All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > General Electronics Chat


General Electronics Chat Discussion forum for general chat about anything electronics related, including asking questions about material in the All About Circuits E-book, Worksheets, and Videos.

Reply   Post New Thread
Thread Tools Display Modes
Old 01-31-2008, 10:56 PM
mechanicalEng mechanicalEng is offline
New Member
Join Date: Jan 2008
Posts: 1
Default 3 phase motor - calculating power


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
Reply With Quote
Old 01-31-2008, 11:19 PM
beenthere's Avatar
beenthere beenthere is offline
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.
Reply With Quote
Old 02-01-2008, 12:40 AM
John Luciani's Avatar
John Luciani John Luciani is offline
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 *)
Reply With Quote
Old 02-01-2008, 09:47 AM
recca02's Avatar
recca02 recca02 is offline
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.
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.
Reply With Quote
Reply   Post New Thread

, , ,

Related Site Pages
Section Title
Worksheet Delta and Wye 3-phase circuits
Worksheet Servo motor systems
Worksheet AC motor theory
Textbook Other specialized motors : Ac Motors
Textbook Single-phase induction motors : Ac Motors
Worksheet AC power
Textbook Brushless DC motor : Ac Motors
Textbook Stepper motors : Ac Motors
Textbook Synchronous Motors : Ac Motors
Textbook Polyphase motor design : Polyphase Ac Circuits

Similar Threads
Thread Thread Starter Forum Replies Last Post
About AC supply LDC3 General Electronics Chat 9 06-26-2013 11:02 PM
Split phase residential power and power factor JamesL The Projects Forum 12 07-04-2012 11:13 PM
is 220 V ac actily more efficent than 110V AC? electronis whiz Homework Help 12 05-29-2012 08:34 PM
Power requirement - 3 phase or single phase? Tdawgg General Electronics Chat 7 05-21-2012 05:15 AM
3 phase - bridge rectifier input/output power Skeebopstop General Electronics Chat 3 03-16-2009 02:51 AM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

All times are GMT. The time now is 05:21 AM.

User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin
Copyright ©2000 - 2014, vBulletin Solutions, Inc.