Caravan Electric Braking
Caravan electric braking efficiency deteriorating over time is not unusual.
Sounds radical; however, if we look at the science, the electric brake circuits needs attention at every service.
Place light pressure on the magnet and rub the metal in one direction only. The magnetisation will take some time to accomplish, so continue rubbing until the iron or steel attracts other pieces of metal. (Source: Science.com)
This information is not new. In fact, search Google for “rubbing metal magnet”, and you will find many reputable writers are reporting on the magnetisation of metals.
Electric Brake Drum Magnetisation
Our testing has found electric drums are not magnetised when an energised magnet engages a drum for several days.
However, by turning the drum and applying a magnet to the surface of the drum, a different result is achieved.
Owners and service mechanics alike continue to be baffled by poor braking,
I have replaced shoes, magnets, checked wire size and all connections. Voltages are 12v, and the current at each magnet is 3.2A.
All perfect; however, if the drums were replaced, braking returns to normal.
New brake drums remove the polarisation of the old drums.
When a magnet is first energised, it attaches itself to the inner drum face. The magnet face may be north or south, depending upon the connection of the wires.
When examining a North Pole connection, the magnet induces a north polarity into the drum face only when the drum is turning.
Remember the statement from science.com?
Place light pressure on the magnet and rub the metal in one direction only.
This action causes the drum face to begin its polarisation journey.
Drum polarisation accelerates during braking (over many journeys), especially in mountainous countries.
How to prove Polarisation
An electric brake magnet draws 3.2Amps +/- 5%, depending on voltage.
In a perfect caravan or camper electrical system, the voltage is 12v (V), and the Magnet current (I) is 3.2A (per magnet)
Therefore the wattage (P=VI) is 12 x 3.2 = 38.4 watts
The same magnet under no-load or unattached draws 1.6 Amps.
Wattage is determined at the factory and never changes. Wattage is a constant.
The Good Oil
So here is the good oil. Polarised magnets repel against the brake drum.
The amount of repelling is determined by the level of polarisation.
Image a drum face polarised N and a magnet wired to produce an N polarity?
Science demands these two components repel each other.
When a magnet repels, it will draw less current than full load or less than 3.2A.
- More than 3.2A currents mean are suffering voltage drop.
- Less than 12v means voltage drop.
- Less than 3.2A means you are suffering polarisation.
If your caravan electrical installation is experiencing a braking problem with 3.2A and 12v per magnet, you do not have any electrical problems.
You have mechanical problems. The problem can be generally traced to heat from bearing failure, heat from misaligned brake shoes or heat from the misaligned weld rings.
For example, if one wheel is hot and the other is not, review both wheels’ brake adjustment.
There is 2 solution to electric drum polarisation.
- Replace the brake drum in severe cases.
- The transposition of electric backing plate wires at every service. Transposing the wires changes the polarity of the magnet face ensuring opposing forces exert when energised.