I've been researching making a driver for a 3 phase induction motor powered by DC source using PWM to approximate sine waves in each phase.
However, the switching speeds of the PWM signal for the rotational speed I want is difficult to obtain with hobby microcontroller boards like the arduino.
Can I just switch the DC directly thru each phase at the correct time only twice per cycle ? Essentially I would be approximating a sine wave with only 2 different voltages thru the coils, ie: +V and -V (using H bridge circuitry here)
Will this create enough of a rotating magnetic field that sufficient current is induced in the rotor cage ?
What will be the losses in power / efficiency for this, or any other drawbacks for this system ?
The bonus is greatly simplified drive electronics.
Best Answer
Yes, using square waves instead of sine waves can be done. As you suspected, it will be less efficient. Think of it in frequency space. The fundamental component of the square wave will do all the work. The remaining harmonics will heat the motor without adding useful torque, will add mechanical stresses, and can cause more audible noise.
You say you can't get a PWM frequency fast enough to produce a reasonable sine wave, but that is hard to believe. Surely the AVR can do 25 kHz or so PWM? I have done this a few times with Microchip PICs, some of which even have a special PWM module intended for this application.