Abstract
Permanent magnet synchronous motors (PMSM) are a type of motors that have permanent magnets mounted on the rotors instead of the rotor windings. This particular design gives the motor a few advantages over ordinary induction synchronous motors. The magnets attached to the rotor do not require excitation, which reduces the power consumption in PMSM. This gives the motor high operational efficiency and reduces the motor active diameter. PMSM are therefore associated with applications that require high torque but have limited operation space. The permanent magnets suffer from flux leakage and are therefore aligned in a pattern that minimizes flux leakage. A steel rim is also used to contain the flux within the motor active diameter. Due the efficiency, the PMSM is finding widespread application as traction motor in electrical vehicles and train. However, the cost and load capacity limitations of PMSM systems curtail their industrial application and more research is required to establish strong and low cost magnet manufacturing raw materials.
Permanent Magnet Synchronous Motor
In the recent years, scientists have developed materials that have enabled the manufacture of high strength permanent magnets. These strong magnets have in turn spurred research in the use of permanent magnets in motors instead of electromagnets. As a result, permanent magnet synchronous motors (PMSM) that have permanent motors on their rotors have been developed and commercially produced. Also, the PMSM machines have already been applied for industrial purposes. For example, large PMSM are being used in windmills to reduce the size of the generator sitting atop of the wind turbine. Also, the motors have previously been used in servo systems to power the servo pumps. Previously, industrial application of PMSM was not possible as they were characterized by very poor demagnetization properties of materials due to the use of poor magnetic materials such as AlNiCo [1]. Such motor used a control parameter, id = 0, to stabilize the polarity of the magnets.
Modern PMSM are magnets are made from materials such as SmCo and NdFeB that allow for the use of a demagnetizing current. The demagnetization current is applied to control the flux generated by the permanent magnet by reducing the flux’s strength. However, this approach has a major drawback occasioned by the lack occurrence of negative current relative to the d-axis within the constant flux range [1]. This negative current, which occurs when the system adjusts itself in a bid to improve its power factor, negates the flux from the permanent magnets thereby reducing the efficiency of the motor.
A permanent magnet synchronous motor is a motor that has an inbuilt permanent magnet. Therefore, the motor does not rely on an electric current to create an electromagnet. PMSM motors incorporate features from both brushless DC motors (BLDC) and AC induction motors (ACIM) [2]. For example, they have rotors similar to BLDC but their stators resemble the ACIM stators [1]. The stator windings, just like in the ACIM motors are designed to produce a sinusoidal flux density between their air spaces, which interacts with the permanent magnets mounted on the rotor. The PMSM motors are advantageous over the induction motors because they deliver a high power density for a relatively small motor, which translates to higher efficiency. This is because the induction motors use some of the input current to generate rotor flux, which is provided by the permanent magnet is the PMSM. Also, some of the flux current is converted to heat due to inefficiencies [2]. Other advantages of PMSM include faster response time, easy control features, rugged composition, and easy maintenance. This paper examines the structure, operation, and applications of the permanent magnet synchronous motor and future areas of development.
A synchronous motor is a motor that runs on AC power and is characterized by steady state operation. The speed of rotor rotation is synchronized to the frequency of the alternating current and its rotation period is proportional to the integral number of the alternating current cycles. Synchronous motors have windings on the stators which when induces produce a multiphase electromagnet whose polarity rotates at the same frequency as the frequency of the input current. The rotor magnets interact with the rotating magnetic field thereby causing it to rotate due to magnetic attraction and repulsion forces [2]. The speed of rotor rotation is the same as that of the stator magnetic field rotation and hence the frequency of the mains current. Therefore, the motor is said to be synchronized.
Types of synchronous motors
The synchronous motors belong to the synchronous family of machines and can operate both as a generator and as a motor. In the generator state, the field poles are pushed ahead of the air gap flux created by the prime mover. Conversely, in the motor state, the field poles run behind the air-gap flux due to the retarding torque posed on the machine by the load [3]. Normally, synchronous motors have both the stator and rotor excited by the mains supply current. However, the PMSMs have permanent magnets instead of the electromagnets resulting to a permanent and constant magnetic field flux. The stator in PMSM motors is similar to other synchronous motors with its windings connected to an AC source.
