Electromechanical relays

Details Electromechanical relays Electromechanical relays

1995-06-05

1997-02-18

Fleming, Fritz

Electricity: electrical systems and devices

Control circuits for electromagnetic devices

For relays or solenoids

361170, 361 92, 307102, 307134, H01H 4700

Patent

active

056046561

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a device for preventing the welding of the switch contacts in a heavy duty electromechanical relay. It is of particular significance to heavy duty relays used in automotive applications where cost considerations preclude the use of materials for the switch contacts which would give a better mechanical resistance to contact welding.
In many cases, heavy duty electromagnetic switches use plain copper switch contacts which have a high tendency to fuse together when breaking inductive, high current, loads. In order to prevent this from occurring, the switch contactor is frequently fitted with a return spring which has a high load versus compression factor. This spring exerts sufficient force to the moving contactor to break any contact fusing which might occur as the relay is de-energized. Provided that the relay coil is well designed and the coil voltage is close to its design parameter then the contactor usually survives its service life without catastrophic welding. However, particularly in the automotive environment, maintenance of the coil voltage at its design parameter is difficult because even under normal conditions, the high currents demanded by heavy duty motors (e.g. starter motors) and lighting circuits (e.g. the cold resistance of halogen lamps) can cause low voltage excursions to occur. In the case of an electromechanical relay controlling the supply of heavy current to a D.C. starter motor, the following situation typically occurs.
The potential difference across the vehicle battery is close to the designed coil voltage, typically 12 volts, but due to a high resistance path in the battery/motor circuit, at the instant that high current is drawn, the potential across the coil is reduced to a value which is below the drop out level of the electromagnetic switch. Consequently, the switch contacts are broken. However, at the instant the switch contacts are broken, the heavy load current is removed and the potential difference across the coil returns to a level at which the switch contacts are closed. This sequence repeats itself and an oscillatory situation therefore arises in which the switch contacts are switched in and out at a relatively high frequency. This is sometimes referred to as "doorbelling" or "fizzing". Of course, when this occurs severe arcing and plasma generation occur at the gap between the switch contacts. This rapidly increases the temperature of the switch contacts and if the condition is allowed to persist failure of the electromechanical relay can occur for any one of the following reasons: generated.
The high resistance path in the battery/motor circuit which gives rise to this condition can be caused by several factors. For example, a high battery internal impedence due to undercharging, low or contaminated acid, sulphation or incorrect specification of battery. It may also be caused by poor installation of wiring, incorrect wire sizes fitted or terminal corrosion.
It is an object of the present invention to provide a device for use with a heavy duty electro-mechanical relay which obviates or at least substantially mitigates the problem of "doorbelling" or "fizzing" described hereinabove, thereby preventing welding of the switch contacts.
According to the present invention there is provided a device for preventing welding of the switch contacts of a heavy duty electromechanical relay, which device comprises means adapted in use for monitoring the voltage supply to the relay coil and detecting low voltage excursions therein, a counter for counting the number of low voltage excursions in the said voltage supply to the relay coil, and switching means for disconnecting the voltage supply to the relay if the number of low voltage excursions counted exceeds a predetermined number.
With the device of the present invention the number of times the relay switch contacts are allowed to oscillate is reduced to a very low number (the preset number set by the counter), thus "doorbelling" or "fizzing" is minimised and welding of the switch contacts is prev

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