Power Formula for 3phase 115v 400Hz 1

[ecampos]

Power Formula for 3phase 115v 400Hz motor Y connection I think.

Is it.....

a) P=Sqrt(3)*E*I*cos(theta)

b) P=3*E*I*cos(theta)

Can anyone explain the difference?

Regards, Edson Campos

 

[busbar]

In (a.) E = ø-ø voltage.

In (b.) E = ø-n voltage.

Both are assumed to be balanced conditions.

 

[Marke]

Well said Busbar.
regards, Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: The formulae should yield the same numerical results. I.e. they are the same formula if the voltages are applied as indicated in busbar's posting.
Notice that the power will be the input power not on the shaft side since the motor efficiency is missing in those equations.
Equation a. can be used for delta or wye motor connection.
Equation b. is more linked to wye motor connection since the phase to neutral voltage is involved.

 

[ecampos]

Thank you for the responses.
Now is (theta) 120 degrees and if not how do I find out what it is?

Reagards,

Edson Campos
Edson Campos

 

[dpc]

No this is the power factor angle - cos(theta) is the power factor. You can use the motor power factor on the nameplate, if given. This is generally given for full load conditions and changes as the load changes.

If you need something exact, you will have to measure the power factor, or get a plot of power factor vs load from motor manuf.

 

[AeroPD805]

Edson,
Or you can try: watts in
---------
1.732 x V x A

Hope this helps...
P.
x103

 

[acmotorengineer]

As written, equation b is incorrect. Presuming E is defined as Line to Line voltage as in the correct equation a, E in equation b should be E-phase.

AND equation b would only be applicable for a WYE connected motor. It would NOT be correct for a delta connected motor.

A dangerous equation to use.

 

[AeroPD805]

Ed,
Oops, you're after power [watts] not power factor - my bad.

 

[d23]

ecampos

In the real world I assume 400 Hertz to be a VSD. VSD controllers offer both a variable torque and constant torque operation. In a constant torque operation they offer a function that is called "extended speed range." In the extended speed range kW still equals 746 kW per HP more or less, but the actual KVA gets very confusing. If you increase the current and frequency while holding the volts constant the drive or motor KVA required is a problem!

That's probably why they have an overload function built into a VSD.

David

 

[jbartos]

Suggestion: Normally,
cos(theta) is defined between and including 0 and 1 since there is no passive element causing a lag or lead greater than 90 degrees.