Estimating Motor I & V under VFD operation
Estimating Motor I & V under VFD operation
(OP)
What is the best way to estimate 3-phase motor current and voltage when driven by a modern VFD at different RPM and torque loads?
Example Motor:
HP - 25
Poles - 4
Volts - 230V
RPM - 1748 rated (1800 SYN)
FLA - 59A
FL Eff - 92%
Torque ~ 75 ft/lbs
VFD input V - 460V (allows full torque above rated speed)
The formulas that I have run across require knowing intimate details about the motor and are usually applied using a sine wave and known line voltage, not a VFD. Is there any way to at least get close when required shaft torque and RPM is known?
Thank-you, Bill
Example Motor:
HP - 25
Poles - 4
Volts - 230V
RPM - 1748 rated (1800 SYN)
FLA - 59A
FL Eff - 92%
Torque ~ 75 ft/lbs
VFD input V - 460V (allows full torque above rated speed)
The formulas that I have run across require knowing intimate details about the motor and are usually applied using a sine wave and known line voltage, not a VFD. Is there any way to at least get close when required shaft torque and RPM is known?
Thank-you, Bill





RE: Estimating Motor I & V under VFD operation
HP = Tq (in ft-lbs) x RPM / 5250, so since you say you know the shaft Tq ad RPMs, you can get HP.
I (current) = 746 x HP / 1.732 x E (Voltage) x Eff. x Pf.
The output voltage of the VFD it will be basically linear to speed percentage (assuming it is a constant torque application), i.e. at 50% speed you should be at 230V output, at 75% speed you should be at 345V etc. etc. You can estimate the Eff. of a standard or Inverter Duty motor at 80%, 90% if it's an energy efficient one, and you can estimate pf at the motor as .7 if you like.
RE: Estimating Motor I & V under VFD operation
The application, sadly, does not have constant torque over the RPM range. From what I understand, the VFD is able to keep the motor in a fairly linear torque vs current mode, except perhaps down at the very low torque levels and RPMs.
If I can solve for voltage or current reasonably under different RPM and Torque levels, then the formula you posted above should get me close. The system has not been built yet, so the estimates are needed for equipment selection / performance purposes.
Bill
RE: Estimating Motor I & V under VFD operation
But now you have me curious as to why you needed this information "for equipment selection / performance purposes." You must always select equipment for the worst case scenario anyway, which would be full speed at full load, not the reduced case.
RE: Estimating Motor I & V under VFD operation
I'm trying to graph what the motor and drive are seeing under various loading conditions and adjusting equipment selection and performance expectations until I reach a happy compromise. While I can do this by treating the drive and motor as a pair sized for worse case, I wanted to estimate what the VFD output voltage and current levels would be under different loading conditions below maximum.
Thank-you, Bill
RE: Estimating Motor I & V under VFD operation
RE: Estimating Motor I & V under VFD operation
I'll give it a go and see what it looks like.
Bill
RE: Estimating Motor I & V under VFD operation
Equation used for speeds below 100%
Hp = k * speed^3 (centrifugal pump)
Hp = 100% = 1
Speed = 100% = 1
1 = k * 1^3
K = 1/1^3 = 1
50hz/60hz=.833, hp = .833^3=57.8%, 57.8%hp at 50hz.
40hz/60hz=.666, hp = .666^3=29.5%, 29.5%hp at 40hz.
30hz/60hz=.500, hp = .500^3=12.5%, 12.5%hp at 30hz.
20hz/60hz=.333, hp = .333^3=3.64%, 3.64%hp at 20hz.
10hz/60hz=.166, hp = .166^3=.457%, .457%hp at 10hz.
You can estimate the motor current based on the calculated HP by using the methods described above by Jaref.
Please note the power factor will decrease as you reduce the HP of the motor via the VFD. You should be able to estimate the PF from the motor data. The data should give PF for 1/4, 2/4, 3/4, 4/4 load. The efficiency should be close throught the speed range.
Also remember the power factor on the input to the VFD will be constant throught the speed range ~0.95