Mechanically Held Contactors and Relays
Mechanically held contactors and relays are often referred to as latching contactors or relays. They employ two electromagnets to operate. One coil is generally called the latch coil and the other is called the unlatch coil (Figure 5 – 34). The latch coil causes the contacts to change position and mechanically hold in position after power is removed from the latch coil. To return the contacts to their normal de-energized position, the unlatch coil must be energized. A circuit using a latching relay is shown in Figure 5 – 35. Power to both coils is provided by momentary contact push buttons. The coils of most mechanically held contactors and relays are intended for momentary use, and continuous power will often cause burnout.
Unlike common magnetic contactors or relays, the contacts of latching relays and contacts do not return to a normal position if power is interrupted. They should be used only where there is not a danger of harm to per- sons or equipment if power is suddenly restored after a power failure.
Sequence of Operation
Many latching type relays and contactors contain contacts that are used to prevent continuous power from being supplied to the coil after it has been energized. These contacts are generally called coil clearing contacts. In Figure 5 – 35, L coil is the latching coil and U coil is the unlatch coil. When the ON push button is pressed, current can flow to L coil, through normally closed L contact to neutral. When the relay changes to the latch position, the normally closed L contact,
connected in series with L coil, opens and disconnects power to L coil. This prevents further power from being supplied to L coil. At the same time, the open U contact, connected in series with U coil, closes to permit operation of U coil when the OFF push button is pressed. When L coil energizes, it also closes the L load contacts, energizing a bank of lamps. The lamps can be turned off by pressing the OFF push button and energizing U coil. This causes the relay to return to the normal position. Notice that the coil clearing contacts prevent power from being supplied continuously to the coils of the mechanically held relay.