telma

[MR WEBER]

Hello!
I have a telma brake but no control to it. Is there anyone who knows where I can find one or how I can build one myself?

[84-1074663779]

First thing you need is a suitable transformer and rectifier to provide the required dc voltage and current. Search the internet to find out which model Telma you have, and its power requirement. I have some specifications here, but not all of them.

An old welder connected to a rectifier, or a very large industrial (electric fork lift) battery charger might be suitable, or it might not. Or you may need to order a brand new tansformer specially wound. If that is the case, I can help you with the specification. I am a retired electronic engineer and have done this before.

Next you will need to power the above transformer from a suitably rated variable voltage transformer or "variac" which will allow you to adjust the retarder power from zero to maximum with a large knob. These turn up fairly frequently on e-bay, but you will require a really large one.

That is the simplest and most basic system that will work.

[MR WEBER]

Now I have recieved my telma and I am thinking about using a weldingtransformer. I am wondering if that is going to work and how I am going to control it? I am building a rolling road and I am wondering if it is the engine speed or the speed on the rollers that I use to calculate the power?

[84-1074663779]

Only use a welding transformer if you already have one, or can get one for free to try out. Remember, welders are not continuously rated. A 150 amp welder will not be able to supply anything like that current for any length of time without quickly overheating. Most other types of transformers are usually continuously rated.

Horsepower at the rollers will be roller Rpm times roller torque divided by 5252. Nothing else really matters.

It will be easier to get traction with a larger roller diameter, and some sort of non slip surface might not be a bad idea.

[MR WEBER]

I have a transformer but I don't know how I am going to control it. I don't know how to adjust the power to the transformer. Is there anyone who has any ideas?

[84-1074663779]

I can certainly help you.

First up, what model Telma do you have, and what can you tell me about your transformer ?

[MR WEBER]

The telma model is TELMA/CC135. And the transformer I am thinkning of using is a welding transformer. The supply to the transformer is 230 V and from the transformer 43 V and is adjustable from 40 Amp to 140 Amp. Or do you have any better idea for a transformer?

[84-1074663779]

The specifications I have here for the CC135 are, 1350 Ft/lb and 4,000 Rpm maximums. Rotor air-gap 1.9mm. It can be connected for either 12v (252 amps) or 24v (126 amps) operation.

That welding transformer is not going to be large enough to reach anywhere near maximum without some serious overheating.

Calculating the required theoretical transformer secondary voltage to give 24v dc works out to 28.5 v RMS, that allows for rectifier drop as well as the RMS/average conversion.

So about (roughly) a 30 volt 125 amp transformer would be the ideal, with four diodes forming a bridge rectifier.

Primary current will reach around 18 to 19 amps. A 230 volt 20 amp variac would be required to control the retarder current from zero up to 126 amps.

[MR WEBER]

Ok, but what should I use to control it with? How am I going to regulate the transformer? And where can I find it?

[86rocco]

Tony seems to know what he's talking about so, assuming he's right, a variac like this one , would probably do nicely. There are always several similar items on eBay at any given time, just do a the keyword search on "variac" or "autotransformer"

[84-1074663779]

The Variac will have a large knob that allows you to adjust the ac mains voltage going into the primary of your transformer.

It will allow you to continuously vary your roller dyno from zero to maximum by simply turning a knob.

E-bay is probably the best place to find a secondhand 20 amp variac. Before you hand over any money, test it by connecting a 230 volt light bulb across the output. You should be able to vary it smoothly from zero to full brightness up and down continuously without any jumps or flickering.

Sometimes part of the winding gets damaged from overloading, and the light bulb test is a good way to spot that. The knob should also feel reasonably smooth in operation.

[MR WEBER]

Thanks for your help I will try to get it to work. Tony, what fun stuff do you have in your garage?

[84-1074663779]

Not a lot these days, the financial realities of retirement have dampened the fun a bit.