Electric Dynamometer

a device that is designed to measure the torque of electric motors. Electric dynamometers are used in bench tests of motors to determine mechanical or electromechanical characteristics of the motors. Such a dynamometer is an electric machine that operates as a generator and is mechanically coupled to a motor to be tested. DC generators are most often used as electric dynamometers.

The torque developed by an electric motor is given by the equation

where U is the voltage across the generator terminals in volts, I is the current in the field winding in amperes, n is the rotation rate in rpm, and η is the efficiency of the generator. The torque is varied by adjusting the load resistance and the current in the generator’s field winding.

Electric dynamometers are used to test high-power traction machines. The torque of low-power electric motors is sometimes measured by means of a very simple dynamometer that consists of a ferromagnetic disk mounted on the shaft of an electric motor and a DC electromagnet with a counterweight. As the disk rotates, a braking torque is produced as a result of the interaction of eddy currents in the disk and the magnetic field of the electromagnet. The angle through which the electromagnet and counterweight swing is proportional to the torque to be measured.