Theory of three phase ac voltage controller for the speed control of squrriel cage induction motor?

introduction,circuit diagram,theory and waveformsTheory of three phase ac voltage controller for the speed control of squrriel cage induction motor?
A principle of frequency conversion is derived from the fact that a dual converter is able


to supply an ac load with a lower frequency than the system frequency. If the control


signal of the dual converter is a function of time, the output voltage will follow this


signal. If this control signal value alters sinusoidally with the desired frequency, then the


waveform depicted in the figure below consists of a single-phase voltage with a large


harmonic current. If the load is inductive, the current will present


less distortion than voltage. The cycloconverter operates in all four quadrants during a


period. A pause (dead time) at least as small as the time required by the switch-over logic


occurs after the current reaches zero, that is, between the transfer to operation in the


quadrant corresponding to the other direction of current flow. Three single-phase


cycloconverters may be combined to build a three-phase cycloconverter. The three-phase


cycloconverters find an application in low-frequency, high-power requirements. Control


speed of large synchronous motors in the low-speed range is one of the most common


applications of three-phase cycloconverters. They are also used to control slip frequency


in wound rotor induction machines, for super-synchronous cascade





Squirrel-cage induction motor with cycloconverter


Consequently, the windings cannot be connected in wye or


delta, but must be isolated from each other. Motor speed is varied by applying


appropriate gate pulses to the thyristors, to vary the output voltage and frequency. For


example, the speed of a 2-pole induction motor can be varied from zero to 1500 r/min, on


a 60 Hz line, by varying the output frequency of the cycloconverter from zero to 25 Hz.