ambient air or transfers to cooling water that dissipates into the air. Regenerative dynamometers, in which the prime mover drives a DC motor as a generator to create load, make excess DC power and potentially - using a DC/AC inverter - can feed AC power back into the commercial electrical power grid. Absorption dynamometers can be equipped with two types of control systems to provide different main test types. Constant force The dynamometer has a "braking" torque regulator - the power absorption unit is configured to provide a set braking force torque load, while the prime mover is configured to operate at whatever throttle opening, fuel delivery rate, or any other variable it is desired to test. The prime mover is then allowed to accelerate the engine through the desired speed or RPM range. Constant force test routines require the PAU to be set slightly torque deficient as referenced to prime mover output to allow some rate of acceleration. Power is calculated based on rotational speed x torque x constant. The constant varies depending on the units used. Constant speed If the dynamometer has a speed regulator (human or computer), the PAU provides a variable amount of braking force (torque) that is necessary to cause the prime mover to operate at the desired single test speed or RPM. The PAU braking load applied to the prime mover can be manually controlled or determined by a computer. Most systems employ eddy current, oil hydraulic, or DC motor produced loads because of their linear and quick load change abilities. The power is calculated as the product of angular velocity and torque. A motoring dynamometer acts as a motor that drives the equipment under test. It must be able to drive the equipment at any speed and develop any level of torque that the test requires. In common usage, AC or DC motors are used to drive the equipment or "load" device. In most dynamometers power (P) is not measured directly, but must be calculated from torque (τ) and angular velocity (ω) values or force (F) and linear velocity (v): or where P is the power in watts τ is the torque in newton metres ω is the angular velocity in radians per second F is the force in newtons v is the linear velocity in metres per second Division by a conversion constant may be required, depending on the units of measure