The TMDS incorporates a dynamometer in order to perform load testing on electric motors.  A dynamometer is essentially a braking mechanism which resists rotation.  This rotational resistance is what forces the motor to work during the test.  The motor and dynamometer are connected by a drive shaft.  The TMDS computer communicates with the dynamometer and instructs the dynamometer to apply varying amounts of rotational resistance to the motor.

Although there are numerous types of dynamometers available, American Railway Technologies recommends the use of only two types. 

Eddy-current Dynamometer
An eddy-current dynamometer is essentially an electric brake.  The rotational energy generated during the test is converted into heat.  The rotating components of an eddy-current dynamometer require cooling.  This cooling is achieved through supplying the dynamometer with water.  Since an eddy-current dynamometer relies on water for cooling, a water storage tank, water pump and heat exchanger system is required.  If the system incorporates an air-to-water heat exchanger located outdoors, the cooling water must be mixed with glycol to prevent freezing.

Regenerative Dynamometer
A regenerative dynamometer is a generator.  A regenerative dynamometer converts rotational energy into electrical energy.  This electrical energy is routed back into the power grid using sophisticated electronics.

Eddy-current Dynamometer vs. Regenerative Dynamometer
The only significant advantage of an eddy-current dynamometer over a regenerative dynamometer is its lower initial cost.  This lower cost is due to its relatively simple construction.  The regenerative dynamometer will prove to be more economical in the long run however.  Since a regenerative dynamometer converts the rotational energy of the motor under test to electrical energy and feeds that electrical energy back into the power grid, the cost of performing a test becomes very low.  Basically, en eddy-current dynamometer transfers the energy generated during the test to the environment in the form of heat and a regenerative dynamometer recycles the test energy.  Over a period of time, the cost of a regenerative dynamometer, when operating costs are considered,  will be lower than the cost of an eddy-current dynamometer.  American Railway Technologies recommends the use of a regenerative dynamometer due to long term cost advantages and environmental considerations.