Customer Questions Acceleration Time on Variable Frequency Drives

Customer Questions Acceleration Time on Variable Frequency Drives

3-VFDTom, a salesperson for a CMCO distributor and recent safety webinar attendee, asked the following question on variable frequency drives:

3-step infinitely variable control is 1st detent slow speed, 2nd detent HOLD, 3rd detent acceleration. If the application absolutely requires less than 2.0 second acceleration (Lodestar) can anything be done to accommodate?

Chris Zgoda, Corporate Trainer and webinar presenter, answers:

Thank you for your question. When the power is on and applied to the inverter, you can access the “U” parameters. The “U” parameters are the monitor parameters that allow the user to see what is happening. Too low of an acceleration time could present the following two issues:

  1. It could pull too much voltage off the DC Bus too quickly and cause a Uv (Under Voltage) fault.  Viewing the “U” parameter that monitors the DC Bus voltage you will see the DC Bus has approximately 340 Volts on it for a 230V hoist.  With an UP run button press you will note the voltage drops significantly at the first button press. With too quick of a ramp-up time, the voltage will drop even more causing an Under Voltage.
  2. Another potential fault is an Oc fault (Over Current) fault, meaning the inrush current that energizes the motor stator. Generally, the inrush current is about 150% of full load amps. The faster you take that motor from 0 RPM to 1725 RPM, the more current “energy” it will need.  So you could have a huge inrush current, especially if you have a heavy load on the hook, which may cause the drive to fault out, too.  You can also monitor the output current from the drive in the “U” parameter.

I might also note that the acceleration times are based on the full scale frequency of operation,  meaning an acceleration time of 5 seconds is from 0 to 60 Hz. If you press the pendant button for the 1st speed, it would not take 5 seconds to reach 6 Hz. It would be approx. 0.5 seconds.

Always follow factory service procedures when making adjustments to products.

For additional information, check out our Variable Frequency Drives Safety Webinar.

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