Synchronous Motors

The construction of the Synchronous Motors is essentially similar to the construction of the salient- pole alternator. In fact, such an alternator may be run as an AC motor. Synchronous motors have the characteristic of constant speed between no load and full load. They are capable of correcting the low power factor of an inductive load when they are operated under certain conditions. They are often used to drive DC generators. Synchronous motors are designed in sizes up to thousands of horsepower. They may be designed as either single-phase or multiphase machines.

To understand how the synchronous motor works, assume that the application of three-phase AC power to the stator causes a rotating magnetic field that gets set up around the rotor. The rotor is energized with DC (it acts like a bar magnet). The strong rotating magnetic field attracts the strong rotor field activated by the DC. This results in a strong turning force on the rotor shaft. The rotor is therefore able to turn the load as it rotates in step with the rotating magnetic field. It works this way once it’s started. However, one of the disadvantages of a synchronous motor is that it cannot be started from a standstill by applying three-phase AC power to the stator. When AC is applied to the stator, a high-speed rotating magnetic field appears immediately. This rotating field rushes past the rotor poles so quickly that the rotor does not have a chance to get started. In effect, the rotor is repelled first in one direction and then in the other. A synchronous motor in its purest form has no starting torque. It has torque only when it is running at synchronous speed. A squirrel-cage type of winding is added to the rotor of a synchronous motor for it to start.

To start a practical synchronous motor, the stator is energized, but the DC supply to the rotor field is not energized. The squirrel-cage windings bring the rotor to near synchronous speed. At that point, the DC field is energized. This locks the rotor in step with the rotating stator field. Full torque is developed and the load is driven. A mechanical switching device that operates on centrifugal force is often used to apply DC to the rotor as synchronous speed is reached. The practical synchronous motor has the disadvantage of requiring a DC exciter voltage for the rotor. This voltage may be obtained either externally or internally, depending on the design of the motor.

A Soft Starter can be used to start synchronous motor in a same way as the Squirrel cage motor is started, but during this, the motor has to be in no load condition, Using Jayashree make HFSR soft starter coupled with the Jayashree make Electronic speed switch, it is possible to know the exact point of bypass to get smooth jerk less reduced current starting for synchronous motors.