Development of Electric Drives in Light Rail Transit (LRT) System

8/3/2019 Development of Electric Drives in Light Rail Transit (LRT) System

1/31

1

Development of Electric Drives in Light Rail Transit (LRT) System

Student Name Arjun Pratap Singh

Discipline Mechatronic Engineering

Module Name Power Electronics and Machines


2/31

2

Table of Contents

CHAPTER

1 Introduction 3

2 Nature of Motor Speed-Torque Characteristics 6

3 DC-DC Converter Drives 11

4 Design Considerations related to the Minimization of Harmonic Problems 17

5 Specifications of the Over-Voltage Protection Circuit 19

6 Improvement of Motor Efficiency 23

7 Calculations of Traction Drive Rating andEnergy Consumption 268 Applications of LRT System 29

9 Proposal for Future Improvements and Conclusion 29

10 References 30


3/31

3

INTRODUCTION

An electric drive can be defined as an electromechanical device for converting electrical energy

into mechanical energy to impart motion to different machines and mechanisms for various kinds

of process control.

Earlier in the days the mode of transportation were animal driven vehicles, as the

technology advanced, came cycles and other modes of manually human driven vehicles, as the

technology advanced more, came in vehicles which were driven by some other mode of energy

than humans energy. The most famous of the late 18th

century being cars, which were driven by

non renewable sources of energy as diesel, petrol, kerosene etc. These vehicles were directed to

people in person; it was directed for ones own need or small group of people or passengers (40-

50) but wasnt catering to large sect of people at once, plus there was another drawback of

pollution being caused as, filtering technology wasnt so advanced back then. By the send of the

19th century, came in the trend of electrical driven vehicles, which were aimed for public use or

larger sect of people, at start they were driven by dc source of power as batteries and dc

transmission lines (low voltage). With the wide spread modernization came in the concept of

largely using AC power for various electrical needs, the main reason being that AC could be

transferred over large distances and large voltages easily generated. Thus, various drives were

designed to move electrical vehicles using the AC as source of power. AC power was used in

directly to drive the vehicles or vehicles had convertors to convert the AC to DC power to drive

the components as motors to move the system.

In present times the most popular mode transportation using electric power are MRTs

and LRTs. They can be found in number of countries all over the world. They use electric drives

to move, than using engines powered by non renewable sources of energies, as there is no

pollution caused by using electricity as source of power plus, electric motors are highly efficient,

there is a better torque control as electric motors can operate in all four quadrants of speed torque plane, then there is better speed control and braking, then electric motors can be started

instantaneously and can be immediately loaded, and last not the least that electrical drives can be

easily controlled and monitored using various types of controllers (e.g. PLC, DCS).

Below is the figure of general block diagram of an electrical drive system:


4/31

4

Below researcher have shown some of the factors taken into consideration while selecting

electrical drives (Kaliammorthy):

y Steady State Operating conditions requirements: Nature of speed torque characteristics, speed regulation, speed range, efficiency, duty

cycle, quadrants of operation, speed fluctuations if any, ratings etc.

y Transient operation requirements:Values of acceleration and deceleration, starting, braking and reversing performance.

y Requirements related to the source:Types of source and its capacity, magnitude of voltage, voltage fluctuations, power

factor, harmonics and their effect on other loads, ability to accept regenerative power.

y Capital and running cost, maintenance needs life.y Space and weight restriction if any.y Environment and location.y Reliability


5/31

5

Both AC and DC drives have been popular for driving electric vehicles over many years.

But, recently the trend of using AC drives has increased due to some of its advantages as

explained above. But, the researcher has chosen DC drive over the AC drive because of 2 main

reasons, the first being that power circuit and control circuit is simple and inexpensive and other

being that fast response and wide spread range of control can be achieved smoothly by

conventional and solid state control.

Thus, researcher has chosen series wound DC motor (LRT) for his case study. The main

advantages being that, it develops large torque and can be operated at low speeds. Heavier loads

are moved slowly while lighter loads are moved rapidly according to Application of Series

Wound Motoravailable from http://www.engineersedge.com/motors [Accessed 29th September,

2011).

