8 minute read
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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Prof. David Walters OBE C.Eng., AEMT life member.
In 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 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 increases. It is this property which enables speed variation by changing only the stator voltage.
The penalty for this clever speed control method is that the pipe cage becomes very hot during prolonged periods of low-speed running. In the Hungarian steelworks, they overcame the problem by cutting large holes in the sides of the cast iron stator frames over the pipe cage area and operating the motors as effectively open drip-proof machines.
I knew this solution would be unacceptable to our UK and European customers. However, we were also developing the Series 7 steel frame motors which had much better heat transfer characteristics than cast iron framed motors, and I thought there might just be a possibility of using this property to build a successful tefc pipe cage motor.
To ascertain this, we needed to build prototype motors using the basic design information from Hungary. The steelworks engineer would cheerfully have given us his designs, but Hungary was a communist country, and all negotiations had to be conducted through the Ministry of Industry who wanted £100,000 for a manufacturing licence. I had no authority to spend this sort of money, nor did I want to, given my concerns about pipe cage heating. After some discussion, it was agreed that under a no-cost confidentiality agreement, they would give us the basic design data for one rating which we could use to design and build not-for-sale prototypes. We agreed to share the results of our prototype tests, and if these were successful, we would return to negotiate a full manufacturing licence. Meanwhile, the Hungarians would not grant a licence to any other manufacturer.
Returning to England with the design data, I started discussions with our engineering and marketing teams. The engineers’ reaction varied from cautious enthusiasm to deep scepticism; the marketing team was more optimistic and produced a flyer to gauge potential customers’ reaction.
Initially, the reaction from the crane and hoist industry was extremely positive. Customers were all exasperated by slipring problems. The only other alternative was an inverter drive, but inverters were then very expensive and not really trusted by the conservative crane and hoist users. Based on this, we ramped up our development programme, but in doing so, we rather lost sight of what was happening elsewhere in the market. This was to prove a costly mistake.
Speed control was not a problem both on and off load; the heat generated in the pipe cage was a different matter. In initial prototypes, the heat from the pipe cage caused the stator winding temperature to rise beyond class H levels and bearing temperatures were also uncomfortably high. We tried many ways to improve cooling and reduce the motor’s temperature. A separately driven external fan did not work (although if we had had the type of fan subsequently developed for the ‘W’ Series motor, it might have done). Eventually, after many experiments, wafters welded to the pipe cage (figure 1) provided the answer. These wafters directed hot air from the pipe cage outwards to the stator cooling fins, but their design proved critical. Axially too long, they did not allow room for cooled air to return inwards and they behaved like a baffled fan; too short and they did not drive enough hot air outwards. Eventually, a satisfactory design was achieved, but it all took considerable time: time we were to find we did not have.
Figure 1. Wafters directed hot air from the pipe cage.
Once we were satisfied that the design worked, our marketing team went back to our customers, but they received a very different response this time. In the time we had taken to get the pipe cage motor ready for market, some four major competitors who had backed inverter drives had moved quickly. Inverter costs had dropped, and more importantly, perceived reliability had improved enormously. Also, the marketplace was shrinking. Whilst some customers were still investing, many were fighting for survival. There were other potential markets for this type of motor such as crushers and some high inertia pumps, but our penetration of these was small compared to the crane and hoist market. Should we go ahead or not? It was a difficult decision.
Eventually, the Board decided that we should proceed if we could obtain better terms with the Hungarians and our sales force gave priority to targeting new high torque variable speed applications. Whilst it was a Board decision, I was the project sponsor and it was clear who would get the blame for failure!
The Hungarians were invited to Guiseley to view our finished product and production facilities and negotiate a final deal. They were treated as VIPs, something the Guiseley workforce excelled at, and we did secure a better deal. I cannot remember the exact terms, but it was a much lower down payment of about £30,000 plus a royalty on each motor sold. At the time, everyone seemed satisfied.
Unfortunately, satisfaction did not last. The 1970s saw the start of the decline in traditional UK manufacturing, which continued for many years. Our second customer survey had already indicated a downturn, but the rate of decline took us by surprise. Some of our crane and hoist customers went out of business; those that were left were too busy fighting for survival to have time to consider anything new. Our penetration of the crusher and high inertia pump markets was disappointing and so, after about ten years, production of pipe cage motors ceased with less than 100 ever sold.
I learned several valuable lessons from this. Firstly, the timing of new developments is crucial. Secondly, don’t get so interested in what you are doing that you fail to notice changes in the market. Thirdly, if a project is running over time and over budget, look at it carefully and do not hesitate to abandon it if you cannot see a way to get it back on track.
The pipe cage motor was a disaster. Fortunately for my career, the much larger Series 7 project for which I was also responsible was a commercial success. I kept my job and lived to fight another day – with important lessons learned.
