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Couplemate™ Theory: Electric Drum Polarisation

by | Dec 27, 2017 | Couplemate Core | 0 comments

Couplemate™ has a working theory currently under empirical testing that highlights the possibility of magnetic polarisation of brake drums.

This theory is in its infancy, however, initial test results suggest that there is merit in further investigation.

Below is an article that focuses on rare problems with 12v caravan and camper electric braking systems.

How electric drum brakes become electrically polarised was unknown until recently. Couplemate™ Australia has found electric drum magnets not to be the problem.

In the past, we have looked at brake shoes, electric magnets, brake controllers, cable size and other trailer parts that have given braking problems. Now we will look deeper into caravan or camper braking problems.

New electric drum brakes work well on new caravans and trailers. However, one side can lose braking power after a few big trips.

It is a common suspicion that the cause for poor braking was limited to poor wiring, connections, or worn magnets.

Problem Solved or Not

On occasions, we have replaced a drum, and braking efficiency has returned. This remedy was one solution and solved the problem; however, it does not identify its cause.

A recent customer had a problem with weak brakes and asked to purchase a new set of backing plates. 

The customer telephoned again and said the new brakes were faulty because they worked the same as his old ones.

We asked for evidence, so he sent us a photo of his ammeter and voltmeter reading 2.8amp and 11.5 volts, respectively. These readings were perfect and should result in ideal braking, but it was still mediocre.

I turned our attention to the magnetic induction of the casting drum. If the magnet had been in continuous use for long periods, it was possible the drum could have become magnetised through a process called magnetic induction.

electric drum magnetsTesting Methodology

Initial Test

Electric brake systems are regulated via the control unit within the towing vehicle. To ascertain the efficacy of the electric brakes, a straightforward procedure involves applying a voltage test.

For tandem trailers, one can execute this test by disengaging the breakaway pin and subsequently employing a compass to determine the magnetic orientation on each of the brake drums. A discerning observation will reveal that some drums exhibit a southerly magnetic polarity, while others exhibit a northerly one. In cases where it is necessary to reverse the polarity of the magnets, this can be achieved by altering the positive and negative connections.

This testing procedure alone does not definitively prove polarisation. Brake drums that remain partially polarised may not exhibit significant enhancements in braking performance.

Secondary Test

Caravan brake magnets draw 3.2 Amps at full power. Less than 3.2 Amps indicates polarisation.

The solution is at the bottom of the page.

Positive and Negative ions flow through trailer electric drums’ brakes

Electromagnetic induction was first discovered by Michael Faraday, who made his discovery public in 1831.

A magnetised drum can have excess positive or negative ions even when the circuit is turned OFF.

This is called residual magnetism.

A NEW non-magnetised drum has the same amount of positive and negative ions.

Here is what is happening on caravan electric brake magnets.

In the longitudinal cross-section of an electric magnet, a constant electrical current is running through it. The magnetic field lines are indicated, with their direction shown by arrows. Magnetic flux corresponds to the ‘density of field lines. The magnetic flux is thus densest in the middle of the solenoid and weakest outside of it.

Once the magnet is switched off, the magnet’s metal housing retains some residual magnetism; however, we found the drum also had residual magnetism.

The magnetism in the drum increases over time — the polarity of the magnetism correctly directly with the polarity of the electric magnet.

One Test

electric drum magnetsHere is a simple test for your electric drums to indicate polarisation.

Point your trailer coupling facing north or south.

Place a compass near the face of your drum. If the N or S of the pointer points to the drum, your electric drum has suffered severe magnetic induction.

The compass is ideal for the handyman to perform. The solution to reversing polarity is down a little further in this article.

The Problem

Initially, we thought the DC electric induction could significantly reduce your capacity to apply the brake effectively.

A non-responsive electric brake magnet attempts to engage a residual magnetic or induced magnetic drum.

Residual magnetism can be excess positive ions(North Pole) or negative ions (South Pole).

A magnet producing a North at the magnet face will repel a drum with the same polarity. Furthermore, this means the magnet is not attracted to the drum, which causes mechanical slippage of the magnet on the drum face and ineffective braking.

However, our test proves this theory wrong.

The Surprise

Drums become magnetised by the consistent application of an energised magnet onto a metallic rotating drum face. As the drum turns, the friction from the interest on the drum surface causes wear and magnetisation.

The Solution

The solution to the problem is two-fold, either

  • replace the magnetised drum or
  • reverse the polarity of the wires to the electric magnet.

Reversing the backing plate polarity is a simple remedy.

How to de-magnetise steel

If this solution fixes your problem, leave a comment, so other folks know how this remedy worked for you.


Re-test with a compass, and the trailer brakes ON. You will notice the polarity has changed, and your brakes will now be active again.


Notice the four holes in the image below? 3 pm, 6 pm. 9 pm and 12?

These are wearing indicators. If any of these holes are missing, your magnets need immediate replacement.

It is wise to replace your brake drum if it has groove lines caused by the electric magnet.

Find adjusting advice here:


Remanence or remanent magnetisation or residual magnetism is the magnetisation left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed.


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