Interior Permanent Magnet Motor (IPM)
An interior permanent magnet motor is a permanent magnet synchronous motor whose magnets are buried inside the rotor core rather than mounted on its surface, with ferromagnetic pole shoes between each magnet and the air gap.
That construction has one electrical consequence, and everything else follows from it: the rotor becomes salient. The magnetic path is not the same in every direction, so the machine produces torque by two mechanisms instead of one.
The alternative construction is SPM — surface permanent magnet — where the magnets sit on the outside of the rotor facing the air gap. Rare-earth magnet material has a relative permeability close to 1, so magnetically it behaves almost like air; a surface rotor therefore looks uniform to the stator field and is non-salient.
Also written: IPM · IPM motor · IPMSM · interior permanent magnet · buried magnet rotor
What the term fixes, and what it leaves open
| Full form | Interior Permanent Magnet |
|---|---|
| Also written | IPM motor · IPMSM · buried-magnet rotor |
| Machine family | Permanent magnet synchronous — the rotor turns with the supply frequency, there is no slip |
| Where the magnets sit | Inside the rotor core, behind ferromagnetic pole shoes |
| Electrical character | Salient. The surface-magnet alternative is non-salientYuan Lei, PMSM Control Principles, ch. 1 |
| Torque produced by | Magnet torque plus reluctance torque |
| Rotor structures in the family | Surface, interior, claw-poleTang Renyuan, Modern Permanent Magnet Motors, §2.2 |
| Starting cage | Can be built into an interior rotor; cannot be fitted to a surface-magnet rotorTang Renyuan, §2.2.1 |
| Needs a converter | Normally yes — the drive has to know where the rotor is, by sensor or by estimation |
What salience buys
On an interior rotor there is iron between adjacent magnet poles, so the magnetic path differs from one rotor axis to the other. That asymmetry produces reluctance torque, which adds to the torque the magnets themselves produce.
The practical gains are higher power density for the same frame, better overload capability, and a usable field-weakening range — meaning the machine can be driven above base speed rather than stopping there. The cost is more magnetic leakage inside the rotor and a more expensive rotor to build.
Why the rotor type appears on a datasheet at all
Two permanent magnet machines with the same output rating can behave very differently above base speed, under overload, and when the supply is removed while the shaft keeps turning. The rotor type is the shortest way to say which of the two you are looking at.
It also decides something binary: whether a starting cage can exist at all. An interior rotor has room for one in the pole shoes; a surface rotor does not. Whether a specific series is actually built with a cage is a property of that series, not of the rotor type — the datasheet is the place to check, not the acronym.
What Interior Permanent Magnet Motor does not tell you
IPM is also an abbreviation for something else entirely
In drive and power-electronics literature, IPM usually means Intelligent Power Module — a packaged inverter stage with its gate drives and protection. On a motor datasheet it means Interior Permanent Magnet. The two appear in the same conversation often enough to be worth separating out loud.
Interior is not simply better than surface
Surface-mount rotors are simpler, cheaper, and easier to give a sinusoidal air-gap flux. Which is right depends on whether the application needs reluctance torque, overload margin and speed above base speed — or does not.
A permanent magnet motor is not a drop-in swap for an induction motor
The field cannot be switched off. Whenever the shaft turns, the terminals are live — even with the supply isolated. And most general-purpose drives cannot control a PM machine at all, because they have no way to track rotor position.
Synchronous does not mean it starts on its own
Without a starting cage, a PM synchronous machine connected straight to a fixed-frequency supply will not pull into step. It draws a heavy current instead, and that current can permanently weaken the magnets.
Which LEADGO series publish an interior rotor
Generated from the published parameter master data. Only series whose rotor construction is stated are listed — for the other permanent magnet series the rotor type is not published on the datasheet, and the honest answer is to ask rather than to infer it from the family name.
GTYP Series
Interior permanent magnet, with reluctance torque component
GTYPS Series
Interior permanent magnet, with reluctance torque component
Frequently asked
What is an interior permanent magnet motor?
A permanent magnet synchronous motor whose magnets are buried inside the rotor core rather than mounted on its surface, with ferromagnetic pole shoes between each magnet and the air gap. That construction makes the rotor magnetically salient, which is what lets the machine produce reluctance torque in addition to magnet torque.
What does IPM stand for on a motor datasheet?
Interior Permanent Magnet. The alternative is SPM, surface permanent magnet, sometimes written surface-mount. Note that in drive literature IPM more often means Intelligent Power Module, which is an inverter component and not a motor at all.
Why does an IPM motor produce reluctance torque and an SPM motor does not?
Rare-earth magnet material has a permeability close to air, so on a surface-mount rotor the magnet behaves magnetically like part of the air gap and the rotor looks uniform to the stator field. On an interior rotor, ferromagnetic material sits between adjacent magnet poles, so the magnetic path differs between the two rotor axes. That asymmetry is what produces reluctance torque.
Can an interior permanent magnet motor start directly on line?
It can if a starting cage is built into the pole shoes, which an interior rotor has room for and a surface-mount rotor does not. Whether a specific series is built that way is a property of that series, not of the rotor type — check the motor documentation before connecting any permanent magnet motor to a fixed-frequency supply.
Do I need a special drive for an IPM motor?
Normally yes. Ordinary industrial practice for an induction motor is scalar V/f control, which can run open-loop with no feedback at all. A permanent magnet synchronous machine needs commutation timed to rotor position, taken either from a position sensor or estimated from motor parameters such as the EMF. A drive that only offers V/f has no way to do that.
Where this is worked through properly
Interior versus surface magnet rotors →
the full comparison, including what salience costs
Where synchronous machines are used →
the wider family this machine belongs to
Line-start PM motors →
what the starting cage makes possible
VFD retrofit: induction or PM →
what changes in an existing installation
PM versus induction in oilfield duty →
the part-load case, worked through