Motor control reverse-phase

The shim-rod motors are three-phase 220-volt 1/6-hp squirrel-cage motors, of 1800 rpm rating. Start, stop, and reverse are the only control actions called for. Reversal is accomplished. by phase, interchange. This simple control picture is, however, slightly complicated by two considerations the desire to have a finer speed control than is obtained by simple on-off switching, and concern as to the possible safety implications of transition from three-phase to single-phase operation by loss (open circuit) of one power wire. 

Solid-state starters usually incorporate additional features and functions providing additional motor control. These may include such things as overvoltage, overcurrent, phase-loss, phase-reversal, and overtemperature protection. 

Saturable reactors, which are adjustable by a small dc signal, have also been used for both primary (stator) and secondary (rotor) control. In the primary they control motor voltage and therefore torque. In combination with fixed secondary resistors and feedback from a tachometer, this system can be used for precise speed and torque control of cranes, hoists, etc. Even reversing can be accomplished by using two saturable reactors in each of two (of three) phases. Other combinations of fixed or saturable reac tors in the primaiy and/or secondaiy, all combined with secondary resistors, provide a wide range of capabiUties and flexibihty for the wound-rotor motor. 

To understand the directional control of the motor, it is important to note that a reversible ac motor has two sets of coils, one of which is fixed (the stator) and the other of which rotates (the rotor). One of these — say. the rotor — is powered from the 110-V power line and thus has a continuously lluctuating magnetic field associated with il. The output from the ac amplifier, on the other hand, is fed to the coils of the slalor. The magnetic field induced here inicracls with the rotor field and causes the rotor to turn. The direction of motion depends on the phase of the stator current with... 

Three-phase motors can be easily reversed electrically, making them useful for applications involving control of direction of rotation or remote positioning. Different combinations of speed-torque characteristics are also available so that motor performance can be specifically matched to an application. 

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