--- title: "Cooling methods for electric machines" subject: "Electric Machine Design" chapter: 2 key: "semester_5/electric_machine_design:cooling-methods-machines" source: https://www.bctengineer.com/q/semester_5/electric_machine_design/cooling-methods-machines site: BCT Engineering --- # Cooling methods for electric machines Electric Machine Design · Chapter 2: Heating and cooling of electric machine - Asked in: 2069 Asar, 8 marks - Video: [Cooling in Electrical Machines Part 1](https://www.youtube.com/watch?v=Bid3kegCb_g) — Pradeep Khanna - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/cooling-methods-machines - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-2 ## Question Explain different cooling methods for an electric machine. ## Short answer Cooling can be natural (TENV), self-ventilated via shaft fan, or forced with separate blower. Airflow paths: radial through ducts, axial through core holes, or mixed. Large machines use closed circuits (TEFC), hydrogen cooling, or direct conductor cooling with water; transformers employ ONAN, ONAF, OFAF, etc. ## Answer **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. ```text 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 | Code | Method | |---|---| | AN / AF | Dry type, natural / forced air | | ONAN | Oil natural, air natural (tank with radiators) | | ONAF | Oil natural, air forced (fans on radiators) | | OFAF | Oil forced by pumps, air forced | | OFWF | Oil forced, water forced (oil-water heat exchanger) | | ODAF | Oil directed into the windings, air forced | 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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In self ventilation, the cooling fan is: A. Driven by a separate motor B. Absent, cooling by radiation C. Mounted on the machine shaft, so cooling depends on speed D. Placed in a heat exchanger only Answer: C. A shaft-mounted fan is common in induction motors and its airflow varies with speed. (Key point: Self ventilation) 2. Which ventilation suits medium and large machines, with air flowing outward through ducts between core packets? A. Axial ventilation B. Natural ventilation C. Open-circuit only D. Radial ventilation Answer: D. Radial ventilation uses radial ducts; axial ventilation is for small, high-speed machines. (Key point: Radial vs axial) 3. Why is hydrogen used in large turbo-alternators? A. It has much higher heat transfer ability and low density, so lower windage loss B. It is denser than air C. It conducts electricity D. It is cheaper than air Answer: A. Hydrogen carries heat 7-14 times better and has 1/14 the density, giving lower windage loss and less noise. (Key point: Hydrogen cooling) 4. In a closed-circuit cooling system, the air: A. Is drawn fresh from the surroundings each time B. Recirculates through a heat exchanger C. Is replaced by oil D. Is not circulated at all Answer: B. Closed circuit recirculates the same air via an air-to-air or air-to-water exchanger, useful for dirty surroundings. 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