Since armature and field are connected in series in the series wound dc motor, the

armature and field currents become identical and torque can be expressed as,

T=KI^2

According to which torque increases greatly as speed reduces and speed increases

drastically as load is removed. The main drawbacks being that it doesnt have very precise speed

control, there might be friction losses (if commutator and brushes present), also that it cant run

on no load and there is instability in motor due to regenerative braking. Using thyristor

convertors the accuracy and efficiency of the system as whole can be improved.

(Series Wound DC Motor) (Accessible from http://www.micromotcontrols.com)


6/31

6

NATURE OF MOTOR SPEED - TORQUE CHARACTERISTICS

Before explaining the speed torque characteristics of the dc series motor, the researcher would

explain working of the dc series motor in brief after which he will blend in the nature of motor

speed torque characteristics.

(Operation of DC Motor) (Available fromhttp://zone.ni.com)

Operation of the series motor is easy to understand. In figure below it can be see that the

field winding is connected in series with the armature winding. This means that power will be

applied to one end of the series field winding and to one end of the armature winding (connected

at the brush). When voltage is applied, current begins to flow from negative power supply

terminals through the series winding and armature winding. The armature is not rotating when

voltage is first applied, and the only resistance in this circuit will be provided by the large

conductors used in the armature and field windings. Since these conductors are so large, they

will have a small amount of resistance. This causes the motor to draw a large amount of current

from the power supply. When the large current begins to flow through the field and armature


7/31

7

windings, it causes a strong magnetic field to be built. Since the current is so large, it will cause

the coils to reach saturation, which will produce the strongest magnetic field possible.

(DC Series Motor) (Available from http://zone.ni.com )

Since the series field winding is connected in series with the armature, it will carry the

same amount of current that passes through the armature. For this reason the field is made from

heavy-gauge wire that is large enough to carry the load. Since the wire gauge is so large, the

winding will have only a few turns of wire. In some larger DC motors, the field winding is made

from copper bar stock rather than the conventional round wire used for power distribution. The

square or rectangular shape of the copper bar stock makes it fit more easily around the field pole

pieces. It can also radiate more easily the heat that has built up in the winding due to the large

amount of current being carried. The amount of current that passes through the winding

determines the amount of torque the motor shaft can produce. Since the series field is made of

large conductors, it can carry large amounts of current and produce large torques.

The strength of these magnetic fields provides the armature shafts with the greatest

amount of torque possible. The large torque causes the armature to begin to spin with the

maximum amount of power (at start of electrical drive with load). When the armature begins

to rotate, it begins to produce voltage. The stronger the magnetic field is or the faster the coil

passes the flux lines, the more current will be generated. When the armature begins to rotate, it

will produce a voltage that is of opposite polarity to that of the power supply. This voltage is

called back voltage; back EMF (electromotive force). The overall effect of this voltage is that it

will be subtracted from the supply voltage so that the motor windings will see a smaller voltage

potential.


8/31

8

When Ohm's law is applied to this circuit, it can be see that when the voltage is slightly

reduced, the current will also be reduced slightly. This means that the series motor will see less

current as its speed is increased. The reduced current will mean that the motor will continue to

lose torque as the motor speed increases. Since the load is moving when the armature begins to

pick up speed, the application will require less torque to keep the load moving. This works to the

motor's advantage by automatically reducing the motor current as soon as the load begins to

move. It also allows the motor to operate with less heat buildup.

