reversing the direction of rotation of the stator field. This is done by interchanging any two of the supply leads to the stator.

Question 5

What type of induction motor is sometimes used to start synchronous motors that have large starting torque requirements?

Answer 5

Wound-rotor induction motor

Question 6

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

Answer 6

because there is no relative motion between the stator field and the amortisseur winding at synchronous speed.

Question 7

Why is it a problem for the synchronous motor to turn at lower-than-synchronous speed for extended periods of time?

Answer 7

The amortisseur winding may overheat

Question 8

Define the term “pull-in torque”

Answer 8

Pull-in torque is the torque required to pull the rotor into synchronism with the rotating stator field once the DC excitation has been applied

Question 9

What is the term used to describe the angle between the centre of the stator poles and the centre of the rotor poles?

Answer 9

torque angle

Question 10

Does the torque angle increase or decrease as load is added to the motor?

Answer 10

increase

Question 11

Define the term “pull-out torque”

Answer 11

Pull-out torque is the maximum sustained torque that the motor can develop at synchronous speed.

Question 12

What happens if the pull-out torque is exceeded?

Answer 12

The motor will pull out of synchronism and slow down

Question 13

As load is added to the synchronous motor, what happens to the power factor of the motor?

Answer 13

It becomes less leading (or more lagging)

Question 14

What controls the magnitude of the current flowing in the stator windings?

Answer 14

the resultant of the applied voltage to the stator and the voltage induced in the stator by the rotor field

Question 15

How does the torque angle of the rotor affect the stator currents?

Answer 15

When the torque angle changes, the angle of the voltage induced in the stator by the rotor also changes. This results in a new resultant emf (ER) in the stator. The new resultant voltage causes a new current to flow in the stator

Question 16

With the load fixed, which of the following cases will cause the lowest stator current?

a. unity power factor operation
b. under-excited field operation
c. over-excited field operation

Answer 16

a. unity power factor operation

Question 17

What happens to the power factor of a synchronous motor when the field current is increased under a fixed load?

Answer 17

Increasing the field current with a fixed load will cause the power factor to become more leading (or less lagging).

Question 18

When a synchronous motor is operated over or under excited, does this change the in-phase component of the stator current?

Answer 18

No.