KNOW ABOUT DIFFERENT TYPES OF ELECTRIC MOTORS USED ON ELECTRIC VEHICLES
vijay tharad
Director Operations at Corporate Professional Academy for Technical Training & Career Development
1. Introduction
Today, the trend toward electric vehicles (EVs), which are alternative solutions, is increasing, due to the fact that internal combustion engine vehicles increase carbon emissions, and countries remain dependent on oil-importing countries. Unlike traditional ICVs, EVs use electric motors instead of internal combustion motor to offer several outstanding features such as wide-torque speed, high power density, higher efficiency, longer range, reduced maintenance costs, and reduction in air pollution.
EVs provide the necessary power to push the vehicle by converting the electrical energy stored in the battery or other energy storage units into mechanical power with one or more electric motors. An electric vehicle consists of subcomponents such as battery, electric motor, power controller, and mechanical transmission
In addition to determining the optimal battery and power management system in order to increase the driving range and provide higher performance and efficiency in electric vehicles, the choice of electric motor is also a very important decision. In the selection of motors used in electric vehicles, simple design, high power density, low maintenance cost, and easy controllability features come to the fore .
In electric vehicles, different types of electric motors are used:
For this presentation we will concentrate on following Electric Motors as they are used on Electric Vehicles.
1. DC Series Motor
High starting torque capability of the DC Series motor makes it a suitable option for traction application. It was the most widely used motor for traction application in the early 1900s. The advantages of this motor are easy speed control and it can also withstand a sudden increase in load. All these characteristics make it an ideal traction motor. The main drawback of DC series motor is high maintenance due to brushes and commutators. These motors are used in Indian railways. This motor comes under the category of DC brushed motors.
The first electric motors used in many prototype electric vehicles were DC series motors in the 1980s and before. DC motors are one of the simplest types of motors that can be connected directly to the vehicle’s battery, which can be controlled simply where speed regulation is provided by voltage change without the need for complex power electronics applications.
However, EVs where brushed DC motors are used meet the demand for high torque at low speeds, but cannot meet the torque demand required for high speeds due to the friction between the brush and collector used to transfer the armature current. In addition, lower efficiency due to lower power density than other types of motors in the same volume and the need for maintenance of brush-collector arrangements (about every 3000 h) are other negative aspects. Although the power density in DC motor is provided by commutation poles and compensation windings in the motor design, this is not preferred because it will lead to an increase in cost and size.
Features of brushed DC motors
2. Brushless direct current motor (BLDC)
BLDC motor provides good control mechanism, and performance during its operation. It has high efficiency and power density which make it favorable for EV use by reducing losses which ultimately increases the driving range of vehicles. Like a DC motor, BLDC motor consists of two parts in construction- rotor and stator. Rotor is the rotating part of the motor which rotates the vehicle and is made up of permanent magnet. Stator is the stationary part of the motor and is made up of copper winding to create artificial magnet poles by passing current.
There are two types of rotors: outer runner and in-runner. In the outer runner, the rotor is outside and the stator is at the center of the rotor while the rotor is inside the stator rotor.
Working principle: We all know that opposite poles of the magnet attract each other. This same principle is used for operation of BLDC motors. A sensor named hall sensor is attached to the stator (mainly) which senses the position of rotor poles and provides a signal to the motor controller to activate the opposite pole in the stator by supplying current to that particular winding. And this process continues in which the rotor is made to chase the stator winding continuously hence giving rotation to the rotor.
Some important features of BLDC motor are:
However, it requires a sensor (hall sensor) and its placement on the stator is a precise work.
BLDC motor is used in Hero Electric Vehicles Pvt. Ltd., TVS iQube, Simple One, Bounce Infinity, Jitendra Electric Vehicle, Komaki Electric Pvt. Ltd, Gemopai electric scooters
And BLDC motor manufacturing companies in India – Spark Motors Pvt. Ltd, GoGoA1.com, Rajamane Industries Pvt. Ltd. A Brushless DC motor is an electric motor that is supplied by a DC power source and commutated electronically, as opposed to ordinary DC motors, which are commutated via brushes. In comparison to types of AC and DC motors,
BLDC motor delivers greater torque,
As a result, due to its traction capabilities, this motor has been the most widely used in EV applications. It has wire-wound poles in the stator and a permanent magnet rotor. Permanent magnets are used to construct the rotor, which may change from two-pole to eight-pole pairs with alternating north and south poles. A uniform flux density is generated in the air gap between the rotor and stator. This allows a consistent DC voltage to be applied to the stator coils. Unlike its counterpart DC brushed motor, it has no maintenance requirements as it doesn’t need commutators or brushes.
