New European Drive Cycle Nedc

Published Date: 02 Nov 2017

Abstract-This paper will describe many kinds of electric motors which used with electric vehicles in past. A new design technique for the electric vehicle (EV) motors have been described in this paper in order to provide a motor with high efficiency and suitable to work with the New European Drive Cycle (NEDC) by satisfying operational torque and other thermal design and volumetric limits.The energy efficiency of the electric motor over the driving cycle can be distinguished against number of exemplary points by the energy distribution analysis of a given driving cycle.Also,design optimization can be achieved respecting with these points.All the factors that influence performance and efficiency of the motor have been discussed with their required calculation equation .

1. Introduction

During the last two decades , most of international car manufacturing companies focus on finding an efficient and environmentally friendly vehicle .Electric vehicles which receive power form the renewable sources like wind, solar and waves are the solution for the future personal mobility [1]. Electric motors are the heart of the electric vehicle tractive system. The electric motor produced in different types, shapes and sizes and consider as the most important mechanical device in our planet .Electric motors considered as an environment friendly device because it has zero emission pollutants .Basically, there are three moving parts in the electric motor which are the rotor and two end bearings [2] .The sole propulsion unit for the electric vehicles (EVs) is the electric motor, whereas electric motor and the internal combustion engine (ICE) work together in series or in parallel on providing the traction power in Hybrid electric vehicles (HEVs).the main benefits from the electric motor are providing full torque at low speed and the instantaneous power rating can be two or three the rated power of the motor which will give the vehicle an excellent acceleration when compare it with ICE . [3]

2. Electric motors

There are many types , shapes and sizes of electric motors, all of them have the same function which is converting received input from the controller to a rotational movement transmitted to the wheel through the differential. Electric motor could be direct current (DC) or alternative current (AC). In 1980s , DC motors used in many prototypes of EVs because of it is easy control and status of development .However, the big size and continuous maintenance which required for the DC motors led to neglect this kind of motors in most automotive and industrial applications. Nowadays, modern EVs and HEVs used AC and brushless motors ,which involve switched reluctance motors, induction motors ,and permanent magnet motors.over the past 50 years , lots of researches and developments were took place on the AC induction motor technology.the main disadvantage of the AC induction motors are their control complexity when compare it with DC motor but with the availability of fast digital processors,this control complexity can be managed easily .New techniques for AC motors drive are carried out in order to make the control of AC motor simple like DC motors through reference frame transformations technique by developing vector control technique. The permanent magnet (PM) is the competitor to the induction motor .On the rotor of the permanent magnet AC motor , there are

magnets, while the structure of the stator still similar to that induction motor.the PM motors classified depending on their flux distribution in the air gap to sinusoidal type or trapezoidal type.Trapezoidal motors have centered three-phase windings , also, are known as brushless DC motors.Simillary with induction motors, the PM motors are driven by six-switch inverter.High power density produced from PM motors a result of using of high density rare earth magnet ,but the high cost of that magnet consider as disadvantage for this kind of motors.the size of the electric motor for EVs and HEVs applications are large compared with other application of PM motors, which increase t effect f cost.However, EV motors are almost larger than HEV motors,and PM motors produce high which overcome the problem of high cost.there are another types of motors which Characterized by simplesty in construction and fault tolerance .the torque inertia rating increases in this type of motors because It have not windings, magnets, or cages n the rotor. The motor-speed features are suitable with road load characteristics , also the performance of SP motors is very high when use them in EV/HEV implementations .On the other hand , there are two problems with SR motors represented by acoustic noise and torque ripple which make the usage of these motors limited with some applications [3].

