Help with high voltage brush motor power control - Back EMF

[NZBen]

I'm designing a drive to control a Brushed DC treadmill motor. The drive is going to be current controlled PWM via comparitors.

I have a 600V 70A IGBT that I plan using to switch the motor (PWM), however I have some issue figuring out how I can protect it from back EMF.

The supply is going to be just over 300Vdc and I'll need to supply pulses of upto 15A to the motor, so back EMF will probably be several thousand volts.

I would ordinarily just put a diode across the motor, however in this application the motor can be driven backwards, and the torque the motor is providing should remain constant, A diode across it will not allow this to happen.

So far my thoughts are to use a gas discharge arrester across the motor (not sure will be fast enough), switch both sides of the motor and use diodes to throw the EMF back to the supply, or actively clamp the emf with a seperate FET or IGBT that triggers when the other turns off (I suppose this could be automatically done with a zener diode onto the gate to the negative side of the motor)

I'd rather stay away from switching the high side if I can,

any thoughts ideas or even bashing my thoughts are welcomed as I'm a little stuck and only half know what I'm doing.

Thanks
Ben

 

[NZBen]

Re: Help with high voltage brush motor power control - Back

I'm not having any luck so far. My dv/dt rise time for my motor back emf is a bit fast at just over 1500V/us which is too fast for gas discharge to work effectively without frying my IGBT.

High voltage FET data shows Vds limits to around +-30V so that idea is not really gonna work - the reverse voltage the motor generates when it's being overrun is around 150-200V

So i'm running to a dead end, any thoughts wouild be appreciated.

Thanks

 

[FvM]

Re: Help with high voltage brush motor power control - Back

All known PWM circuits use free wheeling diodes or synchronous switches for the commutated currents. With these circuits,
there's no back EMF problem. I can't imagine how you want to perform PWM with a single IGBT and no diode?

 

[NZBen]

Re: Help with high voltage brush motor power control - Back

Well, it's not that I have anything against diodes, it's just the diode is not going to make the machine work properly. So i'm trying to find a method I can use that wont hinder the operation of the machine. It may mean that I also have to switch the high side, or just that I get a couple of big Zener diodes to quench it out.

Thanks

 

[FvM]

Re: Help with high voltage brush motor power control - Back

it's just the diode is not going to make the machine work properly

You're talking in riddles. Is there anything special with your machine?
Or is it just the case that you don't undertstand basic PWM operation?

 

[NZBen]

Re: Help with high voltage brush motor power control - Back

Umm - well basically the machine needs to be able to be forced backwards against it's will, and it needs to be able to do this smoothly and efficiently - much like a spring with an electrical tension adjustment.

I think I have a reasonable understanding of PWMing a motor - I've had a reasonable amount of experiance in driving dc brush motors upto 500W for various applications.

If there is a diode across the motor "the machine will not work properly", the motor is geared and pretty much locks up if there is a diode across the motor when the motor is overrun (forced backwards).

I need to come up with a different mechanisim to quench out the back EMF spike so as to not damage my IGBT. Although you are correct in saying most all PWM motor circuits have a diode on them, I'm sure there are other methods of doing the same thing.

So the thoughts I've come up with so far are:

Gas discharge arrestor (too slow - discharge voltage rises rapidly with fast dV/dT)

Switch high side and low side simultaniously then run EMF back to supply through bridge rectifier (complex - would prefer not to drive high side if possible)

Use high voltage zener in series with a standard diode linked across the motor (zener will have to dissipate a lot of heat)

Actively clamping the EMF - (think this might be troublesome - typical Vds is only around +-30V)

So this is where you highly diverse and intelligent bunch of people come in. - Is there a BETTER way?

All I'm tring to do is prevent more than 600V from hitting my IGBT when it turns off. The motor will run upto 200V when run backwards and 300V when running forwards.

 

[FvM]

Re: Help with high voltage brush motor power control - Back

What you observe as "back emf" is a considerable part of the energy you supply to the motor. You are of course able to absorb it
somehow, but the principle of PWM operation is to recover it. That's why I asked, if you are aware of the basic operation. It may
be O.K. to simply discard the stored energy in a low power application. This way, you reduce the PWM efficiency to a linear regulator,
round about. I don't think, that it's a reasonable method for a kW drive.

The problem is however, that a simple single quadrant chopper doesn't allow bipolar motor voltage. You have to go for a
H-bridge or a special design appropriate for your application, e.g. a freewheeling diode with a disconnect switch. If this is all too
much, simply accept the waste of energy. I'm sure, you'll able to figure out a suitable energy absorber.

P.S.: An effective circuit, that allows freewheeling up to the input (respectively bus) voltage, needs two switches and two diodes.
It's often used for arc welding inverters. It's a like a H-bridge with all parts for negative current direction omitted.

 

[NZBen]

Re: Help with high voltage brush motor power control - Back

I've tried various methods so far, I have got it working to a point, however I think the only way to do it is to switch the high side also. Does anyone have any good method for switching the High side of the motor also? I was going to use the same NPN IGBT and same driver so that the turnoff time is close to the same.

So I'll need to create a higher voltage to supply the IGBT Driver, limit it, also suitably trigger it across a 300V potential.

Any thoughts?

 

[dick_freebird]

I think you should consider a simpler control setup. If you
want reversable, why not a commercial H-bridge driver
and 4 power MOSFETs? Your voltage and current are not
so great that you need IGBTs. The bridge driver bootstrap
circuitry will eliminate the need for a higher than main
secondary supply, so long as you have a minimum switch
rate and minimum off time.

Motor back-EMF does not sound like your problem, spikes
are probably just winding LC related. You want continuous
conduction and voltage mode control if you are trying to
make a speed controlled machine. Use a current sense
term to improve speed regulation by removing I*R from
the back-EMF quantity (sum an inverted Vresistive term
into the feedback, appropriately scaled to prevent runaway).