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

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.

Post a Comment:

Plz Calculate (7 * 5) =
(Your comment will show after approved.)

You may also like:

Featured Articles
Non-Enclosure Variable Frequency Drive ... Non-Enclosure Variable Frequency Drive (VFD)No enclosure (cover), reducing installation space and cost effective. Widely used in All-In-One control cabinet. Keep the same ...
Variable frequency drive application ... Variable frequency drive application guideVariable Frequency Drive (VFD) can be used in lots of fields. Variable frequency drives are widely used to control the speed of ...
Variable frequency drive in HVAC ... Variable frequency drive in HVAC systemsVariable frequency drives (VFD) have been used for HVAC systems in buildings for more than 40 years. But only in recent years, ...
Variable Frequency Drive Harmonics and ... A discussion of the benefits of variable frequency drives often leads to a question regarding electrical harmonic distortion ...
Three phase inverters Three phase invertersIn the variable frequency drive rectifier paper, it explains how to go from three phase alternating current voltage to a direct ...
Variable Frequency Drive Service
VFD manufacturers
Variable Frequency Drive Sales Email buy@vfds.org
Variable Frequency Drive Support Email tech@vfds.org