How to measure torque of a motor? | Torque Testing Stand

How to measure torque of a motor? 

A relatively simple method to estimate electrical motor torque (AC - Synchronous or Induction; or DC - Brushed or Brushless) is to measure its electrical power input, the voltage and the current in the power line driving the motor. For most motors, torque is directly proportional to the current and can be derived from it by knowing shaft speed and motor efficiency. This is considered an indirect measurement of torque since it is not the true mechanical torque but instead a relation from electrical power quantities to a theoretical and estimated torque value.

Another alternative and indirect method is to create a transversal arm attached to the motor housing using a known arm length. Let the whole motor housing outside case run freely along the attached arm. The arm tip can then be placed on a load cell. The torque can now be measured using a load cell measurement and the arm length.

However, for a precise and accurate shaft output torque measurement, a rotary torque sensor is the recommended method. Rotary Torque Sensors are frequently used as auditing tools for motor torque testing stand in motors, power tools, turbines, and generators.

In some instances, the user wants to measure the electric motor output power. In that case, one may want to use a rotary torque sensor with encoder and a Universal Signal conditioner to read both torque and angular speed (RPM). Check out this application note on how to measure the electric motor output power.