Motor control locked-rotor

Synchronous-motor rotor frequency can be detected because the rotor field circuit is available. Special control schemes have been devised which take into account both speed and induced rotor current in providing locked-rotor and accelerating protection. 

A rotor fails during start-up, possibly due to a lower supply voltage than desired or a smaller accelerating torque than required or reasons leading to similar conditions. In such cases the rotor tails first, due to higher rotor currents and a prolonged acceleration time or a locked rotor. At this instant, unless the motor control gear trips, the stator may also fail due to excessive heat. Instances can be cited where even... 

Wound-rotor motors may be used as either constant-speed or adjustable-speed motors. With full load, the speed may be reduced by as much as 50 percent of synchronous speed for certain fixed loads such as fans or compressors. These motors are frequently used when high locked-rotor and accelerating torque with low starting current are required. They are also used where heavy or delicate loads must be accelerated gradually and smoothly, as in hoists and elevators. A variety of solid-state control systems are available for use in the rotor circuit of wound-rotor motors. 

The starting torque ciurent is substantially less for an adjustable-frequency controller/motor combination than the locked rotor current for an induction motor started across the line. This results in a soft start for the controller/motor... 

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