On a electric motor electrical power is feed in to get out mechanical power. The product of torque and speed gives the mechanical power. The electrical power, on the other hand, is the product of current and voltage.

Electric dynamo meter(DC generator mounted on pedestal bearing), used to find the torque(shaft) of the machine(engine or motor) under test is the best example, and is the best and very accurate to find shaft torque developed. In this dynamo meter, DC machine mounted on pedestal bearing from both ends, so stator made free(rotate), instead mounting on foundation, rotor shaft is coupled with machine under test, hence when electrically loaded the reaction on stator is measured on spring balance, or torque developd(Ts=(T1-T2)R, R the dist from shaft to spring balance), or it is electrical torque load, you talking...

Yes, exactly

The need for torque measurements has led to several methods of acquiring reliable data from objects moving. A torque sensor or transducer converts torque into an electrical signal.

The most common transducer is a strain gauge that converts torque into a change in electrical resistance. The strain gauge is bonded to a beam or structural member that deforms when a torque or force is applied. Deflection induces a stress that changes its resistance. A Wheatstone bridge converts the resistance change into a calibrated output signal.

The design of a reaction torque cell seeks to eliminate side loading (bending) and axial loading, and is sensitive only to torque loading.

The sensor's output is a function of force and distance, and is usually expressed in inch-pounds, foot-pounds or Newton-meters.

Reaction torque sensors are used when limited rotation won't damage the cable. Its use minimizes the error attributable to the inertia of rotating components. They are used to calibrate torque wrenches, test the breaking torque of aircraft fasteners and monitor the opening torque of child-proof containers.

Other widely accepted methods for measuring torque rely on mounting transducers in the machine train or on the rotating shaft. With few exceptions, these methods use strain gauges. Each method has advantages and limitations, many of which are application dependent. The best solution requires an in-depth understanding of the application.

Two common ways to obtain torque measurements are by strain-gauging the shaft and by using in-line torque cells. Both have two technical obstacles: getting power to the gauges over the stationary/rotating gap and getting the signal back. The methods to bridge the gap are either contact or non-contact.

Gauging the Shaft

With strain gauges bonded to the shaft, the shaft becomes the transducer. The general guideline is that the applied torque must induce at least 150 to 175 micro-strain. The second requirement is for the shaft to be calibrated, a process that usually involves loading the shaft statically and tabulating the results. This is relatively easy to do in small systems, but as loads and shaft size increase, it becomes an onerous task. Selecting a location for the strain gauges, mounting them carefully and protecting them become problematic for users inexperienced in such techniques. Outside contractors are usually available through the torque sensor suppliers for most applications and locations.

Thank you very much for this extensive and very nice explaination.

I would like to add just a view additional remarks from a torque transducer manufacturer:

Gauging the Shaft will be limited in Terms of measurment uncertainty to be reached

The use of a torque transducer with an appropriate condition amplfier allows a measurment chain for torque with only a fraction of a percent as Overall measurment uncertainty.

Also: In a SI-world we should give the Preference to Newton-meters

Thank you again!

Nice

Some words regaring impact in industry by torque Measurement. For us the big torque business are power test stands. Test stands have an important role to play in developing engines, powertrain components and brakes.

Torque transducers are usually at the heart of these test stands. Torque transducers from HBM, especially torque flanges, are impressively accurate, reliable and compact.

Electric Dynamo meter, the easies, accurate and simple method used in most of the machine(engine/motor) testing laboratory, only needed, machine H.P/KW under test, should be lower than dynamo meter. We are using the same at Birla Engg. College.(BVM), V.V.Nagar, Gujarat-388120, India.

You may use a dynamometer to measure and torque, if you want to apply a load, another way if you don't have a specifically a dynamometer, is to use another electric motor of same characteristics, as your shaft is at 480 RPM an 7,5 Nm, that's around 0.38 kW, so you may use another motor of same or superior power, but with an electrical drive with capabilities of speed and torque control (not the basic AC drives, torque control can be measured indirectly by variable speed drive by sensor less vector control techniques), synchronized at same speed of your shaft but controlling applied control, positive or negative, you can achieve this by adjusting torque setpoint on drive or torque limiting on drive, the drive will advance or reduce torque in drive as you need to motorize load or regenerate power from coupled shaft. I hope this helps you.

Also you can use the system: "standard induction motor (IM) which covers the 7.5 Nm, 480 rpm mode + standard frequency converter with IM field-oriented control + brake resistors" as a load system for your case. Torque can be estimated via indirect method (by measuring electric performances of the tested motor). I think this is the cheapest option.

The indirect method, as expressed by the previous answer, would be electrical power, on the other hand, is the product of current and voltage.

Anyway, on a electric motor the direct way is the product of torque and speed gives the mechanical power.

Thank you for your note in the previous answer. I can also clarfy what I meant by "an indirect method": not only electrical power of the tested motor shall be measured but also motor power losses (or alternatively losses of the load machine, the so-called "calibrated-machine test") shall be estimated. So one will have an approximate estimate of mechanical power as "electrcal power - motor power losses". The approach is specified as a standard approuch (IEC 60034-2-1, section 6.1.3) for motors with direct mains supply. It can be also used for motors with PWM converter supply to some extent.