vfd output current rating higher than input current rating

A VFD can be designed to put out close to double the input voltage.
I have controls that are 240V and include 240 In 480 Out VFDs.


The DC buss is charged with each side of the AC phase going to one side. An AC voltage of 480 is actually +480 for half a cycle then -480 the other half. Using input diodes the buss could (almost) be charged to 960V theoretically (with no load being used) Commonly the buss would be at least 600V.


If your VFD does occasionally put out a higher voltage that is not motor regen, then it may have bad IGBT transistors.

I doubt the RMS voltage is 550V. What meter are you getting this from?

The PWM sine wave is an ugly thing for a typical meter to understand. Unless the drive or motor are showing faults or excessive heat, then believe the display on the drive, not external meters.

The peak voltage of the PWM will be around 650-700v, depending on load, and the drive is smart enough to adjust the duty cycle of the transistors to adjust for the changing bus voltage.

Gene Bond said:

I doubt the RMS voltage is 550V. What meter are you getting this from?

The PWM sine wave is an ugly thing for a typical meter to understand. Unless the drive or motor are showing faults or excessive heat, then believe the display on the drive, not external meters.

The peak voltage of the PWM will be around 650-700v, depending on load, and the drive is smart enough to adjust the duty cycle of the transistors to adjust for the changing bus voltage.

Click to expand...

We were using our Dranetz Power Visa.
http://www.dranetz.com/wp-content/uploads/2017/04/Dranetz-PowerVisa-Brochure.pdf

When the output voltage is 540-550, the peaks are 860-870.

The first time we noticed this was when we hooked our meter up. At that moment, we didn't think of checking the keypad. We left our meter there to record because every time since it is putting out 460V when we are there.

Also they just told me that this pump has been running hotter than the other ones.

My suggestion is swap out the VFD and see if it stops.


Along with the transistors it could be the control card firing them wrong.


If it doesn't stop then it would be the pump motor

It sounds as if its regenerating. Odd behavior for a pump, but possible. Is it possible the drive is decelerating too fast for the flow? Or is it possible the pump is being over-driven by the suction-side pressure? (or discharge-side vacuum/low pressure)

Gene Bond said:

It sounds as if its regenerating. Odd behavior for a pump, but possible. Is it possible the drive is decelerating too fast for the flow? Or is it possible the pump is being over-driven by the suction-side pressure? (or discharge-side vacuum/low pressure)

Click to expand...

So this could be possible.
The system is programmed so that as the pit level drops, the hz drop. The hz do not drop abruptly and its directly proportional to the level in the pit.

For your second question, I could run it by the pump engineer who is also involved with this question. There are 5 identical pumps sitting side by side doing the exactly same thing (just not simultaneously) and this is the only one acting like this.