(Relationship between series motor speed and armature current) ( Available from

http://zone.ni.com)

Torque for any given RPM:

T = Ts - (N Ts Nf)

Where, T = Torque at given RPM

N = RPM

Ts = Stall Torque (at 0 RPM)

Nf = Free RPM (torque almost 0 at maximum RPM)


9/31

9

Induced Voltage constant, which relates to back voltage induced in the armature to the speed of

armature is given by:

Ke = V Nf

Where, Ke = Induced Voltage Constant

V = Voltage

Torque constant which relates the torque to the armature current is given by:

Kt = Ts V

Where, Kt = Toque Constant

Ts = Stall Torque

Using the above formulas the motor equation can be written as:

T = Kt (V - (Ke N)

V = (T Kt) + (Ke N)

N = (V - (T Kt)) Ke

Now,the formula relating the torque to the armature current has been provided below, which is:

T = K f Ia

Where, K = Some Constant

F = Flux Density

Ia = Armature Current


10/31

10

At last the formula relating the induced voltage to the speed of the motor and the field strength is

provided by:

Eb = K N f

Where, Eb = Induced Voltage (Momentarily higher than the applied voltage causing motor to act

as a generator, concept of regenerative braking)

F = Flux Density

N = Speed of the motor

It can also been seen from above formula, that when the speed of the motor increases the field

strength decreases.

So, what can be concluded from this chapter in general manner is that, the when the

torque of the dc series motor is at highest the speed or the RPM of the motor is at its lowest, and

when the speed or the RPM of the motor is increased or at absolute maximum the torque of dc

series motor is the lowest. This lower torque at high speed is mainly caused by reducing applied

voltage, because of the back EMF generated by the motor acting as the generator when it cuts the

magnetic field by field windings by causing armature to be rotated at high speeds.

Shaft is connected to motor shaft which is directly controlled by armature movement in

the dc series motor, this shaft is connected by a mechanical system to the wheels. For the case

researcher is studying upon, the shaft would be connected to the LRT wheels, causing them to

turn as the armature turns, and following in same steps of the armature. By which it means

wheels will act in same way as the motor shaft or the armature reacts with varying voltage and

current being supplied (not absolute) to the dc series motor. Below researches has shown the

speed torque chart (4 quadrants) according to which motors (electric drives) can be selected

(specifications operation in quadrants defined) for desired function. Electric drives designed

for having more number of quadrants would have better range of control (speed, direction and

power).


11/31

11

(4 Quadrant Operation) (Available fromhttp://www.enm.com)

DC DC Converter Drives

DC Choppers are used for obtaining variable dc voltage from fixed dc voltage source. DC

voltage source may be a battery or output of diode rectifier connected to ac voltage source. DC

choppers are used for speed control of dc motors used for traction. The following are the

advantages:

yHigh efficiency

y Smooth controly Flexibility in controly Quick responsey Regeneration down to very low speeds

The dc motors speed can be either controlled by armature voltage control or by field flux

control. As the dc chopper circuit is connected between the dc source and dc series motor (field

windings and the armature windings), change in voltages to dc series motor can cause the speed

and torque of the dc series motor to vary (researcher needs this variation control).

Below researches has explained the working of basic dc chopper circuit briefly after

which he has blend in the speed control and braking operations (related to speed change) using

the dc choppers.


12/31

12

(Basic DCChopperCircuit) (Available from Power Semiconductor Drives by P V Rao)

Ede is the dc source voltage, S is switch which is operated at high speed and L is

inductance connected external to dc motor to keep the armature current continuous. Continuous

current operation is preferred to avoid torque pulsations.

(Wave forms of voltage and current) (Available from Power Semiconductor Drives by P V Rao)


13/31

13

S is a solid state switch which can be turned ON or turned OFF, TON is the time for which S is

ON and TOFF is the time for which S is OFF. During ON period output (eo) is equal to input

voltage Ede but during OFF period the output voltage is equal to zero.

The average output voltage eo = Ede [TON / (TON + TOFF)]

TON + TOFF = T the chopping period.

a is called the duty cycle a = TON / T

Then the average output voltage eo = Ede*a

The load voltage (input to the dc series motor) is controlled by varying the duty cycle a. There

are two ways by which this can be done, which are:

(i) Time Ratio Control

a. Constant Frequency System: TON is varied while T is held constant. It is calledpulse width modulation control or PWM control.

b. Variable frequency System: Either TO N or TOFF is held constant while T isvaried. This system is called frequency modulation control.

