CHARACTERISTICS OF ELECTRICAL MACHINES USED IN … · 2020. 6. 10. · A DC machine converts rotary...

e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science

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CHARACTERISTICS OF ELECTRICAL MACHINES USED IN ELECTRIC VEHICLES

Peram Chandra Sekhar Reddy*1

, Gopathi Nikhileswari*2

, Dasari Jyoshna*3

,

Ravi chaitanya Narisetti*4

*1,2,3Master student, Renewable Energy and Electro Mobility, Dept of Electrical Engineering and

Computer Sciences, Hochschule Stralsund, Stralsund, Mecklenburg vorpommern, Germany.

*4Master student, Dept of Mechanical Engineering , Hochschule Stralsund, Stralsund, Mecklenburg

vorpommern, Germany.

ABSTRACT

The mobility in the automotive industry is getting converted from the fossil fuel powered mobility into the

electrical energy powered mobility. The major benefit by this change include the reduction in emission of

harmful environmental substances into atmosphere. By adopting this change, a clean and environmentally

friendly mobility is achieved. In this way of conversion from internal combustion engines to the electrically

powered machines, a lot of challenges have to be faced. The two main parts in the electric mobility are the

electrical machines and the battery pack used in E-Vehicles. The electrical machines are considered as the heart

of the Electric vehicle on contrary to the electric machines used in the industrial and commercial purpose. The

Electric machines used in the electric vehicles are bit different and has to meet more specifications that are

needed by the mobility sector. The requirements and specifications for the electrical machines used in the

electrical vehicles are far more robust than the electrical machines used in the other areas. This paper mainly

deals with the requirements for an electrical machine used for the electric vehicles and compares the

specification of both AC machines and DC machines by means of their performance, cost aspect and many

more. It also gives the practical example of electrical machines that are suitable for E-Vehicles in the mobility

sector.

KEYWORDS: Electric mobility, E –Vehicles, Electrical Machines, Internal Combustion Engines.

I. INTRODUCTION

The main important part of the electrical vehicle is the electrical machine which converts the electrical energy

stored in the battery pack to mechanical energy i.e., Electric Motor and the mechanical energy in the form of

kinetic energy from the wheels into electrical energy and stores into battery pack i.e., Electric Generator. In the

electric vehicle, the motor operation is carried out in the normal operation and the generation operation is

carried out in regenerative braking operation of the vehicle. Hence the electric machines which acts as both

generator and motor are requires by the electric vehicle. In the principle every electric motor can be used as a

generator with the suitable circuit connections, the only difference it makes in the motor is, the electrical energy

make the rotor to rotate in the generator mode, the external force make it to rotate, thus giving back the

electrical energy which is stored in the battery. So instead of calling it as a motor or a generator we call it as

Electrical machine, as it performsboth the operations. The specific requirements of the electrical machines in the

electric vehicle are given below.

1.1) Speed control:

The important aspect of the electric machines is efficient speed control. Electric vehicles require frequent

changing of speed at several driving conditions. Hence more efficient speed control is preferred which is

fulfilled by controller drivers.

1.2) Acceleration:

The rate of velocity with respect to time should be more for an ideal electrical machine. For instance, the

electrical vehicle changes its speed from 0 KMPH to 100 KMPH in 8 seconds.




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Hence the acceleration can be calculated by

Acceleration =

v-u = 100 Kmph =

=

= 3.47 m/sec

2

The more the acceleration of E-vehicle, the more it will be desirable for the consumer. Since everyone likes

their vehicle to be faster.

1.3) Maintenance approach:

The electrical machines used in the electrical vehicles must be of less maintenance and it is preferable. It should

be robust and not be sensitive for little changes in the rating like rated current or voltage. i.e., the tolerant limit

must be more for the machine.

1.4) Noise level:

It is desired to have low noise during the operation of machine. It is evident that noise levels of electric vehicles

are 4-5 dB less compared to internal combustion engines at low speed i.e., (0-30) Kmph and the noise level

further reduces to 2-3 dB than internal combustion engines at higher speed.

1.5) Reliability and Robustness:

Electrical machine for E-Vehicle should be reliable in terms of acceleration, weight of machine, power to weigh

ratio, cost etc., and it should be robust in withstanding the load variation, frequent speed variations, frequent

start and stop of the vehicle, different atmospheric conditions like higher temperatures, freezing temperatures,

humid climate etc.,

1.6) Torque-Speed characteristics:

Torque of an E-Vehicle is the product of the force produced by electric machine and the diameter of the wheel

and speed is the distance covered by vehicle in unit time. The ideal Torque-Speed characteristics is high torque

at low speed for fast acceleration (at places like hill climbing, rough terrain etc., ) and low torque at high speeds

for normal operation of E-Vehicle.

1.7) Power to weight ratio:

It is defined as the ratio of power required by the electrical machine to the weight of the machine. It is preferred

to have high power to weigh ratio. i.e., high power at small weight can be obtained. Lower the weight of the

machine, lower will be the weight of the vehicle, in turn results several advantages.

