# Thread: How much voltage is too much? (Back emf)

1. ## How much voltage is too much? (Back emf)

My IM805 controllers will handle 75V "absolute max... including back-EMF". How do I calculate back EMF?

Reason is this... I have a 70V linear PS which works well with this system (and which I was told by IMS's tech support would "be fine" because the voltage drops a bit under load), but now I want to build a PS into the controller enclosure and it would be 72V linear. Wondering if back-EMF will push this over 75V now, even with a bit of voltage drop.

I can always call IMS again, but I'd like to know how this works, rather than just getting a yes or no answer.

Cheers,
-Neil.

2. ## Back EMF

Back EMF is a function of a ton of things, it would be difficult for you to perdict it.

If your machine has fast rapid moves, lots of inertia, low sliding friction, and you do sudden stops from high linear speeds in multiple axis that would probably push it up the highest.

My guess, that any reasonably designed mechanics in a machine tool with moderate to good performance will easily push your bus voltage up more than 5 volts.

I'd suggest a simple active load / clamp to keep it from going above say 73 volts. You'll need a power npn /n-channel transistor, an op-amp, voltage reference, and a few resistors.

John

3. Hmmm... any links on how to do this? BTW, the 72V is if I use a 50V transformer. My alternate option is to go with 40V transformer to make a 56V PS, but I want to get as much performance out of my setup as possible. If I can find a 45V-48V transformer that fits, that would be ideal.

4. This circuit is designed for lower voltages, but the same applies, just change zener diode voltages. The image on http://www.pminmo.com/wiki/index.php?title=Power It's not mean't to be a 100% duty cycle shunt, it will burn up at 100% duty cycle. It's meant to clamp/drain the back emf energy on a nema 23 motor. As far as looking for the idea transformer to get your voltage, you can also utilize an autotransformer arrange to boost the primary voltage. Here I describe utilizing a 24V transformer to boost the 120V pronary to get a small value boost: http://www.pminmo.com/wiki/index.php?title=40vdcsupply