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#129324 - 01/19/05 01:40 PM Suitable Transistor
rich-wong Offline
Junior Member

Registered: 01/16/05
Posts: 2
Loc: London,NA,UK
Hi all

first thread and all that, it's gonna have to be a question

here is my problem. I want to control an AC induction motor using PWM, so far i think i will Rectify the AC so it becomes a nice DC voltage then generate 20KHz or higher PWM pulse using a PIC16 or PIC18, and an oscilator to control the speed (manual control). then the PWM goes onto the Switches which are going to be a Half Bridge Transistor Bridge. and hopefully it will work like a treat.

Now questions: 1) Are there an Tansistors out there thats say about $10 and can do high power (600watts or so) and can they be switch at 20KHz? 2) if i can avoid it, i really don't want to use MOSFET due to their prices, unless you can find me some really cheap ones. and lastly 3) do i need full bridge? as the motor only really need to go forward and stop.

thanks for any sort of help.


#129325 - 01/22/05 02:04 PM Re: Suitable Transistor
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Hi Rich, and welcome.

Almost any general-purpose power transistor these days will operate fine at 20kHz. For 600W though, you'll probably end up running multiple transistors in parallel.

Are you planning on controlling a regular 120 or 240V motor, or a low-voltage type? That will limit the choice of suitable devices a little.

MOSFETs, though pricey in high power versions, do offer a lower source-drain resistance when saturated than the equivalent emitter-collector resistance of their bipolar counterparts, so you may want to reconsider whether it's worth the extra for the improved efficiency.

There's no need for a full bridge arrangement if you're only ever going to apply power to the windings in one direction.

#129326 - 01/24/05 07:19 AM Re: Suitable Transistor
IanR Offline

Registered: 12/06/04
Posts: 326
Loc: Palm Bay FL USA
600 watts for 10 bucks? Thats kind of a tall order. A single BJT that'll do 600 watts is probably going to cost signifigantly more than that. Maybe using a few smaller devices in parallel with thier associated emitter degeneration resistors might get you close to $10, but it will still probably be difficult.

#129327 - 01/24/05 07:29 AM Re: Suitable Transistor
rich-wong Offline
Junior Member

Registered: 01/16/05
Posts: 2
Loc: London,NA,UK
aye i understand the difficulties in these works. what are the issues i have to be considering while connecting BJTs in parallel?

#129328 - 01/24/05 09:09 PM Re: Suitable Transistor
winnie Offline

Registered: 09/15/03
Posts: 652
Loc: boston, ma
For a system this small, you will see far more cost in the gate drive and balance of system then in the actual transistors.

Say you want to drive a small single phase induction motor, 1 horsepower at 120V. You rectify your supply to get DC at about 170V DC rail, and then you use 4 transistors in a full bridge to synthesize roughly 8A RMS to the motor. You could _borderline_ get away with MOSFETs or IGBTs that have a 20A 200V rating. You could find parts that would get the job done for under $2 (note: you need 4 of these to make the full bridge.)

But in addition to the actual transistors doing the switching, you need to supply gate drive to run the transistors. The gates of large mosfets are capacitors, and you need to charge them quickly. So you have to add a gate drive chip. Then you have to add the fact that the 'high side' switches in the half bridge need to be driven with a floating supply.

If you want to switch at 20 KHz, I'd expect gate drive to swamp the cost of the transistors themselves.



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