Electric vehicle

operation

Normal operation

Hill climbing, rough

terrain surfaces

Low torque at high

speeds

High torque at low speeds




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1.8) Electrical Machine:

The machine which act as a motor when supplied with power and as generator when fed to mechanical (kinetic

energy) energy during the operation. If a machine act as a generator during the regenerative braking, it feed the

energy back to the battery pack thus extending the range of vehicle per charge. Thus the machine which act as

motor as well as generator is suitable for E-Vehicles.

1.9) Machine controllers:

The controllers used for the electric machine must be capable of controlling different torque and speed value as

desired by the driver and should be efficient in handling the variations in power rating. It is desired to have a

machines which has inbuilt controllers thus results in cost reduction which in turn beneficial to customer

purchases.

1.10) Cost approach:

In general the cost of E-Vehicle when compared to internal combustion engine vehicle is more. The two parts

which contribute to higher cost is battery pack and the electrical machine. As this paper dealswith electrical

machines. It is desirable to use the low cost electric machines which results in the reduction of final cost.

1.11) Heating Solutions:

The major drawback of the electrical machine is the heat generation during the operation. This generated heat

cause the loss in armature of the motor. If the heat generated in the electrical machine sis more than the

permissible limit it results in the machinery damage which cannot be replaced. Hence the motor with proper

heating controlling mechanism is to be selected.

The common heating solution to dissipate the heat in the electrical machines are

i) Conductive cooling

ii) Forced air cooling

iii) Liquid cooling

However, the type of cooling adopted entirely depends on the motor type and the amount of heat generated.

1.11.1) Conductive cooling:

In this method the heat generated will be dissipated by means of conduction. i.e., the machine consists of heat

sink which dissipated the heat generated.

1.11.2) Forced air cooling:

In this method the generated heat will be dissipated through externally provided electric fan to blow the air over

the motor which in turn requires a small motor which is not suggested in moving applications like electrical

vehicles.

1.11.3) Liquid cooling:

This method is similar to radiator system un the automobile engine which circulates the liquid cooling oil like

ethylene glycol or other cooling liquids around the engine. In the same way the cooling liquid is circulated in the

machine to dissipate the heat generated in the electrical machine.

1.12) Efficiency:

For an electrical machine, it is defines as the ratio of mechanical output energy to the electrical input energy in

case of motor operation and as ratio of electrical output energy to the mechanical input energy in case of

generator operation. Here the input machinal energy is taken from the kinetic energy acquired by the wheels of

an electric vehicle.




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II. MACHINES SUITABLE FOR ELECTRIC VEHICLE

In order to understand the importance of electrical machine in electrical vehicle, first we should have the basic

knowledge about the process happening in the vehicle. The figure below clearly depicts the brief process of

what is happening in an electrical vehicle.

In battery powered electrical vehicles, the energy is stored in the battery pack. This power is converted into the

suitable form for supplying to the electric motor by power converters and fed to the motor controller which

controls the electrical machine. This electrical machine power the electrical vehicle with the mechanical energy

by converting it from the electrical energy. This paper deals with the various types of electrical machines that

are suitable for electrical vehicles.

Generally, electrical machines are divided into two types based on their power supply

i) AC electrical machines

ii) DC electrical machines

Depending on the customer requirements both machines are used in electrical vehicles and they have their own

advantages and dis-advantages.

DC electrical machine:

A DC machine converts rotary mechanical energy to electrical energy as a generator and as a motor converts

electrical energy to mechanical energy.

Advantages of DC electrical machines:

1) They provide with simple speed control and ideal torque-speed requirements

2) The power inversion is not required unlike the AC electrical machine in which power has to be inverted

from DC energy stored in the battery to AC Power.

3) The cost of machines are relatively cheaper than the AC electrical machines.

4) Higher starting torque can be obtained by using DC series electrical machine.

Dis-advantages of DC electrical machines:

1) Efficient speed control is not possible with frequent start and stop of the vehicle

2) Commentator and brushes are present in DC electrical machines, which requires periodic maintenance

3) Level of efficiency and power density will be low




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AC electrical machine:

AC machines are motors that convert AC electric energy to mechanical energy and generators that convert

mechanical energy to AC electric energy.

Advantages of AC electrical machines:

1) The efficient speed control is possible by frequency variation which is absent in DC machines

2) Construction is simple compared to DC machines

3) It can with stand to frequent load variations and speed variations

4) The required speed-torque characteristics can be obtained more efficiently.

Dis-advantages of AC electrical machines:

1) The main disadvantage by using AC electrical machine is its cost, they are costlier than DC electrical

machines

2) They have low power density when compared to DC electrical machines

With the above brief knowledge of the pros and cons of the AC and DC electrical machines, we can know the

different type of electrical machines used in electric vehicles.

The above mentioned are the types of electrical machines used in electrical vehicle operation. In this paper the

two categories of the machines are explained with respect to the requirements of an electrical vehicle mentioned

in this paper in section 1.

III. DC MACHINES USED IN E-VEHICLE

The Direct current electrical machines that are used in the electrical vehicles can be broadly classified as

brushed and brushless DC motors and a stepper motor. These motors can also act as generators. Hence they are

called as DC electrical machines. The main difference between the brushed and brushless DC machines are the

use of brushes. Brushes are the part of a DC machine through which the rotor of a machine is supplied with

electricity. Since they are in contact with the rotating shaft they require maintenance and wearable as they are

made up of carbon material.




