An electric motor dynamometer is a specialized dynamometer that can measure the power, torque, and rotational speed of a prime mover, such as an engine or another electric motor. It can also drive a load and does not just absorb power under measurement. The electric motor dynamometer can make use of adjustable-speed drives. Power could be direct current (DC) or alternating current (AC). The electric motor dynamometer can either be a load or a prime mover to measure either or both rotational power and loading torque.
Revolution per minute (RPM) is a measure of rotational speed. For instance, a shaft rotating at 3,600 RPM makes 60 revolutions per second. When a motor is loaded by friction or other loads, the RPM usually decreases from the “no load” RPM to the “loaded” RPM. Simple handheld pieces of equipment, such as the tachometer, can measure RPM.
Mechanical loading is measured as torque, usually in newton-meters (Nm). In mathematics, torque is the product of force and distance from the axis. 1 newton is the force exerted to lift a 2.2 lb (1 kg) weight. If a person were to support a 1 kg load at the end of a 39.3-inch (1 m) long pole, the torque to keep everything steady is 1 Nm.
Rotational torque loading is also measured in newton-meters. If a rotating shaft is driven at 1 Nm to maintain a steady flow of water at 2 gallons (7.6 L) per minute, then there is a suggested relationship between the torque and the power of the described system. A simple dynamometer is capable of creating a specified torque, and could replace the water pump in this system. The dynamometer torque just needs to be set to 1 Nm, and tests can be made on the prime mover in any setting.
An electric motor dynamometer can replace the “load side” during tests on the prime mover. The prime mover can be any energy source, such as an electric motor or a turbine driven by wind, water, or steam. This type of dynamometer is relatively simple testing equipment that can provide a variety of torque values.
Measurable torque is a key concept in an electric motor dynamometer. A person wearing protective gloves may introduce a loading torque to a driven shaft, but there is no torque reading in that arrangement. A spring scale connected to a leather belt and partly wrapped halfway to the driven shaft and creating friction to a shaft may be a basic setup for torque measurement. As the driving shaft turns, the force of friction measured as read on the spring scale and is multiplied by the radius of the shaft. From this, the loading torque can be derived.