# AC Motor Output Power

Discussion in 'General Electronics Chat' started by nradam123, Dec 19, 2013.

Dec 3, 2013
10
0
I have to find the output power of an AC motor without using a torque sensor.

My idea was to find the voltage and current through the terminals of the motor and then find V*I to get the input power. Then divide that power by the angular velocity of the motor from the shaft encoder to get the torque.

But this method is wrong because there seems to be lot of losses such as heat losses that will reduce power output of the motor. Also, since it is an AC motor the phase keeps changing and my V-I measurement keeps fluctuating. So maybe i will have to use a rectifier, but wont that further reduce teh accuracy of my measurements??

I dont want to buy a torque sensor. Do you guys know how i can accurately measure the output torque of a motor without using a torque sensor??

Thanks in Advance. I will be forever greatful

2. ### inwo Well-Known Member

Nov 7, 2013
2,416
314
You can get a ball park figure, for a small motor, with a home made sensor if your mechanically inclined.

I've used a lift arm or pulley, lifting a known weight, and a wooden clutch.
Two wooden pulleys. One free on the shaft for lifting. One locked to shaft.
A lever and bearing pushing them together.
Monitor current while tightening the clutch.
Believe I've also used a spring scale and line on the lift pulley.
A 1 foot arm would read lb foot etc.

A known hp motor could be estimated mathematically at rated speed.

3. ### wayneh Expert

Sep 9, 2010
15,892
5,965
A little more context might be helpful. You're asking how to measure something ... without measuring it. Like taking temperature without a thermometer. It raises the obvious question, why?

You could run the motor inside a calorimeter and measure heat production. The input power less the heat produced would leave you with an estimate of the shaft work.

strantor likes this.

Jul 18, 2013
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5. ### crutschow Expert

Mar 14, 2008
21,422
6,137
If you want a good measurement of the output power then you need to measure torque and rpm.

If you want an approximation then you can measure the input power and multiply by an estimated efficiency factor based upon the motor size and type.

Note that to measure input power you need the voltage, current, and the phase between the to determine the power factor. A wattmeter will measure the power accurately since it includes the power factor in its measurement.

6. ### inwo Well-Known Member

Nov 7, 2013
2,416
314

There are ways to get ball park readings with simple equipment.

Such as building a reactive mount for motor.
Hinged mount with spring scale at a fixed point from pivot.
Still need a way to load motor, but not too difficult with small motors.
End to end generator or even a friction block.

As far as watt meter. For small motor use Kilowatt (sp) meter.

7. ### inwo Well-Known Member

Nov 7, 2013
2,416
314
Have one but haven't used it as much as I thought.
Now local library has one to use.

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