It is similar to DC motors with Permanent Magnets. It is called brushless because it does not have the commutator and brush arrangement. The commutation is done electronically in this motor because of this BLDC motors are maintenance free. BLDC motors have traction characteristics like high starting torque, high efficiency around 95-98%, etc. BLDC motors are suitable for high power density design approach. The BLDC motors are the most preferred motors for the electric vehicle application due to its traction characteristics.
BLDC motors further have two types:
i. Out-runner type BLDC Motor:
In this type, the rotor of the motor is present outside and the stator is present inside. It is also called as Hub motors because the wheel is directly connected to the exterior rotor. This type of motors does not require external gear system. In a few cases, the motor itself has inbuilt planetary gears. This motor makes the overall vehicle less bulky as it does not require any gear system. It also eliminates the space required for mounting the motor. There is a restriction on the motor dimensions which limits the power output in the in-runner configuration. This motor is widely preferred by electric cycle manufacturers like Hullikal, Tronx, Spero, light speed bicycles, etc. It is also used by two-wheeler manufacturers like 22 Motors, NDS Eco Motors, etc.
ii. In-runner type BLDC Motor:
The term inrunner refers to an electric motor where the rotor (runner) is inside the stator. The term is in particular used for brushless motors to differentiate them from outrunners that have their rotor outside the stator. The vast majority of electric motors are inrunners.
In this type, the rotor of the motor is present inside and the stator is outside like conventional motors. These motor require an external transmission system to transfer the power to the wheels, because of this the out-runner configuration is little bulky when compared to the in-runner configuration. Many three- wheeler manufacturers like Goenka Electric Motors, Speego Vehicles, Kinetic Green, Volta Automotive use BLDC motors. Low and medium performance scooter manufacturers also use BLDC motors for propulsion.
It is due to these reasons it is widely preferred motor for electric vehicle application. The main drawback is the high cost due to permanent magnets. Overloading the motor beyond a certain limit reduces the life of permanent magnets due to thermal conditions.
Advantages of Inrunner motor
Inrunner motor is more efficient than that of an outrunner motor. Inrunner motor can generate high speed with high Kv value but produce less torque where outrunner generates high torque, but the low speed with low Kv value, because of high winding resistance. Every motor is made of two parts: the stator and the rotor.
Difference in Outrunner and inrunner torque motors
There are two types of direct drive frameless torque motors: the outrunner and the inrunner torque motors. In case of an inrunner motor, the rotor is located on the inside of the stator. In case of the outrunner motor, the rotor is located on the outside of the stator.
Brushless DC motor advantages and disadvantages
Advantages of the brushless DC motor:
Disadvantages of brushless DC Motor:
3. Permanent Magnet Synchronous Motor
The motor that runs at synchronous speed is known as the synchronous motor. The constant speed at which the motor generates the electromotive force is known as synchronous speed. An electromagnet in the rotating magnetic field magnetically locks itself with the rotating magnetic field and rotates simultaneously with the rotating field. This is where the name synchronous motor comes from. This also means that synchronous motors have fixed speeds. The synchronous speed can be calculated using the following formula:
AC Motor Synchronous Speed Formula
Ns = 120 f/ P
where Ns is the synchronous speed, f is the line voltage frequency in Hz, and P is the number of poles.
The synchronous motor works with two electrical inputs provided to it.
The stator is equipped with a 3-phase AC supply, while the rotor is provided with the DC supply.
The stator winding supplied with 3 phase AC supply generates 3 phase rotating magnetic flux. The rotor carrying DC supply produces a constant flux.
At a particular instant, the rotor and the stator poles might be of the same polarity (N-N or S-S), causing a repulsive force and the very next second, it will be N-S causing an attractive force.
Due to this attractive and repulsive force, the motor cannot rotate in any direction and remains in a standstill position.
To overcome this resistance to motion, the rotor is initially fed mechanical input that rotates it in the same direction as the magnetic field. After some time, magnetic locking occurs, and the synchronous motor rotates in synchronism.
Permanent magnet synchronous motors are comparable to brushless DC motors but are powered by a sinusoidal signal to reduce torque ripple. Unlike a brushless DC motor, which has a trapezoidal flux density, a multi-phase stator creates a sinusoidal flux density in the air gap. This motor has features of both a brushless DC motor and an induction motor. The rotor is composed of a permanent magnet, and the stator of the motor is wound. To further enhance its performance, the stator of this motor has been engineered to provide an induction-like sinusoidal flux density. This motor is extremely efficient, exhibits high starting torque and has a better power density than induction motors of equal ratings because the stator power is not allocated to the formation of a magnetic field. But as they are not self-starting, a drive is required to run them. It is also more costly than brushless DC motors. However, permanent magnet synchronous motors are used by the majority of automakers in hybrid and electric cars.
