Motor torque from amps 1

[Barry1961]

I have a 480V, 3 phase, 3.5kw/4.7hp, 3458 rpm, 6.5 FLA motor which is drawing 3.8 amps. What would the torque be given nominal and balanced voltage? This does not have to be exact. If you can give me a range it would really help.

Thanks in advance!!

Barry1961

 

[itsmoked]

P = V * A * 1.73 * PF/1000
= 480 * 3.8 * 1.73 * .85/1000
= 2.6kW

HP = 746W

HP = 2.6kW/746W
= 3.5

T(lbf-ft) = HP x 5252/RPM
T = 3.5 x 5252/3458
= 5.3lbf-ft


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

And for the metrically challenged;

5.3(ft-lbs) = 7.2(N-m)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Barry1961]

I was trying to subtract the no load current from the measured current to get a percent of FLT by the ratio of remaining amps. But I can't seem to find a good rule of thumb for no load current on motors of this size.

Is power factor all I need?

Barry1961

 

[electricpete]

For large 2-pole motors no-load current is about 20%. For 1 - 10hp I'm guessing about 25%.

Knowing no-load amps is equivalent to knowing power factor (recognizing the power factor changes with load).

I think itsmoked is pretty close.

Some minor tweaks that might be considered:
Also address efficiency.
Use 460vac vs 480vac if that is motor nameplate voltage.
Speed is probably closer to 3525 since the motor is only half loaded.

Considering that power factor and efficiency are just a rough guess anyway, I agree with itsmoked answer.


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[electricpete]

Out of curiosity, why do you want to know the torque?

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[Barry1961]

I am trying to guess the weight of the load to size a new drive train.

The existing drive train is explained in a previous post titled, “High cycle….”, a couple of days ago.

The configuration of the existing drive train is not mechanically close enough to the new to just go by horsepower and the existing motors are burning up. They can not easily disconnect the load to weigh it or measure torque and the existing drive does not back drive easily.

The existing drive has two motors, fwd/rev, feeding into a common drive train and is started DOL. The freewheeling motor acts as a flywheel being back driven through the gear train. Since this is high cycle the results are very bad. I can only guess the original idea was to cut the start/stops per minute in half by using two motors.

The new design will use a single 1750, 7.5 hp motor and Vector Drive with braking resistors. The reduction will be a little less than half of the original to allow them to run slightly faster at a rated motor rpm.

Barry1961