![]() ![]() VFDs are designed to maintain a constant V/Hz ratio and torque up to 60 Hz. Such applications need less torque at higher speeds, yet require voltage to be no higher than the rated nameplate voltage because the motor insulation deteriorates exponentially at higher-than-rated voltage. Some applications require a motor to be operated at above base speed. Some examples of constant torque loads are: *Conveyors *Positive-displacement pumps *Extruders *Hydraulic pumps *Packaging machinery 1-Constant torque and constant horsepower (courtesy of Houston Armature Works Inc.). The V/Hz ratio can be kept constant for any speed up to 60 Hz. AC drives change the frequency to vary the speed of the motor and change voltage proportionately to maintain constant flux. As long as a constant V/Hz ratio is maintained, the motor will generate constant torque. An AC drive can operate a motor with a constant flux of from zero to the motor’s rated nameplate frequency (typically 60 Hz), which is the constant-torque range. Motors operated with constant flux are said to have constant torque. ![]() ] running on an AC line operate with a constant flux because voltage and frequency are constant. A VFD maintains a preset V/Hz ratio in supplying power to a motor at the variable speeds. Thus, for a motor to produce its rated torque at variable speeds, it also is necessary to control the voltage and frequency supplied to the motor. The air-gap flux density decrease causes motor torque to decrease because torque is directly proportional to the magnetic flux density in the motor’s air gap. For example, increasing the frequency without increasing the voltage will cause an increase in speed and a decrease in air-gap flux density. Failure to maintain the proper V/Hz ratio will affect motor torque, temperature, speed, noise, and current draw. is rated for 460 VAC and 60Hz, and thus has a 7.67 V/Hz ratio. A typical AC motor manufactured for use in the U.S. The output torque for a motor is determined on the basis of the ratio of the motor’s applied voltage and applied frequency, known as the volts per hertz (V/Hz) ratio. 4 Reduced voltage and frequency starting. ![]()
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