# How to measure shaft torque (or shaft power) for a 20 kW motor?



## Solarsail (Jul 22, 2017)

Electric motor suppliers often claim up to 98% electromechanical efficiency. I need a way to measure the output shaft torque/power of a motor. The motor is rated continuous 20 kW.

The torque (or power) is to be measured at continuous RPMs - from 1,000 to 4,000 RPM.

Are reasonably priced torque meters commercially available? Or do I need to build my own? What kind of torque sensor is needed? How to generate such loads for testing?

Thanks to all.

Solarsail


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## Jack-Lee (Mar 29, 2019)

There are commercially available, but very expencive. Build your own are often cheaper. Strain Gauges are cheap.


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## brian_ (Feb 7, 2017)

The normal approach is to use the motor to drive a generator. You control the load by controlling the generator. To reduce the problem of needing a massive DC power supply to run the motor, you can charge the battery from the generator output while powering the motor, so the battery only provides some smoothing and makes up the energy loss of the inefficiencies of the all of the components.

Ideally the generator characteristics are well known so that you can infer the torque from the current and voltage produced by the generator, but for accuracy you need a more direct measurement of torque.

In theory you can mount the motor or the generator on bearings to allow it to rotate about the shaft, and restrain it with a lever with a load cell measuring the restraining force (torque = force x lever arm length), and that might be okay at constant speed. the load cell can be just a hydraulic cylinder with a pressure gauge on it. This is the principle of basic dynamometers, which is why the power measured by them is called "brake" power... and it's not how accurate modern dynos work.


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## Solarsail (Jul 22, 2017)

brian_ said:


> In theory you can mount the motor or the generator on bearings to allow it to rotate about the shaft, and restrain it with a lever with a load cell measuring the restraining force (torque = force x lever arm length), and that might be okay at constant speed. the load cell can be just a hydraulic cylinder with a pressure gauge on it. This is the principle of basic dynamometers, which is why the power measured by them is called "brake" power... and it's not how accurate modern dynos work.


I am building a motor torque meter using four load cells. The motor stator is mounted on the load cells and the torques are added up. Now if I add a generator and a variable resistive load, do I get a dynamometer? How is this different from accurate commercial dynos? Thanks


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## brian_ (Feb 7, 2017)

Solarsail said:


> I am building a motor torque meter using four load cells. The motor stator is mounted on the load cells and the torques are added up. Now if I add a generator and a variable resistive load, do I get a dynamometer? How is this different from accurate commercial dynos? Thanks


Yes, that's an effective dynamometer. 
This isn't normally done, because it's easier to just secure the motor and hook up a shaft without making a specific torque-measuring cradle for the motor. That cradle isn't trivial to design, because you need to constrain the motor in all directions without putting loads on the load cells in directions that they are not intended to handle, or preventing the load cells from seeing all of the force in the directions that they are intended to handle.


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