This motor is also similar to BLDC motor which has permanent magnets on the rotor. Similar to BLDC motors these motors also have traction characteristics like high power density and high efficiency. The difference is that PMSM has sinusoidal back EMF whereas BLDC has trapezoidal back EMF. Permanent Magnet Synchronous motors are available for higher power ratings. PMSM is the best choice for high performance applications like cars, buses. Despite the high cost, PMSM is providing stiff competition to induction motors due to increased efficiency than the latter. PMSM is also costlier than BLDC motors. Most of the automotive manufacturers use PMSM motors for their hybrid and electric vehicles. For example, Toyota Prius, Chevrolet Bolt EV, Ford Focus Electric, zero motorcycles S/SR, Nissan Leaf, Hinda Accord, BMW i3, etc use PMSM motor for propulsion.PMSM motor is used by Ather (in Ather 450x), Ampere Electric, Tata Nexon, Mahindra XUV 400, MG Comet, and Tata Tiago.
Major manufacturing companies: Bonfiglioli, ABB India Pvt. Ltd., Sumit Engineering Works, Narula Electricals.
PMSM is the most widely used motor in electric vehicles due to its high power density, high efficiency, and absence of torque ripple resulting in less noise during operation. Because of the absence of torque ripples, it is even used in four wheelers.
Like the BLDC motor, it has a permanent magnet made of neodymium-boron-iron on the rotor and winding on the stator. PMSM is also classified as Surface- mounted and interior PMSM based on the magnet placed on the rotor. Interior PMSM is more robust and used in high speed applications.
Working principle: PMSM motor is runned by AC source of power not DC like BLDC motor. When three-phase AC power is given to the stator, a rotating magnetic field is produced by the stator. The constant magnetic field produced by the permanent magnet is synchronized with the rotating stator magnet.
4. Switched reluctance motors
It does not require any permanent magnet on its rotor making it cost effective as compared to the motor containing permanent magnet on its rotor. The absence of winding on rotor reduces the copper losses making SRM more efficient than induction motor. The rotor is a simple steel core with salient poles and does not contain any electrical windings.
Working principle: It works on the principle of magnetic reluctance. It means that the magnetic flux chooses the path which has least magnetic reluctance. The magnetic flux from the stator takes this least magnetic reluctance path. In this way poles are created in rotors which get interlock with the stator rotating magnetic field. But due the inertia of rotor and hysteresis effects the SRM is not self starting. So, a special electronic circuit is designed to make SRM self-starting. This motor runs at synchronous speed making its use in EVs possible for high speed.
Reluctance motors having double saliency are known as switched reluctance motors (SRMs). The construction of these motors is straightforward, and the motors themselves are strong. The Switched reluctance motor's rotor is made up of laminated steel without any windings or permanent magnets. This reduces the rotor's moment of inertia, which aids in fast acceleration. The sturdy quality of SRM makes it suited for the application requiring high speed. SRM also provides great power density, which is required for EVs. It is also simple to cool them since the majority of the heat produced is contained within the stator. The most significant downside of the SRM is the need for complicated control systems. It is also a little noisy. However, in the future, SRM will be able to replace permanent magnet synchronous motors and induction motors.
Switched Reluctance Motors is a category of variable reluctance motor with double saliency. Switched Reluctance motors are simple in construction and robust. The rotor of the SRM is a piece of laminated steel with no windings or permanent magnets on it. This makes the inertia of the rotor less which helps in high acceleration. The robust nature of SRM makes it suitable for the high speed application. SRM also offers high power density which are some required characteristics of Electric Vehicles. Since the heat generated is mostly confined to the stator, it is easier to cool the motor. The biggest drawback of the SRM is the complexity in control and increase in the switching circuit. It also has some noise issues. Once SRM enters the commercial market, it can replace the PMSM and Induction motors in the future
Top OEMs using SRM in EVs- Tesla, Range Rover EV,
Major manufacturing company of SRM Motors: ABB Electric
5. Three Phase AC Induction Motors
Three-phase Induction Motor Construction and
Principle of Operation
1. Introduction:
The induction motors do not have a high starting toque like DC series motors under fixed voltage and fixed frequency operation. But this characteristic can be altered by using various control techniques like FOC or v/f methods. By using these control methods, the maximum torque is made available at the starting of the motor which is suitable for traction application. Squirrel cage induction motors have a long life due to less maintenance. Induction motors can be designed up to an efficiency of 92-95%. The drawback of an induction motor is that it requires complex inverter circuit and control of the motor is difficult.
In permanent magnet motors, the magnets contribute to the flux density B. Therefore, adjusting the value of B in induction motors is easy when compared to permanent magnet motors. It is because in Induction motors the value of B can be adjusted by varying the voltage and frequency (V/f) based on torque requirements. This helps in reducing the losses which in turn improves the efficiency.
