Braking requirements

[IASMike]

I am installing a VFD on a calendar Mill. Osha requires braking during a power failure. I was going to use a dynamic brake with a capacitor bank. The vendor I am dealing with insists that I have to install a mechanical brake. Does anyone know which standard states you need a mechanical brake ? Which standard states I cannot use a dynamic brake with a capcitor bank ?

 

[itsmoked]

How does a capacitor bank offer any braking? Do you mean a resistor bank?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

You can theoretically use capacitors to store a DC charge for dynamic braking into a resistor bank, but don't be mistaken, it would take a MASSIVE amount of capacitors to store any useful amount of power. I once did the calcs to try to prove this out for a 40HP down hole PC (Progressive Cavity) screw pump in the oil industry. It took a cap bank the size of a bus to hold enough charge to brake that motor for 1 minute after a power failure. People have used batteries too, but even that usually proves out to be impractical.

Save yourself the trouble, use a mechanical brake.

 

[LionelHutz]

You can set-up a VFD with a braking resistor and have it brake the motor when the power goes out. Basically, you make sure that a power loss will cut the run signal so that the VFD will decel immediately on power failure. The DC buss will stay energized and keep the electronics running so it can complete the decel cycle. You have to program the VFD to not trip on the power loss.

 

[LionelHutz]

To answer your question on the braking. I'm not aware of a North American standard that states how to do the safety. It seems that for the most part you can read what needs to be done (brake the motor) for safety but it's up to you to figure out how to actually do it.

 

[CJCPE]

The drive vendor's insistance on using a mechanical brake may not be based on any standard or code. It may be based on advice of their product liability law department.