Cooling methods for electric machines

Cooling removes the heat produced by losses so that the temperature of the insulation stays within its class limit. Better cooling allows a higher output from the same machine size. Cooling methods are classified as follows.

1. By the way the coolant moves

• Natural cooling: no fan; heat leaves by natural convection and radiation from the frame. Used for small and totally enclosed non-ventilated (TENV) machines.
• Self ventilation: a fan mounted on the machine's own shaft circulates air. Most common for induction motors. Cooling depends on speed.
• Separate (forced) ventilation: an independently driven fan or blower. Used for variable-speed and large machines.

2. By the path of the cooling air

• Radial ventilation: air enters at the ends, flows radially outward through radial ducts between core packets. Used in medium and large machines (above about 20 kW).
• Axial ventilation: air flows along axial holes in the rotor/stator cores. Used in small, high-speed machines.
• Mixed (radial-axial) ventilation: a combination, used in large machines.

 radial ventilation (section)
  frame |^^^^^^^^^^^^^^^^^^|  air out
 stator |==| |==| |==| |==|
        |  duct  duct  duct|
  rotor |==| |==| |==| |==|
  air in-> --- shaft --- <-air in

3. By the cooling circuit

• Open circuit: fresh air is drawn from the surroundings and thrown out (drip-proof, screen-protected machines).
• Closed circuit: the same air recirculates through a heat exchanger (air-to-air or air-to-water). Used where the air is dirty or for large machines. TEFC (totally enclosed fan-cooled) motors have an external fan blowing over a finned frame.

4. Other coolants

• Hydrogen cooling of large turbo-alternators. Compared with air, hydrogen has about 7 times the thermal conductivity, about 1.5 times the heat transfer coefficient and only about 1/14 the density. This gives lower windage loss, no oxidation of insulation and less noise.
• Direct (inner) conductor cooling: hydrogen or water flows through hollow conductors. Used in very large turbo-alternators.

5. Transformer cooling methods

The choice depends on rating, enclosure, environment and cost. Larger ratings need more intense cooling because losses grow faster (∝ linear dimension³) than surface area (∝ linear dimension²).