Inductive or Transformer Load
Transformers are normally used to provide galvanic isolation between the primary & secondary, or to change the main supply voltage to the nominal load supply voltage i.e. 400V to 60V.
The thyristor power controller can switch either the primary or the secondary of the transformer but will need two completely different ways to control the load. When switching the primary of the transformer attention must be made to the high in-rush current on start-up, which can be 20 times the nominal current and will damage the fuses and/or thyristor semiconductors if not cared for.
Transformers are normally used to step the voltage down to a load. Voltage is typically stepped down because the resistance of an element is limited and the load requires high current with low voltage. Another advantage to stepping down the voltage is that current is increased proportionally. Therefore, the SCR can switch relatively low current on the primary side of the transformer while the current delivered to the load is quite high.Because a transformer is inductive, phase angle firing is used on transformer coupled loads regardless of the element characteristics.
A thyristor controller with phase angle firing, soft-start and current limit is typically employed, or if the load is a fixed resistance then delayed triggering + burst firing can be considered.
Switching the secondary of the transformer is the same as driving a standard resistive load. The issue with this solution is the size of thyristor unit can be 8 times larger than a unit to switch the primary (i.e. 25A v 200A).
Fig. 3 – Switching the Transformer Secondary Winding
So to summarise:
Switching primary of transformer coupled with fixed/variable resistance load?
- Use a Thyristor with phase angle + soft-start + current limit
Switching primary of transformer coupled with fixed resistance load?
- Consider a Thyristor with delayed triggering + burst firing
Switching secondary of transformer coupled with fixed/variable resistance load?
- Use a Thyristor with the same standard considerations as fixed/variable resistance loads