List two reasons why a shaded pole motor is used to power the fan in a portable heater?
any two of:
it can be stalled
its quiet operation
low torque
cost effective
Why is a three phase induction motor more efficient than the single phase equvilant size induction motor?
Three phase induction motors have a balanced rotating magnetic field
single phase have a pulsating one
Give an application where a three phase motor would require phase reversal protection?
Where it may injure people or cause damage to property and equipment
eg, lathe or conveyor
State two reaons for using reduced voltage starting for a three phase induction motor?
Limit mechanical shock
Reduce high starting currents
What is the purpose of the maintaining contacts in a direct online reversing starter?
To maintain supply to the contactor coil allowing user to release start button
To prevent auto restarting when supply is interupted
What is the cheapest type of reduced voltage starter for three phase motors, and what special requirement, if any, does this method of starting impose on the internal connections of the motor winding?
Star-Delta
Needs a six terminal connection
State One important reason for inserting external resistance into the rotor circuit of a slip ring motor at start up
Any one of:
to reduce starting current
To produce a higher starting torque than that availiable from a squirrel cage motor started DOL
To provide a more gentle startup for high inertia loads
To provide a ramped acceleration for high inertia loads
To provide a higher start torque at low speeds starting through improved power factor
What is the main advantage of a single phase synchronous motor over other single phase motors?
It rotates at a fixed speed independant of load
Sketch a Torque Speed graph for a Three phase sqirrel cage induction motor
on graph show:
Starting Torque
Pull out torque
Full load torque
Synchronous speed
Full load speed
A primary resistance starter reduces the motor terminal voltage during starting by inserting resistance into each supply line.
The motor terminal voltage ramps smoothly up during accelaration as a function of the decreasing line current and voltage drop across the line resistances. Explain, in principle, how an electronic soft starter achieves the same thing?