Power dissipation from motors
Deciding on the heat liberated from motors hinges on the following:
(i) The frequency with which the motors will be used, if there is more than one in the conditioned space, so that the maximum simultaneous liberation may be assessed
(ii) The efficiency of the motor
(iii) Whether the motor and its driven machine are in the conditioned space
All the power drawn from the electricity mains is ultimately dissipated as heat. If both the motor and the machine are in the conditioned space then the total amount of power drawn from the mains appears as a heat gain to the room. If only the driven machine is in the room, the motor being outside, then the product of the efficiency (motor plus drive) and the power drawn from the mains is the heat liberated to the space conditioned. Similarly, when only the driving motor is within the room, 100 minus the efficiency is the factor to be used.
Control of motor speed should be done by an efficient method, such as varying the frequency of the input electrical supply. If inefficient methods of speed control are adopted energy is not saved and the inefficiency of the method appears as heat gain in the vicinity of the speed control mechanism.
Posted in Engineering Fifth Edition