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Low Power Factor Motors and VFD

Low Power Factor Motors and VFD

Low Power Factor Motors and VFD

Greetings everyone,

I recently came across a very interesting problem.

We have a motor with the following characteristics.
- 75 HP TEFC
- 900 RPM
- 460V
- Nameplate amps @ 460V - 111amps, with 1.15 SF
- In. Class - F
- Efficiency - 94.5%
- Power Factor - .68

This motor is paired to a VFD that is a typical 75HP VFD rated for 96 amps, per the NEC design criteria. The VFD is a typical Yaskawa Z1000 with an Arteche type KS harmonic filter and a soft start bypass.

When we operate the motor and VFD we are getting a current draw of 120 amps. The VFDs were then upgraded to 100 HP drives so that they can handle the excessive current being generated by the motors.

My question is

- When you have a motor with such a low power factor, how is recommended to size a VFD? I assume we should simply oversize the drive. Would that be correct or is there something else that needs to be done with the VFD in order to make it work with the motor?

We have burned up 1 VFD and 1 motor thus far and I am wondering if anyone has any insight on how to deal with this. I am considering replacing the motors as i have found similar motors that should work that woudl get us within the NEC current drwa guidelines.

If any more information is needed please let me know and thanks in advance for any help.


RE: Low Power Factor Motors and VFD

That's a poor PF for that speed. Having said that, it looks like both your motor and vfd are insufficient for your load.


RE: Low Power Factor Motors and VFD

ALWAYS size a VFD for motor nameplate Full Load Amps, not HP. That was your first mistake. HP is a mechanical value, the VFD is electrical, so is sized for electrical values.

PF of the motor is not directly relevant to the VFD, but the FLA value on the nameplate includes the PF in that calculation, and that’s what the VFD is sized to.

Then you must consider the Overload Capacity of the VFD based on the application you are using the motor on. VFDs come in two values; Variable Torque (VT) / Normal Duty (ND) and Constant Torque (CT) / Heavy Duty (HD). VT rated drives are used ONLY on CENTRIFUGAL pumps and fans where load drops at the cube of the speed change, and it is virtually impossible to overload them under normal operating conditions. So the power devices inside of the VFD are selected with that in mind and are smaller than those in the other type, so they cannot take hardly any OL conditions. Since 70% of all AC induction motors are used in centrifugal machines, that becomes the definition of “Normal”, hence some mfrs using the ND designation. CT / HD ratings are capable of more overload capacity for brief periods. They should be used on ANY load that is NOT a centrifugal machine.

As to your motor having burned up, if it was not specifically designed to run on an inverter drive, that’s not unusual. Since inverter drives only took off about 25 years ago, any motor built before that is not going to be designed for inverter duty. In those cases if you cannot replace the motor, use what’s called a “sine wave filter” between the VFD and the motor.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Low Power Factor Motors and VFD

When it comes down to it, every VFD has a continuously rated current and a short term overload rated current. For example, 132A continuously and 149A for 60 seconds might be the ratings for a 100hp @ 480V VFD. Once you have these numbers then you can then size the VFD to the application. For example, if you need to push your motor to 175% of FLA for short periods of time (say to get a conveyor moving) then the example VFD would be good for an 85A motor (149A/1.75) which probably means it will be a 60HP motor.

You also have to size any VFD filters to the current requirements and not sue the HP tables. Since your motor didn't have the typical 96A FLA then I expect the soft-starter is undersized too.

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