When the PMSM motor is running at the rated synchronous speed, the poles of its permanent magnets lock to the poles of the stator rotating electromagnet. Hence there is no slippage between the rotor and the stator as exhibited by induction motors. PMSM motors are disadvantaged over other motors in that they are not self-starting because the permanent magnets do not allow for induction [1]. As a result, sophisticated equipment such as power inverters, rectifiers, and sensors are required for startup and operation.
Structure of the permanent synchronous motor
The permanent magnets in a PMSM are wrapped around the rotor. As a result, the motor becomes “magnetically round” for smooth interaction with the electromagnets created on the stator. This gives a smoother torque output with a torque angle of 90 degrees. Some type of PMSM motors have round magnets built inside the rotor structure, which gives them the name interior permanent magnet motors (IPM) motors. The radial flux in IPM motor is more concentrated around certain angles resulting to a torque component referred to as reluctant torque [3]. The reluctance torque emanates from the variations in inductance due to the changing flux density.
Figure 1: A PMSM motor rotor showing the unwrapped permanent magnet sections [4].
Figure 2: An exploded view of a PMSM machine [4].
Apart from the AC powered PMSM motor, the motors can also be configured to run on DC power. In a DC PMSM, the stator windings are alternatively switched on and off in a defined sequence to induce a rotating magnetic field in a process referred to as electronic commutation. The permanent magnet then interacts with the commutated field just like in AC PMSM to produce torque [4]. The motor’s frequency is determined by its Hall-device system or the rotor-switching frequency according to its rotor positioning.
The operation characteristics of a permanent magnet largely depend on the rotor design. The rotor can designed from different approaches. For example, when the permanent magnet is designed from modern magnetic materials, the rotor can be fabricated from other materials other than iron products, such as aluminum. In such a case, the permanent magnet is glued to the aluminum rod such that a sinusoidal flux density is created in the air gaps. However, the use of noniron materials leads to the wastage of magnetic flux since the magnetic field travels through air on the rotor side [4]. To mitigate this drawback, a steel rim is placed between the magnets and the rotor shaft. The steel rim can be laminated to reduce the eddy current losses in the motor or it can be in the form of a thin steel tube. However, the tube configuration is not preferable as it leads to high temperatures in the motor due to the harmonics produced by the stator.
The surface-magnet produces very low inductances in the motor. Therefore, a voltage source inverter is used to produce high switching frequency so that the PM machine can operate smoothly. Such a configuration is applied in servo motors which require a certain minimum amount of inertia [4]. This type of setup gives a quadrature inductance that is equal to the direct inductance, which results in a non-salient pole configuration.
Figure 3: Image showing the alignment of magnets, the steel rim, and the magnetic field lines in a PMSM rotor [1].
The inductance in the PMSM can be adjusted through the addition of iron shoes on the poles. For example, pole shoes can be mounted on the magnets to give a sinusoidal flux density in the air gap. The pole shoes are installed in such a manner that the magnets are protected from magnetic and electric stresses to avoid degradation of their magnetic strength. The iron shoes also protect the magnets from mechanical damage especially during assembly [5]. The use of iron shoes in the rotor gives a multi pole PMSM that runs at a slower pace. The pole shoes can also be arranged in such a manner to give a solid rotor. The solid poles in the rotor also serve as damper windings. The shoes shapes are optimized to produce a sinusoidal flux density in the air gap for smooth operation.
The hard magnetic materials used in the motor have a relative permeability that slightly deviates from the unitary. As a result, the materials’ magnetic properties coincide with those of air, which results in a wider air gap in the motor [5]. The large air gap reduces the direct axis reaction from the armature. On the other hand, the harmonics from each slot and each phase do not result in large torque ripples.
Figure 4: The structure of a solid shoe PMSM [1].
In solid pole motors such as the one shown in figure 4 above, the magnetizing inductances are very low, and hence a lot of current is required to bring the rotor rotation speed above the field weakening point. If a high quality magnetic material is used in the rotor, the magnetic flux is spread evenly across the air gap and the resulting flux leakage is negligible [1]. In case the PMSM is equipped with pole shoes, the magnets are shielded from demagnetization. This is because the forces responsible for demagnetization are not transmitted to the magnets but are instead redirected to other motor components by the pole shoes.
