Hello,

I thought someone in this forum might help me.

I need to calculate the eletric power consumption of a motor.

If I remember correctly, for 3 phase motors...

P (watts) = V (Volts) x I (Amperes) x Cos (phi)

what would be the V and I, the line or the phase values?

Thanks

For a 3 phase motor Power (W) = (1.732)(V (phase-phase))(I (any phase))(Power Factor)

The current going thorugh the three phases should be same unless there is a high resistance, etc. type problem. The voltage is the phase - phase voltage (480, 4160, etc.)

Dave

Yup, Dave's got it right.
P = sqrt(3) * VLL * I * P.F. where VLL is the line-to-line = phase to phase voltage.

If you want to derive it for yourself, think of one phase of a balanced 3-phase motor wired phase to neutral. It sees a voltage
VLN = VLL /sqrt(3)

That one phase draws power
P1phase = VLN * I * P.F. = VLL/sqrt(3) * I * P.F.

So all three phases together are 3 times as much
P3phase = 3 * P1phase = 3* VLL/sqrt(3)*I*P.F.

Simplify by noting that 3/sqrt(3) = sqrt(3)
P3phase = sqrt(3)* VLL * I * P.F.

Many thanks to Dave & EPete

Shaft output power is linear with slip.
So when you measure rotor speed and than have a look at the nominal rpm speed and nominal kW power as stamped on the nameplate then you have a good indication of actual motor shaft kW (accurate enough for < 100 kW motors).
Arie Mol

The formulas provided are partially correct as they don't include motor efficiency. Use given formulas and multiply by motor efficiency which depends on frane size and load (from 70% for small motors to 92% for the large ones.)

Many thanks for your replies. I am after the electric power consumed by the motor to arrive at the kWhr and so at the Euros. If voltages and currents are the actual measured, i don't thing efficiencies come into the equation, right?

Right. Calculating electric power consumed by the motor from voltage and current does not require efficiency.

As a matter of fact, you should be able to calculate efficiency from the measurements you made shouldn't you?

Dave

I don't think there is enough info to calculate efficiency from voltage and current and power factor alone.

For the simplest calculation of efficiency, you would need some measure of the motor output power in order to estimate the motor efficiency = (input electrical power) / (output mechanical power)

But there are some instruments and methods that attempt to estimate efficiency without directly measuring output power (which is often difficult to measure) and using instead various models, sometimes with additional input information such as: speed, temperature, multiple load points. As stated at the link below: "Efficiency is output power divided by input power, yet most of the methods and devices attempt to assess losses to circumvent the difficult task of measuring shaft output power. "

http://www1.eere.energy.gov/industry/bestpractices/pdfs..._motor_systemts2.pdf

See also IEEE112 for very detailed discussion of methods of estimating efficiency.

You're right, of course, Epete.
I was thinking of strictly a fully loaded motor, power being used compared to ?

I really don't know what I was thinking last night, other than the blackberry cobbler w/ ice cream was awfully good
Dave

Did you save me some?

quote:

Originally posted by Danny Harvey:
Did you save me some?

I've got about 10 acres of them. I've been mowing paths through the patches. Come on down and dive in. They are as big as your thumb, and man, are they good.

D