ELECTRIC CARAVAN AND TRAILER BRAKES
Proper working Electric Brakes are a very popular topic. Even auto electricians get it wrong sometimes.
Electric magnets have no positive and negative. A copper wire is wound onto a bobbin several thousand time to produce the power of a magnet called EMF (Electromotive force). The two cables you see are the start of the winding and the finish of the winding.
Electric Magnets cannot work backwards if wired incorrectly.
10" Electric Magnets draw 3.2amps and 12" Electric Magnet draw 3.4amps
Car voltage is 12v x 3.2amp = 38.4 watts for one magnet.
(a) The big mistake when wiring electric magnets is to run 3mm cable down one side then run more cable across the axle to the other magnet and connect. This is called a series connection where all the wattage for both magnets runs through one cable. This cable is drawing 38.4w at something less than 12v and something more than
(b) The correct method using 3mm cable is to run a cable down EACH chassis rail and connect each magnet independently. This type of connection is called parallel.
(c) Most brake controllers are proportional controlled. The easiest way to explained proportional control is to think of a pendulum. When your car is moving forward, the pendulum moves and supplies your brakes with current. The more braking you apply, the more current is supplied to your brakes. It is not unusual for your brakes NOT TO WORK when stationary.
Quite often I will get a call about uneven braking or one brake is stronger than the other brake. This problem is often caused by undersize cable wiring magnet in series.
Wiring Electric Magnet in series is OK if your cable size is larger. 4mm, 5mm or 6mm is great for series connections for up to 6 magnets.
So series connections are popular but can cause problems with voltage drop. It is the voltage drop caused by insufficient cross sectional area of copper that causes the voltage problem of uneven braking.
If you are having problems with your brakes then it is most likely to be a cable problem.
- Adjust brakes so wheel spins between 1/2 and 1 revolution.
- Re-adjust after 200klms to allow for the shoes to be properly bedded in.
- Use good brand electric backing plate. Not all backing plates are the same.
- Overheating brakes can often be worn Chinese bearings. If there is not a lot of dust in your drums then hot brakes is likely to be faulty bearings.
- Check out the price for replacing both electric backing plates as opposed to magnets and shoes. You could have a whole lot of pain by replacing the entire backing plate. Just a small tip many folks overlook.
- Last tip, not all backing plates are the same. AL-KO and Couplemate make a good backing plates you can trust and I yes, I am a little bias towards Couplemate.
Steve Wotherspoon ©