📚 Table of Contents
- Why Cooling is Important in Electrical Machines?
- Cooling Methods of Transformer
- Cooling Methods of Alternator
- Comparison Table
- Conclusion
- Frequently Asked Questions (FAQs)
🔌 Why Cooling is Important in Electrical Machines?
- To prevent overheating of winding and core
- To increase machine lifespan
- To maintain insulation resistance
- To reduce the chances of breakdown or fire
- To improve overall efficiency
⚡ Cooling Methods of Transformer
Transformers generate heat due to iron losses (core) and copper losses (winding). The following are the main cooling techniques used for transformers:
1. Air Natural (AN)
Used in small distribution transformers. Cooling is done by natural air circulation around the transformer.
2. Air Forced (AF)
Fans are used to blow air over the transformer body to enhance cooling efficiency. Used for indoor and ventilated transformer rooms.
3. Oil Immersed Air Natural (ONAN)
The core and windings are immersed in insulating oil. The oil transfers heat from the windings to the tank surface, and natural air cools the outer tank.
4. Oil Immersed Air Forced (ONAF)
Same as ONAN but fans are used to circulate air around the tank, improving cooling.
5. Oil Immersed Water Forced (OFWF)
Water is used as a secondary cooling medium. Heat is transferred from oil to water via heat exchanger.
6. Oil Natural Water Forced (ONWF)
Used in high rating transformers. Natural convection of oil with forced water circulation.
🌀 Cooling Methods of Alternator
Alternators also produce significant heat due to I²R losses in stator windings and rotor field windings. Cooling methods include:
1. Air Cooling
- Open Circuit Ventilation: Air is taken from outside, passed through alternator, and then discharged.
- Closed Circuit Ventilation: Air is circulated within the alternator casing. Heat exchangers are used to cool the air.
2. Hydrogen Cooling
Used in large turbo-generators. Hydrogen gas is used due to its superior thermal conductivity and low density, reducing windage losses.
3. Liquid Cooling
In very high-capacity alternators, water is circulated through hollow conductors for direct cooling of stator and rotor.
📊 Comparison Table
| Parameter | Transformer | Alternator |
|---|---|---|
| Common Cooling Medium | Oil, Air, Water | Air, Hydrogen, Water |
| Natural/Forced Circulation | Yes | Yes |
| Used in High Ratings | Oil + Water Cooling | Hydrogen + Water Cooling |
| Application Range | Power Transformers (kVA to MVA) | Generators (MW range) |
🧠Conclusion
Choosing the right cooling method is critical for the safe and efficient operation of transformers and alternators. While small machines may be air-cooled, larger machines require oil, water, or hydrogen-based systems. Proper maintenance and regular monitoring of cooling systems help ensure longevity and performance of these vital electrical machines.
❓ Frequently Asked Questions (FAQs)
Q1. What is ONAN cooling in transformers?
Ans: ONAN stands for Oil Natural Air Natural. It uses natural convection of oil for internal cooling and air circulation around the tank for external cooling.
Q2. Why is hydrogen used in alternator cooling?
Ans: Hydrogen has higher thermal conductivity and lower density than air, which makes it more efficient in removing heat and reduces windage losses in large alternators.
Q3. What is the most commonly used cooling method in power transformers?
Ans: Oil Natural Air Natural (ONAN) is the most commonly used cooling method for medium to large power transformers.
Q4. Which cooling method is used in large alternators?
Ans: Hydrogen cooling and water cooling methods are used in large turbo alternators, often in combination with closed-loop ventilation systems.
Q5. What is the difference between ONAN and ONAF?
Ans: Both use oil for internal cooling. In ONAN, natural air flow is used externally, whereas in ONAF, fans are used to force air over the transformer tank for enhanced cooling.