problem in tacho bearing


Thread Starter


We have a drive with tacho for speed feedback. We once found problem in tacho bearing and so changed it with new one. but then found drive started tripping. on checking finally concluded that the tacho feed back was less compared to earlier feed back. People say that when the armature is taken out of permanent magnet field, the magnetism is reduaced and so is the generated voltage.

what are the reasons for this process. How can this magnetism be resumed? Some suggests that while taking armature out, if the two opposite side of magnet is shorted with metallic loop the chances of reducing magnetism is less.

Michael Griffin

I won't try to describe the physics of what happens, but with certain types of magnetic materials the magnetism is permanently weakened if the magnetic circuit is broken. The severity of this depends upon the material the magnets
are made from.
The best way to prevent this from happening is to not remove the armature. If you have to remove the armature, you need to provide an alternate flux path. How to do this (or if it is even possible) depends too much upon the
design of the particular device to provide a general rule. You should ask the manufacturer of the device whether this is possible, and how to do it.

Restoring the magnetic field generally requires a magnet charger. The charger consists of a fixture which is specially designed for the particular item being charged, and a "power supply" or controller which provides a large
pulse (often in the hundreds or thousands of amperes) of current to the fixture.
The fixture can be thought of as a very powerful electro-magnet which applies a very strong magnetic field to the magnet for a fraction of a second. This aligns the magnetic domains in the magnet material which "magnetises" it. Depending upon various product and manufacturing design criteria, the magnets may be charged outside of the motor (or tach) and then
assembled into it, or the finished product may be fully assembled and then charged, or this may occur at some intermediate step.

In a simple motor application, the magnets are simply charged to "saturation". That is, they are magnetised as much as possible. The resulting magnetic flux strength will vary due to manufacturing tolerances in the magnets (and other factors). Where the exact strength of the magnetic field matters (e.g., in a tach) the magnets can be "calibrated" by charging them to
saturation, measuring the resulting flux, and then "knocking" them back by applying small reverse magnetic pulses to achieve the desired flux.

I hope the above has been of some help to you in understanding your problem.

Michael Griffin
London, Ont. Canada