Motor Soft-Starter Question

[Darktrax]

The device is a thyristor-based item used for soft-starting electric motors.

There are several good brands, but in this case, it is an older Schneider (Telemecanique LH4) type, which can be used for both 3-phase and single phase motors.

Not mentioned at all in the data sheets, but only at the end of an FAQ brochure, it says that a single phase motor can be started "provided it is an asynchronous squirrel cage motor, not a motor with a commutator".

If anyone knows, (or can offer a speculation), what is so special about a single-phase AC brushes+commutator motor as a load, compared to induction motors, that might be a problem for a thyrister based phase-angle switcher?

 

[D.A.(Tony)Stewart]

Synch . AC motors have no starting torque from the rotor current, so a different starting method is used. Often a Squirrel cage type rotor winding is used for self induction until Full speed is reached and then external rotor power is applied.

It wouldnt start.

 

[chuckey]

A squirrel cage motor behaves like an inductor whose value and loss resistance varies with the speed and load on it. A commutator motor has the commutator which switches from pole to pole, so has discontinuities in the current as the switching occurs. If you had a current sensing resistance in series with the motor in the first case you would get a sine wave in the second case certainly not!
Frank