Motor Torque Measurement.

What does the torque/speed characteristic curve of the shaft load it it trying to turn look like?

Where is the intersection point with the motor's curve?

What sort of motor?

Or similar to what Wareagle says, (but you don't have a torque wrench) clamp a lever to the shaft and add weights until it starts to turn, length of lever x weight gives the torque.
Similarly to measure motor toque atach alever to the output shaft and attach it to a spring balance. (Apply common sense safety precautions... e.g get the new guy to do it)
Del

If the motor has a nameplate, it should list the LRA. Locked rotor amps. If the amp draw exceeds this value, and the "rotor won't go", get a bigger motor.

Are you sure that the amp drawn by the motor will exceed the LRA? I thought the locked rotor current indicated by the manufacturer in the data sheet or name plate is the maximum current which can be drawn by the motor in the stand still condition.

If the motor doesn't rotate, the locked rotor current will continue till the motor trips on overload protection (either by a bimetallic type thermal overload relay or an electronic over current relay)

...or it burns out.

...and if it needs a bigger motor, don't forget to look at the motor overload trip device, the motor power cable, the circuit protective device(s) and the upstream power supply to make sure the whole shebang is up to the job.

Well, the principle of operation is pretty easy, but NOT the construction of the testing station, the load is a stationary fixture connected to the motor under test by a magnetically coupled variable torque collar.

The more duty cycle on the variable torque coil, the more load is applied to the motor, therefore more "twisting" effect over the stationary fixture.

The stationary fixture is actually afixed to the chassis plate by one piezo-electric load cell (without it, it would spin freely), so the more torque applied to the load, the higher amplitude from the cell signal, process it as necesary to attain the units of your preference.

Regards

How about a torque measuring coupling or this?

Easy, yes. Cheap, no. (You could probably install a hydrostatic drive and use the hydraulic drive motor pressure to measure torque more cheaply. Actually, depending on your load characteristics, a hydrostatic drive might not be a bad final solution.)

I have to appologize to all for not being clear as to what I was asking. Bigg actually gave me what I needed in the way of a couple of links to suppliers of torque measuring devices. My sincerest thanks to all -- especially Bigg.

As there is no 'easy' way to measure the torque, start by measuring the terminal voltage on the motor at startup and run. This is easy and may show the problem is not in the motor but in the supply. You may have to increase the size of the supply cable.

Which method gives the starting torque? That was the original question, or am I mistaken?

None of them gives the starting torque.

Starting torque has to be seen in relation with acceleration time.

The faster the application is on speed, the higher the starting torque.

The starting torque, also called the locked rotor torque, is typically 3x the motor's rated torque. Rated motor torque = 5250 x hp / speed.

RUDY:Thanks very much for this; I have almost made resolve to simply buy torque measurement devices as were suggested by "bigg", however we will have a closer look at these formulas and how they will appeal. Thank you.

There is a (small ?) mistake in the formula

• [kW] = [Nm] x [rounds per minute] / [seconds per minute]

has to be changed in

• [W] = [Nm] x [rounds per minute] / [seconds per minute]

Additional information:

the calculation is based on a motor with the following data on the motor name plate

• output power P = 0.75 kW / 1 Hp
• 4 pole motor @ 50 Hz synchronous speed 1500 rpm
• full load speed n = 1350 rpm
• full load current I = 1.9 A

The no load current is not mentioned on the name plate and has to be measured.

If this motor has functioned properly in the past, and previously accomplished it's designed task. Check to see if something is impeding the mechanism; something as simple as proper lubrication could be the cause (a problem I have experienced with my Garage Door Opener). If this is a single phase motor with capacitors, (run and/or start) check for a faulty one. A larger motor with greater Hp. may be required when all else has failed.

I have to appologize to all for not being clear as to what I was asking. Bigg actually gave me what I needed in the way of a couple of links to suppliers of torque measuring devices. My sincerest thanks to all -- especially Bigg.

Check the voltage and current on the motor when it does start. You can then work out the starting VA and hence the torque. Your question is answered. Cost nil. Then check what has altered when it does start. Please let me know what you find. Thank you.

@ Rainmain, Please explain the relation between the starting VA and the torque.

Please see the comment by Snave