Electrical Bus

AC Motor Speed - The speed of an AC induction motor depends upon two factors:
1) The number of motor poles
2) The frequency of the applied power.
120 x Frequency
AC Motor Speed Formula:



Example: For example, the speed of a 4-Pole Motor operating at 60 Hz would be:
120 x 60 / 4 = 7200 / 4 = 1800 RPM

Inverter Drives - An inverter is an electronic power unit for generating AC power. By using an inverter-type AC drive, the speed of a conventional AC motor* can be varied through a wide speed range from zero through the base (60 Hz) speed and above (often to 90 or 120 hertz).
Voltage and Frequency Relationship - When the frequency applied to an induction motor is reduced, the applied voltage must also be reduced to limit the current drawn by the motor at reduced frequencies. (The inductive reactance of an AC magnetic circuit is directly proportional to the frequency according to the formula XL = 2 f L. Where: = 3.14, f = frequency in hertz, and L= inductive reactance in Henrys.)

Variable speed AC drives will maintain a constant volts/hertz relationship from 0 - 60 Hertz. For a 460 motor this ratio is 7.6 volts/Hz. To calculate this ratio divide the motor voltage by 60 Hz. At low frequencies the voltage will be low, as the frequency increases the voltage will increase. (Note: this ratio may be varied somewhat to alter the motor performance characteristics such a providing a low-end boost to improve starting torque.)

Depending on the type of AC Drive, the microprocessor control adjusts the output voltage waveform, by one of several methods, to simultaneously change the voltage and frequency to maintain the constant volts/hertz ratio throughout the 0 - 60 Hz range. On most AC variable speed drives the voltage is held constant above the 60 hertz frequency. The diagram below illustrates this voltage/frequency relationship.