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Can I run the motor driven by VFD on any frequency?

If I connect brake resistor to the VFD to absorb the regenerative energy, I can to run the motor driven by VFD below the line frequency. On another way, what about if I connect both motors on VFDs (each motor driven by separate VFD) can I turn the driven motor on any frequency I need?

Yes, you can, however check the wattage ratings for dynamic braking resistors - they are far below the full power rating of the variable frequency drive.

I have done precisely as you describe - connecting two motors together and running them both on VFDs with one in dynamic brake mode. My test setup was to load-test a motor/VFD combination. (The company I work for has a water-brake dynamometer capable of 100% load testing up to 1000 Hp [750kW]. Unfortunately, the minimum load of that dyno is about 25Hp [19kW].)

The test motor was rated 13kW, the variable frequency drive is 15kW. My "load" setup was a 30kW motor on a 37kW VFD. The "heavy duty" rated dynamic braking resistor - catalog item from the VFD manufacturer - was rated 8.0kW. Sufficient to brake a 30kW motor to a stop over a few seconds, then cool down over a minute or two before doing it again. It was not sized for continuous duty.

I was able to dump a maximum of 15kW through that resistor continuously by blowing a LOT of air over it, and it still glowed dull red (without cooling air, it was bright orange), but the 37kW VFD could not handle any more than 15kW of regenerative power. The dynamic braking circuit is not designed for it.

The test procedure was to start the test motor and accelerate it to the test speed, then set the load VFD to match the frequency and start it in "catch-on-the-fly" mode. Once the motors were speed matched, I reduced the frequency reference to the load VFD until the test VFD indicated full load current. Alternately, I could have increased the test VFD reference to accomplish the same thing, but my test procedure was to load the variable frequency drive at different speed settings and observe the VFD and motor temperature rise.

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