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FAQs – Replacement resistors

This article was produced exclusively for IEN (Industrial Equipment News):

Question: As a plant manager of several years standing, one problem I often encounter is replacing damaged power resistors. Often, the original supplier no longer exists in the UK market, or has a presence but doesn’t supply resistors any longer. I’ve had this problem on crane controls, as well as pumps, fans and compressors – what’s the solution?

Answer: Firstly, you’re right about the lack of suppliers in the UK. The power resistors market has consolidated over the years, as the global electrical players have stripped away non-core products. As a result, there are now far fewer active firms. However, if you type the right phrase, such as ‘slip ring starter’ or ‘crane controls’; into Google you should find a trustworthy partner.

Most of the replacement enquiries we get 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 you mention such as Fawcett-Preston, Walshe, BTH, AEI, GEC and Allen West. However, as you point out, there is also a big problem with replacement starting resistors for large pumps, presses, conveyors and fans. 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. However, because large inverter-driven electric motors are much more energy efficient at part load, there are sometimes running-cost savings to be made by moving to inverter systems. In contrast, if your application is safety-critical or infrequently started, 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 your capital expenditure.It is worth stressing that a good resistor manufacturer will give you balanced advice. What we, as suppliers, lose on the motor control roundabout we usually win back on the braking resistor swings!

If you do opt for a replacement you should always send the original to the partner company you have entrusted with the job. They can then ensure that the replacement is completely correct. Beyond the obvious need to match Ohmic 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 you can’t supply the remains for any reason, we have been known to 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 and physical size. The resistor itself may be different but trust me; the laws of thermodynamics remain the same!

For example, we were recently supplied with the remains of the starter and speed control resistor for a DC drive system. We 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 we were 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. We 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 firm will hold large stocks of materials and finished grids.

So, although your original supplier may not exist you don’t need to go back to the drawing board. Help is at hand and your application can be up and running again relatively quickly.

This month’s answer was supplied by Peter Duncan, deputy managing director of Cressall Resistors.

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