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All the specifications of an electrical machines are same for brushed and brushless DC machines except the

usage of brushes in contact with the slip ring of the rotor shaft. In this section we shall discuss the requirements

of an electrical machine to be used as per the requirements mentioned in section 2.

3.1) Brushless DC electrical machine:

For this BLDC machines the different requirements for the usage in electrical vehicles are discussed below

3.1.1) Speed control:

The speed control of electrical machine is to be efficient in order to withstand the frequent stops and starts and

as well as the frequent load variations.

The speed of a DC-motor whether it may be brushed or brushless DC motor, series or shunt motor is given by

Back emf Eb =

Where

Eb= V-IARA

IA Armature current

Ra Armature resistance

N=

x Eb;

N

N α Eb

N α V

The different speed control methods for the Dc motor are given as follows

1) Flux control method

2) Armature and Rheostatic control method

3) Voltage control method

Flux control method:

As we know the speed of a machines is inversely proportional to the flux per pole. As a result of this, by

decreasing the flux, the speed can be increased and vice versa

The flux can be controlled by adding the Rheostat in series with the field winding, as a result the flux will be

reduced and speed gets increased by adjusting the rheostat.

Armature and Rheostat control method:

As we know the speed of electrical machine is directly proportional to the back emf. Thus by changing the back

emf speed can be controlled.

Back emf Eb is given by

Eb= V-IaRa

When the supply voltage(V) and armature resistance(Ra) are kept constant, armature current can be varied, as a

result back emf can be varied which in turn changes the speed.




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Voltage control method:

As we know the speed of electrical machine is directly proportional to the supply voltage, which can be varied

by the machine controller as a result the machine speed will be varied.

These speed controllers are same for all type of Dc machines, the most suitable speed control for an electric

machine in electric vehicle which requires rapid speed control method is flux control method and voltage

control method.

Flux control method is the most commonly used speed control method of an electrical machine in electrical

vehicle.

3.1.2) Acceleration:

The acceleration the rate of change of velocity per time and the velocity is the rate of change of speed per time

and the speed is the distance travelled in a unit time. So thus by calculating the speed we can calculate the

acceleration.

The motor speed calculations can be done by the following ways

N= (Motor speed at no load) – (Motor speed when load is applied)

= NnL – NL

NL = M (

)

Where ΔN = Change in speed

ΔM = Change in Torque

M = Torque constant

=

(

)

Where Km= Motor constant

KM=

Where KM = Torque constant

Therefore, the velocity is given by V =

m/ sec

2

The acceleration required by a motor to be used in electrical vehicle is 27.78 m/sec2, however it depends on the

manufacturer and the customer needs.

3.1.3) Maintenance approach:

If the DC brushed motors are used in electrical vehicles, it requires high maintenance because of the wearing of

the brushes in the rotor part as they are made of carbon material. The main parts of an electrical machine which

require maintenance are:

i) Stator windings

ii) Bearings

iii) Rotor shafts

The characteristics of the motor which are to be maintained for the safer operation ofthe electrical machine are

1) Operational:

a. Operational

b. Loading(Power)




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c. Operating hours

2) Health

a. Rotor health

b. Skin temperature

c. Air gap eccentrical coding condition (temperature levels)

d. Bearing condition

e. Overall vibrations

The electrical machine are equipped with the sensors to detect the motor conditions and gives the information to

the user in user machine interface such that machine maintenance can be ease.

3.1.4) Noise level:

Machines with low noise level are desirable to use in electrical vehicles. As we know that electrical vehicles are

2-3 dB less nosier than internal combustion engine vehicles. In the BLDC motors, the noise level depends on the

ratings of the motor level for different rating of machines are given below.

Power Ratings 600 < n ≤ 960 1320 < n < 1900 2360 < n < 3150

37 kW < p ≤ 55kW 85 dB 92 dB 98 dB

55 kW < p ≤ 100 kW 89 dB 96 dB 99 dB

110 kW < p ≤ 220 kW 91 dB 102 dB 104 dB

These values are approximated values. However the noise value depends on the several operating conditions. In

order to minimize the noise level in the electric vehicle, following methods can be used.

1) Electric machine is fully shielded as a result noise levels can be minimized. If the machine is fully shielded,

the heating solutions are affected, so optimum shielding is employed.

2) Lubricating the moving parts is the most frequently used, as it not only reduces the noise level but also

maintains the machine in proper condition.

3.1.5) Reliability and Robustness:

The BLDC motors are completely reliable in electric vehicles as they are also robust I the following areas.

i) The brushless DC motor provides comparatively higher torque and higher speed which in turn

increases the acceleration, hence that are reliable.

ii) The BLDC motor requires less maintenance due to absence of the brushes on the rotor shaft

iii) The BLDC motors are robust as they withstand high temperatures and freezing temperatures such that

the electric vehicles are more compatible for different variations of temperatures

iv) These motors are robust as they tolerate both frequent load variations and speed variations.

