What is a Permanent Magnet Synchronous Motor?

The permanent magnet synchronous motors are one of the types of AC synchronous motors,
where the field is excited by permanent magnets that generate sinusoidal back EMF. It contains
a rotor and stator same as that of an induction motor, but a permanent magnet is used as a
rotor to create a magnetic field. Hence there is no need to wound field winding on the rotor. It
is also known as a 3-phase brushless permanent sine wave motor. The permanent magnet
synchronous motor diagram is shown below.

Permanent Magnet Synchronous Motor

Permanent Magnet Synchronous Motor Theory
The permanent magnet synchronous motors are very efficient, brushless, very fast, safe, and
give high dynamic performance when compared to the conventional motors. It produces
smooth torque, low noise and mainly used for high-speed applications like robotics. It is a 3-
phase AC synchronous motor that runs at synchronous speed with the applied AC source.
Instead of using winding for the rotor, permanent magnets are mounted to create a rotating
magnetic field. As there is no supply of DC source, these types of motors are very simple and
less cost. It contains a stator with 3 windings installed on it and a rotor with a permanent
magnet mounted to create field poles. The 3-phase input ac supply is given to the stator to start
working.
Working Principle
The permanent magnet synchronous motor working principle is similar to the synchronous
motor. It depends on the rotating magnetic field that generates electromotive force at
synchronous speed. When the stator winding is energized by giving the 3-phase supply, a
rotating magnetic field is created in between the air gaps.
This produces the torque when the rotor field poles hold the rotating magnetic field at
synchronous speed and the rotor rotates continuously. As these motors are not self-starting
motors, it is necessary to provide a variable frequency power supply.
EMF and Torque Equation
In a synchronous machine, the average EMF induced per phase is called dynamic induces EMF in
a synchronous motor, the flux cut by each conductor per revolution is Pϕ Weber
Then the time taken to complete one revolution is 60/N sec
The average EMF induced per conductor can be calculated by using
( PϕN / 60 ) x Zph = ( PϕN / 60 ) x 2Tph
Where Tph = Zph / 2
Therefore, the average EMF per phase is,
= 4 x ϕ x Tph x PN/120 = 4ϕfTph
Where Tph = no. Of turns connected in series per phase
ϕ = flux/pole in weber
P= no. Of poles
F= frequency in Hz
Zph= no. Of conductors connected in series per phase. = Zph/3
The EMF equation depends on the coils and the conductors on the stator. For this motor,
distribution factor Kd and pitch factor Kp is also considered.
Hence, E = 4 x ϕ x f x Tph xKd x Kp
The torque equation of a permanent magnet synchronous motor is given as,
T = (3 x Eph x Iph x sinβ) / ωm
Direct Torque Control of Permanent Magnet Synchronous Motor
To control the permanent magnet synchronous motor, we use different types of control
systems. Depending on the task, the necessary controlling technique is used. The different
controlling methods of permanent magnet synchronous motor are,
Sinusoidal Category
 Scalar
 Vector: Field oriented control (FOC) (with and without position sensor)
 Direct torque control
Trapezoidal Category
 Open-loop
 Closed-loop (with and without position sensor)
Direct torque control technology of this motor is a very simple control circuit with effective
dynamic performance and good control range. It doesn’t require any position sensor for the
rotor. The main disadvantage of using this control method is, it produces high torque and a
current ripple.
Construction
The permanent magnet synchronous motor construction is similar to the basic synchronous
motor, but the only difference is with the rotor. The rotor doesn’t have any field winding, but
the permanent magnets are used to create field poles. The permanent magnets used in the
PMSM are made up of samarium-cobalt and medium, iron, and boron because of their higher
permeability.
The most widely used permanent magnet is neodymium-boron-iron because of its effective cost
and ease of availability. In this type, the permanent magnets are mounted on the rotor. Based
on the mounting of the permanent magnet on the rotor, the construction of a permanent
magnet synchronous motor is divided into two types. They are,
Surface-mounted PMSM
In this construction, the magnet is mounted on the surface of the rotor. It is suited for high-
speed applications, as it is not robust. It provides a uniform air gap because the permeability of
the permanent magnet and the air gap is the same. No reluctance torque, high dynamic
performance, and suitable for high-speed devices like robotics and tool drives.

