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The majority of domestic relay manufacturers only indicate the maximum pure resistance load, which may create two confusions among the customers when selecting the relay loads and consequently cause them to choose the wrong relay.
Confusion 1 : What is useful for the customers is usually not the pure resistance load but the inductive, lamp, motor or capacitive load, and the load is equal or close to the pure resistance load.
Confusion 2 : The load can suit the range from low electric level to rated load.
It must be pointed out that relays capable of switching 10A pure resistance load reliably cannot switch 10A inductive load and is not necessarily able to switch 10mA load reliably because the failure mechanisms of electrical contact under different load conditions are totally different. It must be stressed that contact malfunction is the major cause of relay failure. It is therefore essential to have a proper understanding of the nature, failure and failure mechanism of the contact of different load types under different load conditions and to seek unity of understanding between the manufacturer and the end-user so as to improve the reliability of the operation of the relay. Manufacturers must make improve the marking and content of contact load. Different load types must be marked separately.
▶ Incandescent Lamps
The resistance of tungsten filament in cold state is minimal; however, the surge current at the moment of switching on can be up to 15 times more than the steady-state current. Surge current of this magnitude can easily cause the contact to ablate and even the fusion welding to fail. Surge current can be reduced by putting in current-limiting resistance.
▶ Motor Load
When the motor is inactive, the input impedance is minimal; however, the surge current is tremendous at the instant when the motor is switched on. After the current is in, the current and the magnetic field interact to create a torque. When the motor is switched on, an internal electromotive force is created, causing the contact current to diminish. When the motor is switched off, a back electromotive force occurs between the contacts, creating an arc that causes the contacts to ablate. However, as the motor slows down when it is switched off, part of the electromagnetic energy and kinetic energy accumulated in the motor will be consumed as they are being converted into thermal energy, the back elecctromotive force will not be high.
▶ Inductive Load
The inductor, electromagnet, contactor coil, and choke coil are all inductive loads. When the power is on, the electromagnetic coil tends to suppress the rise of the current to prevent the occurrence of surge current; however, when the power is off, the lectromagnetic energy accumulated in the electromagnetic coil is usually consumed by the arc between the contacts, which causes the contactor to ablaze and results in metal transfer and adhesion. Protective devices such as RC networks, diodes and piezoresistors can reduce the ablation of the contacts.
▶ Capacitive Load
The charging current of the capacitive circuit is likely to be enormous. At the initial stage, the capacitive circuit is like a short circuit; its current is only restricted by the line resistance. There have been instances where the user failed to realize the load is capacitive. In fact, long transmission wires, electrical filter for eliminating magnetic interference, and power source are all highly capacitive. Series limiting resistance can reduce the surge current at the moment when it is connected.
▶ DC Load
The DC load is usually difficult to break off than the AC load, because the voltage is non-zero-crossing and when the contact breaks off, an arc will be created and because the external voltage is maintained continuously, the electric arc is so stretched that it can not
sustain itself and tends to die out. The arc energy can cause the contacts to suffer a serious burning damage. The contact spacing interval of the DC load relay must therefore be large enough. Arc suppressing methods are frequently adopted.
▶ Low Electric Level
Low electric level refers to an open-circuit voltage in the range of 10-100mV. The switching current of the contacts usually ranges from microampere to 10mA. Organic substances and chemical compounds stuck on the contact surface are difficult to eliminate at the time of load switching, which will cause the contact resistance of to be large and unstable, the current to be unstable and the voltage drop of the contact to increase. An effective solution to this issue is to adopt softened contact material with low voltage such as gold plating 1-3m. Technical measures may also be taken to ensure a clean contact surface and to control the content of the noxious gas within the relay. However, the cost of the relay will rise remarkably.