Calculating Torque Constant of small Permanent Magnet Generator

[whitbuzben]

Dear Sirs

I was wondering if anybody could provide me with some information on how to obtain the Torque Constant ,KT, of a small permanent magnet generator. The generator in question can be seen here

http://dvetech.dk/?page=PMG&id=63.

The part number is PMGO-0.2-200. I have also attached the datasheet to this thread. I am aware that it may be possible to obtain the torque from the manufacturer but i would still like to know how i can verify it experimentally.

Is it possible to run the generator as a motor and somehow measure the back emf voltage and the angular velocity. If so how would one physically go about doing this i.e. physically measuring the voltage and driving the motor. I am new to this area and any help or advice would be much appreciated.

Many Thanks

 

[jpts]

Hi

If you know the electric output (0.2 kW), and efficiency (0.84), you can calculate the shaft mechanical power (0.238 kW).

You can calculate the torque by dividing mechanical power by angular velocity (2*pi*n/60), which gives you 0.011 Nm

 

[Clyde38]

What kind of test equipment do you have? How accurate do you want to be able to measure? If you have a DC voltmeter, drive the generator with another motor and measure the the voltage and speed. If you don't have something to measure the speed, drive the motor with a synchronous AC motor of a known speed. Connect a six-diode bridge (see

http://en.wikipedia.org/wiki/Diode_bridge)

to the output of the three phase generator and measure the rectified DC and compare to the Speed-Constant published on the data sheet (190 Vdc/krpm for your example).
See: (

http://www.cartage.org.lb/en/themes...evelopment/MotorConstants/MotorConstants.htm)

1.0000 V/Krpm or mV/rpm
--equals--
1.35229499 ozin/Amp
0.08452 lbin/Amp
0.00704 lbft/Amp
0.00955 NM/Amp
9.5492965 mNM/Amp
0.00955 V/R/S
97.4275928 g-cm/Amp
0.09742759 Kg-cm/Amp


[link

http://www.linkedin.com/in/clydehancock

]

[/url]

 

[whitbuzben]

Thanks for your responses guys.

I think i will have to couple the generator to and induction machine and drive it at a known speed as Clyde 38 suggests.

Just to clarify when you say "measure the voltage" this is obviously the open circuit voltage on the generator terminals with no load connected. By knowing this voltage and the speed i can then apply the following formula,V = K*W where K is the Torque constant and W is the angular speed in Rads/sec. Am i Correct so far?

Having found the torque constant i would then be looking to apply the following formula, T=K * I to get the actual torque at each of these speeds. What i am not clear on is how to obtain I (The armature current) Do i need to load the generator with a resistor?

 

[Clyde38]

I was wondering if anybody could provide me with some information on how to obtain the Torque Constant ,KT, of a small permanent magnet generator.

The voltage constant is provided in the data sheet (190Vdc/krpm). This equals a torque constant of:
256.93604773 ozin/Amp
16.05850 lbin/Amp
1.33821 lbft/Amp
1.81437 NM/Amp *
1814.366351 mNM/Amp
1.81437 V/R/S *
18511.24263 g-cm/Amp
18.51124263 Kg-cm/Amp
*Note that in SI units, torque constant and voltage constant are the same number.

I am aware that it may be possible to obtain the torque from the manufacturer but i would still like to know how i can verify it experimentally.

This is why I suggested driving the generator and measuring the voltage. And yes, this is a "no-load" voltage"

Is it possible to run the generator as a motor and somehow measure the back emf voltage and the angular velocity.

Yes, but this may be beyond your capabilities (test equipment, controls, etc.)

Having found the torque constant i would then be looking to apply the following formula, T=K * I to get the actual torque at each of these speeds. What i am not clear on is how to obtain I (The armature current) Do i need to load the generator with a resistor?

I'm not sure what you are asking ("each of these speeds"), torque constant should remain the same (in therory) at any speed. The torque constant is related to the "torque per amp" in theory. There is no armature current. This is a permenant magnet generator.
I hope this helps

[link

http://www.linkedin.com/in/clydehancock

]

[/url]

 

[dvetech]

Dear whitbuzben

You will be able o find the datasheet for the exact PMG at our site

http://www.dvetech.dk/userfiles/file/Download/90021 PMGO-0_2K-200.pdf

Regards
Rasmus Pedersen
DVE Technologies
Denmark