3.1.6) Torque- Speed characteristics:

The desirable torque-speed characteristics of a electrical vehicle is high torque, low speed at hill areas/ starting

of vehicle and low torque and high speed during the normal operation.

The Torque-Speed characteristics of a BLDC motors can be understood by looking at the following diagram




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Fig: torque speed characteristics of BLDC machine

From the graph, it is clear that in the BLDC machine, at the starting the machine will have higher torque and

lower speed. At running condition, the machine will have lower torque and higher speed which is desirable

characteristic of a electrical vehicle.

3.1.7) Power to weight ratio:

The power to weight ratio should be high such that larger amount if power can be obtained for lower weights of

a machine, here power indicates the electrical power input for the electrical machine.

In general, the power to weight of an BLDC motor is comparatively lower when compared with the other types

of machines that are suitable for electrical vehicles.

3.1.8) Machine:

The BLDC motor can act as both motor and generator in electric vehicle. The power from the battery pack is

given to the electrical motor to convert it from the electrical energy to the mechanical energy and in the

generator the kinetic energy from the moving wheels of an electric vehicle is fed back to the motor to convert

the mechanical energy and store it in the battery pack, This is called regenerative braking. Hence the electric

vehicle requires a machine which act as both motor and generator. As BLDC motor can play the race of

generator and motor it is suitable for electric vehicles.

3.1.9) Machine Controller:

The machine controller plays a key role in electric vehicles, The BLDC machine controller is effective in

handling the frequent torque and speed variations. The cost of the BLDC motor controller is a bit expensive

when compared to the controlling units of other machines used in electrical vehicles. Example of a machine

controllers for BLDC motor is OWO brand controller. Model number: BLSD2410DC-2Q-S

The full details regarding the controller module can be found in the below link.

http://m.brushless-dcmotor.com/sale-10204266-120w-24v-brushless-dc-motor-driver-high-frequency-for-

automatic-machine.htm

3.1.10) Cost approach:

The BLDC motors are very famous for their usage in electric vehicles due to its characteristics. The main

disadvantage for adopting it is its cost. The price is expensive compare to other types of machines used for

electric vehicle. One of the reasons for high cost may be due to absence of carbon brushes and adoption of

BLDC Weight Peak power output Power to weight ratio

25 kg 120 kW (kW/kg) 4.8 kW/kg

55 lb 160 hp (hp/ lb) 2.92 hp/ lb




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commutator circuit by deploying the hall sensors. As this entire setup requires high cost, thisis the major

limitation for the brushless DC motors. However, cost approach can be decreased by careful designing of the

machine and adopting the low cost materials.

3.1.11) Heating solutions:

As discussed earlier there are three types of cooling methods. While using the BLDC motor, forced air-cooling

method is used as a heating solution. Moreover, in the electrical vehicle is moving, the input air can be taken

from the aero-dynamic collectors which can be arranged in the front space of the vehicle as a result it doesn’t

require any external motor to forcefully pump the air into the machine. However, the temperature sensor is to be

arranged such that temperature of incoming air can be sensed and if it is not suitable for cooling it can be

humidified with humidifier.

3.1.12) Efficiency:

It is the ratio of output mechanical power to the input electrical power. In brushless DC motor efficiency is

comparatively better than brushed DC motors and other types of DC motors. The efficiency of a BLDC motor

can be calculated by the formula

Efficiency (η) =

Pout = M.n(

)

Where, M Torque

N Speed

Pin = VI

Where V voltage

I Current

In practical the efficiency of brushless DC motor is 73%

3.2) Brushes DC Motor:

The requirements and specifications of an electrical vehicle for the brushed DC motor are almost similar to

brushless DC motor. The requirements or specifications which are similar to BLDC motor are given below.

1) Speed control

2) Acceleration

3) Power to weight ratio

4) Machine property

5) Heating solutions

The above characteristics of a brushed DC motor are similar as BLDC motor. The remaining characteristics of

brushed DC motor are explained here

3.2.1) Maintenance approach:

Unlike brushless DC motors, the brushed DC motors contain the brushes on the rotor shaft. These brushes as we

know are wearable, it requires higher maintenance when compared with the brushless DC motor and as a result,

these are not preferred in the electric vehicle applications.

3.2.2) Noise level:

The noise levels of an brushed DC motors are comparatively higher than brushless DC motors again this is due

to the brushes. The Noise level for an brushed DC motors are given below.




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Power ratings 600 < n ≤ 960 1320 < n < 1900 2360 < n < 3150

37 kW < p ≤ 55kW 85 dB 92 dB 98 dB

55kW < P ≤100 kW 89 dB 96 dB 99 dB

110 kW < P ≤ 220 kW 91 dB 102 dB 104 dB

These values are approximate values, those are not accurate values. As the machines with less noise levels are

preferred. These are less choosable in the electric vehicle applications.

3.2.3) Reliability and Robustness:

When compared with the brushless DC motors, these are less reliable but they are robust for the ectreme

climatic conditions just like the BLDC motors. These are not reliable as they require high maintenance and high

noise levels.

