SWITCHED RELUCTANCE MOTOR

Switched reluctance motors for electric vehicles

Due to the high volume of mainstream vehicles with internal combustion engines (ICEs), environmental pollution and energy shortages are now a concern. Electric vehicles (EV) can restrict the energy source and the ideal method to save resources and provide zero-emission vehicles.

The main requirements of a Traction Electric Vehicle are therefore summarized as:

• High torque density and power density
• High torque for starting, low speed and hill-climbing, and high power for high-speed cruising
• Wide speed range, with a constant power operating range of around 3–4 times the base speed
• High efficiency over the wide speed and torque ranges, including low torque operation
• Intermittent overload capability, typically twice the rated torque for short durations
• High reliability and robustness appropriate to the vehicle environment
• low acoustic noise
• Acceptable cost

Since the permanent-magnet (PM) machines inherently own the merits of high torque/power density and efficiency due to the utilization of high-energy Permanent Magnetic materials, they are widely used in hybrid vehicles and air conditioners. However, the cost of rare earth permanent magnets is still a problem for mass production. Moreover, the high-speed drive is limited due to the low mechanical strength. Therefore, there is increasing attention in rare-earth-free motors such as induction machines, synchronous reluctances machines, and switched reluctance machines.

Switched reluctance motors (SRMs) are found to be much suitable for electric vehicle (EV) applications due to :

• Simple and rugged motor construction,
• Low weight,
• Low production cost,
• Easy cooling,
• Excellent power-speed characteristics,
• High torque density
• High operating efficiency, and
• Inherent fault tolerance. Switched reluctance motors (SRMs) are coming up as an emerging alternative for e-mobility applications, with decades of reliability testing in zero-fault-tolerant applications, and no rare earth. Recent advances in power electronics and IoT are boosting SRMs to new, highly energy-efficient applications. Some of the key developments in SRM technology include:

• Enedym Inc. Secures $15 Million Investment to Accelerate Business Plan
• Turntide Technologies raised $225 million and acquired three companies to speed up its route to market.
• Advanced Electric Machines (AEM), a spin-out from Newcastle University, developed the HDSRM (High-Density Switched Reluctance Machine), targeting the commercial vehicle segment.

The literature gives evidence that the SRMs are competing with PM motors for electric vehicle propulsion applications.

• For example, SRM has been designed to compete with a PM motor for Toyota Prius vehicle.
• The SRM was designed as the same dimension of the PMSM used in the second-generation Toyota Prius with a 50?kW output power from 1200 to 6000?r/min in Chiba et al.
• The SRM was designed as the same dimension of the PMSM used in the third-generation Toyota Prius with a 60?kW output power from 2768 to 13,900?r/min and 100?kW output power from 5400 to 13,900?r/min in Chiba and Kiyota and that means the shaft output can be enhanced by 160% in SRMs.
• Land Rover unveiled a range of seven electric research vehicles powered by an SRM and drive system developed at the 2013 Geneva Motor Show. The EVs are based on Land Rover’s 110 Defender model in which the standard diesel engine and gearbox have been replaced by a 70?kW switched reluctance motor. It is well-known that SRMs with higher phases generate reduced torque ripple and enhanced fault tolerance in comparison to three-phase SRMs. For example, the torque ripple in the three-phase SRMs is three times more than six-phase SRMs. Moreover, when one phase fault occurs, the output torque in six-phase SRMs is higher than that of three-phase SRMs.
• However, high levels of torque pulsation and acoustic noise are still the major disadvantages of SRMs, especially in the EVs area due to the requirement of comfort.

An electric motor like SRM (switched reluctance motor) runs through reluctance torque. Different from the types of common brushed DC motor, power can be transmitted to windings within the stator instead of the rotor. An alternate name of this motor is VRM (Variable Reluctance Motor). For a better operation of this motor, it uses a switching inverter. The control characteristics of this motor are the same as dc motors which electronically commutated. These motors are applicable where sizing, as well as horsepower (hp) to weight, is critical.

The switched reluctance motor (SRM) is an electric motor that runs by reluctance torque and thus is a subgroup in reluctance motors. Unlike common brushed DC motor types, power is delivered to windings in the stator (case) rather than the rotor. This greatly simplifies mechanical design as power does not have to be delivered to a moving part which eliminates the need for a commutator, but it complicates the electrical design as some sort of switching system needs to be used to deliver power to the different windings. Electronic devices can precisely time the switching of currents, facilitating SRM configurations. Its main drawback is torque ripple Controller technology that limits torque ripple at low speeds has been demonstrated.