(ii) Current limit control

In this control the chopper is switchedON and switched OFF so that the current in the

load is maintained between two limits I max and I min.

Thus using one of the methods explained above the speed of the motor can be varied as

per the need by the researcher, usually PWM technique is used for motors with higher rating, the

ones which produce higher torques and have ability to reach high RPM. Now researcher will

focus briefly on types of DCChoppers which are used to get the power levels from the DC

source to the levels required by the electrical drives.


14/31

14

There are 2 types of DCChoppers:

(i) Step down chopper: The average output voltage eo is less than input voltage Ede.(ii) Step up chopper: The average output voltage eo is greater than input voltage Ede.

Step down chopper

(Step down chopper) (Available from Power Semiconductor Drives by P V Rao)

When S is ON, eo is equal to Ede but when S is OFF eo is equal to zero.

Average output voltage eo = (Ede*TON)/T = Ede*a

Step up chopper

(Step up chopper) (Available from Power Semiconductor Drives by P V Rao)


15/31

15

During TON switch S isOFF, inductor L is connected across Ede and the current increases thus

the stored energy in the inductance increases. Thyristor current is same as the source current and

diode D will not allow current from the load. During TOFF switch S is ON, thus load current is

same as the source current, after a while current decreases as the inductance and load are in

series. The voltage across L would be reversed and load voltage is sum of Ede and eL, this will

be higher than Ede:

eo = Ede + L(dio/dt)

During TON, the energy input to inductor Wi = Edc*Is*TON

During TOFF, the energy released by inductor to lead Wo = (eo Edc)*Is*TOFF

Under steady state these 2 energies are equal thus,

eo = Edc/(1-a)

The main difference which can be seen between the 2 choppers is that, in step up chopper

the switch (SCR) is in series with the DC source, while in step down chopper the switch (SCR) is

in parallel with the DC source. The step down chopper helps to reduce the source dc voltage to

the desired voltage (load), whereas in step up chopper the source dc voltage is increased to the

desired voltage (load). Doing so directly depended on the specifications of the series dc motor

chosen. So, after getting the value of dc source voltage to the level required by the load, various

techniques explained earlier above can be used to control the motors speed and other

parameters.

Now, the researcher will focus (briefly) on the electrical braking of the dc series motors.

According to Vasudevan. K et al, when a motor is switched o it coasts to rest under the action

of frictional forces. Braking is employed when rapid stopping is required. In many cases

mechanical braking is adopted. The electric braking may be done for various reasons such asthose mentioned below:

y To augment the brake power of the mechanical brakes.y To save the life of the mechanical brakes.y To regenerate the electrical power and improve the energy eciency.


16/31

16

y In the case of emergencies to step the machine instantly.y By reducing the stopping time

In many cases electric braking makes more brake power available to the braking process

where mechanical brakes are applied. Where the safety of the personnel or the equipment is at

stake the machine may be required to stop instantly. Extremely large brake power is needed

under those conditions. Electric braking can help in these situations. In processes where frequent

starting and stopping is involved the process time requirement can be reduced if braking time is

reduced. The reduction of the process time improves the throughput.

Basically the electric braking involved is fairly simple. The electric motor can be made to

work as a generator by suitable terminal conditions and absorb mechanical energy. This

converted mechanical power is dissipated on the electrical network suitably.

Electrical Breaking for DC series motor can be done by 2 basic methods which are:

(i) Dynamic or Rheostatic Braking(ii) Regenerative Braking

Dynamic Braking

This is done by removing or cutting the power supply (using SPDT type switch) to the motor,

thus the excitation current becomes zero as soon as the armature is disconnected from the mains

and hence the induced EMF also vanishes. In order to achieve dynamic braking the series eld

must be isolated and connected to a low voltage high current source to provide the eld. Series

connection of all the series elds with parallel connection of all the armatures connected across a

single dynamic braking resistor is used for the braking operation.