Tesla Model S is the best example to prove the high performance capability of induction motors compared to its counterparts. By opting for induction motors, Tesla might have wanted to eliminate the dependency on permanent magnets. Even Mahindra Reva e2o uses a three phase induction motor for its propulsion. Major automotive manufacturers like TATA motors have planned to use Induction motors in their cars and buses. The two-wheeler manufacturer TVS motors will be launching an electric scooter which uses induction motor for its propulsion. Induction motors are the preferred choice for performance oriented electric vehicles due to its cheap cost. The other advantage is that it can withstand rugged environmental conditions. Due to these advantages, the Indian railways has started replacing its DC motors with AC induction motors.
In this type of motor, stator excitation with sinusoidal AC current creates a rotating field that generates current flow in the rotor, which then drives the generation of a magnetic field relative to the rotor. Because of the different frequencies of the rotor and the stator's magnetic fields, torque is generated. Induction motors lack brushes and commutators, are less expensive and need little maintenance. These characteristics make the induction motor an appealing option for EVs. However, the necessity to convert the power source from AC to DC necessitates additional circuitry and sophisticated control techniques.
Three-phase Induction Motor Construction and
Principle of Operation
1. Introduction:
Induction motor (Also called asynchronous motor) is an A.C. motor. The
motor line current flows into the stator windings to set up a flux called
the main flux or the stator flux, which passes through the air gap to be
cut by the conductors of the rotor windings. Consequently, an
electromotive force to be induced in the rotor windings and produces
currents flow in the rotor windings and producing flux called the rotor
flux. The interact between the two fluxes (stator and rotor fluxes)
producing rotation of the rotating part of the motor (rotor). The rotor
receives electrical power in the same way as the secondary winding of
the electrical transformer receiving its power from the primary winding
by means of the electrical induction. That is why an induction motor can
be called as a rotating transformer i.e., in which primary winding is
stationary but the secondary is free to rotate.
Induction Motor
Induction motors are the most commonly used motors. Induction motors are also known as asynchronous motors because they always run slower than synchronous speed.
Based on the type of rotor construction, they are divided into two types as follows:
Squirrel Cage Motor
Slip Ring Motor (Wound Rotor)
Squirrel cage type is more common compared to the wound rotor type due to:
a. Robust, as no brushes, no contacts on the rotor shaft.
b. Simple in construction and easy to manufacture.
c. Almost maintenance-free, except for bearing and other mechanical parts.
d. High efficiency as rotor has very low resistance and thus low copper loss.
Working Principle of Induction Motors
In an induction motor, the stator winding is fed with an AC supply. This causes the stator winding to develop an alternating flux. We call this rotating flux “Rotating Magnetic Field (RMF).”
According to Faraday’s Law of Electromagnetic Induction, the relative speed between the stator RMF and the rotor RMF causes an induced emf in the rotor conductors. Rotor conductors are short-circuited, and a rotor current is produced due to induced emf.
This induced current produces alternating flux around it. It should be noted that the stator flux lags behind the rotor flux.
Due to the relative velocity between the rotating stator flux and the rotor, the rotor rotates in the same direction as that of the stator flux to minimize the relative velocity. This is the basic working principle of the induction motor.
The difference between the synchronous speed (Ns) and the actual speed (N) of the rotor is known as the slip.
Slip = Ns - N/Ns and slip % = (Ns - N)/ Ns×100
I. Squirrel cage induction motor:
Rotors is very simple and consist of bars of aluminum (or copper)
with shorting rings at the ends.
Squirrel cage type is more common compared to the wound rotor type due to:
a. Robust, as no brushes, no contacts on the rotor shaft.
b. Simple in construction and easy to manufacture.
c. Almost maintenance-free, except for bearing and other mechanical
parts.
d. High efficiency as rotor has very low resistance and thus low copper loss.
II. Wound rotor induction motor:
Rotor consists of three phase windings (star connected) with
terminals brought out to slip rings for external connections.
3. Construction:
There are two main types of components which are used in induction
motor manufacturing as follows:
a) Active components: which are classified into two categories:
i. Magnetic materials (0.5 mm electrical steel).
ii. Electrical materials (copper wires, insulations, bars,
end rings, slip rings, brushes, and lead wires).
b) Constructional components: like frame, end shields, shaft, bearings, and
fan. These components are shown in figure 1.
3.1 Stator construction:
The stator is made up of several thin laminations (0.5 mm) of electrical
steel (silicon steel), they are punched and clamped together to form a
hollow cylinder (stator core) with slots, as shown in Figure 2.