3.2.4) Torque-Speed characteristics:

The ideal Torque-speed characteristics of an electric vehicle is the higher torque, lower speed at the start of the

vehicle and the lower torque and high speeds in the normal conditions of the vehicle. The Torque-Speed

characteristics of an Brushed DC motor can be easily understood by the following diagram.

Fig: torque speed characteristics of brushed DC motor

There is a steady decrease in the torque as the speed increases which is the most desirable torque-speed relation

for an electric vehicle. So Brushed DC motor torque-speed characteristic are desirable.

3.3.5) Power to weight ratio:

The power to weight ratio of an electric vehicle is desired to be high but in case of brushed DC motor, the

power to weight ratio is little lower when compared with the BLDC motor. The power to weigh ratio is given

below.

Brushed DC Machine Weight Peak power output Power to weight ratio

29 kg 120 kW (kW/ kg) 4.6 kW/Kg

58 lb 160 hp (hp/lb) 2.84 hp/lb

3.3.6) Cost approach:

The cost of the brushed Dc motor is relatively lower than the BLDC motor. But in order to choose the best




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machine which satisfies all the conditions. The brushes DC motor cannot be selected by depending only on the

cost benefit. All the requirements specifications and characteristics of an electrical machine are taken into

account and depending the requirement electrical machine is selected.

3.3.7) Efficiency:

The Brushed Dc motor are comparatively less efficient than BLDC motor. This makes the Brushed DC motors

less preferred, as efficiency is the main requirement for an electric vehicle machine. Formula for theoretical

calculation of efficiency of the DC machine is given by

η =

=

In general, the efficiency of the DC brushed motor is 65% which is less than BLDC motor.

IV. AC MACHINES USED IN THE ELECTRIC VEHICLES

The type of alternating current machines that are used in the electric vehicles are asynchronous machines also

called as induction machines, synchronous machines and switched reluctance machines. The AC electrical

machines are widely used in electrical vehicles because of their advantages like high efficiency, good speed

regulation and absence of commutators unlike the DC machines

4.1 ) ASYNCHRONOUS MACHINES

In induction machines, three phase AC supply is given to stator which makes the stator as a magnet called

electro magnet and produces the revolving magnetic field. This resulting magnetic field interacts with the

stationary rotor conductors as a result the induced current flows in the rotor conductors which creates it’s own

magnetic field and these two magnetic fields creates the uni directional torque. As speed of rotor is less than the

speed of revolving magnetic field these are also called asynchronous machines

4.2) SYNCHRONOUS MACHINES

In the synchronous motor, the rotor rotates at the synchronous speed that is the speed of revolving field. In these

type of machines, the stator is connected to a three phase AC supply whereas the rotor is excited with a DC

supply , as a result the rotor pole polarities remains same but the stator pole polarities are continuously

changing. As the rotor rotates at the rated speed unless the load variations these type of machines are called

synchronous motors.

4.3 ) SWITCHED RELUCTANCE MACHINES

These types of motors produces the torque by the variable reluctance method, here reluctance is the property of

magnetic coil which opposes the flow of magnetic flux through it. In these type of motors, the stator coils are

excited with the three phase AC supply and the variable reluctance is set up in the air gap between the stator and

rotor. When the variable reluctance is present in the air gap the rotor tends to move to a low reluctance area in

the air gap thus producing the uni directional torque.

Now by having the basic knowledge of types of AC electrical machines, we can get into the requirements,

characteristics and specifications of the each machine in detail

4.1) ASYNCHRONOUS MACHINE (INDUCTION MOTOR)

In this section we shall know the requirements , characteristics and specifications of asynchronous

machine with regard to the electric vehicle application

4.1.1 SPEED CONTROL :

The control methods employed for AC electrical machines are of less difficulty when compared with the

DC electrical machines , AC machines are easily controllable. The speed of machine can be controlled




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either from the stator side or from the rotor side. The speed of an induction motor is given by the

expression

The speed of induction motor N = 120 * F/ P

F is the frequency of the supply

P is the number of poles per conductor

4.1.1.1 SPEED CONTROL WITH RESPECT TO STATOR SIDE:

I) BY CHANGING THE SUPPLY VOLTAGE:

The torque of a induction motor is given by the expression

Ʈ =( Ki * S * E22 * R)/ (R

2+ (SX)

2)

As can be seen from the equation, the torque developed by an induction motor varies as square of the voltage

applied to its stator terminal. So by varying the applied voltage the electromagnetic torque can be varied.

However this method has a very limited speed control

II) CHANGING THE STATOR POLE NUMBER OF THE MACHINE

The speed of an induction machine as discussed earlier is given by

Ns = 120 * F / P

Therefore by changing the number of pole per phase winding the speed can be varied. The poles in motor can be

changed by a DAHLANDER Motor or also called Pole Changing Motor. With this method different speeds of

machine can be obtained by switching the configuration electrical stator winding in the ratio 2:1. Indirectly

adding or removing the poles.

III) CHANGING THE FREQUENCY OF SUPPLY SIDE OF MOTOR

As we know the speed of an induction motor is directly proportional to the frequency of the supply.

Ns α F

Thus by changing the frequency the speed of induction motor can be varied. In this method of speed control by

variable frequency control high efficient speed control is possible. The electrical vehicle speed variation can

easily be controllable by adapting this variable frequency control.

