What happens to the torque and power characteristics of a AC Induction Motor when it is run above its syncronous speed?

 

Torque starts dropping, power (torque*rpm) stays fairly constant. It is typically called the "constant horsepower" range.

Bill

 

Sorry I'm confused. How can we run an AC induction motor above synchronous speed.May somebody please explain? Maybe you want to run it over it's rated speed?

 

When an induction motor is run below base speed it is in the constant torque range. When the motor field is weakened the motor will run above base speed, and is called the constant HP range,

George Younkin
[email protected]

 

AC Induction motor can be run over synchronous speed.It can not be run at synchronous speed as
relative velocity between field flux mmf and rotor induced mmf will become zero, no electromagnetic torque will be produced.
In case of speed over synshronous rotor will start to rotate in reverse direction.

 

This is done all the time in the power generation industry. Hydroelectric and wind power use this and it is called asynchronous generation or induction generation. Say you have an induction motor designed to run with 2% slip, you can drive it over its synchronous by 2 % and it will generate the amount of power it would draw as a motor.

I hope this helps,
William Hinton

 

Eric,

I believe you meant to ask about running above motor nameplate base speed (i.e. RPM at 60Hz), in which case torque drops off and power (HP) remains constant.

You cannot run at or above synchronous speed, as determined by the frequency of waveform delivered to the motor since a certain amount of slip is required for torque generation. If a motor is running above synchronous speed, it is because the motor is being overrun by the connected load, in which case the motor will be regenerating (reverse torque/current).

Chris B

 

Yup... it'll run as an asynchronous generator when operated at speeds above synchronous speed with the help of a prime mover. Now my question is... Can we take slip power from rotor at speed above synch. speed? And if yes, what will be the effect of that on generator?

 

It's is possible using a Varible Frequency Drive, or a Drive, it's an electronic device that can generate other frequencys beyond our power supply