Cyndi Nyberg
Former EASA Technical Support Specialist
There are a number of ways that the shaft of an electric motor can become magnetized in service. The most likely culprit is electric current through the motor and shaft, either from internal dissymmetry, welding or from a variable frequency drive. It can also be caused by electrical faults in the system, or even a lightning strike.
We of course know that shaft voltages and the associated currents can cause bearings to fail. A typical ball bearing failure from shaft currents is shown in Figure 1. when a shaft is magnetized, it can further lead to bearing failures, unless something is done to eliminate the residual magnetism. The first reason for bearing failures is that the residual magnetism can cause shaft currents, which can quickly lead to bearing failures. But in addition, a magnetized shaft will attract bits of metal to the bearings. This reduces bearing life because it damages the bearing surfaces.
The magnetism in the shaft may be strong enough that a screwdriver that sticks to the shaft. In fact, this is the most simple test to check for a magnetized shaft.
What are the effects?
As mentioned before, if a shaft remains magnetized when it is put into service, the effect can be damage to or failure of the bearings. You don’t want a recently repaired motor to come back again with another failed bearing.
How is magnetism measured?
A gaussmeter (also called a magnetometer or fluxmeter) is a simple-to-operate device that measures the amount of magnetic flux (expressed in units of Gauss) density in an object. Small, hand-held units are commercially available, with varying degrees of accuracy. If you have a Gaussmeter, measure the magnetic strength along the shaft extensions.
How much is too much?
There is no industry standard regarding how much magnetism is acceptable for a motor shaft. However, the general rule is that for a motor shaft, no more than 2 Gauss should be measured in the bearing area, and not more than 8 Gauss for the rest of the shaft.
What is degaussing?
Degaussing/demagnetizing is defined as the process of removing magnetism from an object. In electric motors, we are specifically looking at the shaft. The magnetism is removed by passing the object through an AC magnetic field. The reason we use AC is that the polarity of the field must be reversing for the process to work.
How to degauss/demagnetize?
There are a number of different methods to demagnetize a motor shaft. Most of us know that a sharp impact can demagnetize a magnet. The problem with that is an impact (from a mallet or by letting a shaft fall to the ground) can scratch, crack or bend a motor shaft.
There are a number of non-damaging ways to demagnetize a shaft. First, there are commercially available demagnetizers, some that include a built-in gaussmeter.
A second option for demagnetizing a shaft is to make your own demagnetizing coil. The strength of any electric field is measured in ampere-turns, so many turns of low amperes, or few turns of high amperes will give you the same magnetic strength. Using that principle, we can design a coil with a relatively high number of turns, to keep the amperes down, and therefore the wire size relatively small. This is going to be a normal donut-shaped coil, large enough that it can be passed over the shaft end-to-end. See Figure 2.
Here is a design that has been used successfully on many shafts: Use a #14 AWG magnet wire. The coil must be properly sized for the current so it does not overheat. If the coil is only for occasional use, the short duty cycle means the wire can safely be sized for 200-250 cm/amp. For commercial enterprise, where degaussing might be a near-continuous process, use at least 400 cm/amp.
Use about 200-250 turns on the donut coil. You will want to make the coil about 2” radius larger than shaft—that is, 2” larger on each side, so about 4” larger coil inside diameter than the shaft outside diameter.
When the coil is energized, you will want to limit current to 200 CMA. For a #14 wire, this will be about 15 amps. To energize the coil, apply up to 115 volts AC with variable voltage source, but monitor the coil temperature so you don’t burn up the coil.
Next, simply pass the coil over the entire length of the shaft. See Figure 3.
Ideally, the shaft should be pressed out of the rotor or armature to ensure that the entire shaft is demagnetized. If you pass just the ends of the shaft through a magnetic field, the ends of the shaft will be demagnetized; however, the part of the shaft within the rotor or armature bore will remain magnetized, and the magnetism will slowly be restored to the shaft extensions.
Another method that can be used to demagnetize a shaft is with a bearing induction heater or a growler. The principle here is the same as with the donut coil: you will pass the shaft through an
AC field. Energize the bearing heater or growler and pass the shaft slowly through the field.
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