26 Motor Design
The Pipe Cage Motor: Clever Idea to Commercial failure AEMT life member, Prof. David Walters OBE C.Eng., was responsible for designing a motor that was technically very interesting, but not commercially successful, with less than 100 ever sold. David explained to Renew how something which seemed so promising ended so badly.
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n 2018, Matt Fletcher of Fletcher Moorland, assisted by Steve Cockerham of HPC Laser, wrote an excellent article in Renew’s predecessor, the Journal, about repairing a Brook Crompton pipe cage motor, the like of which he and most others had never seen before. David explains how the motor came to be, and where it all went wrong… The story starts in the early 1970s when I was Motors Product Manager at Crompton Parkinson’s Guiseley factory near Leeds. At that time, there were about six UK manufacturers of LV Industrial AC motors, and we were the second largest with about 15% of the market. Brook Motors, with whom we later merged to form Brook Crompton, was the market leader with about 25%. Others included GEC, BTH, Lawrence Scott and Newman – all names familiar to the older readers of Renew. Most of our production was in cage motors up to frame 315, but about 10% was in slipring motors mostly for the crane and hoist industry, where variable speed was important. These motors sold at a considerable premium to cage motors and in theory our profits from them should have been excellent. However, the theory hid a nasty cost, very high slipring and/or brush wear, often within the warranty period, on many but not all machines. Sometimes all three rings would wear rapidly, sometimes only one or two. Despite attempts to analyse the failure pattern, it seemed completely random. We tried alternative copper alloys for the rings and many different grades of brushes; some worked some of the time; none worked all the time. We knew from informal contacts with our competitors that they all experienced the same problems, but this was cold comfort. FINDING A SOLUTION It was against this background that a brief trade press article about a cage motor capable of speed variation by stator voltage control alone caught my interest. The motor had been invented by a Hungarian steelworks engineer, details were sketchy, but eventually, with the help of the Foreign Office and the Hungarian Embassy in London, we had enough information to justify a trip to Budapest for myself and a colleague. Arriving at the steelworks just outside Budapest, we witnessed the pipe cage motors working and spent time discussing them with their inventor, the works’ chief engineer. Whilst the motor certainly gave
Quarter 1 2021
Prof. David Walters OBE C.Eng., AEMT life member.
seamless variable speed by stator voltage control alone, I was immediately concerned by the high temperatures generated in the pipe cage itself. It is perhaps useful to recap on the pipe cage motor and its method of operation to understand this. In a pipe cage motor, the rotor bars are extended to more than twice their normal length, and a steel pipe is fitted over part of each bar extension. These pipes are then welded to steel rings and so form a single turn short-circuited tertiary winding. The rotor bars themselves are brazed to a normal cage endring outboard of the pipe cage. In operation, when the stator voltage is reduced, slip increases, rotor current increases at slip frequency and a corresponding current flows by transformer action in the pipe cage. This effectively adds impedance to the rotor. The amount of added impedance depends on rotor frequency, it is greatest at standstill (which gives the motor excellent starting characteristics) and automatically reduces as motor speed
“A brief trade press article about a cage motor capable of speed variation by stator voltage control alone caught my interest.”
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