4.1.1.2 SPEED CONTROL WITH RESPECT TO ROTOR SIDE:

With respect to the rotor side the speed control of an induction motor can be done by the following methods.

They are

1) Adding an external resistance to the rotor side circuit.

2) Applying cascade connection.

3) Injecting EMF into rotor side circuit of motor.

All these methods of speed control is only limited to sensible speed variations. These are not suggestible for the

usage in electrical vehicle speed control methods.

4.1.2 ACCELERATION

The acceleration of an induction motor is a bit higher than DC machines. The calculation of acceleration of an

induction motor is same as that of a DC machine.

4.1.3 MAINTENANCE APPROACH

The electrical machines which require is preferred in electrical vehicles. The maintenance of the induction

motor is comparatively lower than the BLDC machines because of the absence of the wearable parts in the

induction motor. This is one of the advantage of choosing an induction motor when compared to choosing a DC

machine which requires high maintenance due to the presence of brushes and commutators. However the motor

has to be monitored for general parameters which are the examples mentioned in BLDC machine maintenance

approach.




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4.1.4 NOISE LEVEL

Induction motors have lower noise levels also because of the absence of brushes and commutators which cause

higher noise levels. Having lower noise levels is a desirable characteristic for an electrical vehicle as smooth and

silent operation is always preferred. The noise levels for different ratings of an induction motor at different

speeds are given below.

Power ratings 600 < n ≤ 960 1320 < n < 1900 2360 < n < 3150

37 kW < p ≤ 55kW 78 dB 87 dB 92 dB

55kW < P ≤100 kW 81 dB 92 dB 93 dB

110 kW < P ≤ 220 kW 85 dB 94 dB 96 dB

These values are only approximate . Practical values may differ a bit from these values.

4.1.5 RELIABILITY AND ROBUSTNESS

The induction motors in the electrical vehicle perception are very reliable in the following ways.

Efficient speed control for any load variations and frequent start or stop of the vehicle is possible.

The overall efficiency of an induction motor is very high and maintenance required is low which makes it

more reliable and suitable for an electrical vehicle application.

The noise levels are comparatively lower than BLDC motor.

Induction motors are not robust as of BLDC motors because they are not suitable for extreme climatic

conditions. It uses the electromagnetic principle in which magnetization of stator is done by the current from the

supply and if the supply has harmonics the magnetic field thus produced is not desirable and it also affects the

torque produced.

4.1.6 TORQUE SPEED CHARACTERISTICS

One of the concerning aspect of an induction motor is the torque speed characteristics. As we know that the

ideal torque speed characteristics of an electrical vehicle are higher torque and lower speed at the starting of the

vehicle and lower torque and higher speed at normal operation of the vehicle. The torque speed characteristics

of an induction motor can be understood by observing the following graph-plot between torque and speed.

Fig : torque speed characteristics of asynchronous motors

By looking at this we can understand that the induction motor doesn’t have the desirable torque speed

characteristics.

4.1.7 POWER TO WEIGHT RATIO




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The power to weight ratio of an induction motor is higher than that of a BLDC motor. It is also an added

advantage for the application of induction motor as a machine in electrical vehicle.

Asynchronous Motor Weight Peak power output Power to weight ratio

24 kg 165 kW (kW/ kg) 6.875 kW/Kg

53 lb 221 hp (hp/lb) 4.18 p/lb

4.1.8 MACHINE

The induction motor can act as both motor, which delivers mechanical power by consuming energy from the

battery during normal operation and generator which stores the electrical energy in the battery pack by taking

the mechanical energy from the wheels of the vehicle during the regenerative breaking mode.

4.1.9 MACHINE CONTROLLER

The machine controller of an induction motor is simple as compared to that of a BLDC machine controller

which requires complex circuit. For the speed control of an induction motor of electrical vehicle application

variable frequency controller method is required. Hence machine controller is also a simple circuit. As a result

of which the cost of controller is low. The practical example of an induction motor controller is Z-LOG PMSM

controller. The detailed information can be found on the link given below.

https://m.alibaba.com/product/62464089811/Electric-vehicle-motor-controller.html?s=p

4.1.10 COST APPROACH

In the electrical vehicle application for using an induction motor cost is the main drawback as the cost of

induction motor is a bit high when compared to a BLDC motor or any other DC machines which comprise of

complex circuits like brushes and commutators. However the other parameters for an induction motor are

favorable for electrical vehicle application. Therefore despite of the high cost induction motors are still widely

used in electrical vehicles.

4.1.11 HEATING SOLUTIONS

As we already know the different types of cooling solutions available, the induction motor mostly employs

liquid cooling which used the liquid coolant to cool the machine unlike other types of machines which use

forced air cooling. From the cost analysis the liquid cooling is a bit more costlier than the forced air cooling.

Hence this might be one of the minor drawback for using induction motor in electrical vehicle.

4.1.12 EFFICIENCY

Induction machine also called asynchronous machine’s overall efficiency is very high when compared with all

the other types of machines. The practical efficiency of an induction motor in an electrical vehicle application is

around 90 percent.

