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It depends what you want to do with it.Often, such setups are used to develop and test motor control hardware and software. Normally, one motor acts as the load and is speed-controlled, while the second motor is the unit under test and torque-controlled.Instead of a speed-controlled load motor, you can also simply short-out the three phases of a PM machine. This will provide a speed-dependent load torque, and will allow the two motors to stabilize at a certain speed.
Desktop Motor Test Stand (D...View Instructable »