Help with unusual transformer

[Ironica]

Hi

I have a very unusual problem that I need help with. I have a quarter scale model showmans traction engine, which is run by a 12v car wiper motor in the Dynamo housing. The power is supplied by what appears to be a home made transformer. This engine and unit are nearly 40 years old. It has run perfectly for 2 years and suddenly it stopped. The only thing I know is it has a 240v input, two 2 amp coils and gives out just under 12v. I put my car battery on the wiper motor and it runs too fast, I think there is some form of voltage restrictor that leaves it to run at a gentle speed.

I could Post photos if anyone can help.

alternative would be a new variable transformer - but I have no idea exactly what I need nor who might supply one!

over to the experts...

 

[HandySparks]

A photo of the transformer unit would help. It presumably incorporates a rectifier somewhere to convert the AC transformer output into DC for the motor.

(Ask the moderators for permission to post pics or make a few more posts to do this automatically.)

 

[ruston]

It maybe that the wiper motor has a potentiometer built in and that has failed , and you juicing it direct is causing it to run on high speed

 

[Paul.M]

I've already asked Admin if he can post photos. Just give the lads a bit of time.

 

[Adam W]

Is it a 2 speed wiper motor?

 

[ruston]

That is what I think may be wrong.

 

[Lucien Nunes]

There are lots of easy solutions to this - normal wiper motors are permanent-magnet types that run at a speed almost proportional to voltage. Thus, a variable voltage supply will give variable speed more or less independent of torque (i.e. motion work friction in this application). The main question is whether you want to repair or replace the power unit, which will depend on what is wrong and how well it was built in the first place. If it is a nice thing made with care, it might be worth troubleshooting and repairing. If it was a make-do solution, we can do better without much cost. Please post pics!

 

[Ironica]

Thank you for your considered replies. I will post photos later this week, please look out of them

Ron

i would prefer a more modern variable transformer that would allow me to control the speed of the motor.

There are lots of easy solutions to this - normal wiper motors are permanent-magnet types that run at a speed almost proportional to voltage. Thus, a variable voltage supply will give variable speed more or less independent of torque (i.e. motion work friction in this application). The main question is whether you want to repair or replace the power unit, which will depend on what is wrong and how well it was built in the first place. If it is a nice thing made with care, it might be worth troubleshooting and repairing. If it was a make-do solution, we can do better without much cost. Please post pics!

 

[Marvo]

Permissions given, you should be able to attach photos as long as the file size isn't too large.

 

[Ironica]

Please find photos of the transformer.... it was working fine until I moved the engine than it stopped. There was no burning or smell. I have changed the fuse and the switch. The light will not light up. I am able to get a reading of around 5.5v when it is plugged in and turned on. However, when attached to the motor I get no reading at all. Not much help...

While the transformer was built for purpose and works perfectly well I would prefer a more modern solution
any suggestions as to what I need and how to go about obtaining one.. thank you all in advance

also include pic of the engine

 

[Lucien Nunes]

OK, not the best or safest piece of electrical construction. As fas as I can tell, the mains feeds two transformers of which the secondaries are paralleled. Two feeds are taken off to the terminal block rear left which I assume lead to the model. One is AC (for lighting?) from one end and what I take to be the centre-tap, and one unsmoothed DC from the other end and CT via a bridge rectifier (4-lead device on bottom) and variable wirewound resistor (green device on back).

As I say the construction is not the safest but the fault is quite likely the resistor or its connections having gone high-resistance e.g. at the connection to the movable strap which looks corroded, or the winding at the point the strap is in contact with it, and potientially fixable in minutes. The failure of the neon to light sounds like a separate thing (just a dead neon) otherwise there would likely be no output at all even when open-circuit. The rectifier might have failed but they usually short-circuit which would probably blow the fuse. How handy are you with a multimeter? How about disconnecting the lead from the rectifier to resistor and measuring the resistance from that lead to the output terminal associated with the other resistor lead?

Regarding replacement, apart from constructional quality, the speed variation obtainable from that resistor might not be very great, plus the speed will vary more with torque than it needs to. A variable voltage supply would be better than this fixed voltage supply with series resistance, however we need to confirm what the load current is and the necessary voltage range before one can be chosen. Is it genuinely a normal car wiper motor of nominal 12V? Can you read anything on the resistor?

 

[Ironica]

OK, not the best or safest piece of electrical construction. As fas as I can tell, the mains feeds two transformers of which the secondaries are paralleled. Two feeds are taken off to the terminal block rear left which I assume lead to the model. One is AC (for lighting?) from one end and what I take to be the centre-tap, and one unsmoothed DC from the other end and CT via a bridge rectifier (4-lead device on bottom) and variable wirewound resistor (green device on back).

As I say the construction is not the safest but the fault is quite likely the resistor or its connections having gone high-resistance e.g. at the connection to the movable strap which looks corroded, or the winding at the point the strap is in contact with it, and potientially fixable in minutes. The failure of the neon to light sounds like a separate thing (just a dead neon) otherwise there would likely be no output at all even when open-circuit. The rectifier might have failed but they usually short-circuit which would probably blow the fuse. How handy are you with a multimeter? How about disconnecting the lead from the rectifier to resistor and measuring the resistance from that lead to the output terminal associated with the other resistor lead?

