Return Magazine List
 

Scammell

 

The Automotive Voltage Regulator & Cut-Out.

 

My attempt to unravel the secrets of this mysterious little black box. Having completed the wiring on the Scammell Mechanical Horse, Mick asked if I could write something about the experience. Just how interesting can I make it sound? Well there are lots of pretty coloured wires connected to lots of things that need electricity to make them work. Mick always gives a lot of detail in his reports, almost to the point of causing some readers who don't wear anoraks to suffer from heavy eyelids. I will follow his example so be warned.

 

I have chosen a black box called the 'regulator & cut out' for an in depth description as this was one of the items that caused furrowed brows.

 

Why is it needed?

 

1 - The output of the dynamo will vary with speed.

 

A road going vehicle's engine speed will vary, hence the need to regulate the dynamo's output to prevent too high a voltage from causing damage.

 

2 - A dynamo can run as a motor if connected directly to the battery.

 

So to prevent this happening when the dynamo's output drops below the battery voltage, the cut-out will disconnect the battery from the dynamo.

 

What follows will take you to the mind numbing world of electrical wonderment. If you are satisfied with that explanation above, and are easily bored, then stop reading now!

 

This is how that little magic box regulates the voltage, and when appropriate disconnects the battery from the dynamo.

 

Please note that this is a good old fashioned electromechanical device with none of those funny solid state electronics.

 

Basically it consists of two wound cores each acting on separate armatures with moving contacts.

 

The regulator winding is connected across the dynamo output.

 

As the dynamo speed and voltage rise, the strength of the magnetic pull on the armature opens a pair of contacts which inserts a resistor in series with the dynamo field coils. The effect is to weaken the magnetic field and reduce output voltage. But as the voltage is reduced, so the pull on the armature is reduced and the contacts close, shorting out the resistor. This causes the output voltage to rise again repeating the process over and over so that the contacts vibrate.

 

Voltage can be adjusted by varying the tension on the armature return spring.

 

If you're still reading this, and aren't too bored, then I'll try and give you some inkling as to how the cut-out functions.

 

With no, or low output from the dynamo, the cut-out contacts will be open, disconnecting the battery from the dynamo. But when the dynamo voltage exceeds the battery voltage the cut-out shunt coil will pull the armature down closing a pair of contacts.

 

This connects the battery so the charging current flows through the series coil increasing the magnetic pull of the shunt coil and keeping the contacts closed.

 

The cut-out has no influence on the charging rate. So long as the voltage across the shunt winding is equal to, or greater than, the closing voltage, the cut-out will maintain the circuit between the dynamo and the battery, irrespective of load.

 

Adjustment is more mechanical than electrical as the closing voltage can be altered by varying armature spring tension.

 

Having solved that mystery I was able to connect some of those coloured wires that were mentioned at the beginning of this somewhat over complex ramble. Apologies if your head hurts, but a great deal of thought, planning, blood, sweat & tea goes into every restoration project.

 

Steve McCabe.

 

 

Editor's note: It is very fortunate that Steve agreed to sort out the Scammell wiring. The voltage regulator on the lorry is not the same as that on our wiring diagram. If I had connected it up the result could have been an expensive smell!

 

Steve has followed on from his wiring exploits by taking the carburettor home to give it a good service. He cleaned out some nasty gritty mud and had the parts ultrasonically cleaned before reassembling it with new gaskets. His reward was that the engine started and ran for a few seconds on the third turn of the handle - much to Steve's surprise. It still needs a little more work but this is most encouraging.

 

 

 

Light Duties

 

Some times the 'Friends' are taken away from their heritage preservation and restoration projects. Such an occasion occurred when I realised that I wasn't the only one stumbling around bumping into things in the storage shed. When Mick painted part of his jacket while working on some display boards it was decided that it would be sensible to take another look at the lighting in the working area.

 

After raiding Bob's piggy bank, Mick & I took a trip to a local electrical wholesaler to purchase some more twin fluorescent fittings along with a few steel conduit oddments.

 

Once again, Mick demonstrated his skills by producing some elegant curves in the uninspiring straight steel tubing which then fitted into the contours of the shed just as though it grew there. After I'd added the cable the result was a twin metal clad 13Amp socket appearing in the lean-to extension. A much safer alternative than the extension leads trailing across the floor.

 

When the lean-to lighting was installed, a couple of years ago, steel conduit was used. This meant that in order to add another fitting we would have to withdraw the cable and cut the conduit at the point where the new fitting would be inserted.

 

This is when we made a shocking discovery!

 

All of the fluorescent fittings that were installed at that time had cracked and split in a most alarming and dangerous manner.

 

If one fitting had broken then it might have been inadvertently knocked, but surely not all of them. So to cut a long story short, it was decided that the high temperature near the roof had caused the polycarbonate casing to expand at a greater amount than the conduit to which it was securely attached. Any replacement would have to be hung to allow free movement.

 

Repair of the damaged units seemed impractical until Bob came up with the ingenious solution of stitching the plastic by making wire staples. He heated the staples to melt through the plastic either side of the break, then on the underside folded the legs over to pull the broken edges firmly together, finishing off by running a hot wire along the crack to seal it. I hung the repaired units up and so far it seems to be a successful cost effective repair.

 

Because an electrician had installed a plug & socket system for the lighting in the main shed, it was a relatively easy job to add two more twin tube fittings. The effect was instant, Bob's glasses darkened, Mick could see the clutter and had a tidy up, while I rediscovered all the items that I'd dropped under the Scammell while battling with it's rewire. Charlie was speechless, or perhaps he hasn't noticed the extra fittings as he always keeps his head down and gets stuck in with the job in hand.

 

Please note that all electrical installation work has to be carried out in accordance with The IEE Wiring Regulations BS 7671 & inspected & tested by a registered part P electrician.

 

Although this work is not heritage, it does make the restoration and maintenance of the important projects possible.

 

So when the opportunity to combine light duties with some heritage work arose, I found that I couldn't resist Mick's enthusiastic, infectious and persuasive aura.

 

True to form, from what appeared to be a shelf of dubious scrap, Mick was able to produce a little gem, a classic light fitting from a period that matches our station.

 

With hands trembling with excitement, clutching the treasured find we rushed out of the shed and looked up at the modern incongruous light attached to the apex. It spoiled the ambiance so it had to be removed as soon as possible to make way for the more appropriate period fitting. Not too sure how often we need to switch on outside lighting because if it's dark and we're still working then it means we're late home for tea. But as all the external features of the shed are rescued heritage, apart from the modern light fitting, we decided to go ahead with the swap.

 

I disconnected and isolated all the wiring supplying the photocell (the clever little device that prevents the light being illuminated in daylight) and removed the wiring from the lamp. We erected a scaffold tower and while I pretended to be busy Mick was at the top of the tower and with a nervous whistle the modern lamp was on the ground. What followed was a period of intense activity with drill, saw, grinder and file as Mick fashioned a customised mounting plate in order to secure the 'new' old lamp in it's rightful place.

 

After having our heritage lamp tested to make sure that it was safe to install, I stood on the ground holding the lamp and while looking up at Mick's splendid mounting plate I couldn't help but think "it's a bit high". However, I made it to the top of the tower and the task was completed successfully without incident and the lamp now looks totally at home.

Return Magazine List