In the past the overload torque requirement has been met by using a large frame size than necessary to meet full load torque requirements, the torque being proportional to the product of the AC supply voltage and the DC field produced by the excitation current.

With thyristor control of excitation current it is possible to use a smaller frame size for a given horse power rating and arrange to boost the excitation by means of a controller to avoid loss synchronism under torque overload conditions.

The excitation current of a synchronous motor may be controlled by supplying the motor field winding from a static thyristor bridge, using the motor supply current to control the firing angle, figure below. A pulse generator varies the firing angle of the thyristors in proportion to a DC control signal from a diode function generator. Variable elements in the function generator enable a reasonable approximation to be made to any of a wide range of compensating characteristics.

simple_compensated_excitation_circuitSimple compensated excitation circuit
When the motor operates a synchronously, i.e. during starting, a high emf is induced in the field winding, and the resulting voltage appearing across the bridge must be limited to prevent the destruction of the bridge elements. This may be done by using a shunt resistor connected as shown.

Where more exacting requirements have to be met, current feed back can be applied to eliminate effects of non-linearity in the pulse generator and rectifier bridge and it will also improve the response of the system to sudden changes of load. Automatic synchronizing is possible without relays by incorporating a slip frequency sensing circuit to control the gate which supplies the control signal to the pulse generator.

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