Measuring Magnets and Motors
Almost every number in this series is measured on one of about ten instruments. Knowing which one produced a figure tells you how much to trust it.
Magnets and Motors · Part 4 — Limits and Supply · Chapter 11 · 14 min read
Almost every number in this series is measured on one of about ten instruments. Knowing which one produced a figure tells you how much to trust it — and which numbers a supplier has quietly not measured at all.
Hysteresigraph
The definitive test
Back-EMF
Best health check on a built rotor
Helmholtz
100 % production screening
Dynamometer
Where claims become measurements
11.1 — Eleven instruments and what each is for
| Instrument | Measures | What it is really for |
|---|---|---|
| Hysteresigraph | Full B–H loop | The definitive test. B_r, H_cB, H_cJ and (BH)max on a real sample, at temperature. Incoming acceptance for magnet lots. |
| VSM | Moment vs field, small samples | Research and failure analysis; works on milligram fragments a hysteresigraph cannot hold. |
| Helmholtz coil + fluxmeter | Total magnetic moment | Fast 100 % production screening. Catches under-magnetised, wrongly-oriented or wrong-grade parts before assembly. |
| Gaussmeter / Hall probe | Local B at a point | Airgap flux, surface field maps, verifying magnetisation pattern and polarity. |
| Back-EMF test | Voltage vs speed, spun open-circuit | The single best health check on an assembled rotor. Falling K_e is direct evidence of demagnetisation. |
| Locked-rotor / standstill test | L_d, L_q vs current | Extracts the inductances the controller needs. Saturation makes them current-dependent, so a single number is never enough. |
| Dynamometer | Torque, speed, power in and out | The efficiency map. The only place claimed efficiency becomes measured efficiency. |
| Search coil / Epstein frame | Core loss in the steel itself | Validates the iron-loss model separately from the machine — the only way to separate iron from copper loss cleanly. |
| Thermocouples · fibre-optic · IR | Winding, iron and magnet temperature | Fibre-optic is what gets you rotor and magnet temperature on a spinning rotor without slip rings. |
| Kerr microscopy | Domain structure at the surface | Seeing the domains and grain boundaries that coercivity actually depends on — chapter 4’s subject, made visible. |
| XRD · SEM-EDS | Phase and elemental distribution | Confirming the Nd₂Fe₁₄B phase, grain size, and whether the Dy really stayed at the grain boundaries. |
11.2 — What to ask a supplier for
Important
If you buy magnets or motors, ask for two things. A hysteresigraph trace for your lot at your maximum operating temperature — not a datasheet curve at 20 °C — and the demagnetisation analysis at the combined worst case of maximum temperature and maximum fault current.
A supplier who has neither has not done the work.
The reason those two specifically is that both of the failure modes in chapter 9 are invisible to every other test. A magnet that will cross its knee in service passes every room-temperature measurement, and a rotor that has already crossed it passes a capacity check while showing up only as reduced back-EMF.
Quick check: test yourself
1.A motor comes back from the field with reduced torque per amp and no other symptom. What do you measure?
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2.Why is a datasheet demagnetisation curve at 20 °C insufficient?
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3.Why can a single number for L_d and L_q not be trusted?
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Chapter summary
- ✓The hysteresigraph is the definitive magnet test and the basis of incoming acceptance; everything else measures one aspect more cheaply or more locally.
- ✓Back-EMF against speed is the best health check on an assembled rotor, because K_e equals K_t and a fall in it is direct evidence of demagnetisation.
- ✓Helmholtz coil and fluxmeter give fast 100 per cent screening, catching under-magnetised or wrong-grade parts before they are assembled.
- ✓L_d and L_q are current-dependent because of saturation, so a single number is never enough for a controller.
- ✓Ask suppliers for a hysteresigraph trace on your lot at your maximum temperature and a combined worst-case demagnetisation analysis — the two failure modes that every other test misses.
Frequently asked questions
What is the definitive test for a magnet?+
A hysteresigraph, which measures the full B–H loop and yields B_r, H_cB, H_cJ and (BH)max on a real sample at temperature. It is the basis of incoming acceptance for magnet lots. A VSM measures moment against field on much smaller samples and is used for research and failure analysis, since it works on milligram fragments a hysteresigraph cannot hold.
How do you tell whether a rotor has been demagnetised?+
Spin it open-circuit and measure voltage against speed. The back-EMF test is the single best health check on an assembled rotor, because a falling K_e is direct evidence of demagnetisation — and since K_e equals K_t, it is also direct evidence of lost torque per amp. Nothing in normal drive telemetry sees demagnetisation happening; it shows up afterwards as reduced back-EMF.
What should you ask a magnet or motor supplier for?+
Two things. A hysteresigraph trace for your lot at your maximum operating temperature — not a datasheet curve at 20 °C — and the demagnetisation analysis at the combined worst case of maximum temperature and maximum fault current. A supplier who has neither has not done the work.
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Magnets and Motors is an original educational series on permanent magnets and electric machines. Datasheet ranges, temperature coefficients and efficiency figures are representative standard-condition literature values, not measurements of a specific product, and sources differ on several. Always verify against the specific magnet and lamination datasheets in use before making design, procurement or certification decisions.