How to size a Thyristor Power Controller

Single Phase (1PH)

Thyristor current (I) = Power (WATTS)

                                      V

Example: A 9kW load across 240V AC

              I = 9000 = 37.5A

                    240

If we apply the safety margin of 20%, the thyristor current equals:

37.7 x 1.2 = 45A

We can therefore use a 45A, 240V AC thyristor unit

Three Phase (3 or 4 wire load, 2PH or 3PH)

Thyristor current (I) = Power (WATTS)

                                    1.73 x V

Example: A 53kW load across 415V AC, 3PH supply

              I =      53000     = 73.8A

                    1.73 x 415

If we apply the safety margin of 20%, the thyristor current equals:

73.8 x 1.2 = 88.6A

We can therefore use a 90A, 415V AC 3PH thyristor unit