4.2) SYNCHRONOUS MACHINE

The induction motor ( asynchronous machine ) , synchronous motors and switched reluctance motors almost

have the same characteristics, required and specifications. The list of same characteristics for three kinds of

motors are mentioned below. For this same characteristics of synchronous motors and switched reluctance

motors please refer the characteristics of asynchronous motors or induction motors. The common parameters

are as follows.

1. Speed control

2. Acceleration

3. Maintenance approach

4. Noise level

5. Reliability and Robustness




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6. Machine requirement

7. Heating solution

The remaining parameters of synchronous and switched reluctance motors with regard to an electrical vehicle

are explained below.

4.2.1 TORQUE SPEED CHARACTERISTICS

For synchronous and switched reluctance motors the torque speed characteristics can be easily understood by

looking at the below graph plot of Torque vs. Speed.

Fig : torque speed characteristics of synchronous motors

Fig : torque speed characteristics of switched reluctance motor

By looking at the graph we can clearly say that torque speed characteristics of an ideal electrical vehicle are

same as that of both synchronous and switched reluctance motors.

4.2.2 POWERTO WEIGHT RATIO

The power to weight ratio of the induction motor and that of switched reluctance motor is almost same but

synchronous motor has slightly less power to weight ratio when compared to induction or switched reluctance

motor

Fig : comparison of power to weight ration of AC electrical machines

4.2.3 MACHINE CONTROLLER




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For both the synchronous motor and switched reluctance motor the machine controller are same in the cost

approach and the induction motor is less than synchronous motor or switched reluctance motor.

Fig : cost of controllers comparison for AC electrical machines

4.2.4 COST APPROACH

In between the synchronous motor and switched reluctance motor, the cost of the switched reluctance motor is

very high when compared to the cost of the induction motor or the cost of synchronous motor.

Fig: cost comparison of AC electrical machines

4.2.5 EFFICIENCY

The efficiency of switched reluctance motor and synchronous motor is slightly higher than the efficiency of the

induction motor but by comparing with all the properties. The induction motor will be appropriate for the

electric vehicle application.

The practical efficiency of synchronous motor is around 92%.

The practical efficiency of switched reluctance is around 95%.

V. PRACTICAL EXAMPLES OF ELECTRICAL MACHINES USED IN THE ELECTRIC

VEHICLE

The famous German car maker company Audi had recently built a new Electric car known as Audi e-tron. In

order to practically understand the types of machines used, their characteristics specifications and requirements

for an Electric vehicles machine. It is wise to understand the parameters of a practical example. So, we shall

cover the technology used in this car such that basic characteristics of an electric machine are fully understood.




[359]

The type of machine used in Audi e-tron is the Asynchronous electrical motor or the Induction motor. By

comparing the DC machines and AC machines for the usage the speed control for an AC machines is far more

superior than that of the DC machines. In the AC machines the electrical machines suitable for the electric

vehicles are:

1. Asynchronous motors

2. Synchronous Motors

3. Switched reluctance motor.

Considering all the parameters mentioned in (2), Asynchronous motors offers better characteristics like

efficiency, speed regulation for the Electric Vehicles among the above three motors. The features and

specifications of the Audi e-tron are as follows

5.1 SPEED CONTROL:

The speed control method used in the Induction motor of an Audi e-tron is the frequency variable method. It

employs the adaptive cruise control (ACC). It is a control method which employs change in Frequency value to

adjust the speed of car automatically in all situations from congested motorways, averaged speed zones to slip

roads and round abouts etc.

The two Asynchronous motors drive the electric e-tron powerfully and almost silently with a system output of

upto 300KW and 664Nm of torque. The maximum torque available within few milliseconds and provides

enormous pulling power. The Audi e-tron completes the standard sprint in 5.7 secs. Top speed is electronically

limited to 200kmph (124.3 mph).

5.2) ACCELERATION:

Audi e-tron speeds up to 100 Kmph in 5.7 secs. Therefore the acceleration of the car can be calculated by

change in Velocity = v-u

= 100-0 Kmph

= 100÷(60 x 60) m/sec

= 27.78 m/sec

Acceleration of e-tron = v÷t

= 27.78÷5.7

= 4.873 m/sec 2

5.3) MAINTENANCE APPROACH:

The maintenance with regard to the electric machine is very less because of the usage of the Asynchronous

(Induction) motor. As we know the maintenance approach for Induction motor is very low when compared with

the all other types of motors. This is also one of the reason for selecting the Induction motor in Electric

Vehicles.

5.4)NOISE LEVEL:

The Noise level of the Induction motor used in the Electric vehicle is greatly reduced than that of the

conventional noise level for the Induction motor. The conventional noise level of the Induction motor

(110<;p<=220 KW) is 94 dB; at 1320<n<=19000 speed. The noise level of the improved Induction motor in

Audi e-tron is 63 dB.

5.5)RELIABILITY AND ROBUSTNESS:

It is of no doubt to say that the Induction motor used in Audi e-tron is reliable and robust in all aspects.

5.6) TORQUE-SPEED CHARACTERISTICS:




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The following curve depicts the Torque-Speed Characteristics of an Induction motor in the Audi e-tron.

