Brake Resistor Failure in Variable Speed Drive


Thread Starter


We are using LTi PMII+ drives for pitching of wind turbines blade. we are facing failure of Brake resistors leading to dc link over voltages.


Voltage: 3*400V
Current: 40A
Power rating of drive: 22kW

Chopper activation when DC Link voltage >650V
Over voltage error when DC Link voltage >=700V
Brake resistor: 300W, 20Ohms

We are using pitch drive with 7.5kW motor to pitch the blades.
Rated speed of motor 1445rpm
Rated current: 27A
eff: 0.86

max deceleration: 20deg/sec2, deceleration time: 0.4sec,
at max speed of 8 deg/sec (blades) where speed of motor is 1516 rpm of motor, taking into consideration GB ratio
max Braking duty cycle = 1.83%

We are facing brake resistor failure in this drive due to which we get over voltages on DC link and the entire system (Turbine) trip. I need help to understand what can be reason for the failure of the resistor.

I have calculated average brake power Pavg = 1072.57W. As per datasheet of Resistor we can load it 20 time *300W (rated power) = 6000W.

So required Brake resistor = 1072.57W/20 = 53W and we have used 300W Resistor.

Curt Wuollet

I'm not a wind turbine pitching expert, but If your resistor is rated at 300 watts your average power should not exceed that.

Your peak power can perhaps be 20 X your rated power. But an average power of 1kW will probably toast a 300 watt resistor.

The 20 X rating is to allow for a low duty cycle. And that's the story your faults and burnt resistor tell. In any case, I'd say you need a resistor with a higher power rating so it doesn't burn out, regardless of any math:^). I like to oversize these as well so you don't see light coming out and they don't burn open and fault your system.


Bob Peterson

If the resistor is actually failing that is a sign that there is much more energy involved than what your calculations show.

There are a few things you can try. One is to force air cool the resistor. The other is to increase the rating of the resistor so it can handle the actual load.