The latching relay in energy management

Despite technological advances in power semiconductors, the need for disconnection specifically in areas like energy management, mains supply disconnection and prepayment has created an increasing demand for latching relays.

The reasons are obvious. No semiconductor-based disconnection device can tolerate the harsh environment in mains supply systems without expensive protection. In addition, most applications in these areas require physical disconnection because of the protection of human life (`safe disconnection').

The state-of-the-art disconnection device today is the magnetically latching relay. It requires only a short current pulse through its driving coil to switch to the required position (open or closed). A permanent magnet in a magnetically closed loop keeps the position. Driving power is only required for the change (open/close) of the relay status no holding current through the coil is required. The desired switching position depends on the direction of the current through the driving coil.

 

Figure 1
Figure 2
Two transistors, one for on and one for off, drive a centre tapped coil which is supplied with DC. Figure 2 shows the driving of a single coil relay through a triac. The centre tapped coil arrangement has the advantage that the energy for the coil can be taken from a storage capacitor, whereas the circuit in Figure 2 can only be driven when the AC voltage from the mains is present. In addition, the circuit is connected to the neutral potential.

The low energy requirement for switching permits very low power supply circuitry in the system. This reduces component costs, space and cooling requirements.

Two types of magnetically latching systems are dominating the market - the latching solenoid and the H-armature system. The H-armature system is not sensitive to vibration and shock, because of its rotating anchor. Figure 3 shows the open and close operation. The actuating part (H-armature) contains a permanent magnet. As soon as the magnetic loop is closed, the current through the coil is switched off to save consumption. For rated contact currents of 100A, a contact force of approximately 400 cN is achieved with this system.

The contacts carrying the load current have to comply with the IEC and ANSI specifications regarding over-current, isolation, endurance and contact resistance. Some countries such as South Africa require fault current handling capabilities in excess of IEC 1037. Here, in prepayment applications, fault current handling capabilities independent of the rated current are specified.

In addition, it must be possible to switch repetitively into fault currents of 2100A peak, without welding or destroying the contact. The optimal composition of the contact material for this specific requirement would cause a reduced lifetime and higher contact resistance. Materials with better performance as regards contact resistance and lifetime tend to weld more easily in over-current conditions. The contact material, the shape and the contact force are optimised for best overall performance.

High fault currents through the contact generate magnetic forces which push the contacts apart. If they open, the immediately developing arc would destroy the contact. Newest state-of-the-art products use the magnetic forces in the current loop to the contact to counteract the opening forces. The contact bounce is kept as low as possible to improve the switching performance and the lifetime.

100 A Latching Relay 

The following table lists the most important technical data for a 100A latching relay.

Rated current: 100A
Max current for 10ms: 4200A peak
Max current for 1s: 1200A
Max switching current (cos phi = 0.8): 2100A peak
Min switching operations at 100A: >10000
Coil driving power: typ. 4.5W for 20ms
Contact resistance: <0.5mOhm

Further integration of the system electronics, less physical space available and tighter limits for the power consumption across the contacts are keeping a continuous pressure on the suppliers of latching relays. Field experiences, new materials and designs, together with modern CAD systems, allow more optimised products to be developed. Precision production equipment with tight quality control is essential in order to achieve the required performance.

The first samples of latching relays with integrated current sensor (shunt) have recently been introduced into the market another step in the evolution towards lower costs, smaller dimensions and increased reliability.