Fig : torque speed characteristics of induction motor used in audi e tron

The peak torque of Audi e--tron is 247/314 N-m. However is dependent on the car variant. The torque for 300

KW output is around 664 N-m.

5.7) POWER TO WEIGHT RATIO:

Audi e-tron has a power to weight ratio of 1.78 kg/hp with curb weight of 1450 kgs with ideal 50:50 percent

weight distribution between the front and the rear axle.

5.8) MACHINE:

The regenerative braking system used in Audi e-tron makes the induction motor to act as a generator in the

braking state and generates the energy which will store in HVDC pack specially built for Audi e-tron.

5.9 )MACHINE CONTROLLER:

The machine controller also known as the power electronics unit is built on the motor. It follows the Intelligent

Integration concept of the electric drive. Inside the power electronics unit, three latest generation silicon power

modules are configured to create a conventional B6 pulse inverter and installed in a rack. In rack, the power

electronics are cooled on both sides and held so that the gate driver board can be plugged directly on to their

contact pins and contacted.

5.10) HEATING SOLUTIONS:

For the cooling of electric axle drives an innovative combination of water and convection cooling is used. The

cooling concept is divided into four sections for which heating solutions are needed.

i) Cooling of power electronics

ii) Conventional motor cooling on the circumference of the stator.

iii) Bearing plate cooling

iv) Rotor internal cooling

The clear details on how the cooling concept is applied on Audi e-tron can be checked out in the provided at the

bottom of this section .

5.11) EFFICIENCY:

The efficiency of the induction motor used in Audi e-tron is around 93%. As it is a four wheel motion, the back

wheels induction is more efficient that that of the front wheels induction motor.

This is due to some of the practical mechanical stability issues.




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VI. CONCLUSION

In this paper we have illustrated the different characteristics, parameters and specifications of five different

electrical machines that are suitable for the usage in Electric Vehicles in brief. The conclusions we draw by this

study are as follows:

1. The BLDC machines are more suitable in the DC machines sector for Electric Vehicles since they have high

power to weight ratio, but the cost of controllers, machines are very high.

2. The Brushed DC motors provide high torque at lower speeds and speed control is also easy but these motors

have high maintenance cost, large size and it's efficiency is also very low.

3. The Induction motors are more efficient and all the other parameters are also in favorable with Electric

Vehicles applications. As a result, these motors are widely used in Electric Vehicle's.

4. Synchronous motors are constant speed motors. Hence, these are preferred where constant torque is required.

5. Switched reluctance motor provides a high efficiency and these are also reliable as a result these motors are

also considered as great alternative for the BLDC motors and Induction motors.

However, since each motor has its own advantages and disadvantages, at the end it is based up on the consumer

requirements, but in general the BLDC motors in DC machining category and the Induction motors in AC

machines category are the better options for the usage in Electric Vehicles.

VII. REFERENCES

[1] https://irjmets.com/forms/uploads/control-of-an-electric-vehical.

[2] https://www.irejournals.com/paper-details/1702221.

[3] Zhang, Y.; Wang, W.; Kobayashi, Y.; Shirai, K. Remaining driving range estimation of electric

vehicle. In Proceedings of the 2012 IEEE International Electric Vehicle Conference, Greenville,

SC, USA, 4–8 March 2012; pp. 1–7..

[4] Shi, R.; Li, S.; Zhang, P.; Lee, K.Y. Integration of Renewable Energy Sources and Electric

Vehicles in V2G Network with Adjustable Robust Optimization. Renew. Energy 2020, 153,

1067– 1080.

[5] Wang, D.; Saxena, S.; Coignard, J.; Iosifidou, E.A.; Guan, X. Quantifying electric vehicle battery

degradation from driving vs. V2G services. In Proceedings of the 2016 IEEE Power and Energy

Society General Meeting (PESGM), Boston, MA, USA, 17–21 July 2016; pp. 1–5....

[6] https://mail.google.com/mail/u/1/#inbox/FMfcgxwHNgldcvFttJhlPwfhXzfKGSnK?projector=1&

messagePartId=0.1.

[7] https://www.faulhaber.com/en/support/technical-support/motors/tutorials/dc-motor-tutorial-dc-motor-

calculation

[8] Dongbin Lu, Minggao Ouyang, Jianqiu Li, Liangfei Xu, “Economic Operating Characteristics of

Permanent Magnet Synchronous Motor in Electric Vehicle”, 2012 IEEE Vehicle Power and

Propulsion Conference, Oct. 9-12, 2012, Seoul, Korea

https://mail.google.com/mail/u/1/#inbox/FMfcgxwHNgldcvFttJhlPwfhXzfKGSnK?projector=1&messagePartId=0.1

https://mail.google.com/mail/u/1/#inbox/FMfcgxwHNgldcvFttJhlPwfhXzfKGSnK?projector=1&messagePartId=0.1

https://www.faulhaber.com/en/support/technical-support/motors/tutorials/dc-motor-tutorial-dc-motor-calculation

https://www.faulhaber.com/en/support/technical-support/motors/tutorials/dc-motor-tutorial-dc-motor-calculation

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