The same electromechanical design can be used in a generator. The load is switched to the coils in sequence to synchronize the current flow with the rotation. Such generators can be run at much higher speeds than conventional types as the armature can be made as one piece of magnetisable material, as a slotted cylinder. In this case the abbreviation SRM is extended to mean Switched Reluctance Machine, (along with SRG, Switched Reluctance Generator). A topology that is both motor and generator is useful for starting the prime mover, as it saves a dedicated starter motor.

Operating principle

The SRM has wound field coils as in a DC motor for the stator windings. The rotor however has no magnets or coils attached. It is a solid salient-pole rotor (having projecting magnetic poles) made of soft magnetic material (often laminated steel). When power is applied to the stator windings, the rotor's magnetic reluctance creates a force that attempts to align the rotor pole with the nearest stator pole. In order to maintain rotation, an electronic control system switches on the windings of successive stator poles in sequence so that the magnetic field of the stator "leads" the rotor pole, pulling it forward. Rather than using a mechanical commutator to switch the winding current as in traditional motors, the switched-reluctance motor uses an electronic position sensor to determine the angle of the rotor shaft and solid state electronics to switch the stator windings, which enables dynamic control of pulse timing and shaping. This differs from the apparently similar induction motor that also energizes windings in a rotating phased sequence. In an SRM the rotor magnetization is static (a salient 'North' pole remains so as the motor rotates) while an induction motor has slip (rotates at slightly less than synchronous speed). SRM's absence of slip makes it possible to know the rotor position exactly, allowing the motor to be stepped arbitrarily slowly.

Simple switching

If the poles A0 and A1 are energized then the rotor will align itself with these poles. Once this has occurred it is possible for the stator poles to be de-energized before the stator poles of B0 and B1 are energized. The rotor is now positioned at the stator poles b. This sequence continues through c before arriving back at the start. This sequence can also be reversed to achieve motion in the opposite direction. High loads and/or high de/acceleration can destabilize this sequence, causing a step to be missed, such that the rotor jumps to wrong angle, perhaps going back one step instead of forward three.

Quadrature

A much more stable system can be found by using a "quadrature" sequence in which two coils are energised at any time. First, stator poles A0 and A1 are energized. Then stator poles B0 and B1 are energized which, pulls the rotor so that it is aligned in between A and B. Following this A's stator poles are de-energized and the rotor continues on to be aligned with B. The sequence continues through BC, C and CA to complete a full rotation. This sequence can be reversed to achieve motion in the opposite direction. More steps between positions with identical magnetisation, so the onset of missed steps occurs at higher speeds or loads. In addition to more stable operation, this approach leads to a duty cycle of each phase of 1/2, rather than 1/3 as in the simpler sequence.

Control

The control system is responsible for giving the required sequential pulses to the power circuitry. It is possible to do this using electro-mechanical means such as commutators or analog or digital timing circuits.

Many controllers incorporate programmable logic controllers (PLCs) rather than electromechanical components. A microcontroller can enable precise phase activation timing. It also enables a soft start function in software form, in order to reduce the amount of required hardware. A feedback loop enhances the control system.

Power circuitry

The most common approach to powering an SRM is to use an asymmetric bridge converter. The switching frequency can be 10 times lower than for AC motors.

The phases in an asymmetric bridge converter correspond to the motor phases. If both of the power switches on either side of the phase are turned on, then that corresponding phase is actuated. Once the current has risen above the set value, the switch turns off. The energy now stored within the winding maintains the current in the same direction, the so-called back EMF (BEMF). This BEMF is fed back through the diodes to the capacitor for re-use, thus improving efficiency.

This basic circuitry may be altered so that fewer components are required although the circuit performs the same action. This efficient circuit is known as the (n+1) switch and diode configuration.

A capacitor, in either configuration, is used for storing BEMF for re-use and to suppress electrical and acoustic noise by limiting fluctuations in the supply voltage.

If a phase is disconnected, an SR motor may continue to operate at lower torque, unlike an AC induction motor which turns off.

The two primary components of a reluctance motor are the outer stationary stator and the inner rotor, separated by a small air gap. Depending upon the type of reluctance motor, the construction of these two parts change, but the basic principles of operation remain the same.

