Large electro-magnets are extensively used for handling iron and steel in a variety of sectors.
Power is fed to the magnets through trailing cables from a control box. These magnets, with ratings of tens of kilowatts, possess considerable stored magnetic energy, and it is therefore essential to provide a discharge varistor to absorb this energy when the magnet is switched off.
The discharge varistor is usually located in the control box. Lifting magnets are often subjected to rough treatment and it is not uncommon for the cable to be broken. If this happens while it is switched on, the magnet is deprived of its discharge varistor and the final result is a breakdown of the magnet insulation, followed by a costly re-wind.
A special range of Metrosil discharge varistors, intended for mounting on the magnet itself, have been designed. These varistors are not intended to replace the metal linear discharge resistor in the control box; their purpose is to protect the magnet in the event of cable breakage. Of robust construction, they are permanently connected across the magnet coil and, due to their non-linear characteristics, their power consumption is negligible.
Extensive tests and many years of service have shown that these units will provide ample protection against voltage surges caused by cable breakage. They will also offer protection against surges caused by contact bounce as the contactor closes (the metal varistor is not in circuit at this time). Similarly, protection is given if the supply should for any reason be interrupted before the control gear has time to operate.