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RISE AND MOTOR LIFE
The increase in temperature of a motor is due to the losses that occur in the motor. These losses are mainly made up of copper and iron losses. The temperature inside the motor will depend on how effectively this heat can be removed by the cooling system of the motor. It should not be assumed that a motor that appears to be hot externally is not internally.
If the cooling system is efficient, the thermal gradient through the motor will be small and the difference between the winding temperature and the external temperature low.
Some standards estimate the life of the insulation materials as 25,000 hours if operated continuously at their rated temperature and the external temperature low.
Western Electric motors are built with Class F insulation and designed for Class B rise, and most of the motors only have a Class E rise. This 'Thermal Reserve' greatly increases the life of the motor, so that it is not of concern, especially when most motors do not operate at less than full load, are not not in a continuous ambient temperature of 40 degrees. A life of 20 to 30 years under normal conditions can confidently be expected.
The difference between the 'Maximum Temperature of the Winding' and the 'Temperature Limit' is because there will be hot spots in the winding which are not measured by the 'Resistance Method' which only measures the mean temperature of the whole winding. An allowance is made for this difference to ensure that no part of the winding is operating at its full thermal rating. It is not considered practical to try to locate and measure the hottest spot in the winding.
The temperature rise of the winding is measured by the resistance method using the following formula:
For a winding to comply with Class F insulation requirements, all the materials must be to Class F specification or better.
* Service factor: this is really an American (NEMA) term that is not covered in IEC standards. It means that the motor can be overloaded without serious damage of overheating.
Typically, NEMA specifications call for Service Factor of 1.1 or 1.15, meaning a 10% or 15% overload Service Factor is in fact using up the thermal reserve of the motor and allowing it to operate at its full Class temperature rise.
Although IEC does not acknowledge Service Factor in the same way, it certainly allows motors to operate to their full class rating and in fact most motors with a generous thermal reserve will easily match the NEMA requirement for 1.1 or 1.15.
Service Factor and duty rating (eg S1, S2 etc) should not be confused. Duty ratings are clearly covered in IEC standards for different repetitive short term overloads which can be defined and simulated to ensure the motor still meets the requirements for temperature rise.
*Voltage of Frequency Variations: In some installations, especially with their own power generation, or a very weak grid, large fluctuations in voltage and/or frequency are possible which can cause increases in temperature rise of the motor. Motors with a large thermal reserve can operate in these conditions, usually without exceeding their Class rating, by using some or all of their thermal reserve, depending on the size of the fluctuation.
It is a fact that you could take just about any electric motor and hook it up to any appliance and it would work. As would be expected though, some motors would perform better and last longer than others.
The one fundamental characteristic which determines whether a motor survives or dies is quite simply how hot it gets. Motor overheating can be caused in a number of ways including:
All of these can seriously affect motor performance and/or life.
On the other hand, a motor operating in an ambient temperature below freezing could be overloaded and run quite happily for years.
The controlling factor is always the temperature of the motor.
Perhaps the best starting point is to understand that any motor is designed such that when operating at its rated output under known conditions, its windings will experience a set increase in temperature (temperature rise) above the ambient temperature around the motor.
The temperature rise is a function of two criteria:
1. The amount of heat
generated in the rotor and stator per unit time
Providing the motor output, power supply, location and other relevant conditions remain constant, then the resulting temperature rise will also remain constant.
The motor manufacturer having established the design temperature rise and assuming a maximum ambient temperature that is likely to be encountered, then decides upon the use of particular grade or 'class' of insulating material which will be sufficient to ensure no thermal breakdown that would lead toshort circuiting between phases or between phase and earth as well as providing for an acceptable lifetime for the motor.
Each 'class' of insulation has its own maximum recommended temperature. If this critical temperature is drastically exceeded, failure will occur in a very short time. If it is marginally exceeded, then the lifetime of the insulation (and the motor) will be reduced by the order of 50%, leading to premature burn-out.
An increase in load of about 4% will result in an increase in temperature rise of 10%, which makes selection of the correct motors for the job absolutely vital for long term cost-effective operation.
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