Regenerative Braking

In regenerative braking as the name suggests the energy recovered from the rotating masses is

fed back into the dc power source (dc motor acts like a generator). Thus this type of braking

improves the energy eciency of the machine. The armature current can be made to reverse for


17/31

17

a constant voltage operation by increase in speed/excitation only. Increase in speed does not

result in braking and the increase in excitation is feasible only over a small range, which may be

of the order of 10 to 15%. Hence the best method for obtaining the regenerative braking is to

operate, the machine on a variable voltage supply. As the voltage is continuously pulled below

the value of the induced EMF the speed steadily comes down. The eld current is held constant

by means of separate excitation. Braking torque can be obtained right up to zero speed. However

it suers from drawbacks like large ripple at low voltage levels, unidirectional power ow and

low over load capacity.

Thus, from the research done by the researcher he has found out (concluded) that neither

of the braking methods described above can be used solely for braking of the dc series motors

(LRT) alone. There is a need of combination of both the methods described above for better

efficiency along with the mechanical braking system. Whereas, for sure the regenerative braking

method cant be used alone without the mechanical braking system as, regenerative braking

characteristics arent applicable for the second quadrant. Still some work and research has to be

done for absolute electrical braking system for the dc series motor without using any mechanical

system (or researcher has not researched properly to find out if any such system or method

already exists).

Design Considerations related to Minimization of Harmonics Problems

A harmonic is a signal or wave whose frequency is an integral (whole-number) multiple of the

frequency of some reference signal or wave. The term can also refer to the ratio of the frequency

of such a signal or wave to the frequency of the reference signal or wave, according to

http://whatis.techtarget.com [Accessed 29th

September, 2011]. The effect of harmonics can be

seen clearly in voltage and current in power circuits.

For the case of LRT (dc choppers), the source of harmonics can be said to be the

switching action of thyristor (SCR). As, the switch goes ON and OFF 1000s of time in a second,

small ripples are produced in the output due to abrupt change of state from ON to OFF and vice

versa, which in turn effect the efficiency of motor as, most of parameters of motor as speed and

torque are directly related to the input voltage source.


18/31

18

For controlling or reducing the harmonics on the output of the load, for this case the dc

series motor, researcher uses feedback control system. For the case of the LRT system, the

harmonics mainly affect the dc series motor by changing its value of speed in sudden surges,

which directly affect other parameters. So, for minimizing this problem the researcher plan to use

a control system composed of speed proportional integral (PI) regulator, a current PI controller, a

speed measurement sensor, and a pulse width modulation sensor as can be seen below:

(Speed and Current DCChopperControl) ( Available from High Performance DC Chopper

Speed and Current Control of Universal Motors Using a Microcontroller by King. K et al)

The system has two control loops: an inner current loop and an outer speed loop. Both the

speed and current controller use a typical PI control loop format. The speed PI loop outputs the

reference current required for the proper torque. The current PI control loop generates the

voltage necessary to maintain the torque. Whenever a reference speed S* is given, the control

system automatically compares that reference speed to the motors actual speed S, which isdirectly measured from the speed sensor. Based on the equation for the motors mechanical

motion, the speed error S is the torque profile. Therefore, the output of the speed controller can

be considered as the torque or the current reference value . An adjustment is made so that the

motor speed S follows the given reference value S*, and the motor achieves a steady state

quickly. The current PI controller regulates the motors real current I and the reference current ,


19/31

19

and tracks the reference current and load in time. The output of the current controller is the

PWM duty ratio, which directly controls the MOSFET ON orOFF states.

The current control loop is designed to be much faster than the speed control, so that the

proposed DC chopper control system has a fast dynamic response to load changes and system

variations. For this reason the addition of current control is crucial to the application according to

King. K et al. This proposed speed and current dc chopper for dc series motor has been

implemented using 16 bit, low cost and low pin count microcontroller. For the sped detection

and measurement Hall Effect sensors are used whose output acts as interrupts for the

microcontroller.

Specifications of Over Voltage Circuit Protection

The least one can do to protect the output load is by doing 2 basic things at start while

constructing an electro mechanical system, which are choosing or designing the load (dc series

motor) with higher power rating than the input power rating and using fuse to prevent the load

from sudden rise in voltage.

