High voltage brushed motor control problem - back EMF

[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.

[George]

Exactly Henrik, There is going to be power supplied to the motor when it's driven backwards (overrun) so unfortunately the relay idea wont work. It's also going to be run it at high freq PWM so would guess the IGBT would see a few thousand plus volt spikes before the relay made contact on it's first switching.

My idea of using a Fet to quench it out is not looking that hopeful looking at some datasheets, the Vgs is only around +-30V on some high voltage FETs. I suppose tho I could use some more complex circuitry to activate the FET

Gas discharge arrestors look like they might be too slow - their spark over voltage dramatically increases with dv/dt speed.

Maybe an LC quench between IGBT and Motor?

Thanks for your input so far guys

[George]

BTW my dv/dt rise time for my motor back emf is around 1500V/us - also the motor is geared so when mechanically overrun is generating some 200V in reverse direction so that's well over the Vf of the diode and causes significant drag - pretty much locks up!

[mtripoli]

This is interesting... I worked at Powerex years ago doing "smart" power module design. Normally we'd use a bridge to drive motors so I can't think of a way out of this one... One thing comes to mind; clutch the gear train so it can't drive backwards...(sorry, had to throw that out there).