Moir PMT-2

Permanent Magnet Tester

Check motor field magnets for partial demagnetization

Quickly determine if a field must be remagnetized or replaced

Provide better ‘Repair or Replace’ recommendations

Improve accuracy of repair estimates

Permanent magnet motors can fail with bad magnets, bad armatures, or both. It is not sufficient to check a magnet’s condition by seeing if it attracts (pulls) on a screwdriver! In our experience, most ‘bad’ magnets have been demagnetized at one side of each pole. In some instances, the field polarity will even be reversed in that portion of the magnets. This has the dual effect of weakening the strength of the field poles and of shifting the neutral. If the motor can be run, it will overspeed, will not develop the designed torque, and will probably run at different speeds in opposite rotations. The brushes will probably arc badly. Similarly, if the motor is driven through its shaft as a generator, the output voltage will be low and will be different in each rotation. There will probably not be sufficient travel in the brush rigging to reset the neutral.

If the magnets are good, rewinding the armature is probably the most time and cost effective solution. If the magnets are bad, rewinding the armature will not solve the problem. At this point, you will have to decide whether to repair the motor or to scrap it. New fields are sometimes available from the manufacturer. Otherwise, the manufacturer or any of a number of independent shops (including Moir) may be able to economically re-magnetize the fields.

The motor magnets do not wear out and can be recharged as often as necessary. However, if a current spike – whether from a motor stall, a plug reverse, a controller malfunction or some other cause – has once damaged a motor, it will likely happen again unless the power source is upgraded to include active (electronic) current limiting. A simple fuse or circuit breaker will not act fast enough. The motor will have been damaged long before a fuse can blow.

Several years ago, we designed a permanent magnet tester and published the circuit. Over the years, we have found it extremely useful. We have sent copies of the schematic to many of our customers who have built their own units.

If you do not have a tester of your own, we will be happy to test your field rather than take the chance of ruining a good rewind by installing it in a bad field. Send the field packed separately from the armature and without a keeper. (The magnets are fragile and may be cracked or chipped if you ship them with the armature loose inside the field.) If the magnets test 'bad', we will advise you before rewinding the armature. We will also either give you a quote on recharging the magnets ourselves or will recommend a number of other shops that can do it. You will also want to check with the manufacturer to see if replacement fields are available.

We recently developed a slightly revised circuit of our permanent magnet tester (PMT-2) and have completed a limited production run. Unfortunately, there are now none left for sale. We encourage any of our customers to make their own if they have the resources. The schematic, parts list, and pictures of the interior of the instrument may be found below.

The PMT-2 is a simple integrating gauss meter built around a single integrated circuit operational amplifier. The probe is a pickup coil of fine wire that is placed against the field pole and moved over the surface of the magnet. A zero center meter indicates the magnet polarity and the strength of the field under the pickup coil.

The probe is simply a plastic sewing machine bobbin filled with 30 gauge magnet wire. It is fastened to the end of a phenolic handle with a brass screw. The connecting cable can be any two conductor cable - it does not need to be shielded. It is important to avoid using any iron or steel components because they will become magnetized and throw off the reading of the meter.

If the meter swings too far for the magnets you are testing, you can reduce the number of turns of wire on the probe or increase the value of the resistor in series with the meter (R9). If the meter doesn't swing far enough, increase the turns or reduce the resistance of R9.

Before beginning a test, hold the probe well away from any magnetic field and set the switch to the Zero & Balance position. Adjust the potentiometer knob until the amplifier is balanced as indicated by a zero deflection of the meter. The closer you can set it to zero, the longer the instrument will maintain its balance. During a long test, occasionally remove the probe from the motor and check for zero deflection. If the amplifier has drifted, zero the charge on the integrating capacitor by flipping the switch back to the Zero & Balance position. You can also readjust the balance if necessary.

To test a magnet, set the switch to the Test position and place the probe’s pickup coil as flat as possible against a pole. The meter deflection is proportional to the magnet’s field strength. Each pole should have quite uniform field strength over its entire surface. Poles must alternate north and south around the frame and should all have close to the same strength. Field strength will vary from motor to motor but will be roughly proportional to the magnet thickness. The tester is not calibrated to any particular standard, but similar motors should yield similar results.

The PMT2 Schematic and Parts List is available as a PDF file by clicking this link. The following pictures show the simplicity of the instrument. The circuit board was designed to mount directly on the back of a standard 2 1/2" zero center panel meter with terminals on 1.2" centers. The circuit board could easily be mounted on standoffs and connected to the meter by wires. The interconnections to the panel mounted components are via a 14 pin DIP connector and ribbon cable.

Side View of PMT2

Rear View of PMT2


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