There are three major sources of transient which may affect thyristor circuits:

y The mains supply (e.g. lightning)y Other mains and load switches (opening and closing)y The rectifying and load circuit (commutation)

In order to ensure reliable circuit operation these transients must be suppressed by

additional components, removed at source or allowed for in component ratings. Three types of

circuit are commonly employed to suppress voltage transients; a snubber network across the

device, a choke between the power device and external circuit or an overvoltage protection suchas a varistor.

For dc chopper circuits, researcher found snubber network to be most useful to minimize

over voltage surges or transients. Thus, researcher is going to explain briefly the working of the

snubber network to minimize the overshoot or the over voltage for a thyristor circuit.


20/31

20

Snubbers are circuits which are placed across semiconductor devices for protection and to

improve performance. Snubbers can do many things:

y Reduce or eliminate voltage or current spikes.y Limit dI/dt or dV/dt.y Shape the load line to keep it within the safe operating area (SOA).y Transfer power dissipation from the switch to a resistor or a useful load.y Reduce total losses due to switching.y Reduce EMI by damping voltage and current ringing.

As, most of the circuits in power electronics have common networks and waveforms

associated with it, researching with be focusing of easiest to understand of them all the boost

converter. For snubber design we are concerned with circuit behavior during the switch transition

time which is much shorter than the switching period. This allows us to simplify the analysis. In

normal operation the output voltage is DC with very little ripple. This means that we can replace

the load and filter capacitor with a battery since the output voltage changes very little during

switch transitions. The current in the inductor will also change very little during a transition and

we can replace the inductor with a current source. Typical boost converter along with the

simplified circuit is shown below:

(Typical Boost Converter) (Available from Design of Snubbers for Power Circuit by Severns)


21/31

21

(SimplifiedC

ircuit for BoostC

onverter) (Available from Design of Snubbers for Power Circuitby Severns)

At the beginning of the switching cycle the switch is open and all of the current (Io) will be

flowing through the diode into the battery. As the switch turns on, the current will gradually shift

from the diode to the switch. However, as long as there is current in the diode, the switch voltage

will remain at Eo. Once all of the current has been transferred to the switch, the switch voltage

can begin to fall. At turn-off the situation is reversed. As the switch turns off, the voltage across

it will rise. The current in the switch will however, not begin to fall until the switch voltagereaches Eo because the diode will be reverse biased until that point. Once the diode begins to

conduct the current in the switch can fall.

This type of switching commonly referred to as hard switching, exposes the switch to

high stress because the maximum voltage and maximum current must be supported

simultaneously. This also leads to high switching loss. In practical circuits the switch stress will

be even higher due to the unavoidable presence of parasitic inductance Lp and capacitance Cp as

shown below. Cp includes the junction capacitance of the switch and stray capacitance due to

circuit layout and mounting. Lp is due to the finite size of the circuit layout and lead inductance.

Lp can be minimized with good layout practice but there may be some residual inductance which

may cause a ringing voltage spike at turn-off.


22/31

22

(Practical Boost ConverterCircuit) (Available from Design of Snubbers for Power Circuit by

Severns)

The most common reasons for using a snubber are to limit the peak voltage across the

switch and to reduce the switching loss during turn-off. An RC snubber, placed across the switch

as shown in figure below, can be used to reduce the peak voltage at turn-off and to damp the

ringing. In most cases a very simple design technique can be used to determine suitable values

for the snubber components Rs and Cs.

(RC SnubberCircuit) (Available from Design of Snubbers for Power Circuit by Severns)


23/31

23

According to Severns to achieve significant damping Cs > Cp. A good first choice is to make Cs

equal to twice the sum of the output capacitance of the switch and the estimated mounting

capacitance. Rs is selected so that Rs =Eo/Io. This means that the initial voltage step due to the

current flowing in Rs is no greater than the clamped output voltage. The power dissipated in Rs

can be estimated from peak energy stored in Cs:

Up = CsEo^2/2

This is the amount of energy dissipated in Rs when Cs is charged and discharged so that the

average power dissipation at a given switching frequency (fs) is given by:

P dissipation Cs (Eo^2) fs

This is how using RC snubber network the over voltage can be minimized in the dc chopper

circuit, the one being used by the researcher for the LRT system (dc series motors).

