PWM Drive for AC Induction Motors

636

08:32:49

Bridge and drive power arrangement for PWM power control of AC Induction motors.  Switching AC at high speed with fets is done by placing a bridge rectifier into the AC circuit, then switching the rectified current,  creating a Bidirectional Switch.

change the pulse width of the square wave in the centre to control the power through the load.  longer pulses mean more power. 

in this circuit there are two bidirectional switches.  the top switch controls main drive current through the AC Motor, and the bottom switch handles the emf current generated when the motor is deenergised.  PWM then  controls the on time of each switch. 

the gate drive power supply for each circuit has to be referenced to the source terminal of each fet to create the voltage difference necessary to drive the gate while also keeping that voltage within the Vgs of the fet.  this means that both power supplies to the gates are floating and can reach line voltages.  this is especially prevalent when switching the top drive switch during the negative phase of the AC cycle.  the gate drive signal has the be referenced to the source terminal, not to the circuit ground.

The PWM signal is fully isolated from the gates using optocouplers.  there must not be any dead time between both switches, or very large EMF spikes start happening. 

notes here:

https://www.google.com.au/url?sa=t&source=web&rct=j&url=https://www.nxp.com/files/microcontrollers/doc/ref_manual/DRM039.pdf&ved=0ahUKEwiIhImc29bRAhXLjLwKHS6aBFsQFghdMAw&usg=AFQjCNFon2xgKOMSITM6jNS4NQ667h_MXQ&sig2=Ls6pRg15lKgBPZKsNiODJw

published 9 years ago

hurz

9 years ago

Vgs drop a little to much, below 5V. So the peak detector needs bigger caps 47uF or the mos is also with a bigger pullup than 1kOhm happy.

faceblast

9 years ago

OK will change it is there a better way to drive the fet? floating both supplies seems dangerous

hurz

9 years ago

No, the floating is ok, it just looks strange on scope to the standard zero volt reference point. But you can move it to any other position in this circuit to see better whats happen there.

faceblast

9 years ago

I mean the whole circuit from each transformer is at AC line voltage

faceblast

9 years ago

it seems like a huge waste of parts to make a linear power supply just to create the potential difference to run the Vgs; is there a better way to do it with bootstrapping

hurz

9 years ago

A cheap double 9V transformer is not a to bad solution. The cap bootstrap is not working for a 100% pulse width ratio, cuz it need activity to chargeup the cap periodically. So give this a try.

hamilton1300

9 years ago

Love it

ranaakbah

9 years ago

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