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#1
01-31-2008, 10:56 PM
 mechanicalEng New Member Join Date: Jan 2008 Posts: 1
3 phase motor - calculating power

Hi,

Mechanical guy here with little electrical knowledge.

I have the amps reading on each phase of a 50hp rated motor. The line voltage is know to be 460V. Don't know if it is a Y or Delta circuit (don't think it makes any difference).

Is the following an acceptable way to estimate the power usage of the drive?
Ip1 = 27
Ip2 = 28
Ip3 = 27

P drive = avg(27,28,27) * 460V * sqrt3 = 21.78 kW

Or am I way out in the left field?

Many thanks
#2
01-31-2008, 11:19 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

Approaching it from the rated HP - 1 HP is 768 watts. 50 HP, then, is 38,400 watts. That would be a maximum power usage. Your figure is in the ball park.
#3
02-01-2008, 12:40 AM
 John Luciani Senior Member Join Date: Apr 2007 Location: Massachusetts Posts: 477

The power for a three phase circuit is

Ptotal = sqrt3 * Vline * Iline * cos(theta) where

Vline is the line-to-line voltage, Iline is the current through a line (assmuming
the lines are balanced) and theta is the phase angle between the voltage and current.
The maximum power is when the phase angle between the voltage and current
is zero (cos(0) = 1). You should check to see if the motor specifies the
phase angle (or power factor).

(* jcl *)
#4
02-01-2008, 09:47 AM
 recca02 Senior Member Join Date: Apr 2007 Location: India-22.35° N 82.68° E. Posts: 1,211

or power = sum of power in each phase.
= 3*v(phase)i(phase)
so it does matter to know if its a y or a delta.
or simply measure voltage across each phase(phase and neutral) or phase to phase current.
Quote:
 Ip1 = 27 Ip2 = 28 Ip3 = 27 P drive = avg(27,28,27) * 460V * sqrt3 = 21.78 kW
this seems correct for Y.
again for active power from drive u shud know abt the power factor.
also as already stated 50 HP shud be the full load power from the motor.

 Tags calculating, motor, phase, power

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