Temperature Rise And Insulation Systems

"Temperature has an effect upon electrical resistance, magnetic characteristics, viscosity of lubricants, rate of volatilization of lubricants, dielectric strength of insulation and life of physical components in general. These effects must be taken into consideration in each motor application. 

The temperature rise of a motor is the difference between the measured temperature of the motor winding and the ambient temperature. Electrical insulation systems are classified according to the maximum temperature that they can withstand. Hurst motors have either Class A or Class B insulation systems which are rated at 105° C and 130° C respectively for the hottest-spot temperature. A hot-spot allowance must be made for the difference between the measured temperature of the winding and the actual temperature of the hottest spot within the winding, usually 50° C to 150° C depending upon the type of motor construction. The sum of the temperature rise, the hot-spot allowance, and the temperature of the ambient must not exceed the temperature rating of the insulation.

The temperature rise of a motor should also be specified at a particular operating point, e.g., no-load, full-load, or locked-rotor. Many of the Hurst motors are ""impedance protected,"" that is, they are designed with enough impedance in the windings to limit the locked rotor currents to values that do not cause the motor to overheat beyond a safe temperature. Other motors are available with a thermal protector, a device installed adjacent to the stator winding which will disconnect the motor from the line should the winding temperature increase beyond a safe value. 

The life of the electrical insulation and of the lubricants are adversely affected by high temperatures. A generally accepted ""rule of thumb"" is that for every 10° C increase in operating temperature, life is halved."

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