High voltage brushed motor control problem - back EMF

[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).

[Sneaky-geek]

Hi,
Just my 2C. Would not TVS work? I have used them in series to increase trigger voltage.
Some models can switch in just a few nanoseconds. Nice thing is they can also dissipate a few KW. Cheap TOO. Check out Digikey and the other usual suspects.

Cheers,

Terry

[PMController]

I think Speed controller is sense rotor's position based on back-EMF to manage acceleration, control speed.

[Acetronics2]

Hi, George

For relays, I sometimes use a RC series ladder to kill the back EMF ... but never tried it with such voltage and power levels.

also seen use of 2 Power Avalanche controlled diodes ( connected in series by anodes or cathodes ) in the high power welding equipment ...

Alain