AC Motors

DC Motors

Advantages

High Torque/Horsepower

Capabilities

Less Rotor Heat produced

No Permanent Magnet

Wide Spectrum of Optimal Power Setting

Magnetic Field Strength Adjustable

No Efficiency Losses due to DC to AC

Conversion

Light in weight

Multi-speed transmission

Low cost

The Efficiency of the motor is between 85-95% at full load

The efficiency of the motor is 95% at full load

Controller for this motor is simple

Motor and controller are cheaper than AC

Disadvantages

Optimal Power Factor: 85 percent

Heavy in weight at same power

Cumbersome to Control

More expensive

Single speed transmission

Permanent Magnet Expensive

Complex controller

Motor and controller are more expensive than DC

More rotor heat produced

Table : AC/DC motors Comparison [6] [7]

In general , a comparison was carried on between these two kinds of motors as it is shown in table 1. DC motors require at least two permanent magnets that generate a magnetic field, whereas the AC motor has no magnets but stacks of steel lamination with secondary conductors. These permanent magnets cause the construction of the DC motor to be more expensive than the AC motor .The DC motor , however , can operate uniformly across a wider spectrum of power settings, while the optimum power setting for the AC motor is approximately 85% of the motor total capability. Even though the AC motor has a cost and a maximum torque advantage over the DC motor, the efficiency loss of converting the DC battery power to AC current for a passenger car is a primary reason that a DC motor is selected for these kinds of EVs or HEVs [5]. Also ,there are many factors should be taken in order to choose the best motor for the application whether EVs or HEVs which are [3] :

• Ruggedness

• High torque-to-inertia ratio (Te/J); large Te/J results in "good" acceleration capabilities

• Peak torque capability of about 200 to 300% of continuous torque rating

• High power-to-weight ratio (Pe/w)

• High-speed operation, ease of control

• Low acoustic noise, low electromagnetic interference (EMI), low maintenance, and low cost

• Extended constant power region of operation

3. DC motors :

The torque in DC motors is generated by using one of the two electromagnetic basic rules theory: by Lorentz force precept, where the torque is generated by the reciprocal interaction of two orthogonal magnetomotive forces (mmf); and by reluctance precept, where the rotor produces torque when moving into the minimum reluctance position in a varying reluctance path. There are many types of DC motors classified to brush DC motors and brushless DC motors.

3.1 Brushed DC motors :

3.1.1

The brushed DC motor is the simplest type of electric motor. It is used in many applications such as portable tools, toys, electrically operated windows in cars , and in other small applications like hair dryers. Even if they have an AC power supply. Also , the simple design of DC motor used for traction purposes .Figure 1 shows the main component of the classic simplified DC motor which contain permanent magnets , brushes and one coil .The force which generated in the coil come from passing the current through the wire near the magnet.

Figure : the operation of the simple permanent magnet DC motor [5]

The current pass through brush X, commutator half ring A , about the coil and go out through the other commutator half ring B and brush Y (XABY).the current is passing back towards the brushes and commutator so , the force on one side is upward and downward on the other side as it shown in the figure 1 . These two forces giving rotation motion to the coil. The coil and the commutator rotate together, when the wire pass from the magnet the momentum will carry it on round till the half ring of the commutator connect again with the brush. During that, the current will flow in the same direction relative to the magnet , and hence the forces are in the same direction, continuing to rotate the motor as before. However the current will pass through brush X, half ring B, rotate the coil to A and go out through Y, so the current will flowing in the opposite direction through the coil (XBAY). The objective of he commutator inside the DC motor is to ensure that the current in the coil continue changing the direction so the force is in the same direction even the coil has moved.

Clearly, in a real DC motor there are many refinements over the arrangement of

Figure 6.1. The most important of these are as follows.

• The rotating wire coil, often called the armature, is wound round a piece of iron, so

that the magnetic field of the magnets does not have to cross a large air gap, which

would weaken the magnetic field.

• More than one coil will be used, so that a current-carrying wire is near the magnets

for a higher proportion of the time. This means that the commutator does not consist

of two half rings (as in Figure 6.1) but several segments, two segments for each coil.

• Each coil will consist of several wires, so that the torque is increased (more wires,more force).

• More than one pair of magnets may be used, to further increase the turning force.

