WINDING INSULATION IN AC MACHINES
In ac machine design, one of the most critical parts is the insulation of the windings. When the insulation breaks down, the machine shorts out. The repair of machines with shorted insulation is expensive and sometimes impossible.
The temperature of the windings should be limited to prevent the insulation from breaking down due to overheating. This can be done by providing circulation of cool air over the windings. The continuous power supplied by the machine is usually limited by the maxi- mum temperature of the windings. The increase in temperature usually degrades the insulation, causing it to fail by another cause such as shock, vibration, or electrical stress. A rule of thumb indicates that the life of an ac machine is halved for a temperature rise of 10 per- cent above the rated temperature of the windings.
The temperature limits of machine insulation have been standardized by the National Electrical Manufacturers Association (NEMA). A series of insulation system classes have been defined. Each insulation system class specifies the maximum temperature rise allowed for the insulation. The most common NEMA insulation classes for ac motors are B, F, and H. Each class has a higher permissible winding temperature than the one before it. For example, the temperature rise above ambient of the armature windings in contin ously operating induction machines is limited to 80°C for class B, 105°C for class F, and 125°C for class H insulation.
Similar standards have been defined by the International Electrotechnical Commission (IEC) and by other national standards organizations.