The Effect of Above Rated Frequency for Induction Motors
Operating alternating current, general purpose induction motors at frequencies above motor rating results in a variety of effects including increased speed, increased operating temperatures, decreased functional horse power and potentially shortened motor life. Over frequency can occur when a variable frequency drive is used to power the motor and also due to momentary spikes from normal power sources.
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Speed Variation
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Electric motor speed directly related to frequency and is determined using the formula:
Speed = Frequency * 120 / number of poles
Because the multiplier and the number of poles are fixed, the direct correlation is clear. An increase in frequency of 5 Hz will result in a speed change of 300 rpm in a 2-pole motor.
Reduced Effective Horsepower
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The National Electrical Manufacturers Association's (NEMA) Motors and Generators Standards state that an increase in frequency results in a direct decrease in locked rotor torque and effective horsepower of the motor. If the load remains constant, this results in an increase in amperage drawn by the motor.
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Temperature Variation
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Temperature can rise as much as ten degrees Celsius per every 10 percent increase in the motor frequency. This is also dependent on the type of cooling used in the motor design. An air over-motor will dispel more heat due to the increased speed and air volume. An open-drip-proof motor can still dispel heat but is more limited in the heat it can remove. Totally-enclosed, fan-cooled motors are most susceptible to overheating This temperature rise can cause winding damage as well as the secondary effect of damage to roller bearings.
Shortened Motor Life
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Extended increase in heat can cause insulation breakdown and eventual motor failure. For every increase of 10 degrees Celsius, the motor winding's expected life is decreased by half. In addition to temperature variations in the windings, frequency surges contribute to coil movement that can cause insulation breakdown, turn-to-turn shorts and grounding. Like voltage surges, frequency surges are momentary but destructive.
Variable Frequency Drives and Inverters
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Variable frequency drives and inverters are designed to alter operating frequencies on motors. They are typically used in manufacturing processes that require variations in operating speed. General purpose induction motors should not be used in conjunction with variable frequency drives or inverters due to the high failure rate. Inverter duty and high-E rated motors are designed with heavier insulation to withstand the temperatures and spikes produced by the variable frequencies. They are also frequently designed with secondary cooling fans that are not dependent on the drive motor's speed.
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