Motor Magnets

Which Motor Type Are You Building?

The table below covers the types we see most. Each row gives the shape we usually make, the grade range that fits it, and the one constraint that decides the design.

Ranges overlap on purpose. The same motor can be built several ways, and the cheapest one usually comes from the constraint at the top of its column.

What Changes the Spec for Motor Magnets?

Four things move the specification. Temperature decides which neodymium grade you can use. The enclosure decides the coating. Speed and air gap decide the demagnetization margin. The tolerance on the air-gap dimensions decides how much of the field you actually get.

Temperature sets the grade. Use the temperature the magnet reaches in service, not the ambient temperature of the room the motor stands in. That figure is the continuous operating temperature of the motor plus the rise from the windings and the rotor losses. It is why motor magnets are often ordered in H, SH or UH grade even when the application looks like a room-temperature job from the outside.

The enclosure sets the coating. A sealed, dry motor is served by standard nickel-copper-nickel. A motor that sees condensation, salt spray, coolant mist or repeated thermal cycling is usually ordered with epoxy. A coating that survives humidity also survives the handling that comes with rotor assembly. We compare the options in our guide to neodymium magnet coating.

Speed and air gap set the demagnetization margin. A wider gap lowers the field the magnet sees from its own circuit. A magnet working below its knee keeps its strength. One working above it loses strength for good, and the loss shows up first at the pole edges.

Tolerance becomes a magnetic tolerance. Where the air gap is short, a few hundredths of a millimetre on the mounting surface changes the field at the winding. That is why we agree the tolerance plan before the first part is made.

Magnetization for Motor Duty

Segments are usually magnetized radially, so the field crosses the gap. In precision motors a multipole pattern is written across each arc instead. That divides one segment into several poles, which cuts cogging and harmonic loss.

Magnetizing before bonding is the usual choice. Unmagnetized parts are easier to handle, and nothing pulls the segments out of position while the rotor goes together. It also keeps steel chips off the magnet. The trade-off is that a rotor magnetized after assembly needs a fixture that can reach around the shaft.

Pole Pitch Is Not the Magnet Arc

Pole pitch is the full span of one pole: 360 degrees divided by the pole count. The arc the magnet occupies is shorter. The ratio between the two is the pole-arc coefficient, usually written as alpha, and values between 0.67 and 0.86 are common.

The gap is deliberate. It shapes the flux at the air gap and it sets the cogging. A longer arc gives more flux. It also gives more cogging and more fringing between neighbouring poles.

Geometry, grinding method and the dimensional tolerances on a curved part are covered under arc and segment magnets. This page stays with the motor side.

Thickness or Area?

When a motor needs more flux there are two ways to get it, and they do not behave the same way.

Adding area adds flux in proportion. A longer arc or a longer stack gives more flux at the same thickness, and the gain stays close to linear.

Adding thickness behaves differently. A thicker magnet drives more flux into the gap, but the steel in the circuit can only carry so much. Past the point where it saturates, extra thickness mostly adds to the magnet's own path and little to the gap.

Which route is cheaper depends on where the crossover sits in your circuit, and that moves with the air gap. Send us the air gap and the working point and we will look at both.

How We Confirm the Spec Before You Commit

For a new shape we agree the inspection plan before the first part is made. That fixes the sampling frequency and the method used to measure a curved part, which is not the same as measuring a block.

The plan then runs in three stages: incoming material is checked, dimensions and magnetic readings are taken during production, and a pre-shipment spot check closes the batch. Where the specification is not settled yet, we send both sides of the data and wait for your confirmation before production starts.

Common Questions

Which grade do I need for a hot motor?
Pick the grade from the temperature at the magnet, not from the motor rating. That is the running temperature plus the rise from the windings and the rotor losses. The M, H, SH and UH suffixes cover higher working temperatures. Tell us the temperature and we will pick the grade.

Should the magnets be magnetized before or after bonding?
Before, in most rotors. Unmagnetized segments are easier to handle and keep steel chips off the magnet. Magnetizing after assembly is possible, but the fixture has to reach around the rotor.

How do I work out the arc angle for my motor?
Start from the pole pitch: 360 degrees divided by the pole count. The magnet arc is a fraction of that, usually 0.67 to 0.86 of it, set by the cogging target. Send the pole count and the air gap and we will confirm the arc.

Can one arc be magnetized with several poles?
Yes. A multipole pattern across the arc gives you more poles from the same segment, which suits precision motors. It also takes more magnetizing work, so we agree the pattern before tooling.

What tolerance do you hold on a motor magnet?
Our standard band on arc dimensions and thickness is ±0.05 mm. Where a dimension sets the air gap and the assembly needs more, we work to ±0.02 mm, and to ±0.01 mm on the critical dimensions. We agree the plan before the first part.

Do you build the rotor or the motor?
We make the magnets, and the bonded sub-assembly where that helps. We do not build the finished motor, so the rotor design and the winding stay with you.

Send the drawing, the working temperature at the magnet, the magnetization direction or pole count, and the quantity. If a drawing is not ready yet, the pole count and the air gap are enough to start.

Send Your Drawing