Three-Phase Motor Current Calculation

[JoshBosh]

I have a three phase motor rated at 30KW, 400V, 55A with a p.f of 0.85.
From this does this mean that the efficiency of the motor equals 92% ? (based on P= 1.73*U*I*p.f*eff).
The motor is rated at 30KW, is this value the physical rated mechanical output rating of the motor, and thus the electrical input power = 32.388KW ? (based on P= 1.73*U*I*p.f).
Does this simply mean that current taken from the supply will still be 55 amps/phase ?

If I fit a VFD to the motor, does this have an affect on the increasing the p.f it to a higher value, thus the current drawn is much less ?. If so, how do i know what this p.f and current reduction is ?

Gratefull for any help received...

 

[aolalde]

Yes, at full load the efficiency (EFF%) is 92.62% because:
EFF =100* kWo / kWin , were:
kWo = The mechanical power out put at the motor shaft.
kWi =1.732*kV*IL*PF/100
kV = the line to line voltage
IL = the line current
PF= the electric supply power factor.

The total electrical input power in kVA is:
KVAi = 1.732*kV*IL = 38.105 kVA

The active electric power (kW) is kWi = kVA*PF = kWo/EFF
KWi = 38.105*.85 = 32.389 (kW)

As you see above, the nominal 30 kW is the mechanical power delivered at the shaft output (kWo)

When a VFD is added, the input to the VFD will change since it will convert AC power to DC power normally at the VFD input the PF will be close to unity. Then the VFD will feed the motor with a generated AC system, which is not necessarily a pure sinus wave, some difference in the electrical parameters could be expected depending on the type of driver selected. Expect some harmonics and possible surges on the motor lines.

 

[LionelHutz]

The VFD will only draw the active or real power from the power line. The reactive motor power only flows between the VFD and the motor.

If running the motor with no load then the VFD input kVA would be very low, being just the power required to keep the shaft turning, while the output kVA would be somewhere around 10kVA. If running at full load the input to the VFD would be closer to 32kVA than the 38kVA that you would see if the motor was connected directly. Unfortunately, the input harmonics of a VFD negatively affect the input current and kVA so don't plan on using lower rated input cables with a VFD.