When pole shoes are incorporated into a PMSM, the magnets used can be rectangular. This is because the pole shoe can be designed to any required form. Also, the rotors of PMSM can be fabricated from electric sheets just like in asynchronous motors. Furthermore, the electric sheets can be laminated in several different ways to give a PMSM with the required characteristics [6]. Likewise, the stator in PMSM machine can have a plate rotor just like in an induction motor.
The permanent magnets are attached to the rotor by gluing them directly to the rotor surface. Also, the magnets can be embedded completely or partly on the rotor [7]. If the magnets are embedded, they can be placed in different positions depending on the desired motor characteristics.
Permanent magnet alignment in the rotor
Since the use of iron shoes allows for various magnet arrangements on the rotor, the assembly can be varied to suit a particular motor application. For example, the magnets can be set around the rotor to give a rectangle as shown in figure 5 below. In the rectangular arrangement, the air gap remains relatively constant. Also, the emf induced in the stator of PMSM with such an arrangement can cause harmonics, which can be transferred to the induced torque resulting to noise and vibrations in the motor [7]. To eliminate the harmonics and smoothen out the torque, either the rotor or stator mmf has to be sinusoidal. The harmonic cause torque ripples when they fluctuate with the same wave length as the flux in the air gap.
Figure 5: Rectangular arrangements of the permanent magnets on the rotor [1].
Figure 6: Magnet arrangement in the rotor optimized to reduce harmonics on the stator [1].
PMSM with plate rotors exhibit different characteristics which depend on the nature of the rotor. For example, the rotor on figure 5 above gives a hybrid motor which operates to a limited extent as a reluctance machine without the help of the magnets. In this type of a PMSM, a certain percentage of the overall torque is produced by the various inductances resulting from the quadrature and direct directions. The torque generated by this difference in torque is referred to as reluctance torque. Adding magnets to hybrid PMSM improves its characteristics as compared to those of a reluctance motor [7]. For example, the running efficiency, start-up characteristics, and the power factor are better in a reluctance machines.
Application of rectangular magnets, such as the one shown in figure 5, requires installation of flux barriers within its axis to contain and prevent the flux from spilling through the motor axis. The arrangement is mechanically challenging to achieve and also the structure enhances armature reaction, which has negative effects on the motor assembly. In the assembly shown in figure 6, the permanent magnets are embedded onto the rotor surface [7]. This arrangement enhances a reluctance difference between the quadrature and direct axes. As a result of the reluctance difference, the motor produces the maximum possible torque at pole angles above 90 degrees. PMSM machines often achieve maximum torque at pole angles that are at angles above 90 degrees. This is because the resulting inductance in the q-direction is always more than inductance in the d-direction.
Also, the magnet arrangement in figure 6 has been optimized to operate with a smooth torque and low noise at low speeds. A PMSM machine can be designed to have a low speed output and a high efficiency and near unitary power factor. As such, the need of mechanical gear is often eliminated. An inverter can be used to regulate the frequency of the supply current and hence the speed of the motor [1]. However, it’s advantageous to use a rotor with a high number of magnetic poles as it will lead to a reduced stator yoke thickness. As a result, the rotor can be designed with a bigger diameter when there are space constraints.
The slots in a pole and in a phase can be set to one or below one. As a result, the stator mmf is associated with a lot of harmonics. To cancel out the effects of these stator harmonics on the torque, the rotor mmf is made sinusoidal. The magnet set up in figure 6 may give rise to torque ripples due to reluctance differences. But such ripples can be eliminated by tweaking the design. PMSM equipped with plate rotors exhibit flux leakage and therefore requires flux leakage guides to contain the leakage [5]. The leakage guides used can essentially be air or any other nonmagnetic material. Also, the rotor poles can be designed to shape the rotor flux into a sinusoidal wave form and to reduce flux leakage at the same time. The utilization factor of magnets is lower in embedded magnet rotors than in salient pole motors because the flux in the salient machines flows through a larger percentage of the air gap.