Surface Mounted
Buried PMSM or Interior PMSM
In this type of construction, the permanent magnet is embedded into the rotor as shown in the
figure below. It is suitable for high-speed applications and gets robustness. Reluctance torque is
due to the saliency of the motor.

Buried PMSM
Working of Permanent Magnet Synchronous Motor
The working of the permanent magnet synchronous motor is very simple, fast, and effective
when compared to conventional motors. The working of PMSM depends on the rotating
magnetic field of the stator and the constant magnetic field of the rotor. The permanent
magnets are used as the rotor to create constant magnetic flux, operates and locks at
synchronous speed. These types of motors are similar to brushless DC motors.
The phasor groups are formed by joining the windings of the stator with one another. These
phasor groups are joined together to form different connections like a star, Delta, double and
single phases. To reduce harmonic voltages, the windings should be wound shortly with each
other.
When the 3-phase AC supply is given to the stator, it creates a rotating magnetic field and the
constant magnetic field is induced due to the permanent magnet of the rotor. This rotor
operates in synchronism with the synchronous speed. The whole working of the PMSM depends
on the air gap between the stator and rotor with no load.
If the air gap is large, then the windage losses of the motor will be reduced. The field poles
created by the permanent magnet are salient. The permanent magnet synchronous motors are
not self-starting motors. So, it is necessary to control the variable frequency of the stator
electronically.
WHAT IS TORQUE RIPPLE?
Torque ripple refers to the periodic or fluctuating variations in the output torque of a motor
during its operation. In the context of electric motors, torque ripple is characterized by changes
in the torque produced by the motor as it rotates, creating a repetitive pattern over time. This
phenomenon is particularly relevant in Permanent Magnet Synchronous Motors (PMSMs) and
other types of motors where the torque output is influenced by the interaction between the
stator and rotor magnetic fields.
Several factors contribute to torque ripple:
1. Cogging Torque: Cogging torque is a result of the interaction between the permanent
magnets on the rotor and the teeth of the stator. This interaction causes periodic
resistance as the rotor rotates, leading to torque fluctuations.
2. Magnetic Saturation: When the magnetic material in the motor core reaches its
saturation point, the relationship between magnetic flux and magnetic field strength
becomes nonlinear. This nonlinearity can contribute to variations in torque output.
3. Harmonic Components: Imperfections in motor construction or non-uniformities in
magnetic materials can introduce harmonic components in the magnetic field. These
harmonics contribute to torque ripple.
4. Rotor and Stator Slot Effects: The design of the rotor and stator slots can influence
torque ripple. Non-uniformities in the shape and arrangement of slots may introduce
variations in the magnetic field, affecting torque production.
5. Electromagnetic Forces: Variations in the magnitude and direction of electromagnetic
forces acting on the rotor during operation can contribute to torque ripple
Reducing torque ripple is essential for improving the overall performance, efficiency, and
reliability of electric motors in various industrial applications.
How to minimize torque ripple?
Minimizing torque ripple in Permanent Magnet Synchronous Motors (PMSMs) is essential for
improving the motor's performance, efficiency, and overall reliability. Here are several strategies
and techniques commonly employed to reduce torque ripple:
Magnet Segmentation:
 Radial Segmentation: Dividing the permanent magnets along the radial direction can
alter the radial flux distribution and reduce torque ripple.
 Tangential Segmentation: Dividing the magnets along the tangential direction modifies
the tangential flux distribution, providing another dimension for torque ripple reduction.
 Hybrid Segmentation: Combining both radial and tangential segmentation techniques
can achieve a more comprehensive reduction in torque ripple.

How Magnet Segmentation can minimize torque ripple?

