Page 135 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SPEED CONTROL OF INDUCTION MOTORS

7.2 CHAPTER SEVEN

If the electrical frequency decreases by 10 percent while the voltage remains constant, the
flux in the core will increase by 10 percent. The magnetization current will also increase
by 10 percent in the unsaturated region of the motor’s magnetization curve. The magneti-
zation current will increase by much more than 10 percent in the saturated region.
Since induction motors are designed to operate near saturation, the increase in flux due
to the decrease in frequency will cause a large magnetization current to flow. The stator

FIGURE 7.1 Variable-frequency speed control in an induction motor: (a) The family
of torque-speed characteristic curves for speeds below base speed, assuming that the line
voltage is derated linearly with frequency. (b) The family of torque-speed characteristic
curves for speeds above base speed, assuming that the line voltage is held constant.

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