A plate rotor can be used to increase the flux density in the air gap through the application of a configuration with two magnets per pole. In such a machine, the air gap increases with increase in the pole area [5]. However, the volume of the required magnetic material increases leading to higher motor prices.
The per unit (pu) value of permanent magnet motors is different from those of traditional synchronous and induction motors used in industrial applications. The pu values of PM machines is over three times that of induction motors and two times that of synchronous machines. The flux leakage in all types of motors is 0.1. However, the synchronous inductance of a servo motor equipped with a surface magnet is 0.3. Likewise, multi-pole motors with embedded magnets in their rotors have synchronous ranges between 0.4 to 0.6 for direct axis and 0.6 to 0.9 for the quadrature axis. PMSM machines with high number of poles exhibit increased flux leakage which can be as high as a half of the overall synchronous inductance.
Operation principles of permanent magnet motors
The development of high energy permanent magnets using rare earth elements has spurred the application of permanent magnets in motors. Motors designed with permanent magnets have high speed, high operation efficiency, and high power density. The high efficiency in permanent magnet (PM) machines emanates from lack of brushes and commutators that create mechanical friction. Also, no rotor excitation current is required in the machines thereby lowering the electrical power input per unit of horse power produced. In addition, the use of high energy density magnets results in high flux density which in turn enhances a high torque output from the motor [3]. Furthermore, the PMSM machines have easy control variables because they have few control variables which operate in a steady state when the motor is running.
However, despite the numerous advantages of the PM machines, their operation is hampered by various shortcomings inherent to their design. For example, rare earth materials used in the fabrication of the magnets are very costly, which makes PMSM machines expensive [3]. Also, the magnets can get demagnetized over time due to high operational temperatures and magnetomotive forces. In addition, the constant presence of magnetic field in the motor poses a serious safety risk. This is because the rotor is always “magnetized” and a short circuit fault in the inverter can create a very large current in the stator. The induced short circuit current can created a blocking force that brakes the rotation of the rotor forcing it to stall.
PMSM machines can be classified into two categories based on wave form of the back emf produced by the stator windings as shown in figure 7 below. The two categories are PMSM with sinusoidal back emf and brushless PM DC motors with trapezoidal back emf as shown in figure 8 below.
Figure 7: Sinusoidal back e.m.f. [4].
Figure 8: Trapezoidal e.m.f. [4].
PMSM can also be categorized based on the structure of the rotor in relation to the positioning of the permanent magnets. By using this approach, the PM machines can broadly be termed as inner rotor machine, interior rotor machine, and outer rotor machine [3]. The three categories are illustrated in figure 9 below.
Figure 9: From left to right; inner, outer, and exterior rotor machine [4].
The flux linkage between the stator and the rotor cause a change in flux in the stator which induces an e.m.f. in the stator whose wave form depends on the nature of flux linkage. Therefore, if the air gap flux change is sinusoidal, then the induced emf is given as shown in figure 10 below.
Figure 10: Relationship between stator induced emf and the air gap flux change [4].
Circuit and vector diagram of PMSM machines
Just as in other synchronous motors with an excited rotor, permanent magnet machines are analyzed using the dq reference frame attached to the rotor. The resulting circuit is shown in the figure below.
Figure 11: The equivalent circuit of a PM machine in the d direction [1].
Figure 12: The equivalent circuit in the q direction [1].
The equivalent circuits are in the q and d directions whereby the magnets on the rotor are represented by a current source iPM in the circuit. The current source creates the flux linkage in the air gap equal to the one produced by the permanent magnets. If damper windings are also included in the circuit, the resulting voltage equations are different from the inductance synchronous machine due to the fact that there are no equations for the rotor windings [1]. The voltage equation of a PM machine, based on the diagrams above, is given as below.
Figure 13: Vector diagram of a PMSM [1].
The XY axes represent the stator reference while dq represents the rotor reference.