Coils of insulated wires are inserted into these slots. Each group of coils,
together with the core that it surrounds, forms an electromagnet, forms
an electromagnet (a pair of poles). The number of poles of an induction
motor depends on the internal connection of the stator windings.
3.2 Rotor construction:
The squirrel cage rotor is made up of several thin electrical steel
lamination (0.5mm) with evenly spaced bars , which are made up of
aluminum or copper , along the periphery .In the most popular type of
rotor (squirrel cage rotor), +these bars are connected at ends
mechanically and electrically by the use of end rings as in Figure 3 (A).
Almost 90 % of induction motors have squirrel cage rotors. The rotor
slots are not exactly parallel to the shaft. Instead, they are given a skew
for two main reasons, firstly to make the motor run quietly by reducing
magnetic hum and to decrease slot harmonics, secondly to help
reducing the locking tendency of the rotor (the rotor teeth tend to
remain locked under the stator teeth due to direct magnetic attraction
between the two). The rotor is mounted on the shaft using bearings on
both ends.
The wound rotor has a set of windings on the rotor slots which are not
short circuited, but they are terminated to a set of slip rings. These are
helpful in adding external resistors and contactors, as in Figure 3 (B).
The typical squirrel cage rotor circuit is shown in figure 4 (A),
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while the typical wound rotor circuit with an external rotor resistor circuit is
shown in figure 4 (B).
4. Typical name plate of induction motor:
A typical name plate of induction motor is shown in Figure 5,
All the information of the above table is according to the motor standards:
NEMA: National Electrical Manufactures Association
IEC: International Electrotechnical Commission
5. Motor insulation class:
Insulation Classes for Electric Motors
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NEMA motor insulation classes describes the ability of motor insulation in the windings to handle heat. There are four insulation classes in use namely: A, B, F, and H. All four classes identify the allowable temperature rise from an ambient temperature of 40° C (104° F). Classes B and F are the most common in many applications.
Temperature rises in the motor windings as soon as the AC motor is started. As shown in the table below, the combination of ambient temperature and allowed temperature rise equals the maximum rated winding temperature. Allowable temperature rise is made up of the maximum temperature rise for each insulation class plus a hot-spot over-temperature allowance. If the motor is operated at a higher winding temperature, service life will be reduced. As a rule, a 10° C increase in the operating temperature above the allowed maximum can cut the motor’s insulation life expectancy in half.
The table below shows the different insulation classes as defined by NEMA:Hot-spot Over Temperature Allowance
Each insulation class has a margin allowed to compensate for the motor’s hot spot. The hot-spot is a point at the center of the motor’s windings where the temperature is higher. As can be seen from the above table, hot-spot over temperature allowance for A, B, F and H are respectively 5°C, 10°C, 10°C and 15°C
When replacing a motor, extreme care must be taken not to choose a motor with the wrong insulation class. It is advisable therefore to replace a motor with one having an equal or higher insulation class. Replacement with one of lower temperature rating could result in premature failure of the motor.
Insulations have been standardized and graded by their resistance to thermal
aging and failure. Four insulation classes are in common use, they have been
designated by the letters A, B, F, and H .The temperature capabilities of these
classes are separated from each other by 25 °C increments. The temperature
capabilities of each insulation class are defined as being the maximum
temperature at which the insulation can be operated to yield an average life
of 20,000 hours, as in Table 2 below.
Table 2 Motor Insulation Classes
Insulation Class Temperature Rating
A 105° C
B 130° C
F 155° C
H 180° C
6. Induction Motor Protection System
The circuit diagram of the three-phase induction motors is shown in the figure below. Magnetic contactor starter essentially consists of a set of start and stop push buttons with associated contacts, overload and under load protective devices. The start push button is a momentary contact switch that is held normally open by a spring. The stop push button is held normally closed by a spring.
Types of Protections Needed for Induction Motor
Three-phase induction motors are accountable for 85 percent of the installed capacity of the industrial driving systems. Therefore, the protection of these motors is necessary for the reliable operation of loads. Motor failures are mainly divided into three groups: electrical, mechanical, and environmental. Mechanical stresses cause overheating resulting in the rotor bearings’ wear and tear, whereas the over mechanical load causes heavy currents to draw, and thus results in increasing temperatures. Electrical failures are caused by various faults like Phase-to-phase and phase-to-ground faults, single phasing, over and under-voltage, voltage and current unbalance, under frequency, etc.
In addition to the motor protection systems for the above-mentioned faults, it is also necessary to use a three-phase motor starter to limit the staring current of the induction motor. As we know – in every electrical machine, when supply is provided, there is opposition to this supply by an induced EMF – which is called back EMF. This limits the current drawing by the machine, but at the beginning, the EMF is zero because it is directly proportional to the speed of the motor. And therefore, the zero back EMF’s huge current will be drawn by the motor at the start, and this will be 8-12 times the full-load current as shown in the figure.