Improvement of Motor Efficiency

Basically motor efficiency can be defined as output power by input power. This is 1 for ideal

case, such that there arent any loses which transmitting power from input to output or electrical

energy to mechanical energy. But this isnt true in practical, as there are various factors by which

there are energy losses when the power is transmitted from input to the output. Thus the motor

efficiency in practical conditions is usually less than 1.

(Motor Losses) (Available from Electrical Energy Equipment: Electric Motors)


24/31

24

Improving the motor efficiency means reducing costs for running the motors, saving

energy and higher productivity. These motor losses (2-20%) can be separated into various

categorized (causes) which are:

y Stator Copper Loss (I2R) that are result of current in windings. Two ways to reducestator copper losses are by reducing the electric current flowing through the stator and

reducing the windings ohmic resistance. A certain amount of current is needed to

magnetize the stator steel while the balance produces the output power. The efficiency is

raised by reducing the magnetizing current. Motor manufacturers accomplished this by

increasing the steel stack length (core length). The copper losses also can be reduced by

reducing the winding resistance. This requires a larger wire size. The slot sizes are fixed

for a given stator lamination configuration, so theres a limit on how much wire can be

inserted into it. It would be possible to exceed the slot size limit only by redesigning the

lamination to increase the slot size.

y Rotor Copper Losses (I2R) that are a result of current in the rotor. One can reduce rotorlosses by decreasing the rotors ohmic resistance. Motor manufacturers increase the rotor

end ring, redesign the rotor slot and use copper rotor conductors instead of aluminum. All

these options cost money. Theres a lower limit on rotor resistance because as it drops,

the full-load speed increases. Changing end rings is a relatively small expense compared

to redesigning the rotor slot and converting to copper conductors, but it adds expense

nonetheless.

y Core Losses that result from magnetizing the steel. These consist of hysteresis and eddycurrent losses. Core loss can be lowered by reducing the magnetic flux density and by

changing the steel type. The hysteresis component of core loss is determined by the

characteristics of the steel when magnetized in one direction then demagnetized and

remagnetized in the other direction. This is largely a function of the steel type and

processing.

The eddy current component of the core loss results from the changing magnetic

field inducing voltages and currents in the steel core. Its a function of the steel type,


25/31

25

thickness, and processing and can be reduced by upgrading the steel type and using

thinner laminations. This alloy is more costly and more laminations are needed for a

given core length to produce the same power.

y Friction and Windage Losses that result from bearing friction and the rotor moving theair inside the motor. These losses have two major components: bearing friction and

cooling system losses. Bearing friction is a function of the bearing size, lubricant and the

approach used to seal the bearing. Because most bearings are chosen as a function of the

load and application, it leaves little room for change, other than using better grease and

seals.

Windage loss results from having to move cooling air through the motor. The

cooling fan constitutes the largest part of the loss. Motors have cooling fans to maintain

temperatures within the limitations of the insulation system. The fan can be removed if

its possible to control the temperature within safe limits in other ways. In most cases,

however, some means of cooling is required. Improving the entire cooling system to

reduce air flow losses is a reasonable goal. Improved cooling systems usually require the

addition of baffles and venturis, which add cost.

y Stray Load Losses are those not accounted for in the other loss categories. Theresevidence that some stray load losses are caused by eddy currents in the tooth tips and

rotor surface, as well as rotor bars being shorted to the lamination steel. These losses can

be reduced somewhat by changing the rotor manufacturing methods. But, these additional

operations, too, lead to increased costs as according to www.plantservices.com [Accessed

on 29th

September, 2011].


