Electrokinetica The Electro-mechanical Museum

Battery charging Motor-Generator set

Background

In the 1930s, if a DC supply was needed capable of delivering hundreds of amps at a variable voltage, one of the most satisfactory ways of producing it from the fixed voltage electrical supply was using a motor-generator set. Not to be confused with a generator driven by a prime mover such as a steam engine, the motor-generator uses an electric motor to convert energy from the electrical supply into mechanical energy, which in turn is used to drive a generator to convert it back into electrical energy. This may seem a little longwinded and inefficient but there was sound logic behind it. Firstly, the motor could be made for whatever kind of power was available. DC or AC, high voltage or low, the only function of the motor was to turn the shaft at a steady speed. The generator likewise could be made for exactly the type of output desired, be it DC, AC, unusual frequencies or voltages, even multiple independent supplies could be produced by driving two or more generators from a single motor. The static transformer provided a more efficient solution to convert AC from one voltage to another, but the motor-generator set offered the more exotic scenario of frequency conversion, say from the 50 hertz supply network to 400 hertz for instrumentation, which a conventional transformer cannot do. In the field of heavy-current supply for electroplating, the generator could be designed to provide many thousands of amps at just a few volts; at this they were more or less unrivalled. Another advantage was the ease with which a variable output voltage could be achieved, as a suitably wound dynamo can be controlled easily by a small field rheostat to vary the voltage down to nearly zero. Thus they found widespread application for variable-speed motor drive (often called Ward-Leonard drive after the inventor), battery charging, carbon arc supply etc.

The Secret Motor-Generator

Making a peephole

Making a peephole

What they saw

What they saw

We were on site in the electric vehicle maintenance and charging area of an old factory collecting a mercury arc rectifier and its associated charging regulator panels. This had long since been superseded by newer electronic chargers but was physically still in situ. At the end of the line of regulator panels was a DC circuit breaker connected to an ammeter scaled up to 1500A, evidently fed via a two pairs of 0.2 sq. in. lead-sheathed cables laid in a cable trench. Below the circuit breaker was a panel carrying a small rheostat looking suspiciously like a dynamo field control, suggesting that some sort of generating set might be at the far end. "We must remember to follow those cables" remarked Lucien while packing the mercury-arc rectifier bulb. "There might be something interesting connected to them." A quick check with a meter revealed that the cables might have been severed underground as a strong potential was present between them and to earth, making it impossible to gauge the resistance. Bill took a more direct approach by following the trench and knocking a small hole through a partition into a concealed area through which the trench passed; he pointed a torch into the blackness and shouted "I think I've found it!" Everyone took turns looking through the hole; time was running out and did not allow further investigation that day, but clearly a motor-generator set was lurking there, walled in perhaps for decades since it was last used. The electrochemical cause of the potential was obvious too; a leak in the roof had escaped notice and flooded the area and condensation was dripping from every part of the machine.

Scratching of heads

How do you test a wet generator? Any good book will tell you how to calculate the acceptable minimum insulation resistance of a machine once it has been carefully dried out; that would take weeks even with power on site. We had no power and a few hours. From what we had seen, any attempt to ‘Megger’ the winding would just bend the needle on the Megger. What about the dynamo armature? Evidently it was wound for a current of a thousand amps or so, which at perhaps 50 volts would make for a very low resistance winding on which shorts would be difficult to detect reliably with a simple bar-to-bar drop test. Maybe the damage would be so major and so obvious that it would not be worth further investigation. Perhaps some of us secretly hoped it would be a write-off, as there was an unspoken recognition that recovering it would be a bit of a slog!

A secret pile of junk, perhaps?