To protect the motor from the high-staring current, there are different staring methods available like the reduced voltage, rotor resistance, DOL, star-delta starter, autotransformer, soft starter, etc. And, for protecting the motor from the above-discussed faults; various protection equipment like relays, circuit breakers, contactors and various drives are implemented. These are some of the protection systems for three-phase induction motors against starting inrush currents, overheating, and single phasing faults with the use of a microcontroller for low-level applications for better understanding.
Electronic Soft Start for 3-Phase Induction Motor
This soft start of induction motor is the modern method of starting that reduces the mechanical and electrical stresses caused in the DOL and star-delta starters. This limits the starting current to the induction motor by using thyristors.
This 3-phase motor starter consists of two major units: one is the power unit and the other control unit. The power unit consists of back to back SCRs for each phase, and these are controlled by the logic implemented in the control circuit. This control unit consists of a zero voltage crossing circuit with capacitors for producing delay time. In the above block diagram, when a three-phase supply is given to the system, the control circuit rectifies each phase supply, regulates it, and compares for zero-crossing voltage by the operational amplifier. This Op-Amp output drives the transistor, which is responsible for producing time delay with the use of a capacitor. This capacitor discharging enables another Op-Amp output for a certain time so that Opto-isolators are driven for this elapsed time. During this time, the opto-isolator output triggers back-to-back thyristors; and, the output applied to the motor is reduced during this time. After this starting time, a full voltage is applied to the induction motor, and hence, the motor runs at full speed. In this way, zero voltage triggering for a certain time period at the starting of an induction motor deliberately reduces the starting inrush current of the induction motor.
Induction Motor Protection System
This system protects the 3 phase AC motor from single phasing and overheating. When any of the phases is out, then this system recognizes it and immediately turns off the motor, which is powered by the mains.
All three phases are rectified, filtered, and regulated and given to an operational amplifier where this supply voltage is compared with a certain voltage. If any of the phases is missed, then it gives zero voltage at the Op-amp input, and therefore, it gives low logic to the transistor which further de-energizes the relay. Hence, the main relay gets turned off and the power to the motor is interrupted.
Similarly, when the temperature of the motor exceeds a certain limit, the operational amplifier output de-energizes the appropriate relay; even then also the main relay gets turned off. In this way, the single phasing faults and over-temperature conditions can be overcome in the induction motor.
This is all about three-phase motor protection systems against starting inrush currents, single phasing, and overheating.
7. Principle of Operation:
In order to clarify the principles of operations, consider a portion of 3-phase
induction motor as shown in Figure.
The operation of the motor can be explained below.
I. When 3-phase stator winding is energized from a 3-phase supply,
a rotating magnetic field is set up which rotates round the stator
at synchronous speed Ns (= 120 f/P).
II. The rotating field passes through the air gap and cuts the rotor
conductors, which still in stationary condition. Due to the relative
speed between the rotating flux and the stationary rotor, an
electromotive force (e.m.f.) is induced in the rotor conductors.
Since the rotor circuit is short-circuited, currents start flowing in
the rotor conductors.
III. The current-carrying rotor conductors are placed in the magnetic
field produced by the stator. Consequently, mechanical force acts
on the rotor conductors. The sum of the mechanical forces on all
the rotor conductors produces a torque which tends to move the
rotor in the same direction as the rotating magnetic field.
IV. The fact that rotor is urged to follow the stator field (i.e., rotor
moves in the direction of stator field) can be explained by Lenz’s
law. According to this law, the direction of rotor currents will be
such that they tend to oppose the cause producing them.
V. Now, the cause producing the rotor currents is the relative speed
between the rotating magnetic field and the stationary rotor
conductors. Hence to reduce this relative speed, the rotor starts
running in the same direction as that of stator field and tries to
catch its speed.
We have seen above that rotor rapidly accelerates in the direction
of rotating field. In practice, the rotor can never reach the speed
of stator flux. If it did, there would be no relative speed between
the stator field and rotor conductors, no induced rotor currents
and, therefore, no torque to drive the rotor. The friction and
windage would immediately cause the rotor to slow down. Hence,
the rotor speed (Nr) is always less than the suitor field speed (Ns).
This difference in speed depends upon load on the motor.
VII. The difference between the synchronous speed (Ns) of the
rotating stator field and the actual rotor speed (Nr) is called slip. It
is usually expressed as a percentage of synchronous speed i.e.,
Slip = S = Ns − Nr /Ns× 100 %
Where, the quantity (Ns – Nr) is called slip speed, and the slip at
the stationary situation is unity or 100%.
