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Posts: 21
Topic starter


I’m thinking to build an Eddy current retarder capable of handling around 120kW perhaps a CC80 or CC100 would be a good choice. I’m undecided about the Chinese units and tried searching for a second hand unit here in Thailand but this is not so easy to find, the only one that pops up is a Telma Focal unit FN72-00 (F2000) that’s gearbox mounted and would require fabrication to provide a bearing support.

Are there any advantages / disadvantages to using a Telma retarder that’s too big for the job ?


If I do roll with the DIY version one feature I’d like to implement is a water cooled disc.

The one area I have no idea about is the coil specs, I guess I could buy Coils or a DIY option to wind your own. If anyone has any experience and knowledge with rewinding a coil I’d appreciate your input to my questions below?

  1. What was the wire diameter used
  2. How many meters of wire per coil.
  3. Coil Resistance
  4. Dimensions of the coil ID x OD x Height


I found this on the tractor Sport forum…maybe it’s a good starting point.

The central iron magnetic core was 80mm in diameter and 90mm long (face to face). The coil consisted of 380 turns of 2.0mm diameter copper wire. Outside diameter of this coil 125mm. This is a twelve volt coil, the electrical resistance ends up being around 0.695 ohms, and it draws 17 amps at 12 volts. This is for a fairly large retarder. Sixteen of these coils will hold 900 Ft/Lb of torque at 4,000 rpm (685 Hp)


Posted : 20/06/2021 6:10 pm
Posts: 15

A useful site for coil calculations is

I've just done some for a CC160 Telma using 2.12mm diameter aluminium wire on a bobbin diameter of 93 mm, 35 turns per layer and 8 layers giving a total of 280 turns, approx 97 meters with a resistance around 0.75 ohms per coil, same as the originals


Posted : 24/06/2021 10:53 pm
Posts: 21
Topic starter

Thanks GSE_UK, That's useful information ? 

BTW do you know if the Core material used is Magnetic Iron (ARMCO IRON) I understand that a high permeability is best.

I'm looking for a supplier for magnetic iron Round Bar here in Thailand, there's plenty of sheet metal suppliers and places that stamp out the laminated variety for transformers and electric motors.



Posted : 08/07/2021 7:51 am
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