Plant managers across the UK have struggled for years to replace damaged power resistors, because often, the original supplier no longer exists in the UK market, or does not supply resistors any longer, so managers are forced to look for the required components elsewhere. Whether the application is crane control, pumps, fans or compressors, replacement resistors are still needed for legacy systems.
Most of the replacement enquiries handled by Cressall are for motor control resistors on cranes, especially overhead cranes in severe environments such as steel works. Older cranes almost invariably have cast iron, grid or folded strip resistors, probably from one of the many now-defunct British suppliers such as Fawcett-Preston, Walshe, BTH, AEI, GEC and Allenwest.
Also in high demand are braking and control resistors for traction applications used in metro trains and electric locomotives. For reasons of speed, simplicity and cost, it is usually more economical to replace old resistors rather than to take out a whole drive system and replace it with squirrel cage motors and modern drives.
It is worth remembering that although resistors are certainly not energy saving devices in themselves, they can be part of overall good plant maintenance and energy saving strategies.
On the other hand, if the application is safety-critical or infrequent, such as a motor on a drainage pump, the proven reliability of a resistor-started wound-rotor motor is worth retaining. Of course, the replacement cost is normally lower than fitting a new system so it does no harm to capital expenditure.
If one does opt for a replacement resistor, the original should always be sent to the partner company that has been entrusted with the job. It can then ensure that the replacement is completely correct. Beyond the obvious need to match resistance values, it can be equally important to ensure that the active mass, type of material used and the electrical creepages and clearances are all appropriate.