VIII. The frequency of a voltage or current induced due to the relative
speed between a vending and a magnetic field is given by the
general formula:
Frequency of the rotor circuit = Fr = N P/ 120
Where, N is the slip speed (Ns - Nr) , and by the substitution of
the slip speed and the slip in the above equation we will get:
Fr = S F
Where, S is the slip, F is the supply frequency
IX. When the rotor is at standstill or stationery (i.e., s = 1), the
frequency of rotor current is the same as that of supply frequency.
As the rotor picks up speed, the relative speed between the
rotating flux and the rotor decreases. Consequently, the slip s and
hence rotor current frequency decreases.
8. Rotating Magnetic Field (RMF):
Three phase induction motor have a symmetric three phase stator windings
displayed 120 degree in space, so each winding sets up a field that varies
sinusoidally around the circumference of the air gap and varies sinusoidally
with time. These fields are displayed from one another by 120 degree in
both time and space. The flux density from phase A (for example) is maximum
in certain point and drops sinusoidally to zero, ninety degree away from this
point. So, the stator field can be visualized as a set of north and south poles
rotating around the circumference of the stator as shown in the figure 7 below.
For a phase sequence ABC , the phase the magnetic motive force (m.m.f.) as
functions of time are as follows:
F a = F max cos wt (1)
F b = F max cos ( wt - 120°) (2)
Fc = F max cos ( wt +120°) (3)
Where ( F max) is the maximum ( m.m.f.) of any one phase. The resultant stator
(m.m.f.) is ( Fs ) along an axis at an angle of ( β ) to the horizontal is found by
summing up the projections of the three-phase (m.m.f's)along this line:
That means the magnitude of the resultant rotating magnetic field (RMF) is
(1.5 times) the field produced by any one phase.
2. The use of induction motors in electric vehicles
2.1 Dynamic equation of the EVs with IMs
The three-phase AC supply is connected to the stator winding for the purpose of establishing the rotating magnetic field. This rotating magnetic field interacts with the constant rotor conductors, and the induced current flows through the rotor conductors. The induced current creates its own magnetic field. The interaction between the rotating magnetic field and the field due to induced currents leads to unidirectional torque that provides EVs movement.
Since the movement in terrestrial EVs will be one-way, one-dimensional lateral dynamics are taken into account in the calculations. There are five forces: inertial force, longitudinal traction force, air drag, tire rolling resistance, and gravity force responsible for the movement of an EV going up or down an inclined road. In Figure 2, the mathematical expression of the traction force of the EV vehicle in relation to other forces is given. In this figure, Fg = mv.g is the gravity force. In this equation, mv and g refer to the mass including passenger loads and acceleration of gravity, respectively.
In Eq.(1), the mathematical expression of the traction force of the EVs in relation to other forces is given. V is the vehicle speed on the x axis, and α is the angle of inclination of the road . The main source of traction force is the electric motor, which rotates the clutch shaft with a T torque .
Fx=mv.d/dt.Vx+Faero+Froll+mv.g.sinα Eq1
α in Eq.(1) is defined as the percentile degree of the horizontal (d) and vertical (Δh) lengths of the slope of the road.
%grade=Δh/d.100 Eq2
α=arctan*%grade/100 Eq3
The aerodynamic force, Faero, generated by a wind at the speed of Vwind on the moving vehicle is calculated by using Eq.(4). In this equation, ρ, Cd, and AF refer to the mass density of air, aerodynamic friction coefficient, and front surface area of the vehicle, respectively. In standard cases, the air mass density is usually equal to 1225 kg/m3. The aerodynamic drag coefficient, Cd, is a constant ranging between 0.2 and 0.4 in the literature. However, the value of Cd is approximately 0.3 for passenger vehicles and 0.4 for sports vehicles [9]
Faero=ρCdAF2(Vx+Vwind)2 Eq4
The rolling resistance varies depending on the rotational dynamics of the wheel and the relationship between the road and the surface area. The total rolling resistance is calculated by using Eq.(5). The rolling coefficient of a radial wheel typically takes a value between 0.009 and 0.015 .
Froll=fr.mv.g.cosα Eq5
2.2 Performance evaluation of EVs with IMs
2.2.1 Power-to-weight ratio (P/W)
In electric motors, Power-to-Weight Ratio (P/W) is the expression of the ratio of the output power of the motor to the weight of the motor, as seen in the Eq.(6).
P/W=P[kW]/W[kg] Eq6
The high P/W ratio of the motors used in electric vehicles increases vehicle performance by providing higher power density. Just as different types of motors with the same power, voltage and speed values have different P/W values, the P/W ratio may vary in motors with the same type and label values produced by different companies.
