Cooling and Ventilation of Electric Motors (IC)

Cooling and ventilation of electric motors (IC)
The surface cooling using flat ribs combined with a defined inner cooling circuit with shaft-mounted fan inside the motor ensure optimum motor utilization.

Electrical and Mechanical Losses

All rotating electrical machines generate heat as a result of the electrical and mechanical losses inside the machine. Losses are high during starting or dynamic braking.

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.

The most common type of AC motor is the totally enclosed fan cooled (TEFC) motor, which is provided with an external forced cooling fan mounted on the non-drive end (NDE) of the shaft, with cooling ribs running axially along the outer surface of the motor frame.

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:
    1. The cooling circuit layout
    2. The way in which the power is supplied to the circulation of the cooling fluid, fan, no fan, separate forced ventilation, etc
Figure 1 - Designation of the most common methods of cooling
Figure 1 - Designation of the most common methods of cooling

SOURCE: Practical Variable Speed Drives – Malcolm B.

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on


  1. Atul Verma
    Nov 20, 2015

    What is the impact of additional cover (normally made of GI sheet) on electrical motor???
    Isi ti actually required or just to protect from external environment????

  2. Raghu
    Dec 06, 2014

    Guys I’m a electrical engineer working in a plant, which has both ac and dc motors. I just want to see how many of u can answet this….
    Why even a small ac motor(induction) have ribs on its body for cooling but even the largest of dc motors(we in our plant have upto 1MW DC MOTOR) don’t have ribs for cooling on their body?

    • db
      Jul 02, 2015

      DC machines see a DC field in the gap! The stator mounted magnets do not generate any eddy currents which could generate heat. No other heat generating source in the stator (no windings).

      I am not sure if you asked this because you don’t know ( which is OK to ask ) or, if you trying to sound like a smart ass – in which cases, mission accomplished!

  3. Julius Were
    Aug 06, 2014

    I like your explanation ,it is short and very vivid

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