Dynamic Braking Resistor 3

[rockman7892]

I have a 60hp 480V motor which is being driven by a 75hp Siemens MicroMaster 440 VFD. The application for this motor is a grizzly feeder (crusher shaker) which presents a high inertia load to the motor. Whenever the motor is VFD is stopped or ramped down, the VFD is tripping on a bus Overvoltage condition. This tripping has become a persistant problem and I need to find a solution.

I wanted to go ahead and install a braking resistor on this drive to dissipate this excess voltage when the motor is being ramped down and regenerating voltage back onto the drive. Can anyone help me determine what size braking resistor I will need for this drive. Is there a formula or some other means of calculating the size of the resistor need for this application?

 

[cswilson]

rockman:

Try this link for a basic understanding of the different drive technologies:

http://www.ab.com/drives/techpapers/PWMDrives01.pdf

Fundamentally, a classic VFD is open loop -- that is, the waveforms it outputs are not dependent on what the motor is actually doing.

By contrast, "vector" and "field-oriented" control drives are closed-loop -- they use feedback in the calculation of the output waveforms to optimize the performance, including (but not limited to) tightly controlling the slip frequency for optimum efficiency.

"Sensorless" vector/field-oriented control is a bit of a misnomer. The technical papers use the term "shaft- sensorless". That is, there is no encoder, resolver, or tachometer on the motor shaft for position/velocity measurement, so these quantities must be estimated from the drive's electrical voltage and current sensors through a sophisticated motor model (which must be tuned).

As computational costs come down, VFDs increasingly are able to run as sensorless vector controllers; you just need some better internal sensors and better algorithms. From the outside, you don't really know the difference.