Figure 3 shows the P/W ratio of different types of electric motors comparatively. Compared with other types of motors, it is concluded that a good vehicle performance can be achieved in EV vehicles with the P/W ratio that IM has above average.
2.2.2 Speed-torque characteristics of EVs with IMs
The torque-speed characteristic is a deterministic criterion in the selection of the electric motor with the most suitable performance to be used in a particular application. An ideal EV is expected to have the torque-speed characteristic given in Figure 4.
The ideal electric vehicle motor torque-speed curve consists of two different operating zones: a constant power zone and a fixed torque zone. This is due to the fact that the torque-speed changes are different according to the driving demands of electric vehicles. In order to provide the necessary torque in EV uses, where continuous stop/start, that is, speed needs to be constantly increased/decreased in heavy urban traffic, the motor must operate in the constant torque zone. In order to meet the demand for high speed in out-of-town use, the motor must operate in the constant power zone in areas that are not often stop-and-go areas that are expected to work.
Although DC motors provide high starting torque, the speed decreases as the torque increases. Therefore, these motors are suitable for urban EV use where there is no expectation of high speed. Similarly, it is suitable for constant speed applications in synchronous motors that are operated at synchronous speeds that vary depending on the load. Since there is no winding or permanent magnet in the rotors of the switched reluctance motors, it is possible to reach very high speeds in electric vehicles where such motors are used. However, torque fluctuation, which is one of the most important disadvantages of ARM motors, causes undesired speed fluctuations at high speeds and errors in motion control at low speeds .
IMs have a torque-speed characteristic very close to the mechanical characteristics expected from EVs. When the motor is loaded up to the breakdown torque, high speeds can be reached because the speed will increase as the torque increases linearly. The breakdown torque limits the value of the maximum speed or the constant power zone.
An ideal electric motor with EVs must have high efficiency, strong overload capacity, and a wide speed range in all modes of operation. The mechanical characteristics of Induction Motors can provide stable operation in both the constant power and constant speed zones expected from EVs, which has made it stand out as the optimal type of motor .
2.2.3 Efficiency of EVs with Induction Motors
Efficiency in electric motors is a magnitude that describes the amount the electrical energy at the input of the motor conversion into mechanical energy at the output. A high-efficiency motor means that it provides maximum mechanical power with less loss.
η=Pout/Pin*100 Eq7
The electric motors used in EVs operate under different loading conditions. Therefore, when choosing an electric motor in EVs, the efficiency values of the motor should be taken into account under all load conditions from no-load to over load operation.
In Table 1, the efficiency values of different types of electric motors are presented comparatively. According to the relevant table, it has been concluded that asynchronous motors will exhibit an efficient working performance under different loading conditions from minimum load to maximum load in the use of electric vehicles.
Types of motor Under maximum load Under minimum load
DCM 85–90 80–85
BLDC >95 70–80
IM >90 >90
PM >92 80–85
SRM <95 >90
Table 1. Comparison of the efficiency of different types of electric motors.
2.2.4 Cost of EVs with IMs
EV manufacturers must choose a low-cost motor and controller if they want to produce low-cost EVs. IMs stand out in the market as low-cost and low-maintenance motors. Figure 5 shows the cost comparison of different types of motors and controllers. The data in the table confirm that IM is the optimal choice for manufacturers’ choice of low-cost motors and controllers.
Advantages of Induction Motor
Disadvantages of Induction Motor
Both AC and DC motors cater to the same purpose of powering up electric cars and contributing to a healthier planet. But there’re some key advantages of using the former over the latter, which is perhaps the key reason why major electric car manufacturers prefer AC electric car motors. Let’s have quick look at them.
Many car buyers often overlook the importance of permanent magnet motors and induction motors, but these electric motors are capable of enhancing the performance of an electric car as a whole.
Some leading electric car manufacturers using AC and DC motors in their models include Tesla, Nissan, and Chevrolet, among others.
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bahriyedeteknoloji.com - Retired from Turkish Naval Forces
4mowonderfull article covering every concievable aspect of the subject..congradulations and many thanks !.. one byproduct question comes to my mind ; how the halftwist and fulltwist throttles effect the operation of these motors?in other words , which is better for motor speed and torque control realy ?
BMW Master Technician & Customer Service Technical Support AGMC Dubai // Sharjah
10moThanks for sharing this information ℹ️🙏
An Executive, with 2 Decades of experience in After-sales | Technical trainings | Technical Support |Customer Trainings | MNC Certified Trainer - competence and Skill development | Driver Training | Skill assessments.
1yExcellent Information, to understand the basic concepts
Student at Yuvarajas College Mysore
1yWell said sir
Student at Yuvarajas College Mysore
1yNice