About Break resistor

Thread Starter


Joined Jan 9, 2012

I am having one 22kw motor which is used for hoisting purpose i am running motor through VFD

In this i want to connect break resistor

Can any one can help me how to calculate the break resistor value for my 22kw motor
What is the formula for calculating value
My hoist on time is - 25s to reach down off time is per cycle 4 min


Joined Oct 26, 2011
A braking resistor (DB or Dynamic Braking) is used on DC motors only.

Your application appears to be using an AC motor with a Variable Frequency AC Drive.
The AC drive can provide regenerative braking, but such energy goes into the bus capacitor rather than back into the power mains.

What you need is an "Electronic Dissipator" or "AC Drive Braking Unit" that throws a load resistor across the bus capacitor when the bus voltage exceeds a specific voltage. It is far more complex than adding a simple resistor and contactor. Many VFD controls offer this as an option. I suggest that you check out your drive supplier.

This is one offered by Automation Direct:

John P

Joined Oct 14, 2008
22KW implies a large system. What will be the result if any device that you install fails? People dying, expensive damage? You could get in the newspapers. The police might come. It would be interesting.

I assume "break" really means "brake".

A motor of that size couldn't possibly be a permanent magnet type. It wouldn't actually run as a generator unless you can drive the field coils somehow. But then you could modulate the power it would produce by changing the coil excitation. As far as selecting a resistor is concerned, I'd start by calculating the power produced (descent rate multiplied by moving mass) to get the resistor power rating. Getting a resistance is more tricky. Can you spin the motor (with power off) and measure the generated voltage? If you do that, and measure its resistance, you could make a guess at what the operating condition would be during descent, i.e. theoretical voltage generated for a given speed, voltage drop in motor, voltage drop in resistor. Then you'd have an idea of what the resistance should be.

If you need to experiment, you could get a box of identical resistors and play with them in various series and parallel combinations until you get the syetem working the way you want. But you have to keep it safe.

Edited to say, if it's really an a.c. motor, unless you're really an expert, don't fool around with it. It's going to be very complex running as a generator.