Magnet segmentation is a technique that involves dividing the permanent magnets in the rotor
of a Permanent Magnet Synchronous Motor (PMSM) into distinct segments. This segmentation
aims to modify the magnetic field distribution in the air gap between the rotor and stator,
providing a means to reduce torque ripple. Here's how magnet segmentation can minimize
torque ripple:
1. Altering Magnetic Field Distribution:
 By dividing the magnets into segments, magnet segmentation changes the
distribution of the magnetic field in the air gap. This alteration can help mitigate
the effects of cogging torque and other factors contributing to torque ripple.
2. Reducing Cogging Torque:
 Cogging torque, which is the torque required to overcome the reluctance of the
motor to start rotating, is a significant contributor to torque ripple. Magnet
segmentation helps to reduce cogging torque by breaking up the symmetry in
the magnetic field, resulting in a more even distribution of forces during rotor
movement.
3. Harmonizing Magnetic Flux:
 The segmentation of magnets can be designed to reduce the presence of
harmonic components in the magnetic field. This harmonization contributes to a
smoother and more consistent torque output, minimizing torque ripple.
4. Optimizing Rotor Design:
 Magnet segmentation allows for the optimization of the rotor design by
strategically placing segmented magnets. This optimization can be tailored to
specific applications and operational conditions to achieve the best compromise
between torque production and torque ripple reduction.
5. Addressing Demagnetization Risks:
 Magnet segmentation can help mitigate the risk of demagnetization by allowing
for more effective thermal management. The segmented structure can enhance
heat dissipation, reducing the likelihood of temperature-related demagnetization
issues.
6. Hybrid Segmentation:
 Combining both radial and tangential segmentation techniques (hybrid
segmentation) provides a more comprehensive approach to torque ripple
reduction. This synergistic combination addresses torque ripple from different
directions, offering improved overall performance.
7. Simulation and Optimization:
 Advanced simulation tools can be used to analyze the impact of different magnet
segmentation patterns on torque characteristics. This enables engineers to
optimize the segmentation design based on the specific requirements of the
motor and the intended application.
8. Experimental Validation:
 Implementing magnet segmentation designs in prototype motors and conducting
experimental studies can validate the simulation results. Experimental validation
is crucial to ensuring that the chosen segmentation pattern effectively reduces
torque ripple in real-world operating conditions.
What is the function of air gap for permanent magnet synchronous motor?
The air gap in a Permanent Magnet Synchronous Motor (PMSM) plays a crucial role in
determining the motor's performance, efficiency, and overall functionality. The air gap is the
physical space between the rotor (which contains permanent magnets) and the stator (which
houses the motor windings). The primary functions of the air gap in a PMSM are as follows:
1. Magnetic Field Interaction:
 The air gap serves as the medium through which the magnetic fields of the rotor
and stator interact. The permanent magnets on the rotor generate a magnetic
field, and this field interacts with the magnetic field produced by the stator
windings when an electric current is applied. The air gap distance influences the
strength and efficiency of this magnetic interaction.
2. Torque Production:
 The torque produced in a PMSM is a result of the interaction between the
magnetic fields of the rotor and stator. The air gap distance affects the flux
linkage between the rotor and stator, influencing the torque output. A smaller air
gap generally leads to stronger magnetic coupling, resulting in higher torque
production.
3. Efficiency and Power Density:
 The efficiency of a PMSM is influenced by the air gap design. A well-optimized air
gap helps minimize energy losses, contributing to higher overall motor efficiency.
Additionally, the air gap dimensions impact the power density of the motor; a
properly designed air gap allows for a more compact motor design without
compromising performance.
4. Cogging Torque:
 The air gap plays a role in determining cogging torque, which is the torque
required to overcome the reluctance of the motor to start rotating. A well-
designed air gap helps minimize cogging torque, ensuring smoother motor
operation, especially at low speeds.
5. Demagnetization Risk:
 The air gap distance is a critical factor in preventing demagnetization of the
permanent magnets on the rotor. A larger air gap reduces the risk of
demagnetization caused by high temperatures, overcurrent conditions, or other
factors that may affect the magnetic properties of the rotor.
6. Thermal Considerations:
 The air gap provides a thermal barrier between the rotor and stator, allowing for
efficient heat dissipation. Proper cooling is essential to maintain the temperature
within a safe range and prevent thermal degradation of motor components.