High voltage brushed motor control problem - back EMF

[George]

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

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 needs to 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 ideas are welcomed as I'm a little stuck and only half know what I'm doing.

Thanks

[mackrackit]

Look at the pdf on page #3. Replace the DPDT with a relay for direction. Replace the SPST with your IGBT for speed.


http://www.picbasic.co.uk/forum/atta...4&d=1228220843

[George]

Thanks, but I'm not sure that's going to solve my problem. Electrically the motor is going to be only driven in one direction, however it will occasionally be forced backwards mechanically, and I dont want the freewheeling diode to get in the way and provide increased torque when motor is being 'overrun'.

So how to cope with the back EMF without a freewheeling diode is my question - at the moment I'm thinking of using 2 x 200V zener diodes onto the gate of a FET that's wired across the motor, when the voltage gets reverse above 400V, the FET switches on and quenches it out, the potential then changes and the charge on the FET gate should drain back through the zeners turning the FET off quickly.

The IGBT is good for 600V so the 400V FET trigger should be enough margin?

I'm hoping at the moment the tail on the turnoff of the IGBT will limit the back EMF to managable levels.

[mackrackit]

Sorry I misunderstood your first post. I was thinking the motor would be under power when running in reverse.

I do not know of any good way to not use a Fly-back diode, I never noticed much resistance using one either.

[amgen]

Hi,
is there any driving current to the motor when it is driven backwords by the mechanism ?
If not, the motor will develop voltage in reverse of the drive, but unless there is some load resistance , I don't think the motor will present any torque.. ?

don
amgen

[HenrikOlsson]

Don,
The problem, as I understand is it that if he places a flyback diode across the motor (or across the switch) IT will be the load when the motor is backdriven - in one direction but not in the other. As soon as the motor is backdriven fast enough to produce an EMF larger than Vf of the diode it will get forward biased and start conducting current.

George,
Crude solution but anyway: If the motor is only backdriven when power is off can't you put the diode in series with a relay that is activated when the system is powered. That way the diode is "in circuit" when the power is on and disconnected when the power is off.

/Henrik.