Have you ever seen one of these motors before? It turns out most people haven't either!
A rare Brook Crompton pipe cage rotor in for repair at Fletcher Moorland.
I've written this article in response to the interest shown from the LinkedIn community to a photo I posted. The photo was of a rather strange looking rotor. It was from a two speed 4 & 8 pole, Brook Crompton 7-series AC machine. On first viewing you can see the rotor lamination pack on the left-hand side, then the cage extended to a length greater then the lamination pack, there are air agitation fins on the extended cage and what looks to be another alloy cladding the extended cage. I've had 35 years in and around the motor repair business and this was the first time I've seen such a machine. So, how does one find out about such a machine in this day and age? Well turn to the LinkedIn community of course!
'Many comments were made about the rotor and it was quite clear that this was a rather rare and unusual machine indeed!'
A handful of people recognised that it was a Brook Crompton pipe-cage-rotor machine. It's the first time I've heard 'pipe-cage-rotor' too.
With the networking power of LinkedIn, there was one person who knew these machines very well indeed. Steve Cockerham (now Director at HPC Laser Ltd), who used to work for Brook Crompton. Steve provided a wonderful description of the motor and how the pipe-cage works provided in the description below:
'That's a Brook Crompton pipe cage rotor, or as they were otherwise known an "integral eddy current inductor" which was essentially a relatively unsuccessful, if very novel attempt at a cage motor which replicates the starting performance of a wound rotor or slipring machine. Typically low starting current and high starting torque, figures like 225% LRT (Locked Rotor Torque) and 400% LRC (Locked Rotor Current) were typical and they were intended for mills, crushers etc.
The silvery core on the left is the rotor core and there will be an equivalent stator core in alignment. The section on the right is a series of steel pipes which extend from the core and are both supported and shorted out by spidered steel discs under the pipes.
The stator will be half empty with a very short core, this accommodates all the pipe arrangement on the right. The flaps of steel welded to the pipes are wafters purely for stirring the air around the stator.
So the copper bars pass right from the shorting-ring on the left, through the core, through the pipes and to another shorting-ring on the far right.
The clever thing is that the resistance of the steel pipes is proportional to the rotor frequency and so directly proportional to speed. So with high resistance at standstill, as the rotor accelerates the resistance drops and the torque characteristic gradually drops to that of the copper cage with minimal influence from the steel pipes.
A really novel design which never really took off, I'd be surprised if they managed to get into 3 figures of units in production. The pipe cage assembly acts as a series of individual short circuited transformers with the rotor bars as their respective primary windings. The pipe forms both the core and secondary winding of these transformers which are short circuited by the supporting discs.
The pipes are made of steel and therefore exhibit a high electrical resistance which is further increased when the primary (ie rotor) current is at mains frequency. This is due to skin effect, which makes the effective pipe resistance dependent upon rotor frequency and hence motor speed.
At start, with rotor current at mains frequency, a longitudinal current flows in the pipes and adds impedance to the rotor circuit. Both the resistance and inductive components of this impedance are dependent upon rotor current and frequency, and therefore decrease as the motor accelerates to full speed.
It is this automatic adjustment of impedance that gives the pipe cage rotor its torque, current and speed characteristics. Starting current is decreased at start in comparison with a normal cage machine and the additional resistance increases the starting torque.’
Thank you Steve Cockerham for your knowledge and thank you LinkedIn for connecting engineers to share knowledge.
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Matt Fletcher can be found on Linkedin and regularly posts articles similar to this. Follow matt on this link: https://www.linkedin.com/in/mattkfletcher/