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### Motor Power Measurement

05/18/2012 12:51 PM

Hi

I am currently assigned to measure the motor power rating for a machine. The name plate of the motor is rated at 37kw

The measurement is via power logger (fluke 1735)

The question is why the result of the measurement did not show the power rating according to name plate? The result I got: At AC drive input: 12kW, At drive output to motor:5kW

As I understand , if the motor is not run in full load, the efficiency of the motor will drop and this will increase current, but the motor power should be the same, why is the result from the power logger shows differently?

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#1

### Re: Motor Power Measurement

05/18/2012 2:24 PM

The nameplate lists the MAXIMUM power that the motor can deliver safely. The LOAD determines how much power it will consume, plus any internal losses. The Efficiency rating of the motor is what tells you what losses to expect, as a percentage of LOAD on the motor. If you are measuring the input power to the motor (or drive), the motor efficiency is already in that value.

Measuring kW on the output of a VFD is a pointless exercise unless you spent upwards of \$10,000US for that Fluke meter.

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#2

### Re: Motor Power Measurement

05/18/2012 4:12 PM

It sounds like you have a 50 HP motor, when all you need is a 10HP motor.

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#3

### Re: Motor Power Measurement

05/18/2012 10:43 PM

A new toy? Learn how to use it first. You might make sense of the readings then.

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#4

### Re: Motor Power Measurement

05/18/2012 11:34 PM

Wow! That's a horribly inefficient drive!

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#5

### Re: Motor Power Measurement

05/19/2012 1:38 AM

Measure the LRC and divide by 6,you will get approximate rated current.

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#6

### Re: Motor Power Measurement

05/19/2012 8:44 AM

whats that to mean?

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#7

### Re: Motor Power Measurement

05/19/2012 11:15 AM

If you apply P=sq rt 3 x V x I x cos phi,you will get power assuming reasonable value for cos phi & V or by measuring cos phi & V

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#9

### Re: Motor Power Measurement

05/21/2012 3:29 AM

Codswallop. The original poster knows nothing about the mechanical load that the motor is driving.

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#10

### Re: Motor Power Measurement

05/21/2012 5:06 AM

Use a dynamometer,tachometer,clampon ammeter,pf meter and voltmeter(power analyser) and keep on increasing the load and record the speed,V,I,pf and draw graphs of N,I,V,pf against Load, from which extrapolate the approximate power rating of the motor.

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#8

### Re: Motor Power Measurement

05/20/2012 11:41 AM

Check whether there are kW measured in both points - not kVar's. 7 kW lost in the drive is in any case too much. (I assume you do not have them in the form of scorching heat, deafening explosions, deadly radiation waves)

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#11

### Re: Motor Power Measurement

05/21/2012 6:37 AM

The rating of the motor is 37kW, because it is stated on the plate. It doesn't draw 37kW all the time. It will draw whatever the load requires, in terms of torque multiplied by angular velocity, up to a limit of 37kW. Above 37kW the motor is at risk, and the overload protection device(s) is/are there to protect it.

A measurement of 5kW going to a 37kW motor indicates that the motor is loaded to around 14% of its rating. This is no big deal, apart from the motor being 7 times bigger than the load needed at the time of the measurement, which may be either normal or an over-sizing error.

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#12

### Re: Motor Power Measurement

05/21/2012 11:13 AM

Since the motor is connected to a drive (VFD), shouldn't the drive be able to tell you the current power output to the motor making the use of the fluke redundant?

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