Background

While building my new garage, I decided to start a project that covers both of my hobbies - cars and electronics. I have a long time been thinking of having by own power dyno, and a device based on an eddy-current brake sounded the best alternative. In addition, most of the best power dynos come with this type of brake. It has no wearing parts in addition to the bearings, it does not need any coolant and the braking force is easy to control.

There are some links at the bottom of this page that I have found useful. This project is not going to be put through very fast, but I will publish a detailed description of the device on these pages - including the schematics of the brake controller - if I ever get this ready.

The retarder

After a long seeking, I purchased an old eddy-current brake, also called as a retarder. This Telma is from a bus. Those retarders are common on the newish buses only, hence their availability was very limited for a hobbyist with a limited budget. Also, older vehicles equipped with automatic or semi-automatic transmission can have this type of a retarder.

My Telma is of type AC61-35 (or I think so, because there were no markings left...) that equals to type CC135. This brake can handle maximum 650 horsepower with the maximum RPM rating of 4000. The maximum braking torque is 1350 Nm. The unit weighs about 230 kg. Some information can be behind from the links at the bottom of this page.

Despite of its appearance, the retarder seems to be in moderate condition. The rotors and stators were rusty but the bearings were intact. The spacer ring shown in the pictures below was missing from one side, so a new one must be made.

The retarder before reconditioning, the bearing runner mentioned in context, and the unit with the rotors dismantled.

After some investigations, I decided to strip down all the wirings of the retarder. They differ much from all the schematics found from manuals - this unit seems to be some custom-made version. In addition, the wires and connection blocks are very corroded and look dodgy. I am also planning to re-arrange the coil connections, so I can use higher voltage for exciting - the current needed will be lower then. The brake will need over 3 kW of electrical power at full braking capacity, so the currents needed will be very large, if low voltage is used for exciting. The original circuitry is often built for 24V or even 12V, depending on the model.

At least partial dismantling of wiring is also necessary to check and measure the condition of the coils.

Corroded wirings before dismantling.

Condition of the windings was tested with multimeter and battery charger. Some of the coils worked well but over half of the 16 coils had broken wires. Aluminum wire is not very resistant against corrosion. Since the spare parts are almost unavailable,the only option is to fix the windings.

A working coil with a pair of pliers and corroded windings of another coil.

Pole shoes needed to come off, for repairing the windings.

Some of the coil windings were still repairable. However, reliability of this kind of repair is questionable because there will be extra junctions. They must be done by crimping because the aluminium is not solderable. However, the coils must be at least re-tested with maximum current before putting the parts of the dyno bench together.

Four coils were not possible to repair in place. The windings were uncoiled and a new, replacing coils must be manufactured. I have ordered new, 2,36 mm diameter wire for this. As the aluminum wire is quite hard to find in small amounts, I will have some excess.

A pole without winding. All the rust was removed from the retarder frame and a new paint was applied.

A crude drawing below shows the basic structure of the dyno bench I am designing. The retarder is mounted into a inner frame, that is hung to the outer (main) frame with a pair of bearings. The inner frame would be free to rotate if it was not held in place with an arm that is connected to the load cell. This measures the twisting force of the inner frame, and the engine torque can be calculated from this. In the same frame with the retarder is the transmission, that is needed because the maximum allowable RPM of the retarder is 4000. The engine under test is mounted to the outer frame.

A basic diagram of the engine dyno bench.