Using latched contactors to switch transformers

Control Circuit Considerations

A conventional magnetically-held contactor, once it is closed, remains closed as long as the main coil is energized. Most commonly, the control supply for the main coil circuit is a control power transformer supplied as an integral part of the complete controller.

Thus, for motor loads, the motor is automatically disconnected when system voltage collapses, thus preventing damage to the motor.

By contrast, the latched contactor remains closed when the system voltage collapses.

A magnetically-held contactor opens when a contact in the main coil control circuit opens. On the other hand, a latched contactor is unlatched by the closing of a contact in the latch circuit, allowing the contactor to open. Thus, the control required for a latched contactor somewhat resembles that of a medium voltage circuit breaker.

The latched contactor requires a reliable source of control power for tripping. A dc (battery) source is preferred, but if the only source of control power is a control power transformer connected to the primary voltage source, an ac capacitor trip device is appropriate.

The close circuit should use a momentary contact pushbutton, so that the main coil is energized only during closing. Similarly, the trip (latch release) circuit should use a momentary contact pushbutton. For automatic tripping from the protective relays, a normally open contact should be connected in the trip (latch release) circuit, and a normally closed contact from the protective relay should be connected in the close circuit.

It is also desirable to include a lockout (86) relay function, either by means of the 86 function incorporated in some multifunction microprocessor relays, or by means of a separate lockout relay.

It is important that the user’s external control circuit not include a maintained contact in the close circuit. A latched contactor operates in the same manner as a magnetically-held contactor, and in addition, includes a mechanical latch.

If the main coil circuit is continuously energized, the contactor will remain closed even if the trip latch is operated.