Thyristor switching circuits are used on Brushless Exciters for Synchronous Motors to control the starting process. Figure # 1 shows the Trigger-2 control connected in a typical Excitation System.

Trigger-2 Exciter System

This control must provide the following functions:

  1. During across-the-line starting of the synchronous motor, an induced, AC field current flows out of the motor field.  This current is 120 Hz with the motor at rest and is reduced to a direct current with the motor at synchronous speed.  A discharge resistor increases the synchronous motor starting torque by the amount of losses dissipated in the discharge resistor.
  2. The rectifier trigger circuit must trigger SCR1 to allow the reverse induced motor field current to flow through the discharge resistor.  The diode D7 allows the forward induced field current to flow current portion of the induced, ac, motor field current during across-the- line starting.  A voltage sensitive circuit in Trigger-2 will trigger Thyristor SCR1 into conduction above a particular voltage level, allowing the reverse current to flow without damaging the exciter or motor components.
  3. When the motor has reached at least 95% of synchronous speed, The trigger-2 circuits must trigger SCR2 into conduction at the “optimum angle” to obtain the best synchronizing capability for high load torques or inertia.  If the starting load is relatively light, then the motor may reach synchronous speed more quickly than the control can provide “optimum angle” triggering and then the control must supply continuous triggering until the motor synchronizes.
  4. If, after synchronizing, the discharge resistor and the motor field are both energized, then the control circuit must trigger SCR3 to provide an alternate path for current which bypasses SCR1 and allows SCR1 to turn off.  This removes the discharge resistor from the field circuit and avoids damage to the resistor or a reduction of the available field current for the motor.

Trigger – 2 uses state of the art components to reduce the power consumption and size of the control.  The result is a small, rugged package, simply mounted, which can be easily integrated into a rectifier assembly.  The circuit is potted and enclosed in a cast aluminum case.  The barrier terminal strips provide for rectifier circuit connections.

The nominal discharge resistor triggering voltage is 250 vdc but can be provided in voltages up to 360 vdc.

Complete instructions are included for a simple means of rectifier assembly testing to ensure correct operation of the exciter rectifier.

Elastic Stop Nuts are supplied with each unit and can be used for the recommended mounting, with the proper length bolts.

Trigger-2 Enclosure and label

The actual dimensions are 7 7/8 x 2 x 3 3/8 (Including terminals).

The approximate weight is 2.0 pounds.