The stator consists of protruding, “salient” pole-pairs created by running current through a wire which is wound around these protrusions. The rotor is created using ferromagnetic metal and contains its own poles that follow the contours of the stator’s magnetic field (either with protrusions or air gaps/notches). When a rotor’s salient pole lines up with the stator’s salient pole, it is said that the rotor is in the minimum reluctance position – that is, the amount of magnetic “resistance” is lowest at this point, and is “fully aligned”. When a stator pole lines up with the notches/barriers/slots of the rotor, it is said that the rotor is in the maximum reluctance position, or “fully un-aligned”. Due to the conservation of energy, the rotor will always move to the position of least reluctance, and so there is a “reluctance” torque produced when the rotor is fully unaligned. This torque will pull the rotor to the nearest salient stator pole, and cause rotation. If timed correctly using control systems equipment or specific rotor geometry, this effect can create continuous, rotational output.

The magnetic circuit developed between rotor and stator has high reluctance when they both are out of alignment. At this time, the stator pole pairs get energized, and the rotor tries to get in line with the powered stator poles, which decreases the magnetic reluctance. This ability of rotor to reach the minimum point of reluctance produces a torque, known as reluctance torque. Excitation of the stator poles must be accurately timed to make sure that it happens only when the rotor is trying to be aligned with the excited pole. For this purpose, SRM may need positive feedback from Hall effect sensors or encoders to control the excitation of stator based on an accurate rotor position.

This motor simplifies its mechanical design to restrict the flow of current toward a rotary part; however, it complicates the design because some kind of switching system must be employed to transmit the power toward the different windings. This mechanical design can also be used for a generator. The load can be switched toward the coils within the sequence to coordinate the flow of current through the rotation. So these generators can also run at high speed as compared with conventional types of motors because the armature is made like a single piece of magnetizable material like a slotted cylinder.

The working principle of the switched reluctance motor is, it works on the principle of variable reluctance that means, the rotor of this motor constantly tries to align through the lowest reluctance lane.

The formation of the rotary magnetic field can be done using the circuit of power electronics switching. In this, the magnetic circuit’s reluctance can mainly depend on the air gap. Therefore, by modifying the air gap among the rotor as well as a stator, we can also modify the reluctance of this motor. Here, reluctance can be defined as resistance toward the magnetic flux. For Electrical circuits, reluctance is the combination of resistance as well as the magnetic circuit.

Switched Reluctance Motor Construction

The construction of the switched reluctance motor is shown below. This motor includes 6- stator poles as well as 4 rotor poles. The design of the stator can be done using silicon steel stampings inside projected poles. The poles in the stator are either an odd number or even number. Most of the electric motors have an even number of poles within the stator which have field coils.

When the poles are opposite then the field coils will be connected in series. So their magnetomotive forces are additive which are called phase windings. A set of coils or a single-coil can comprise phase windings. Each winding can be connected to the motor terminal and these are properly connected toward the o/p terminals of a switching circuitry of power semiconductor. The input of this is a DC supply.

The designing of the rotor can be done with Si steel stampings through externally projected poles. The rotor poles are dissimilar as compared with the stator poles.? In most of the existing motors, the rotor poles are 4 otherwise 6 based on the number of stator poles like 6 8. The shaft of the rotor holds a position sensor. So the operating of various devices in the power semiconductor circuitry is mainly controlled through the signals attained from this sensor.

In this motor, both the stator as well as rotor includes a projected pole that is designed with a soft iron as well as silicon stampings which are used for reducing hysteresis losses. The stator of the motor includes a field winding whereas the rotor doesn’t. In the stator, each winding can be connected within the series through the opposite poles for increasing the magnetomotive force of the circuit.

Types of Switched Reluctance Motor

The categorization of switched reluctance motor can be done based on the construction like linear SRM as well as rotary SRM.

Linear SRM

The linear SRM or linear switched reluctance motors are known as servos in the market. It includes a single-step stator as well as the rotor.

Rotary SRM

The rotary SRM or rotary switched reluctance motors are available in two types like radial field as well as the axial field. Axial field SRMs are classified into two types like single stack and multi-stack. This rotary SRM includes more than one rotor and stator.

Switched Reluctance Motor Working

The operating of SRM (switched reluctance motor) can be done through switching currents within the stator windings of the motor by making changes within the magnetic circuit. This circuit can be formed through the stator as well as the rotor of the motor.

The stator of this motor includes windings that are related to a BLDC motor; however, the design of the rotor can be done with steel that is turned into salient poles without magnets or windings. Once the poles of the stator & the rotor are out of position, then the magnetic circuit among them includes a high reluctance.

