3.18 AC Motors Flashcards
AC motors can be divided into two main categories, what are these categories?
-synchronous motors (single)
-asynchronous motors
What are asynchronous motors usually called?
Induction motors
What is a synchronous motor?
A machine where the rotor speed and stator speed are equal
What is an asynchronous motor?
A machine where the rotors rotate at slower speed than a synchronous speed
Slip is required to make?
Torque
What are the two types of rotating field machines?
-salient pole machines
-non-salient pole machines
When are salient pole machines used over non-salient pole machines?
At low rotational speeds
When are non-salient pole machines used over salient pole machines?
At high rotational speeds
What are non-salient pole machines also referred to as?
-cylindrical rotor machines
-round rotor machines
-drum-type machines
Why cant a synchronous motor start by itself?
Because of inertia (needs help to get movement)
What is the most common method of starting a synchronous motor?
Damper windings
If a synchronous motor doesn’t have a damper winding how does it get up to speed?
It will need an induction motor
If you exceed the load angle what will happen??
The motor will reach the breakdown torque and stop
What determines the speed of a synchronous motor?
The number of poles and supply frequency
What is hunting?
The oscillation of the rotor about its steady state position or equilibrium state in a synchronous motor
How do synchronous motors correct the power factor?
By varying their excitation
Why are asynchronous motors the most important used motors?
-they’re reliable
-little maintenance
-cheap to make
In most three phase induction it looks like there is 12 phases, why is this?
There is 4 As, 4 Bs, 4 Cs
What is a squirrel cage?
It is used in an C motor to hold the rotor bars and end rings together
What is usually attached to the end rings of a squirrel cage and what are they used for?
Fan blades for motor cooling
How do you increase the electromotive force in a AC Motor?
By increasing the magnetic field relative to the circuit
What happens if the rotor speed equals the same as the stator speed?
No electromagnetic force is produced
The difference between the stator and rotor speeds is called?
The slip
Is the three-phase AC motor self starting?
Yes
What are the two ways of controlling speed of an AC motor?
-change the frequency
-use a wound rotor
Why is changing frequency not really viable for a good way to alter the speed of an AC motor?
Because you need to add a frequency change unit
How does a wound rotor induction motor work?
It’s the same as a normal motor but has a variation in speed by connecting an external resistance to the rotor circuit
What are the two method usually used to start a single phase induction motor?
-capacitor start
-shaded pole
What percentage does the starting capacitor turn off?
70-80%
If a motor requires a high starting torque what can be used?
A second starting capacitor
What is a simple way of reversing the direction of a single-phase induction?
By changing it from RYB to RBY
