PMSM Permanent Magnet Synchronous Motors are a type of permanent magnet motor widely used in electric vehicles. PMSM motors are 15% more efficient than induction motors and represent the highest power density traction motors available.

  1 What is a Permanent Magnet Synchronous Motor (PMSM)?

  A permanent magnet synchronous motor is a type of AC synchronous motor whose magnetic field is generated by permanent magnets producing a sinusoidal counter-rotating electromagnetic field. It contains the same rotor and stator as an induction motor, but the rotor uses permanent magnets to generate the magnetic field. Therefore, no field windings are required on the rotor. It is also known as a three-phase brushless permanent magnet sinusoidal motor.

  Compared to traditional motors, permanent magnet synchronous motors offer high efficiency, brushless operation, high speed, safety, and superior dynamic performance. They produce smooth torque with low noise and are primarily used in high-speed applications such as robotics. It is a three-phase AC synchronous motor that operates in synchronization with an applied AC power source.

  The rotor uses no windings but instead incorporates permanent magnets to generate a rotating magnetic field. Without requiring a DC power supply, this type of motor is highly simple and cost-effective. It consists of a stator with three windings and a rotor equipped with permanent magnets to form magnetic poles. Operation commences when a three-phase AC input is supplied to the stator.

  The operating principle of permanent magnet synchronous motors is similar to that of synchronous motors. It relies on a rotating magnetic field to generate electromotive force at synchronous speed. When the stator windings are energized by a three-phase power source, a rotating magnetic field is produced in the air gap.

  Torque is generated when the rotor poles maintain the rotating magnetic field at synchronous speed while the rotor rotates continuously. Since these motors are not self-starting, it is necessary to provide a variable-frequency power supply.

  2 Structure of PMSM Motors

  Similar to conventional AC induction motors, power is supplied through the stator windings. PMSM stator windings are typically distributed across multiple slots in a near-sinusoidal pattern to generate a sinusoidal back-EMF waveform.

  The structure of a permanent magnet synchronous motor resembles that of a basic synchronous motor, differing only in the rotor. The rotor lacks any field windings, instead utilizing permanent magnets to create magnetic poles. The permanent magnets used in PMSMs are composed of samarium cobalt and a medium, iron, and boron, due to their high magnetic permeability.

  The most widely used permanent magnet is neodymium boron iron, owing to its low cost and availability. In this type, the permanent magnet is mounted on the rotor. Based on how the permanent magnet is mounted on the rotor, the structure of permanent magnet synchronous motors can be divided into two types.

  If magnets are mounted on the surface of the motor rotor, the PMSM is termed a Surface-mounted Permanent Magnet (SPM) motor.

  If magnets are embedded within the rotor, the PMSM is termed an Interior Permanent Magnet (IPM) motor. Motors with internal permanent magnets (IPM rotors) can achieve exceptionally high efficiency.

  3 PMSM Control Principles

  The PMSM drive is a classic vector control drive for permanent magnet synchronous motors. This drive employs closed-loop speed control based on vector control methods. The closed-loop configuration provides speed feedback. Through this feedback, the drive tracks the rotor's precise position, delivering true stepless speed regulation—including full torque at zero speed.

  PMSM motors require a position sensor mounted on the rotor shaft. The most common sensors for these motors are encoders and resolvers.

  These permanent magnet drives calculate rotor position using motor data and current measurements; the calculations performed by the Digital Signal Processor (DSP) are highly precise. Within each sampling interval, the three-phase AC system, related to time and speed, is transformed into a rotating dual-coordinate system where each current is expressed and controlled as the sum of two vectors.

  Based on the vector control strategy, the stator current reference dq (direct and quadrature) components corresponding to the command torque are derived. The required gate drive signals for the inverter are then obtained using the reference dq components of the stator current.

  The primary advantage of this drive lies in its rapid dynamic response. The inherent coupling effect between torque and flux in the machine can be managed through decoupled control (stator flux orientation), enabling independent control of torque and flux. However, due to its computational complexity, implementing this drive requires a fast computing processor or DSP.

  4 Advantages and Disadvantages of PMSM

  PMSM exhibits strong overload capability. PMSM offers higher power density than induction motors.

  Compared to induction motors, it delivers higher efficiency and smaller size (permanent magnet motors are only one-third the size of most AC motors, facilitating easier installation and maintenance).

  PMSMs maintain full torque at low speeds.

  PMSM motors generate the rotor magnetic field using magnets, not the magnetizing component of stator current as in induction motors. These magnets consume negligible power, resulting in negligible rotor copper losses unlike AC induction motors and excited synchronous motors.

  Compared to induction motors, PMSMs exhibit lower rotor electrical losses and reduced heat dissipation. Additionally, they feature low friction and high durability since they do not require wear-prone mechanical commutators and brushes.

  PMSMs offer low maintenance costs, high durability, and exceptional reliability. Costs for periodic maintenance are significantly reduced for brushless and mechanical switches, while eliminating the risk of sparking in hazardous environments.

  PMSM motors operate at a higher power factor, thereby improving the overall system power factor. This enhanced power factor also reduces system voltage drops and motor terminal voltage drops.

  Provides smooth torque and dynamic performance

  Disadvantages

  These motors are very expensive compared to induction motors.Difficult to start since they are not self-starting motors,A complex control system is required to regulate stator current.