High voltage brushed motor control problem - back EMF

[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

[George]

Thanks for the comments, I'm not sure a TVS would do the job - unless it was huge. It would be dissipating a lot of power near enough to continuously? Arent they only made for transient voltages?

What do you guys think of an SCR triggered by a Zener?

I'm unfamiliar with them although I have set a pic to trigger triacs before for light dimming. I guess a Triac is jst 2 SCRs? So rather than dissipating 350-400V like a TVS would be doing it would only dissipate around 2V?

My understanding of them is when they see a voltage, they latch. They then unlatch when the voltage across them returns through zero.

So the main IGBT will turn on, charge up the motor to set flux (full flux is 10ms) then turn off for a set period. The SCR gate would be set with a zener - probably a standard diode also to prevent main power going through it. Then EMF spike rises, latches SCR when it reaches Zener voltage, this bypasses the voltage back upto the top rail. Voltage then drops and SCR unlatches?

Not sure - confirmation would be nice.

Thanks

[aratti]

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.

George, I think Herink's suggestion is still the best one!

You can activate the relay via a dedicated pic port (hope you have a spare one) so you can control the timing. Activate the relay, wait few millisecs and then fire your IGBT via pwm.

Reverse the concept when you will stop the motor: set pwm duty cycle to zero, wait a while till the motor stops turning, then open the relay.

All very easy to implement using PBP.

Al.