When the pairs of the pole in the stator are switched, the rotor switches to connect through the activated stator poles to reduce the reluctance of the circuit. When the stator poles are switched then they should be exactly timed to make sure that it happens because the rotor pole is moving toward to connect with the activated stator pole.

Not like stepper motors, these motors need position feedback using Hall Effect sensors otherwise an encoder to manage the stator currents commutation depending on the exact position of the rotor.

As compared with stepper motors, these motors include fewer poles as well as a larger stepping angle. Stepper motors are mainly used for placing applications, wherever high resolution, as well as step integrity, is significant. But SRMs are applicable where a primary concern is power density. These motors include rotors without windings, magnets & lower inertia. Thus, they can attain higher speeds as well as accelerations as compared with stepper motors because they have permanent magnet rotors.

The main difference between SRMs (switched reluctance motors) & stepper motors is the construction of stator. In an SRM, the phases are autonomous with each other that means, if one otherwise more phases stop working, then the motor will operable even though by decreased torque output.

Switched reluctance motors generate more clear noise as compared with stepper motors. The main source of noise can be the distortion of the stator because of the radial forces that happen once the pairs of stator poles are activated. These pairs are attracted to cause radial forces to alter the stator.

Switched Reluctance Motor Characteristics

The characteristics of the switched reluctance motor include the following.

• This kind of reluctance motor is a 1-phase or 3-phase
• Speed control of this motor is simple.
• The triggering circuit can be changed to get high speed
• It operates with a DC supply once used with an inverter.
• Once the firing angle of any switching device can be changed then different speeds can be achieved.
• Control of one phase is independent of the other two phases.
• The unutilized energy fed to the motor can be retrieved by using the feedback diodes. This improves efficiency.

Advantages

The advantages of a switched reluctance motor include the following.

• These motors are very simple & the rotors in this motor are extremely strong
• These motors are applicable for high-speed applications.
• The VFDs (variable frequency drives) of this motor are somewhat simpler as compared with conventional VFDs.
• This motor doesn’t use any additional ventilation system when the stator, as well as rotor slots, is projected. So the airflow can be maintained among the slots.
• These are less expensive because of the nonexistence of permanent magnets.
• Fault tolerance is high
• This motor works with a simple two-phase or three-phase pulse generator.
• Phase losses do not change the operation of the motor.
• Once the phase sequence is changed then the motor direction will be changed.
• Inertia Ratio or High Torque
• Self-starting without using additional arrangements

Disadvantages

The disadvantages of a switched reluctance motor include the following.

• Switched reluctance motors have less torque capacity & normally these motors are noisy.
• While operating this motor at high speed, it creates a torque ripple.
• High noise level
• It uses an external rotor position sensor
• These are applicable for medium to high speed, low-cost applications wherever controllability & shaft or noise torque ripple are not dangerous.
• This motor generates harmonics when it operates at high speed, so to reduce this, large size capacitors need to install.
• Since the nonexistence of a permanent magnet, the SRM has to carry a high i/p current to increase the necessity of converter KVA.

Applications

The applications of switched reluctance motors include the following.

• These types of motors are used as an alternative for induction motors in different applications wherever the operating conditions of this motor do not suit them.
• In textile machinery like towel looms, rapier looms, etc
• Used in electric vehicles
• Oilfield machinery like beam pumps, vertical pumps, well testing machinery, etc.
• Mining machinery like conveyors, shearers, winches, ball mills, boring machines, coal crushers, etc.
• Used in all kinds of mechanical presses like screw presses.

These motors are used in miscellaneous applications which include the following.

• Machine tools like vertical lathes, planers, drilling machines, etc.
• Coil winding as well as unwinding equipment
• General machinery like pumps, fans, compressors, etc.
• Equipment used in paper mills
• Machinery used for food mixing
• Rolling mill for metals
• Lifting machines such as winches, lifts, conveyors, etc
• Manufacturing of plastic-like extrusion, injection molding devices
• Power generation device like load control using wind turbine rotor blade
• Used in domestic appliances like vacuum cleaners, washing machines, fans, etc.

Control methods

Switched reluctance motors suffer from high torque ripple because of discrete torque production and independent phases. Thus, it is its major drawback and may restrict its use in EVs. However, many control strategies are currently being used to solve this problem. Broadly speaking, there are two main torque control methods for SRM: direct and indirect torque control.

