AC motors Flashcards
Describe with the aid of a simple diagram, the principles of operation of an AC synchronous motor.
Diagram should show a permanent magnet in the middle with phases around the outside with red start red finish etc.
A rotating magnetic field is produced by a 3 phase stator. The rotor contains a magnet, the magnets field locks onto the field of the stator. Calling it to be dragged around at the same speed.
Define slip and state an equation which links it rotor and field speed.
Slip is the difference between the speed of the rotating field and the squirrel cage rotor
slip= (Ns - Nr)/Ns
Draw and explain the torque/slip characteristics for a double cage rotor. State one advantage and one disadvantage of this type of motor.
Graph will contain torque on the y axis and speed on the x axis. It needs to have a line for the outer iron core, the inner copper core and a line for a combined.
Advantage - Higher starting torque
Disadvantage - Maximum torque is less than a single cage
Define the term synchronous speed as it applies to an AC motor
The rotor and rotating field moving at the same speed.
Identify the 2 parameters that can be altered to change the synchronous speed of an AC motor and state the formula that links them.
Frequency and number of pairs of poles
N = 60f/p
State the effects on a 3 phase induction motor when it is single phasing whilst running and stopped.
Running:
Low load, little effect
Medium load, vibration
High load, stalling and overheating
Stopped:
The motor cannot restart
Explain how an MTPU also provides under voltage protection
If the supply voltage falls below 53% of it’s rated voltage for more than 3 seconds the MTPU will trip the motor out.
State the equation you would use calculate speed in a synchronous motor.
Ns = 60f/p
Briefly describe 2 ways that can be used to start a synchronous motor
Pony motor, spins the permanent magnet up to speed so it can lock onto the rotating field.
Variable frequency supply, The starting frequency for the stator will be lower allowing the slower moving permanent magnet to lock on quicker.
Briefly explain the difference between stator switching and pole changing in an induction motor and state which is normally used in 2 speed motors.
Stator switching, Multiple stator windings are used, one for each speed
Pole changing, All of the windings are used at each speed, but connected in a different way for each speed.
Pole changing is the most common method used.
State 4 common causes of faults from which a motor will need to be protected
- Short circuit
- Overloading
- Stalling
- Under voltage
- Single phasing
Briefly explain the main construction differences between a universal motor and a single phase induction motor. State one advantage of each type of motor.
Universal motor, Has brushes and a commutator. The advantage for this type of motor is that speed control is easy.
Induction motor, Has no brushes and uses a squirrel cage rotor. The advantage for this motor is that it’s quiet.
Briefly explain the role of a capacitor in a capacitive start single phase induction motor.
The capacitor is used to provide a phase shift of 90∘ for one of the motor windings.
Using a simple diagram explain how torque is produced in an induction motor.
Diagram should be a basic cross section of an induction motor it needs to include the following labels:
- Rotating magnetic field
- Current induced in rotor bars
- Field produced around conductors
- Resulting force
The rotating field induces a current in the rotor bars. This current produces it’s own magnetic field which interacts with the rotating field to produce a torque.
Briefly explain why frequency control provides a more flexible way to control the speed of an AC motor than pole changing. State one advantage of pole changing over frequency control.
Frequency control provides complete and smooth control of speed from 0 to max. Pole changing only provides 2 speeds, normally full speed and half speed.
Advantage of pole changing, Simpler and cheaper
