Tips on testing servo motor torque

Below are a few simple suggestions for testing the strength of the motor magnets and the amount of holding torque. 

Most motors will have a torque rating on the nameplate and a corresponding stall current rating right next to it. The idea here is fairly simple, you lock the motor with the rated current and then use a torque wrench on the motor shaft to make sure it is holding the rated torque. In the example below the motor would be locked with 4.85 Amps and the motor shaft should hold up to 1.58 Nm of torque. If the locked rotor breaks free to the next pole pair before reaching the rated holding torque, then the motor may not be meeting specification.  

Another test that can check magnet strength is back driving the motor and recording the Ke voltage coming from the windings. This value is called a voltage constant, often labeled as Ke or BEMF, and is typically rated at 1,000 RPM. Some motors print this info on the motor nameplate or in their corresponding manuals, but certainly not all. To test the Ke you simply spin up the motor under test with another motor at 1,000rpm and read the AC voltage generated from the motor windings (U reference to V phase). Weak magnets, which result in weak holding torque, may have Ke values that vary greatly from their technical rating.