Direct torque control strategy includes a simple control system with a hysteresis controller. The non-linear characteristics of SRM are taken into account to compensate for the output torque ripple based on current and rotor position. Its main benefits are simplicity, high performance, and fast torque response. In many advanced strategies, these characteristics are utilized to fluctuate the reference current depending upon rotor position and reference torque. This type of torque control is also subdivided into advanced direct instantaneous torque control (ADITC) and direct instantaneous torque control (DITC).

In DITC, the average torque may be controlled within a certain bandwidth using estimation of the instantaneous torque. However, it is not possible to control the instantaneous torque and performance is dependent upon sampling time. In the ADITC method, the different phase currents in a single sample time are controlled by regulating the average phase voltage. Compared with DITC, it can get a smaller torque ripple by increasing the sampling time. But, if the sampling time is the same, ADITC switching frequency will be almost double, and thus there will be more electromagnetic emissions and switching losses than DITC.

There are three types of indirect torque control: nonlinear, cosine and linear logical torque sharing function. The nonlinear TSF method has given promising performance in comparison to the other two. The torque is controlled by directly controlling the phase currents, which in turn control the torque. The torque reference is converted into a corresponding phase current reference. However, this conversion is complicated as the torque is dependent upon the rotor position. The relationship between rotor position, torque and current is nonlinear, and thus it cannot be expressed as an analytical expression. However, it can be formulated using artificial neural networks or a lookup table.

Introduction to Switched Reluctant Motors (SRM)?

The switched reluctance motor (SRM) is a reluctance torque-driven DC electric motor. Unlike most brushed DC motors, power is routed to stator (case) windings rather than the rotor. It emulates the behavior of an AC motor by switching the DC between the stator windings.

It complicates electrical design because power must be delivered to the various windings via a switching mechanism. Electronic devices can accurately time current switching, making SRM designs easier.

The switched reluctance motor operates on the changing reluctance principle, which implies that the rotor of this motor is constantly aiming to align through the lowest reluctance lane. The rotational magnetic field can be produced with the help of a power electronics switching circuit.

?Advantages of Switched Reluctant Motors (SRM)?

• Construction?is straightforward?and robust.?
• No windings, slip rings, brushes, etc are included in the rotor as a result it is easier to maintain than other types of motors.?
• There’s?no?static magnet.?
• Ventilation is easier as losses take place mostly within the stator.?
• Power semiconductor switching circuitry is easier.?
• It’s?possible?to urge?very high?speed.?

?Disadvantages of Switched Reluctant Motors (SRM)?

• Torque-rippling cause noise pollution at medium to high speeds?
• Stator phase winding should be capable of carrying magnetizing current.?
• It requires position sensors.?

?Introduction to Permanent magnet synchronous motors (PMSM)?

Permanent magnet synchronous motors are a form of AC synchronous motor in which the field is stimulated by permanent magnets, which produce sinusoidal back EMF.?

It has the same rotor and stator as an induction motor, but the rotor is a permanent magnet that creates a magnetic field.?As a result, the rotor’s field winding does not need to be wound.?It is also known as 3-Phase Brushless permanent sine-wave motor.

It is regulated by the synchronously rotating magnetic field that generates electromotive force.?A rotating magnetic field is formed in between the air gaps when the stator winding is activated by a 3-phase supply.?

When the rotor field poles?maintain?the revolving magnetic field at synchronous speed and the rotor rotates continuously, the torque is produced.?Given this?fact,?these motors are not self-starting, a variable frequency power supply is?required.?

??Advantages of Permanent magnet synchronous motors (PMSM)

• Torque ripples are absent.?
• High-Performance efficiency (output to input ratio)?
• Availability in?different sizes.?
• Smooth torque.??

?Disadvantages of Permanent magnet synchronous motors (PMSM)

• We need a control system as we can only change the stator current.?
• There are no self-starting motors.?
• There is a high?initial?cost involved.?
• Performance on PMSM depends on the quality of the magnet used.?

Switched Reluctance Motor Vs Permanent Magnet Synchronous Motors in Electric Vehicle

The invention was brought up by Nikola Tesla in 1883. The basic principle of the working of induction motors is electromagnetic induction where the magnetic field generated around the?conductor induces current due to continuous rotation.

This induction motor functions with the flow of AC induction.??

The above-discussed principle of electromagnetic induction creates a minute delay which is known as slip. Due to the induced current flow, there is a noticeable difference between the motor speed due to the magnetic field and the rotating shaft speed.?

?Therefore, to remember in an easier sense, these induction motors are termed as “Asynchronous Motors”.?

