A Permanent Magnet Synchronous Motor (PMSM) is an AC electric motor that uses magnetic interaction to convert electrical energy to mechanical energy. It comprises an excitation system consisting of permanent magnets. The rotation speed of a PMSM is synchronized with the electric frequency of the alternating current. This motor rotates at constant speed irrespective of the load acting on it. The constant speed characteristics are achieved by interaction between a rotating magnetic field and a constant magnetic field. PMSMs are highly efficient and are mainly used by automotive manufacturers for electric and hybrid vehicles.

Construction and Working of PMSM

A permanent magnet synchronous motor consists of a rotor and a stator. The rotor is the rotating part and the stator is the fixed part. Usually, the rotor is placed inside the stator of the electric motor.

The working of a PMSM is based on the interaction of the rotating magnetic field of the stator and the constant magnetic field of the rotor. 

When a three-phase AC supply is applied to the windings of the stator coils, a rotating magnetic field is generated that rotates at a speed proportional to the frequency of the supply voltage. The permanent magnets on the PMSM rotor create a constant magnetic field. The interaction between the rotating magnetic field of the stator and the constant magnetic field of the rotor creates a torque, according to Ampere’s Law, thereby forcing the rotor to rotate. Suppose an initial rotation is given to the rotor in the same direction as that of the rotating magnetic field. In that case, the opposite poles of the rotating magnetic field and the rotor will be attracted to each other leading to the interlocking of rotor poles with the rotating magnetic field of the stator. Thus, a PMSM cannot start itself when it is connected directly to the three-phase current network.

Synchronous Speed:  The speed of magnetic field rotation may be given by:

Where

• Ns is the frequency of rotation of the magnetic field in RPM
• f is the frequency of stator current in Hz
• p is the number of pole pairs.

This implies that the speed of a PMSM can be controlled by varying the frequency of the supply current making them suitable for high-precision applications.

Types of PMSM

In a PMSM, the rotor consists of permanent magnets. High-coercive force materials are used as permanent magnets. Based on the rotor design, PMSMs are classified into two categories:

• PMSM with salient pole rotor
• PMSM with non-salient pole rotor.

For a PMSM with a salient pole rotor, the quadrature inductance is not equal to direct inductance, Lq ≠ Ld. Whereas, for a PMSM with a non-salient pole rotor, the quadrature inductance is equal to direct inductance, Lq = Ld.

Another classification of PMSM based on the rotor design is:

• Surface permanent magnet synchronous motor
• Interior permanent magnet synchronous motor.

Rotor of a (a) surface permanent magnet synchronous motor (b) interior permanent magnet synchronous motor

The stator consists of an outer frame and core with windings. Two-phase or three-phase windings are usually employed. Based on the stator design, PMSM may be classified as:

• PMSM with distributed winding
• PMSM with a concentrated winding

PMSM Stator with (a) distributed winding and (b) concentrated winding 

A distributed winding consists of several coils inserted into the slots of the motor stator, otherwise known as stator teeth. In the case of concentrated winding, only one stator tooth is wound. The type of winding is chosen based on the dimension of the motor and its application. The major difference between the distributed and concentrated winding is in the shapes of the back electromotive force (back emf) generated by the permanent magnets. The back emf is the voltage that is generated in electric motors when there is a relative motion between the stator windings and the rotor’s magnetic field. The geometric properties of the rotor will determine the shape of the back-emf waveform. These waveforms can be sinusoidal, trapezoidal, or triangular. The back emf of a PMSM with distributed winding is sinusoidal, whereas the back emf of a PMSM with concentrated winding is trapezoidal. More torque is generated due to trapezoidal back emf at the cost of losses in the copper and laminations. Higher efficiency is achieved with sinusoidal back emf making it ideal for electric vehicles.

Advantages of PMSM

• Simple construction that is easy to maintain
• Brushless and has very high reliability and efficiency
• Due to its permanent magnet rotor, it has high torque with a small frame size and no rotor current
• The absence of a DC source makes the construction simple and cost-effective
• Highly efficient as there are no losses on the rotor
• The permanent magnet enables the PMSM to generate torque at zero speed.
• Capable of maintaining full torque at low speeds
• Less noisy

Disadvantages of PMSM

• High initial cost
• Difficult to start as it is not a self-starting motor

Applications of PMSM

• Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs): PMSMs are commonly used in electric and hybrid vehicles due to their high efficiency, compact size, and ability to provide high torque at low speeds. They contribute to improving the overall performance and range of electric vehicles while enabling regenerative braking for energy recovery.
• Industrial Automation: PMSMs find extensive use in industrial applications, such as robotics, CNC machines, conveyors, and various machinery used in manufacturing processes. Their precise speed control, high torque, and efficiency make them suitable for different automation tasks.
• HVAC Systems: Permanent Magnet Synchronous Motors are employed in heating, ventilation, and air conditioning (HVAC) systems, including fans and pumps, due to their energy efficiency and ability to operate at variable speeds, matching the load requirements accurately.
• Renewable Energy Generation: PMSMs are utilized in wind turbines and hydroelectric generators due to their ability to efficiently convert mechanical energy into electrical energy. Their high efficiency is beneficial in renewable energy systems where maximizing energy conversion is essential.
• Consumer Appliances: Certain household appliances like washing machines, refrigerators, and dishwashers also use PMSMs for their energy efficiency, reduced noise levels, and improved performance.
• Aerospace: PMSMs are employed in aerospace systems, including electric propulsion systems in aircraft and satellites, owing to their lightweight design, high power density, and efficiency.
• High-Precision Motion Control Systems: Applications requiring high-precision control, such as in robotics, CNC machining, and medical equipment, benefit from the use of PMSMs due to their precise speed and position control capabilities.
• Pump and Fan Systems: PMSMs are used in various pump and fan applications, like water pumps, vacuum pumps, and industrial fans, due to their ability to operate efficiently at different speeds and loads.

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