Regarding replacement, apart from constructional quality, the speed variation obtainable from that resistor might not be very great, plus the speed will vary more with torque than it needs to. A variable voltage supply would be better than this fixed voltage supply with series resistance, however we need to confirm what the load current is and the necessary voltage range before one can be chosen. Is it genuinely a normal car wiper motor of nominal 12V? Can you read anything on the resistor?

thank you for your detailed reply. I will take a look at some of your suggestions, but sadly I am not all handy with a multimeter... As far as I know, I am told the wiper motor was a 1960s car a ford Anglia I think. This was overhauled about 3 years ago. I put my car battery on it in situ and once the belt was fitted over the flywheel the engine turned over very fast. It was a little too fast for my liking and of course it has only one speed. I am thinking this is not very helpful!

as I said in my original post its a rather unusual problem...

thanks again!

 

[Besoeker]

Hi

I have a very unusual problem that I need help with. I have a quarter scale model showmans traction engine, which is run by a 12v car wiper motor in the Dynamo housing. The power is supplied by what appears to be a home made transformer. This engine and unit are nearly 40 years old. It has run perfectly for 2 years and suddenly it stopped. The only thing I know is it has a 240v input, two 2 amp coils and gives out just under 12v. I put my car battery on the wiper motor and it runs too fast, I think there is some form of voltage restrictor that leaves it to run at a gentle speed.

I could Post photos if anyone can help.

It ought to be a relatively simple circuit. Transformer, rectifier, possibly a regulator, and a DC motor.

Have you measured anything since it stopped working? Is the transformer still giving an output? Note that 12Vac will result in about 9Vdc for a full wave rectifier, lower for others. That would account for the DC motor running faster on your battery.

What voltage is being applied to the motor?
If there is voltage and the motor doesn't run, it could be a brush wear problem. Or the brushes sticking.
If you can't read continuity through the motor, brushes could be the thing to look at.

If you need a new transformer, you can try here.
Chassis Mounting Transformers | RS Components

For the rectifier and regulator may be more difficult if they are 40 years old.
A circuit daigram would be most helpful.

Good luck!

 

[Besoeker]

@ Luvien Nunes

You could well be right about the resistor tapping band - I hadn't seen the images when I posted.

 

[davesparks]

From luciens description of the AC output and DC controlled output it sounds similar in function to the basic model train controllers with a 16V AC output and variable 12C DC output.
Which is what I have for a couple of model fairground rides.

 

[UKMeterman]

Ok,
A better way forward would be a 12V PWM controller for the speed, fed from a safe 12V powersupply, just need to know how much current the motor takes when running. At a rough guess I would suggest giving this a try with the car battery as a power source https://www.amazon.co.uk/Motor-Control-Controller-Professional-Regulator/dp/B00H8JRXKG as a quick test. Please power the unit via some fuses and proper clips, short out a car battery and it will start a fire.

 

[Lucien Nunes]

Agreed, a PWM speed controller is the way forward. The many options on that Amazon page and many more that you get from googling '12V PWM controller' might all be OK but they are mostly unknowns.

The motor probably only consumes a couple of amps, maybe 5, I doubt very much it reaches 10A (does it get warm / hot?) If we say hypothetically that a 10A supply and speed controller will be ample (there's no harm in having them a bit over rated, they will run cooler and last longer) then as an example of units from a reputable source you could try this power supply:
STONTRONICS - PD-150-12 - Single Output Internal Enclosed SMPSU, 150W, 12V, 12.5A | CPC UK
feeding this speed controller:
KEMO ELECTRONIC - M171 - MODULE, PWM CONTROLLER, 10A | CPC UK

Mount the two in your chassis, connect mains in, 12V to controller and controller to motor and you're sorted. The motor may run slightly smoother and quieter too as there will no longer be the 100Hz ripple from the unsmoothed DC.

 

[Ironica]

Agreed, a PWM speed controller is the way forward. The many options on that Amazon page and many more that you get from googling '12V PWM controller' might all be OK but they are mostly unknowns.

The motor probably only consumes a couple of amps, maybe 5, I doubt very much it reaches 10A (does it get warm / hot?) If we say hypothetically that a 10A supply and speed controller will be ample (there's no harm in having them a bit over rated, they will run cooler and last longer) then as an example of units from a reputable source you could try this power supply:
STONTRONICS - PD-150-12 - Single Output Internal Enclosed SMPSU, 150W, 12V, 12.5A | CPC UK
feeding this speed controller:
KEMO ELECTRONIC - M171 - MODULE, PWM CONTROLLER, 10A | CPC UK

Mount the two in your chassis, connect mains in, 12V to controller and controller to motor and you're sorted. The motor may run slightly smoother and quieter too as there will no longer be the 100Hz ripple from the unsmoothed DC.

I had the most extraordinary turn of events yesterday. I work for Whipsnade Zoo steam railway and I went to London Zoo to assess the clearing of an old workshop they have. Amongst the lathes, milling machines and various machinery was a load of old electrical equipment, clearly not been used for many years, destined for the tip. There it sat. A dual tracking regulated power supply, made by maplin. This has solved my problem - I am able to fine adjust the running of the wiper motor so my engine can turn from very very slowly to fast speed. Quite an amazing find. I have downloaded a pic Thank you all very much for your replies, it was appreciated.

 

[Lucien Nunes]

That's a result, well spotted!

Looks like the output is running at max current in your pics. That's usually OK for a bench supply with variable current limiting, but if this model permits its outputs to be paralleled (some do, many don't) then that might be preferable to reduce stress. Give it lots of ventilation though, as its linear regulator dissipates heat that wasn't there in the old supply unit.

 

[UKMeterman]

Hi,
Filp the switch on the front to trackiing and use a pair of these http://www.maplin.co.uk/p/p600j-6a-silicon-rectifier-uk61r to common up the output of the power supply, thus the heat gets dumped in both sides. The side away from the band goes to the red side of the power supplies, the side with the band gets commoned up and then connect to your motor. Connect both negative supply terminals together and to your motor.
One question what would happen if there was a severe overspeed of the motor?