Now, as we have an idea about Induction Motors, let us understand why PMSM is better than induction motors.

Firstly, PMSM is a synchronous motor whereas the induction motor is an asynchronous motor.?Slips produce torque in induction motors, sometimes slips are unnecessary and lead to energy losses and decreasing efficiency of the induction motors.?

Secondly, PMSM is designed in such a way that it has a higher performance output compared to Induction Motors. (You can refer to the graph given below when both motors are compared based on power factor).

The above graph indicates PMSM’s consistent efficiency under different load ranges. This eventually indicates energy saving during its EV’s running span.?

Thirdly, Permanent magnet synchronous motors PMSM can generate torque even at zero speed ensuring smooth pickup and braking operations compared to induction motors.?

Lastly, EVs are being manufactured in large masses where the more efficient and sustainable?motor?will lead the motors’ demand?in the EV market.?

One of the most controversial and complicated motors amongst different motors. SRM has been understood for a long time yet this wasn’t much in use for electronic advancements.

So why do we use PMSM Motor??

When we compare SRM and IM, they are just better and worse in some situations.?

SRM has more power density but lacks in keeping up the power factor against IM. There are more vibrations caused by SRM due to torque ripples while it can generate a huge amount of torque in the shortest time when compared with other electric motors.?

It needs to be a better arrangement for operating SRMs (starting and sync). There have been optimization taking place but those are just minor improvements in SRM advancements.?

The recent noticeable advancements have been observed in the Tesla Model 3 3 and Toyota Prius. about the IPM-SynRM electric motor, Toyota Prius is an HEV with a single magnet piece but it wasn’t revolutionary as the Tesla Model 3.?

The EV Markets are rising like never before and everyone is just so excited to notice and learn about the different advancements going on recently.?

Advantages of Reluctance Motor

1. Simple motor construction.
2. Low rotor inertia facilitates high starting torque.
3. These motors produce high self-inductance.
4. Can produce variable speeds at constant power outputs.

Switched Reluctance motor vs BLDC for EV

1. BLDC motors perform better when compared to SRMs at high speeds.
2. SRM is more cost-effective than BLDC motors.
3. When both these motors are under high power applications, SRM motors perform much better than BLDC motors as SRM are cooler during their operations.

Switched Reluctance Motor vs Induction Motor

1. As discussed in this article, SRM has a higher power density than IM.
2. Due to high power density, SRM performs operations more efficiently than Induction Motor.

Induction Motor is more cost-effective than SRM.

Top 5 Electric Motor Manufacturers in India

The Rebel EV industry has a large number of electric motor manufacturers: here is the list of the top 5 electric car motor manufacturers in India.

1. Tata AutoCorp,
2. General Motors,
3. Virya Mobility,
4. Nidec Corp.,
5. Aisin Seiki Co. Ltd., etc.

types of EVs motors

Today electric vehicles are definitely not the new topic, it is the most popular and we can say a trendy word. As electric vehicles are becoming the boon in the country, people are much interested to know more about EVs whether they buy it one for them or not but they are looking for some wonderful specifications for the best performance of the vehicle such as speed, range, motor capacity or power, etc, so we have got some of the advantages and disadvantages of diff types of EVs motors.

Advantages & disadvantages of different types of EVs motors

Before we go into the motor manufacturers, let us see what are the types of motors used in EVs and which motors are widely used in EVs.

There are many types of motors but in that, there are 5 different types of motors used widely and currently, most of the EVs manufacturers use BLDC motors, Induction, SRM, Permanent magnet motors.

Different types of electric vehicles motors

• Permanent Magnet Synchronous Motor(PMSM),
• Brushless DC Motor(BLDCM),
• AC Induction Motor(ACIM),
• Permanent Magnet Switched Reluctance Motor(PMSRM)
• Interior Permanent Magnet Motor(IPMM),

Types of Electric Motors

The electric motors are available in 3 main segments like AC motor, DC motor, and special purpose motors.

DC & AC Motors

DC motors may run from 96 to 192 volts and AC motor, three-phase AC motor running at 240 volts AC with a 300-volt battery pack. A typical motor will be in the 20,000 watts to the 30,000-watt range.

Different types of EVs motors

Brushed DC Motor

Advantage

• It gives the maximum torque at low speed.

Disadvantages

• Bulky structure
• Low efficiency
• Heat generation at brushes

Usage

It is used by the Fiat Panda Elettra.

