IPM

An IPM (interior permanent magnet) motor embeds permanent magnets inside the rotor structure rather than gluing them to its outer surface. The magnets sit recessed in slots or cavities within the rotor iron itself, typically oriented to create both radial and circumferential flux paths. This geometry lets the motor develop torque from two sources: the interaction between stator field and rotor magnets, plus reluctance torque from the rotor's varying magnetic resistance as it rotates.

The buried magnet design trades manufacturing complexity for performance gains. Because magnets are protected within the rotor body, IPM motors tolerate higher speeds and temperatures than surface-mounted permanent magnet (SPM) types, where magnets are vulnerable to centrifugal stress and thermal degradation. Rotor mechanical strength improves because iron surrounds the magnets. However, assembly requires precision: magnets must be positioned and secured accurately inside cavities, and the rotor must be dynamically balanced afterward to avoid vibration.

Efficiency and operating range

IPM motors operate efficiently across a wider speed range than SPM motors because the rotor inductance provides a second torque pathway at high speeds. Once a motor spins fast enough that back-EMF approaches supply voltage, reluctance torque becomes dominant, allowing the motor to maintain useful power output even as magnet flux weakens. This property makes IPM drives suitable for variable-frequency applications where constant-power operation is required, such as electric vehicle drivetrains and machine tool spindles.

Demagnetization remains a concern if the motor overheats or experiences high reverse current. The buried magnet geometry helps here by surrounding magnets with iron, which supports the magnetic circuit and reduces exposure to destabilizing fields. Designers choose magnet grades (typically NdFeB at 120 to 150 degrees Celsius rated) and rotor structure to balance cost against thermal and electrical safety margins.

IPM motors dominate in high-volume automotive and industrial servo applications where efficiency, power density, and speed range justify the rotor design cost. They compete against induction motors (which need no magnets) for applications requiring constant-torque output across a modest speed range, and against synchronous reluctance motors (which have no magnets at all but lower torque density) where magnet supply or cost is critical.