Comparison between permanent magnet synchronous motors and induction synchronous motors
As noted, PMSM machine outperform induction motors because they do not require external excitation of the rotor. This attribute gives PMSM properties that make it favorable for certain industrial applications. Such properties include electrical stability, reliable performance, and durability. These properties have enabled the application of PMSM as standalone machines in industries in applications such as spindle, articulation of ways and parts, and in provision of both rotary and linear motions [1]. Also, PMSM machine have been widely applied in emerging sectors such as robotics to enhance precision in areas such as material handling, work piece and tool changing, and loading and unloading functions. The motors are also found in oil reservoirs, coolant pumps, and hydraulic manifolds. In the production industry, the motors are applied in areas such as rubber and plastic molds extrusion, packaging, papermaking, woodworking, and in molding ceramics among others [1]. Therefore, the PMSM machines are important machines in the engineering sector. Table 1 below compares PMSM machines and induction motors
Other properties that give the PMSM machine an edge over the induction motors include high torque density from a relatively light motor, low inertia, high efficiency, higher and constant torque over a wide range of speeds, high dynamic performance in loaded conditions, low rotor inertia, and the lack of heat load generated by the rotor windings. Other qualities of the motors include better power factor and drive utilization and robust performance than DC motors. On the other hand, PMSM exhibit a few shortcomings such as creation of a back voltage by the magnetic fields that degrade the magnets and a limited loading capacity [1].
The future of permanent magnets: Application in the transport industry
The future of PMSM machines lies in the transport industry. The motor are preferred in the manufacture of electric trains and vehicles due to their high efficiency and high torque density compared to other types of motors. These applications will be discussed in detail below.
Application of PMSM in electric trains
The PMSM has all the required performance attributes for application as a traction motor. In electric railway lines, the machine is used as a direct drive motor. In railway vehicles, gear systems are used to reduce the size of the traction motor. The torque produced by the PMSM traction machine is delivered to the wheel axle to drive the vehicle. The gear system in the vehicle introduces some inefficiency such as transmission losses, maintenance work, and noise. However, these inefficiencies can be solved through the use of a direct drive system. Unfortunately, the direct drive strategy requires bigger motors and increases the motor load and unsprung masses [8]. In the adoption of PMSM for traction operations, the noise from the machine is reduced to about 14dB when motors are installed in direct drive systems for train vehicles operating on narrow gauge rails. Also, the direct drive system can be installed on an adjustable gauge and low riding vehicles.
When the PMSM is applied as a traction motor in trains, provisions are set to incorporate ventilation cooling systems for the railway vehicles. However, the ventilation air contains dust and other particle which soil the inside of the motor and as such, frequent disassembly is required to clean the machine. Also, a cooling fan, which is directly coupled to the rotor shaft, produces a lot of noise when the machine is operating at high speeds. To eliminate the noise and the need for frequent disassembly for cleaning operations, the PMSM traction motor is completely sealed off. However, the ventilation cooling is dramatically reduced leading to overheating and hence reduced performance [8]. Therefore, the enclosed PMSM machine requires a new system that can cool itself without the need of external air while still maintaining temperature within allowable limits. This is a developing area of research aimed and ensuring application of PMSMs in long distance trains without sacrificing performance and durability.
Application of PMSM in electric vehicles
In electric vehicles, motors are used as prime movers and are powered by an onboard rechargeable battery. The motor produces the torque which is then transmitted to the wheels by a system of gears. Essentially, electric cars are designed as a means of reducing environmental impact of vehicles on the road through reduction of greenhouse emissions [9]. Therefore, the vehicles must reduce energy consumption to the minimum possible value as the electric energy is also generated using unclean fossil fuels. Therefore, the prime mover used in electric vehicles must consume the minimum amount of energy [9]. PMSM offers an ideal option due to their high energy efficiency and torque density. The motor can be primed to operate on the power supplied by the battery, which has an average voltage of around 300volts.
Also, the space occupied by the electric motor is limited and therefore its active and outer diameters are maintained at the minimum level possible to generate the required torque in line with the mass of the vehicle [9]. In addition, the torque oscillation must be maintained below 3% to reduce the electromagnetic noise.