Permanent Magnet Brushless DC Motor (BLDC)

Advantages

• No rooter copper less
• More efficiency than the induction motors
• It is lighter
• Smaller
• Better heat dissipation
• More reliable

Disadvantages

• Short constant power range
• When speed increases torque decreases
• Expensive

Usage

This type of motor is used in the Toyota Prius (2005).

Permanent Magnet Synchronous Motor (PMSM)

Advantages

• can be used without using gear systems in a different speed range
• More efficient
• Compact
• Best suitable for in-wheel application
• Gives high torque even at least speeds

Disadvantages

• During the in-wheel operation at high speed it losses the high amount of iron

Usage

Toyota Prius, Nissan Leaf, Soul EV

Induction Motor (IM)

Advantages

• It is the most mature commutator less motor drive system
• By employing the field operation control it can be operated like separately excited DC motor

Usage

Tesla Model S, Tesla Model X, Toyota RAV4, GM EV1

Switched Reluctance Motor and (SRM)Synchronous Reluctance Motor (SynRM)

Advantages

• It is simple
• Robust construction
• High speed
• Low cost
• Constant long power range
• Fault-tolerant
• Higher power density

Disadvantages

• Makes so much of noise
• Low efficiency
• Larger
• Heavy
• Complex and design control
• Problem creating in manufacturing and controllability
• Low power

Usage

Chloride Lucas

PM assisted Synchronous Reluctance Motor

• More power than SynRms
• Free from demagnetizing problems seen in IPM

Usage

BMWi3

Axial Flux Ironless Permanent Magnet Motor

Advantages

• Iron is not used in the outer rotor
• No stator core
• Weightless
• More power density
• Minimize copper loss
• Good efficiency
• Variable speed machines
• Rotor is capable of being filtered to the lateral side of the wheel

Usage

Renovo Coupe

Electric Vehicles

CONCLUSION :

The benefit of SRMs is their high torque component, enabling their use in many applications such as wind energy, generator starter systems in gas turbine engines, and high-performance aerospace applications. Moreover, their advantages in EVs include their robustness, simple control, high efficiency, wide constant power operation region, fault tolerance, and effective torque-speed characteristics. Since they do not contain brushes, collectors, or magnets, the maintenance of SRMs is very simple and effective and their price is very competitive.

The absence of magnets eliminates the problem with mechanical forces, enabling the motor to operate at a high speed. Since the motor's windings are not used, there are no copper losses in the rotor ensuring the rotor temperature is lower than other motor types. Since the phases are not connected, SRM motors can continue their operation even when one of the phases disconnects. SRM rotors have a lower inertia than other motor types. The drawbacks of this motor type are increased vibration and acoustic noise. In addition, the salient-pole rotor and stator construction cause high torque ripple. The high rotor inductance ratio allows sensor-less control to perform.

Proper motor design enables the wide constant power operation region, which in turn allows operation at high speeds. SRMs have a suitable torque/power speed characteristic for EV applications.

Table 1: Comparing the relevant features of different motor types

CharacteristicsMotor DC IM PM SRM

Power density Low Medium Very High Medium Efficiency Low Medium Very High Medium Controllability Very high Very high High Medium

Reliability Medium Very high Very High high

Technological maturity Very high Very high High High Cost Low Very low High Low

It is noticeable that the IM motor type has all the characteristics suitable for EVs. In this application, safety is one of the most important considerations and the SRM and IM types provide driving safety. However, the rated speed of IM is relatively low. PM has a higher power factor and efficiency in low-speed region.

The SRM type does not use brush collectors and magnets and thus has fewer maintenance requirements. This type also has lower power losses than other types. This is because of the short winding ends and their total length. The rotor does not contain conductors enabling low rotor temperature and easy cooling, which is one of the main advantages of SRM type motors. SRM operates at high speeds in a wide constant power region and allows the extremely high-speed operation. Besides this, the motor is lightweight, competitive and has high efficiency. If all characteristics are considered, SRM is the most suitable motor type for EVs.

Even with their relatively high power density and efficiency, BLDC motors are not commonly used in EV applications, mostly because of their limited constant power range.

The physical principles behind the reluctance motor are fairly simple. The first is that the magnetic analog of current, called flux, wants to travel the path of least magnetic resistance, called reluctance. The second is that low reluctance materials like iron and its alloys, nickel, cobalt, etc., tend to strongly align to an incident magnetic field.

