VFDs and strange motor measurements

[MD88]

Hello,

I was given some site data about a motor running on VFD...

RPM: 1391
Hz: 45 (measured)
Amps: 185 (measured)
HP: 100 (nameplate)
Voltage: 480 (nameplate)
Power Factor: 0.65

On the same spreadsheet, they calculated shaft output power on the motor to be:

P= Amps*Voltage*1.73*Power Factor*(45Hz/60Hz)

Does this seem right to you guys?

 

[steve66]

No.

Steve

 

[Jraef]

...
Amps: 185 (measured)
...
Power Factor: 0.65

What does "measured" mean? If the data was not taken from the VFD display, it is probably meaningless. Only the most expensive sophisticated meters can accurately measure, filter and interpret the output of a VFD correctly.

And is that .65 pf value from the motor's nameplate? That's a horrible motor or a high slip motor if it is. If they measured the pf on the output of the VFD however, same issue: it's meaningless.

 

[Besoeker]

Hello,

I was given some site data about a motor running on VFD...

RPM: 1391
Hz: 45 (measured)
Amps: 185 (measured)
HP: 100 (nameplate)
Voltage: 480 (nameplate)
Power Factor: 0.65

On the same spreadsheet, they calculated shaft output power on the motor to be:

P= Amps*Voltage*1.73*Power Factor*(45Hz/60Hz)

Does this seem right to you guys?

If the voltage used to calculate power was nameplate voltage this would be an obvious error. At 45Hz the voltage applied to the motor would be 360V (or possibly lower.

To a good approximation the power motor can produce is proportional to applied frequency. At 45Hz you could get 75 hp unless constrained by the reduced cooling.

 

[winnie]

IMHO this equation is a correct but poor approximation to the _input_ power to the motor.

480*(45/60) is _approximately_ the voltage being supplied to the motor.

While voltage measurements on the output of an inverter are almost meaningless because you see the switching of the DC rail voltage, current measurements are more likely to be useful. The motor filters much of the high frequency current, so an ordinary clamp on current meter will provide reasonable (but noisy) numbers.

It looks like power factor came right off the nameplate, which is pretty inaccurate unless you are operating at near full torque.

Thus the equation is Pin = (approximate voltage) * (measured current) * (guessed power factor) * root(3)

-Jon

 

[steve66]

I was assuming they measured the input voltage and current to the VFD.

The Power In would be Amps * Volts * 1.73 * PF.

P out = P in - P losses.

The P losses should be pretty small for the VFD and motor. But, at any rate, I don't see any reason to include the frequency conversion of 60 Hz to 45 Hz in the equation (again assuming we are using input voltage and current, and a faily accurate RMS Meter.)

Steve

 

[Jraef]

I was assuming they measured the input voltage and current to the VFD.

The Power In would be Amps * Volts * 1.73 * PF.

P out = P in - P losses.

The P losses should be pretty small for the VFD and motor. But, at any rate, I don't see any reason to include the frequency conversion of 60 Hz to 45 Hz in the equation (again assuming we are using input voltage and current, and a faily accurate RMS Meter.)

Steve

Depends on where and why the "P" is being looked at. If it is Power consumption as it relates to cost of operation, then I would agree; what takes place between the VFD and motor is largely irrelevant. But if the "P" is Power consumed by the motor for comparison to input power in order to determine efficiency, or shaft power at any given speed to determine input power expressed on drive train components, then the output current and voltage becomes relevant and the speed is therefore relevant as well. At 45Hz, the shaft power will be lower by the ratio of full speed to operating speed, so if you are basing your calcs on nameplate data (which assumes full fixed speed), then you must compensate.

 

[weressl]

What does "measured" mean? If the data was not taken from the VFD display, it is probably meaningless. Only the most expensive sophisticated meters can accurately measure, filter and interpret the output of a VFD correctly.

And is that .65 pf value from the motor's nameplate? That's a horrible motor or a high slip motor if it is. If they measured the pf on the output of the VFD however, same issue: it's meaningless.

Actually the ASD's nameplate represent the valuas pretty accurately, they are RMS values within the display accuracy. Note: the voltage was given as nameplate, not measured.

 

[weressl]

If the voltage used to calculate power was nameplate voltage this would be an obvious error. At 45Hz the voltage applied to the motor would be 360V (or possibly lower.

To a good approximation the power motor can produce is proportional to applied frequency. At 45Hz you could get 75 hp unless constrained by the reduced cooling.

That of course would depend what mode is selected on the drive.Linear or Squared V/F ratios will produce different resullts.