Chapter 17 Three-Phase Motors

True

A synchronous motor must never be started with DC current connected to the rotor.

False

An induction motor can always reach synchronous speed.

True

Induction motors can act as a generator.

True

Maximum torque is developed when stater and rotor flux are in phase with each other.

True

Single-phasing occurs when one line of a three-phase system is open.

S = 120•f/P S = Speed in rpm f = Frequency in Hz P = Number of stator poles (per phase)

Synchronous speed can be calculated using the formula ____

Wound-rotor

The _____ induction motor was very popular in industry for many years because of its high starting torque and low starting current.

True

The direction of rotation of any three-phase motor can be changed by reversing two of its stater leads.

rotating magnetic field

The principle of operation for all three-phase motors is the _______

f = P x Sr/120 f= Frequency in Hz P = Number of stator poles Sr = Rotor slip in rpm

The rotor frequency can be determined by using the formula ____

Amortisseur winding

The set of squirrel cage bars on a synchronous motor, used to start the motor, is known as the ______

synchronous speed

The speed at which the magnetic field rotates is called the ____

Percent slip

The speed performance of an induction motor is measured in ____

Squirrel-cage, wound rotor, synchronous

The three basic types of three phase motors are _____

Consequent pole

____ squirrel-cage motors permit the synchronous speed to be changed by changing the number of stator poles.