Thus a reluctance motor merely has a rotor with alternating regions of high and low reluctance on it, and a stator with several electromagnets that when energized in sequence (and regardless of polarity!) will pull the low reluctance regions, or poles, along.

This is quite a bit different from the way more familiar motors like series DC or AC induction work; in both of those motors torque is produced from the interaction of two separate magnetic fields (with both an attraction and repulsion component), while in the reluctance motor torque is strictly from magnetic attraction. Figure 1 shows a typical SRM design (called “6/4,” for the number of stator and rotor poles, respectively) that would be appropriate for traction applications. You can see that the stator has six windings spaced equidistantly while the rotor has four “salient poles,” which is an engineering term for areas of higher magnetic flux concentration. In this case, the salient poles are those parts of the rotor that are closest to the stator and are formed by simply cutting away parts of the rotor.

SRM should be very inexpensive to manufacture for two main reasons. The first is that the SRM rotor is very simple and can be made out of a solid block of steel with notches for the salient poles, or even built up from a stack of thin steel stampings. The SRM rotor does not carry any current, nor is an alternating field induced in it, so there is no need for a commutator and armature coils as in a DC motor, nor is there need of a cast metal “squirrel cage” as in an induction motor. The second reason it’s cheaper to make is that the stator is comprised of simple solenoid-wound electromagnets spaced evenly around it, much like the field coils in a DC motor. This is in stark contrast to the stator windings in an AC induction motor which must be wound in a complex distributed pattern into slots in the stator housing.

One of the application advantages of the SRM is that the rotor does not experience flux reversals, so there are no “iron losses” to deal with inside it; all of the losses occur in the stator, which is much easier to cool. This unipolar fluxing of the rotor also means it is perfectly acceptable to stall the SRM without damaging it, useful for holding a vehicle still on an incline or “proving” a load on a crane.

SRM requires unipolar excitation, not the bipolar (AC) excitation required by induction motors. This means a different power stage design is required for the SRM controller.

One typical configuration is to wire each phase between the output terminals of two complementary switch/diode chopper modules (i.e., one module with the switch on top and the other with the switch on the bottom). The upshot of this is that you end up using twice as many modules as compared to the single half-bridge needed for each phase in an AC inverter, but you can drive each phase of the SRM with the full DC supply voltage.

A worse problem for the SRM controller is that the inductance of each phase is proportional to the degree of alignment with the salient poles of the rotor. As one or more rotor poles line up with a given stator winding, the inductance of that winding shoots up, making it harder to push the correct amount of current through it at the correct time. Conversely, as the rotor pole moves away from the winding, its inductance once again drops. The worst thing about this is that rotor torque will only be positive as current is supplied to the winding when inductance is increasing; the torque turns negative – i.e., regeneration occurs – when the inductance is falling. Thus, small timing errors in the delivery of current to each winding can result in less torque than expected, vibrations from the torque being inconsistent from phase winding to phase winding, or even from it going negative every so often.

The rotor position must be known (or predicted) with a high degree of accuracy, and the current control loop for each winding must be very fast – much faster than in an AC induction motor inverter – to get the best performance from the SRM.

Also, the effects of the change in the resistance of the windings with temperature, as well as the non-linear relationship of phase angle and winding current must be accommodated.

All of these demands add up to a very computationally-intensive control strategy for the SRM, which more or less explains why they have been sitting on the dusty shelves since the 1840s – there simply wasn’t enough computing power in programmable logic or microcontroller ICs to operate them until very recently.

The final disadvantage to the SRM, and perhaps the most difficult to address without also increasing the manufacturing cost and/or the controller complexity, is its tendency to emit a lot of noise in operation.

One of the main sources of noise is the stator being squeezed towards the rotor by the attractive force exerted by each phase pole pair as it is energized. An obvious solution to this is to make the stator stronger – i.e., use more material, which, of course, costs more. A less-obvious approach is to inject current into “inactive” windings at precise points in time to partially cancel the force vector from the active windings.

Some of the acoustic noise and vibration from the SRM is the result of the torque output having a lot of “ripple.” Especially unfortunate is the fact that the more one optimizes the SRM for high average torque output (by using a lower number of stator and rotor poles, which is unintuitive for those familiar with AC induction motors), the more torque ripple results.

All in all, the pluses of the SRM design are pretty compelling – cheap to manufacture, low to non-existent rotor losses, robust power stage topology – while the disadvantages, such as control algorithm complexity and high vibration and noise, do not seem insurmountable, especially when used as traction motors for electric vehicles.

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