Danger sign

Danger sign

Dynamo plate

Dynamo plate

Two weeks later, Bill, Dave and Lucien returned to inspect the motor-generator set. The partitions were taken down and debris cleared from around the machine. 1000 amps had been right on the mark, at 60 volts according to the dynamo plate which proclaimed W. Canning to be the maker. In fact the styling of the cast iron frames is unmistakeably Crompton in origin; Canning were a local electroplating equipment supplier who would have purchased machines of this type and in this instance rebadged them. It is an interesting reminder that battery charging and electroplating are rather similar activities and can be done with the same machinery! The motor was found to be a synchronous machine of 91 horsepower with overhung exciter, which had not been visible from the original peephole. The oil-filled starter stood at the motor end apparently complete except for the wire ropes for lowering the tank. Behind the middle of the machine bed stood the motor excitation control pillar with rheostat and ammeter. Yes, the machine was rusty and the insulation degraded in places, mostly in the motor, but the brush springs had not rusted through nor was any verdigris visible around the winding connections. We did the best we could with a meter testing for obvious shorts or open-circuits having disconnected some of the cables from the machine. The lowest resistance we could reliably measure was the dynamo armature at around 7000 ohms, although clearly the surface leakage was as likely to be responsible for this as insulation breakdown. Dave took the guard off the shaft coupling and turned the machine by hand, easily and without clonks or graunches. In the absence of any further sensible tests to make, Lucien suggested it might be run at low speed using the dynamo as a motor. Could we get enough current from a car battery?

Clunk, Trundle, Whirr...

Because the dynamo was made for battery charging under hand regulation, it was wound with a simple shunt field. This configuration makes a machine difficult to run effectively at reduced voltage because the field is weak and the armature current therefore heavy. Lucien's first experiment was to connect the field directly across a 12V car battery and the armature through a long piece of heavy scrap cable to limit the current. Lots of current went in but the machine gave no sign of movement. Another check revealed the shunt field resistance to be 52 ohms. That seemed rather high for a 60 volt machine so an attempt was made to test the four coils individually. The first one measured at exactly 13 ohms, suggesting that 52 was indeed correct. Then the penny dropped. It would be separately excited, even the dynamo plate hinted at this by showing the rated voltage as 0/60. Following the conduit from the machine to the rheostat in the next room revealed a branch leading to a distribution board. On the front, scribbled in pencil, was a note: "Alive! 110V DC". Alive it certainly wasn't but the need for a higher field supply voltage was clear. We had amongst our site equipment two worklights powered by small 12 volt lead acid batteries. Two in series on top of the 9-ish we could expect from the heavily loaded armature supply battery might do the trick, we thought. This arrangement was rigged up, and to give it the benefit of the doubt Bill turned the machine over by hand and built up a few volts of back-EMF before Lucien made the armature connection. And off it went, blowing out dust and cobwebs as it gathered speed to a leisurely 200 rpm.

Bill examining machine

Bill examining machine

Running on batteries

Running on batteries

Dynamo brushgear

Dynamo brushgear

See the test run, but mind out for the sparks and arcs as Lucien connects the final cable! Watch video: Testing the motor-generator

A phone call to Edward...

From the excitement of the test run back to the irritating reality of the recovery. No access wide enough for Edward's crane truck existed within reach of the machine. The nearest lift point seemed to be from a service road outside the site perimeter fence, to which the machine would have to be dragged on skates by a winch. While we surveyed and investigated, a man from the local council walked past and handed us a flyer through the bars of the fence, advising that the road was to be closed for an unspecified time pending bridge repairs. Would the closure stop the truck getting through? The chap wasn't forthcoming on the details!

Recovery day

Winching machine out

Winching machine out

Craning machine onto truck

Craning machine onto truck

Edward, Bill and Lucien did this one. We waited until after the roadworks were completed, then Tom Thurrell had the access coned off so that there was enough room for the truck. We jacked the machine up, put it on wheels, winched it to the truck and craned it on. Yes, it really was that simple; no hitches or glitches, barely an hour's work to shift it. The starter, main cables, field control pillar and all the accessories came out with no hassle either. Draining the starter tank was amusing though. You've heard of oil-filled switchgear; it's simply switchgear filled with oil in order to improve insulation, arc-quenching and cooling. Water-filled switchgear doesn't have so much to recommend it but we got over two gallons of clean water out of the starter before the first oil appeared; it had dripped in through an open conduit entry in the top of the casing. After ten gallons of oil, the flow stopped but the tank still wasn't quite empty; it contained a layer of something like soft cheese formed at the interface between the oil and water. Now cheese-filled switchgear sounds quite promising, if one could arrange to insert bread through a slot and have it toasted automatically by the starting resistances, but I digress. More to follow...

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