Setting up duty cycle for braking resistors

[rockman7892]

I am installing a brake resistor on a 480V vfd, with the brake resistor having a continuous rating of 5000W. I understand that once the resistor is installed across the chopper on the drive, the chopper must be set up to recognize that the braking resistor is installed.

One of the parameters for setting up this chopper, is setting the duty cycle for this chopper controlling the on time of the resistor.

When setting up the duty cycle, is it typically set as a function of the resistor rating weather continous or max rating, or is it a function of how often the drive is stopped or ramped down. I belive it is a function of how long the chopper keeps the resistor in the circuit, but I'm not sure what vaules to look at as the limiting value for determing duty cycle.

What factors need to be taken into consideration when setting this duty cycle?

 

[DickDV]

The chopper does indeed control the power to the resistors but it does that based upon the DC bus voltage it sees.

On a 480V drive, once the DC bus voltage sees more than about 800VDC, it turns on to divert the excess energy to the resistors.

The chopper will do this mercilessly, even burning up the resistors if the high voltage conditions persist. It is the braking conditions in the application that determine how often and for how long the bus voltage stays high and, consequently, how often and for how long the resistors are energized.

Generally, the resistor wattage is a continuous rating. If the resistors are cold, they can usually take a five or ten second "hit" up to 10 times there continuous wattage rating and be ok.

You will need to analyze the frequency, duration, and intensity of your application's braking requirements to determine if your resistor set is large enough.

 

[ozmosis]

rockman
What type of VFD is it? (I recall you have used Siemens Micromasters in the past)
Another factor determining the brake duty is the current rating of the brake chopper. This will respond differently depending on the braking duty required.
As DickDV points out, the chopper will come into operation once the DC voltage reaches (approx) 800vDC and so this voltage tied in with the ohm value of resistors will then be dependant on the current rating of chopper.
Once the current rating of chopper is deemed ok for continuous duty then the thermal characteristic of the resistor should be checked and this is usually a thermostatic device mounted to the resistor and then connected into the VFD or into the mains supply feeding the VFD to shutdown if it gets too hot.
A useful document is attached (from Siemens) to highlight what this all means.