# Three-Phase Motors- Test 2 Flashcards

The flux created by any winding will be at a maximum when the current through that winding is at a ___________

maximum

When the current through a winding reverses, the flux direction created by that winding

_____________

reverses

What effect does increasing the frequency of the supply have on the speed of the rotating magnetic field?

Increasing the frequency of the supply increases the speed of the rotating magnetic field.

What effect does increasing the number of poles have on the speed of the rotating magnetic field?

Increasing the number of poles in the machine decreases the speed of the rotating magnetic field.

Define the term synchronous speed.

The synchronous speed is the speed of the rotating magnetic field

What would be the synchronous speed of an eight-pole motor connected to a 60-hertz supply?

n = f x 60/P

n - 60 Hz x 60 / 4

= 900 rpm

What would be the number of poles in a motor with a synchronous speed of 720 rpm when the motor is connected to a 60-hertz supply?

P = f x 60/n

P = 60 Hz x 60/ 720 rpm

= 5 pairs of poles

= 10 poles

Since there is no electrical connection between the stator and the rotor, how is current made to flow in the rotor of a squirrel-cage induction motor?

through electromagnetic induction

Define the term slip speed.

Slip speed is the difference between synchronous speed and rotor speed.

Define the term percent slip.

Percent slip is the slip speed expressed as a percentage of the synchronous speed.

Define the term speed regulation.

Speed regulation is the difference between no-load speed and full-load speed of the rotor.

How is the direction of rotation reversed in a squirrel-cage induction motor?

By reversing the direction of the rotating field. This is accomplished by interchanging any two of the stator line connections to the source.

Define the term full-load torque.

The full-load torque of a motor is the torque necessary to produce rated horsepower at rated full-load speed.

Define the term breakdown torque

The breakdown torque of a motor is the maximum torque the motor will produce. It is sometimes referred to as the stalling torque because, if the motor load is increased beyond that point, the motor will quickly slow down and stall.

Define the term locked rotor torque

The locked rotor torque of a motor is the maximum torque the motor will develop at rest for all angular positions of the rotor with rated voltage and frequency applied to the stator.