Electrical and Mechanical Losses
Also, losses usually increase with increased loading. Cooling is necessary to continuously transfer the heat to a cooling medium, such as the air. The different methods of cooling rotating machines are classified in the standards IEC 34.6 and AS 1359.21.
For AC induction motors, cooling air is usually circulated internally and externally by one or more fans mounted on the rotor shaft. To allow for operation of the machine in either direction of rotation, fans are usually of the bi-directional type and made of a strong plastic material, aluminum, or steel.
In addition, the external frames of the motor are usually provided with cooling ribs to increase the surface area for heat radiation.
These are designed to keep the air flow close to the surface of the motor along its entire length, thus improving the cooling and self-cleaning of the ribs. An air-gap is usually left between the ribs and the fan cover for this purpose.
Internally, on smaller TEFC motors, the rotor end-rings are usually constructed with ribs to provide additional agitation of the internal air for even distribution of temperature and to allow the radiation of heat from the end shields and frame.
Special precautions need to be taken when standard TEFC induction motors are used with AC variable speed drives, powered by VVVF converters. For operation at speeds below the rated frequency of 50 Hz, the shaft mounted fan cooling efficiency is lost. For constant torque loads, it is sometimes necessary to install a separately powered forced cooling fan (IC 43) to maintain adequate cooling at low speeds. On the other hand, for prolonged operation at high speeds above 50 Hz, the shaft mounted fan works well but may make excessive noise. Again, it may be advisable to fit a separately powered cooling fan.
Larger rotating machines can have more elaborate cooling systems with heat exchangers.
The system used to describe the method of cooling is currently being changed by IEC, but the designation system currently in use is as follows:
- A prefix comprising the letters IC (index of cooling)
- A letter designating the cooling medium, this is omitted if only air is used
- Two numerals which represent:
- The cooling circuit layout
- The way in which the power is supplied to the circulation of the cooling fluid, fan, no fan, separate forced ventilation, etc
SOURCE: Practical Variable Speed Drives – Malcolm B.