Measuring Torque on an Induction motor

[dmccric]

I want to measure the torque produced from my 3-phase 40W induction motor when running at 60rpm. My motor gives me a rated torque of 3.6Nm. I want to know at what operating percentage my motor is running at. If I am operating at only 10% then I will use a small motor/gear head, etc

I am powering my inverter with 230V AC and it is drawing 80mA. So my inverter is using 18.4W.

Is it correct that it is simply 18.4W / 60 rpm to give 0.31Nm?

Many thanks

 

[Prototyp_V1.0]

Hi.

It is possible to make your motor swing freely at the rotor axis? At least some few degrees to put a weight cell between?

The calculation you show will not be correct. First of all, RPM is not a SI-unit, you have to convert it to ω (radians pr second).
Second problem is that the formula doesn't take acount for wasted energy in the motor and gears.

 

[dick_freebird]

One easy way is to use a larger-diameter wheel (known r) and a
friction bar with a weight and pull-scale. The motor is fixed. The
friction bar lays on top of the wheel, and is kept from rolling along
the wheel by the scale (horizontal). The scale gives you force and
with the wheel's r you can figure torque.

Now you can place a series of weights on the bar over the wheel
and measure motor armature currents, and derive a torque-current
curve.

 

[dmccric]

I have measured the current of each phase with an AC current clamp.

250mA @ 150V per phase at 60rpm.

I think that this is right. I thought that since torque is almost proportional to current that I would be able to calculate torque easily but I just dont know with 3 phase.

- - - Updated - - -

Also the efficiency of the motor gear head is 81% with a ratio of 1:18

 

[Prototyp_V1.0]

Hi.

You should expect that the phase (I vs. V) will change too by different speed and loads. Not only the measured value of the current.

 

[FvM]

There's a large magnetizing current part, rated cos phi should be printed on the type plate, something like 0.7 - 0.8 at rated current and much lower at partial load. You need to measure consumed real power with a power meter. A good industry standard VFD inverter (e.g. Simatic) will display torque or real power.