Figure 6.2(a) is the cross-section diagram of a DC motor several steps nearer reality than that of Figure 6.1. Since we are in cross-section, the electric current is flowing in the wires either up out of the page, or down into the page. Figure 6.2(b) shows the convention used when using such diagrams. It can be seen that most of the wires are both carrying a current and in a magnetic field. Furthermore, all the wires are turning the motor in the same direction.

Figure (a) Cross-section through a four-pole DC motor. The dotted lines show the magnetic (a) Cross-section through a four-pole DC motor. The dotted lines shows the magnetic

flux. The motor torque is clockwise. (b) shows the convention used to indicate the direction of

current flow in wires drawn in cross-section

3.1.2 Torque speed characteristics

If a wire in an electric motor has a length l metres, carries a current I amps, and is in a

magnetic field of strength B Wb.m−2, then the force on the wire is:

............3.

If the radius of the coil is r, and the armature consists of n turns, then the motor torque

T is given by the equation:

............3.

The term 2Blr = B × area can be replaced by ɸ the total flux passing through the coil.

This gives:

............3.

However, this is the peak torque, when the coil is fully in the flux, which is perfectly

radial. In practice this will not always be so. Also, it does not take into account the

fact that there may be more than one pair of magnetic poles, as in Figure 2. So we

use a constant Km, known as the motor constant, to connect the average torque with the

current and the magnetic flux. The value of Km clearly depends on the number of turns

in each coil, but also on the number of pole pairs, and other aspects of motor design.

Thus we have:

............3.

We thus see that the motor torque is directly proportional to the rotor (also called armature)

current I . However, what controls this current? Clearly it depends on the supply

voltage ES to the motor. It will also depend on the electrical resistance of the armature

coil Ra. As the motor turns the armature will be moving in a magnetic field. This

means it will be working as a generator or dynamo. If we consider the basic machine of

Figure.1, and consider one side of the coil, the voltage generated is expressed by the

basic equation:

............3.

This equation is the generator form of equation (3.1). The voltage generated is usually

called the back EMF, hence the symbol Eb. It depends on the velocity v of the wire moving

through the magnetic field. To develop this further, the velocity of the wire moving in

the magnetic field depends on ω the angular velocity and r the radius according to the

simple equation v = rω. Also, the armature has two sides, so equation (3.5) becomes:

Eb = 2Blrω

However, as there are many turns, we have:

Eb = 2nrBlω

This equation should be compared with equation (3.2). By similar reasoning we simplify

it to an equation like (3.4). Since it is the same motor, the constant Km can be used

again, and it obviously has the same value. The equation gives the voltage or back EMF

generated by the dynamo effect of the motor as it turns.

............3.

This voltage opposes the supply voltage ES and acts to reduce the current in the motor.

The net voltage across the armature is the difference between the supply voltage ES and

the back EMF Eb. The armature current is thus:

............3.

This equation shows that the current falls with increasing angular speed. We can substitute

it into equation (3.4) to get the equation connecting the torque and the rotational speed.

............3.

This important equation shows that the torque from this type of motor has a maximum

value at zero speed, when stalled, and it then falls steadily with increasing speed. In this

analysis we have ignored the losses in the form of torque needed to overcome friction in

bearings, and at the commutator, and windage losses. This torque is generally assumed

to be constant, which means the general form of equation (3.7) still holds true, and gives

the characteristic graph of Figure.3.

The simple linear relationship between speed and torque, implied by equation (3.7), is

replicated in practice for this type of constant magnetic flux DC motor. However, except

in the case of very small motors, the low speed torque is reduced, either by the electronic

controller, or by the internal resistance of the battery supplying the motor. Otherwise the

currents would be extremely high, and would damage the motor

Figure Torque/speed graph for a brushed DC motor

3.1.3 DC motor efficiency

The major sources of loss in the brushed DC electric motor are the same as for all types

of electric motor, and can be divided into four main types, as follows.

Firstly there are the copper losses. These are caused by the electrical resistance of the

wires (and brushes) of the motor. This causes heating, and some of the electrical energy

supplied is turned into heat energy rather than electrical work. The heating effect of an

electrical current is proportional to the square of the current:

............3.

