Motor & speed control

[zacky]

Hi guys,
I think DC motors' speed can be cotrolled by reducing the DC rating voltage- If this is right, does that will effect the motor's torgue available ?

And what about the AC motors, can the speed also controlled by reducing the applied AC voltage , by using variable variac for example?
Does that will effect the torque rating of the AC motor?

Thanks in advance

 

[spargher]

DC motors' speed can be reduced by reducing the DC voltage. In this case you will have the same torque but you will reduce the power.
DC motors can also be controlled by changing the field current and the speed-torque-power carachteristic depends on motor design. You can reduce spedd by rising field current, but if motor is not designed for a high DC current for example you can burn the field coil. You should ask motor manufacturer.

In the case of AC motor, it depends on motor type, but for induction and synchronous motor the only way for reduce speed is reducing the voltage frequency.

 

[Skogsgurra]

There is a class of asynchronous induction motors than can be speed controlled by reducing the stator voltage. The have a high rotor resistance and the principle is that lower voltage results in higher slip - or lower speed.

The can be used in low power applications where power loss can be toleratde. Fans are an example. I have seen units up to about 5 kW running fans with a simple triac control. Look for Ziehl/Abegg. They specialise in such high slip motors.

Gunnar Englund

http://www.gke.org