Design of a 300-kW Calorimeter for Electrical Motor Loss Measurement

This paper describes the design of a calorimeter that is suitable for measuring power losses in electrical motors of up to 300 kW (402 hp). Refined from the two previous generations, this calorimeter is of air-cooled open type. Accurate control of the air flow rate and temperature gradients between the inside and outside walls of the calorimeter ensures that a repeatable and durable measurement environment is achieved. During the design stage, attention is focused on heat leakage prevention and reduction in the length of the testing procedures. Heat leakage is mitigated by appropriate insulation and active temperature control over all walls and connection ports. In selecting the materials for the structure, lightweight aluminum and polystyrene are used in sandwich form to reduce the thermal time constant and, thus, the operational time for the calorimeter to reach its equilibrium. After the calorimeter is commissioned, further reduction in the lengthy test procedures is realized by boosting the initial power loss value of the motor under test. That is, some additional resistive heaters are installed inside the calorimeter and operate in conjunction with the test motor. This push-pull technique can guarantee a constant combined power loss during operation, which is assumed to be the final value of motor loss. By predefining a suitable motor loss and a tolerance, it is possible to dynamically control the input power to the dc heaters from the start and to shut down the heaters once the motor loss value enters the tolerance selected. Calibration results confirm the effectiveness and accuracy of the calorimeter.