3 phase asynchronous motor torque

[Shark96]

Does the torque of a motor change considerably from full load to no load and vice versa even though the voltage and frequency are the same?

 

[ScottyUK]

Yes, there's usually an initial peak followed by a prolonged dip before rising to a maximum around 85% of rated speed, then falling toward zero torque at synchronous speed. Rotor design is a big factor in the shape of the curve, and with a wound rotor it is possible to change the shape of the curve 'on the fly' by adjusting the resistance in the rotor circuit. Plenty of examples on the internet!

 

[LionelHutz]

The output torque would change from rated torque to zero torque as the load changes from rated load to no load.

 

[electricpete]

Yup.
Scotty discussed the shape of the torque speed curve (useful in #3 below)
Lionel gave the direct answer

To op:
1 - Motor output torque must match load torque during steady state (otherwise it would not be steady state)
2 - Motor output power is the product of motor output torque and speed (this is a mechanical first principle)
3 - For an induction motor fed with fixed-frequency, motor speed changes very little (as a fraction) from no-load to full load (5% max slip at rated load for NEMA Design B, usually much less).... you can see this from the steep slope of Scotty's torque-speed curve between rated speed and sync speed.
4 - Putting above together (since power is proportional to torque times speed and speed is roughly constant), torque is roughly proportional to motor output power (for a fixed-frequency induction motor)


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(2B)+(2B)' ?

 

[waross]

This misunderstanding about torque has come around a couple of times.
The rated torque of a motor is a limit.
The load determines the torque delivered.
The motor develops whatever torque the load demands up to a point.
If the load demands more than rated torque, the motor will overheat.
If the load demands even more torque the motor will stall.
If there is no load then there is no external torque. (There will be enough torque developed internally to overcome bearing friction and windage.)
The torque curves referenced by Scotty show how much how much the slows down below synchronous speed with a given given load, and the maximum torque developed as the motor accelerates when starting.
During normal starting the torque developed by the motor does exceed the torque demanded by the load. When the motor develops more torque than the load demands, the motor accelerates.
That motor starting torque must exceed the torque demanded by the load at all speeds or the motor will run slow and overheat or stall.
A compressor typically depends on flywheel effect to smooth out the torque demands of the compressor.
It is common on compressors for an unloader to hold the valves open until the motor has gainec enough speed for the flywheel effect to become effective.
If the unloader fails to operate properly the motor will stall due to the excessive torque demands of the load.

Bill
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