How to implement a thermal protection of braking resistor?

[Steven01]

Let's say I have an inverter fed three phase induction motor drive where in the braking phase (when the motor operates in a generator mode) the generated electrical power is dissipated in the braking resistor controlled via simple chopper. Below is a record of the braking chopper operation.



I need to implement a software based thermal protection of the braking resistor. Unfortunately there is no temperature sensor installed in the braking resistor. So I has to somehow estimate the temperature.

My first idea how to do that was to exploit the measured dc link voltage and the current through the braking resistor and calculate resistance of the braking resistor via the Ohm's law. Then I can derive its temperature from the temperature dependancy of the resistance. I have attempted to experiment with this approach but I have received huge spikes of the resistance value. The results aren't much more better neither in case I filter the resistance values via the simple first order low pass filter with time constant 1 s (from my point of view reasonable value)



Does anybody see any way how to get more reliable resistance values via the method I have described above?

 

[joseph_e2]

I understand the desire to calculate this out, but it's almost certainly going to be more reliable and definitely simpler to install a temperature switch. One step fancier would be a temperature sensor with an analog output so you can monitor the actual temperature over time.

The last drive I worked on was an Allen-Bradley PowerFlex 525. They have software to help you select the correct resistor based on expected duty cycle and a parameter in the drive to tell it what resistor it has. The drive then has its own algorithms inside to protect the resistor.

I'm not sure what you're calculating there, but the brake resistance is constant so if you're measuring the actual brake current (I've never seen a drive present this information to the user but it would be cool information to have), then calculating the dissipated watts should be fairly straightforward (I^2 x R). The brake resistor should have some specifications for dissipated power (duty cycle, wattage, etc.). I don't think you're going to be able to get synchronized samples of the brake current and DC Bus voltage and have them match well enough and be fast enough to accurately calculate the brake resistance...but the resistance of a resistor doesn't change.

 

[JesperMP]

Maybe just a circuit breaker ?
The circuit breaker will do the averaging of the thermal load.
And with an aux contact to inhibit the VFD if the breaker is tripped.