What is PM Motor?

What is a PM motor?

A motor family that uses permanent magnets for excitation

PM stands for Permanent Magnet. In these electric motors, permanent magnets provide all or part of the excitation field. PM does not identify a single supply arrangement or control method. PMSM, BLDC and brushed PMDC motors have different constructions; magnet location and current commutation depend on the motor type.

Are PMSM, PMAC, BLDC and PMDC the same?

PMSM means permanent magnet synchronous motor. Common three-phase designs have stator windings and rotor magnets. PMAC refers to permanent magnet AC motors and often describes sinusoidally driven PMSM products in industrial use. Product terminology alone is insufficient: check the manufacturer's motor and drive documentation.

BLDC motors also use electronic commutation and a permanent magnet rotor. In the conventional distinction, PMSMs are associated with sinusoidal back electromotive force and sinusoidal phase currents, while BLDC motors are associated with approximately trapezoidal back EMF and six-step commutation. Ideal six-step phase currents are block-shaped; saying that BLDC current must be trapezoidal is inaccurate. Back-EMF shape, current waveform and control algorithm are separate characteristics. Suitable BLDC motors can also operate with FOC.

A common brushed PMDC construction has stator magnets and rotor windings, with brushes and a commutator switching current direction. Consequently, not every PM motor is brushless or three-phase. PM and hybrid stepper motors also contain magnets, whereas variable-reluctance stepper motors can operate without them. “Servo” describes use in a feedback-controlled motion system rather than specifying the magnet construction alone.

How does synchronous rotation determine speed?

A PMSM drive regulates stator phase currents in relation to rotor position, creating a rotating magnetic field. The rotor runs at synchronous speed. In steady operation, rotor and field speeds are equal, although their angular difference need not be zero. A load change alters the required current and appropriate angular relationship. Continuous slip speed, as used by an induction motor, is unnecessary; exceeding operating limits can cause loss of synchronism or a drive trip.

Synchronous mechanical speed follows n = 60 × f_e / p, where n is rpm, f_e is the fundamental electrical frequency and p is the number of pole pairs. For example, an eight-pole motor with four pole pairs reaches a synchronous speed of 1500 rpm at 100 Hz. The drive's PWM carrier frequency is not the frequency used in this equation. One electrical cycle does not necessarily equal one mechanical revolution.

The diagram illustrates synchronous motion in a PMSM with one pole pair. Start enables motion; Stop and reset returns it to the initial position. The angle offset is fixed; load transients, phase currents and actual torque are not simulated. A static drawing appears when reduced motion is requested.

SPM and IPM construction affect the control strategy

SPM motors have surface-mounted rotor magnets; IPM motors have magnets embedded within the rotor. Direction-dependent inductance in an IPM motor can generate reluctance torque in addition to magnet torque. Field-oriented control regulates current along the rotor's rotating d and q axes. An ideal surface-magnet motor can use approximately zero d-axis current below base speed, but maximum torque per ampere, or MTPA, in an IPM motor can require a different combination of d-q currents.

Above base speed, many PMSM controllers use negative d-axis current for field weakening, extending the speed range within the available voltage limit. Available torque can decrease in return. This does not mean permanently weakening the magnets. FOC implementation must respect current, voltage, temperature and magnet demagnetisation limits; the same current strategy does not suit every PM motor.

Which drive is suitable, and are position sensors essential?

A PMSM needs a drive that supports the appropriate PM motor control mode. Matching the power rating of a drive intended only for induction motors is insufficient. Enter nameplate data, pole count, stator resistance, inductances and magnet flux or back-EMF constant in the form required by the drive. Follow the motor-drive documentation for commissioning and tuning. Three motor terminals do not establish that direct mains connection is permitted.

Rotor position can come from an encoder, resolver or Hall sensors, or be estimated by a suitable sensorless algorithm. Back-EMF-based methods face greater difficulty at standstill and very low speed; special starting routines or methods such as high-frequency signal injection may be needed. Sensorless operation does not guarantee full torque at zero speed and identical positioning accuracy in every application. A brushed PMDC controller and a PMSM drive are not interchangeable.

Editor's note: “It is a PM motor and the drive has the same kW rating, so it will work” is an incomplete assessment. Identify whether the motor is PMSM, BLDC or PMDC, then verify voltage, current, back-EMF constant and feedback compatibility. PM identifies a motor family, not universal drive compatibility.

What are the advantages over induction motors?

A PMSM needs no separate electrical field winding for rotor excitation and has no squirrel-cage rotor losses of the type found in an induction motor. This can provide high efficiency and torque density. Stator copper losses, iron losses, mechanical losses and drive losses still remain. Energy efficiency in a PM servo system depends on the load profile and correct sizing. A PM motor is not necessarily faster, quieter or more economical in every operating condition.

An induction motor can be attractive for robustness, service availability and purchase cost. PM motors can offer advantages where compact size, high torque at low speed or frequent dynamic motion matters. Compare IE efficiency classes under equivalent power, speed, test scope and supply conditions. Magnet material and temperature limits also matter: “permanent” does not mean immune to excessive temperature or inappropriate current.

Select from the torque-speed envelope as well as the kW rating

PM servo motor selection requires continuous and peak torque, speed range, duty cycle, load inertia, ambient temperature, protection rating and feedback to be assessed together. Use a torque-speed curve for the actual motor-drive combination. The relationship between mechanical power and torque is approximately T = 9550 × P / n, with P in kW, n in rpm and T in N·m. A shaft output of 2 kW at 1500 rpm therefore corresponds to approximately 12.7 N·m; this calculation does not establish peak torque or standstill holding capability.

Account for heating and generated voltage

Thermal assessment must include time spent holding a load as well as moving it. External shaft rotation can generate voltage in a PM motor; switching off the drive does not automatically make the motor terminals voltage-free. During deceleration, regenerative energy can return to the DC bus. Depending on system design, it is reused, returned to the mains through a suitable unit, or dissipated in a braking resistor. Follow the manufacturer's stopping, mechanical restraint and energy-isolation procedures during maintenance.