Synchronous motor

Synchronous motor

A motor that rotates at exactly the same speed as the rotating magnetic field of the stator

Field windings

Magnets or stationary windings used to produce the magnetic field in an alternator or motor

the rotor winding is excited by an external_____

A) DC power source

why is the rotor connected to an external DC source?

establishes electromagnets in the rotor with north-south poles that enable the rotor to synchronize with the rotating stator flux

stator construction of synchronous motor

consists of laminated iron cores and is equipped with winding coils that create a rotating magnetic field when powered by alternating current.

Rotor construction of synchronous motor

two part:

• induction-motor rotor section

  • synchronous motor starts with this first

• wound rotor section

  • provides excitation for synchronization

Why does a synchronous motor not develop starting torque?

C. Rotor and stator fields are stationary relative to each other at standstill

What provides starting torque in a synchronous motor?

B. Damper windings

What happens to torque if rotor excitation is increased beyond normal?

C. Torque increases and then destabilizes

What is "pull-out torque" in a synchronous motor?

C. Maximum torque before loss of synchronism

At synchronous speed, what is the torque developed by rotor induced current?

A. Zero

How is synchronous speed (Ns) calculated?

Ns = 120 × f / P

At synchronous speed, the torque in a synchronous motor is produced by:

B. Interaction of rotor’s DC field and stator field

What happens if load increases beyond pull-out torque?

C. Motor falls out of synchronism and stops

Why does a synchronous motor run at constant speed?

C. Rotor locks with stator field

Can a synchronous motor change speed with load?

B. No, it always runs at synchronous speed

What determines the synchronous speed of a motor?

C. Number of poles and supply frequency

Why is a synchronous motor more efficient at full load?

B. Rotor losses are minimized at synchronism

Which of the following contributes to losses in a synchronous motor?

D. All of the above

How can a synchronous motor’s efficiency be improved?

C. Optimize excitation and loading

What is the main source of rotor loss in a synchronous motor

C. DC excitation losses

Which part supplies DC to the rotor?

C. Slip rings and brushes

Which rotor type is best suited for low-speed applications?

C. Salient pole rotor

What is the purpose of damper windings?

C. Assist in starting torque and damp oscillations

How does brushless excitation work?

B. Using onboard rotating rectifiers and an AC exciter

How is synchronism achieved in a synchronous motor?

B. By locking rotor field with rotating stator field

What happens when the excitation is increased beyond normal?

C. Power factor becomes leading