However, we know from equations (3.3) and (3.4) that the current is proportional to

the torque T provided by the motor, so we can say that:

............3.

where kc is a constant depending on the resistance of the brushes and the coil, and also the

magnetic flux ɸ. These copper losses are probably the most straightforward to understand

and, especially in smaller motors, they are the largest cause of inefficiency.

The second major source of losses is called iron losses, because they are caused by

magnetic effects in the iron of the motor, particularly in the rotor. There are two main

causes of these iron losses, but to understand both it must be understood that the magnetic

field in the rotor is continually changing. Imagine a small ant clinging onto the edge of the rotor of Figure.2. If the rotor turns round one turn then this ant will pass a north pole,

then a south pole, and then a north pole, and so on. As the rotor rotates the magnetic field

supplied by the magnets may be unchanged, but that seen by the turning rotor (or the ant

clinging to it) is always changing. Any one piece of iron on the rotor is thus effectively

in an ever-changing magnetic field. This causes two types of loss. The first is called

‘hysteresis’ loss, and is the energy required to continually magnetise and demagnetise the

iron, aligning and re-aligning the magnetic dipoles of the iron. In a good magnetically

soft iron this should be very small, but will not be zero. The second iron loss results

from the fact that the changing magnetic field will generate a current in the iron, by the

normal methods of electromagnetic induction. This current will result in heating of the

iron. Because these currents just flow around and within the iron rotor they are called

‘eddy currents’. These eddy currents are minimised by making the iron rotor, not out of

one piece, but using thin sheets all bolted or glued together. Each sheet is separated from

its neighbour by a layer of paint. This greatly reduces the eddy currents by effectively

increasing the electrical resistance of the iron.

It should be clear that these iron losses are proportional to the frequency with which that

magnetic field changes; a higher frequency results in more magnetising and demagnetising,

and hence more hysteresis losses. Higher frequency also results in a greater rate of change

of flux, and hence a greater induced eddy currents. However, the rate of change of

magnetic flux is directly proportional to the speed of the rotor; to how quickly it is

turning. We can thus say that:

............3.

where ki is a constant. In fact, it will not really be constant, as its value will be affected

by the magnetic field strength, among other non-constant factors. However, a single value

can usually be found which gives a good indication of iron losses. The degree to which

we can say ki is constant depends on the way the magnetic field is provided; it is more

constant in the case of the permanent magnet motor than the separately excited.

The third category of loss is that due to friction and windage. There will of course

be a friction torque in the bearings and brushes of the motor. The rotor will also have

a wind resistance, which might be quite large if a fan is fitted to the rotor for cooling.

The friction force will normally be more or less constant. However, the wind resistance

force will increase with the square of the speed. To get at the power associated with these

forces, we must multiply by the speed, as:

power = torque × angular speed

the power involved in these forces will then be:

.......3.

where Tf is the friction torque, and kw is a constant depending mainly on the size and

shape of the rotor, and whether or not a cooling fan is fitted.

Finally, we address those constant losses that occur even if the motor is totally stationary,

and vary neither with speed or torque. In the case of the separately excited motor these are definitely not negligible, as current (and hence power) must be supplied to

the coil providing the magnetic field. In the other types of motor to be described in the

sections that follow, power is needed for the electronic control circuits that operate at all

times. The only type of motor for which this type of loss could be zero is the permanent

magnet motor with brushes. The letter C is used to designate these losses.

It is useful to bring together all these different losses into a single equation that allows

us to model and predict the losses in a motor. When we do this it helps to combine the

terms for the iron losses and the friction losses, as both are proportional to motor speed.

Although we have done this for the brushed DC motor, it is important to note that this

equation is true, to a good approximation, for all types of motor, including the more

sophisticated types to be described in later section.

If we combine equations (3.10), (3.11) and (3.12), we have:

kw....3.

However, it is usually the motor efficiency ηm that we want. This is found as follows:

............3.

This equation will be very useful when modeling the performance of electrical

Vehicles