Application in servo pumps
PMSM are widely applied in servo pumps because they enhance huge energy savings and operational capacity. The PMSM servo pumps are used in applications such as hydraulic oil reservoir and supplanting a normal variable capacity pump. The use of PMSM enables the servo system to operate only when the pumping action is required thereby eliminating the use of a pumping system that runs perpetually [6]. Such systems have enabled reduction of energy consumption by pumping systems by more than a half. Also, servo systems reduce the number of required mechanical units systems such as the gearbox.
Conclusion
The permanent magnet synchronous motor is a type of a motor that does not have rotor windings. Instead, strong and durable permanent magnets made from rare earth materials are mounted on the rotor shaft. The stator of the PMSM machines is similar to that of normal induction motors. The alternating current fed to the stator sets up a rotating magnetic field in the stator whose speed of rotation is equivalent to the frequency of the alternating current. The magnetic field produced by the permanent magnets reacts with the rotating magnet field and the attraction and repulsion forces between the two fields cause the rotor to rotate. The magnetic poles of the permanent magnets are locked to the poles of the stator windings and thus there is no slippage between the rotor and the stator. Because the rotor does not require any external excitation, the PMSM machines exhibit high energy efficiency compared to the induction synchronous motors.
Also, the machines have high torque density per unit volume and are more durable. These properties make the motor ideal for applications such servo pumps and as traction motors in electric vehicles and electric trains. Despite the advantages of PMSM machines they are faced with several shortcomings such as high costs due to the use of rare earth materials in the manufacture of the permanent magnets. Also, the stator in the motor generates a counter emf due to the constant flux posed by the magnets. The voltage causes gradual degradation of the magnets and therefore they require frequent changing, which further raises the maintenance costs. In addition, the motors can only support a limited load due to the constant amount of flux generated by the rotor, which limits the motor’s loading capacity. Therefore, further research is required to device ways of mitigating the weaknesses of the PMSMs to enable widespread industrial applications of the machines. If this is achieved, the industrial inductive power load would be lowered by a half, which would lead to huge environmental and economic benefits.
References
[1] P. Juha. “Permanent magnet synchronous machine (PMSM),” Internet: https://noppa.lut.fi/noppa/opintojakso/bl30a1010/luennot/drives7_en.pdf, n.d. [April 14, 2016].
[2] S. M. Zeid. An analysis of permanent magnet synchronous motor drive. 1998, pp. 45-66
[3] J. F. Gieras & M. Wing. Permanent magnet motor technology: Design and applications. New York: Marcel Dekker, 2002, pp. 45-63
[4] R. Krishnan. Permanent magnet synchronous and brushless DC motor drives. Boca Raton: CRC Press/Taylor & Francis, 2010, pp. 75-92
[5] G. Iit. “Permanent Magnet Motors” Internet: http://nptel.ac.in/courses/108103009/23, 2013. [April 14, 2016].
[6] J. Dan. “How Induction Motors And PM Synchronous Motors Operate.” Internet: http://electronicdesign.com/analog/how-induction-motors-and-pm-synchronous-motors- operate, Nov. 9, 2012. [April 14, 2016].
[7] P. Harald. “Permanent magnet motors outperform induction motors in many applications.” Internet: http://www.controleng.com/single-article/permanent-magnet-motors- outperform-induction-motors-in-many- applications/413305407b2989675983a5babac0ac6d.html, Sept. 24, 2012. [April 14, 2016].
[8] M. Kondo. (2003, Jan.). “Application of permanent magnet synchronous motor for driving railway lines.” Railway Technology Avalanche. [On-line]. 1 pp. 6. Available: www.rtri.or.jp/eng//01/RTA-01.pdf. [April 14, 2016].
[9] M. Khanchoul, G. Krebs, C. Marchand, F. Alves, A. Battelier & M. Roz. (2011, Mar.). “Design and Study of a Permanent Magnet Synchronous Motor for an Electric Compressor.” Progress In Electromagnetics Research Symposium Proceedings. [On- line]. Pp.171-175 Available: https://piers.org/piersproceedings/download.php?file=cGllcnMyMDExTWFycmFrZXNo fDFBOV8wMTcxLnBkZnwxMDExMTYwMzMyMTk=[April 14, 2016].
