Martin Nicholls offers advice to manufacturers and plant managers who need to replace old or obsolete electrical resistors
They say that one man’s junk can be another man’s gold and, although one may think that the saying only applies to car boot sales, the maxim also illustrates the state of the industrial power resistor market.
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.
If the customer cannot supply the remnants for any reason, Cressall can work from photographs and a description, or even just the rating plate details of the motors involved. Although it may not be possible to produce a carbon copy, it is normally feasible to produce resistors that are functionally identical in terms of electrical and thermal performance, as well as physical size. The resistor itself may be different, but the laws of thermodynamics remain the same.
For instance, Cressall was supplied with the remnants of the starter and speed control resistor for a DC drive system. The company duplicated the size, mountings and terminal locations for the customer, who had sent his motor to Lincolnshire Rewinds, a Lincoln based motor rewind specialist.
Finally, many of the older designs, which use low-grade silicon steels or even cast iron, can be readily replicated in modern stainless steel coils, grids or strips. In another recent example, Cressall was sent an old ASEA starting resistor with cast iron grids for a 2.5MW motor. It was used for driving the wind-tunnel fans in a UK research facility. Cressall replaced the failed section and the wind-tunnel was back in operation within days of the failure. To cater for breakdowns of this type a good power resistor supplier will hold large stocks of materials and finished grids.
So, although an original supplier may not exist, end-users don’t have to go back to the drawing board. Help is at hand and their application can be up and running again relatively quickly, thanks to those few manufacturers who have seized the opportunities discarded by other European resistor companies and turned them into asset gold.
Martin Nicholls is sales director at Cressall Resistors.