26/31

26

Beyond motor losses, there are other factors that can impact electric motor efficiency. They

include:

y Proper sizing (appropriate size, motor shouldnt be oversized or undersized)y Electrical power quality (harmonics, over voltage, under voltage, frequency imbalances)y Type of controly Distribution lossesy Type of transmissiony Maintenancey Operating temperature (lesser the temperature, lesser is the winding resistance)y Application (mechanical efficiency of driven equipment)

According to http://www.ohioelectricmotors.com, motor efficiency is improved by reducing

motor losses: power, core and mechanical. This is accomplished in any of the following ways:

y Build with closer tolerancesy Reduce vibrationsy Increase the amount of copper in the stator windingsy Use higher-grade electrical steely Improve the power factor to reduce reactive current heatingy Use high efficiency mechanical loads (pumps, fans, etc.)y Use electronic controllers instead of across-the-line start/stop controlsy Use energy efficient belts and/or gear reducersy Use power conditioning equipment.y Use new dc motor types: Permanent Magnet and Brushless types.y Advances in the brush-commutator area.

Calculations of Traction Drive Rating and Energy Consumption

Traction maybe defined as maximum frictional force between 2 surfaces, without slipping over

each other. Tractive force is the force exerted by a vehicle or object over another object or


27/31

27

surface, to be capable to push or pull. In simpler terms (railways) its the friction force between

the wheels and the railway tracks or roads. Usable traction is defined as coefficient of traction

multiplied by weight (wheels, locomotive). Coefficient of traction is same as limiting friction.

For calculating the traction drive rating and energy consumption researcher has taken

some assumptions as were not provided by the question. The assumptions taken for LRT train

are as following:

Number of coaches = 3

Weight of each coach = 1200 kg

Maximum add on weight = 11500 kg (includes 150 passengers and luggage)

Operating Time = 20 Hrs.

Wheels Diameter = 80 cm

Static coefficient of friction = 0.3 (metal on metal)

Kinetic coefficient of friction = 0.2 (metal on metal)

Acceleration due to gravity = 10 m/s^2

Tractive force (Braking):

F (braking) = Total Weight * kinetic coefficient of friction

= [11500 + 3600] * 10 * 0.2

= 30200 N

F (start) = Total Weight * static coefficient of friction

= [11500 + 3600] * 10 * 0.3

= 45300


28/31

28

T (Torque required to restart the motion) = F (start) * Radius of Wheels

= 45300 * 0.4 m

= 18120 N.m

W (Angular Velocity) = Velocity/ Radius of Wheels

= 120 km/hr. / 0.4 m

= 33 m/sec / 0.4 m

= 82.5 rad/ sec

P (Power required to drive at desired maximum speed) = Torque * Angular Velocity

= 18120 * 82.5

= 1.49 MW

So, now researcher can calculate energy consumed by the LRT in 20 hrs by:

E = Power * Operating Time

= 1.49 MW * 20 hrs.

= 29.8 MW. H

Thus, energy consumption by the LRT train is 29.8 MW. H and the traction drive rating should

be around 1.86 MW considering 80% efficiency of the dc series motor after the motor energy

losses.


29/31

29

Applications of LRT System

LRT systems have been life line of many metropolitan cities around the world as Kuala Lumpur,

New Delhi, New York, Singapore City and many more. The main reason being that LRT systems

are fastest mode of transportation on land, for short distances as due to increasing traffic by

motor vehicles. The other reason LRT systems are supposed to be popular are is because they

can reach congested places where it is difficult for other modes of transportation to reach fast and

safely (LRT systems have been constructed over the highways and underground).

From the environmental point of view, LRT systems cause almost no pollution and helps

in reducing the dependence upon non renewable sources of energy. From the passenger point

of view LRT systems are timesavers as they are fast and have high frequency of operation, to top

that it is easy, convenient and cheap to use.

Proposal for Future Improvements and Conclusion

In above chapters researcher has discussed the basic operation of dc series motor, explained the

motor speed torque characteristics according to which motors speed and torque can be

controlled by varying the input voltage, then dc dc power converter used has been explained

and how are dc choppers able to control the speed and torque of the dc series motor using various

methods, after which researcher focused upon the braking methods which can be applied for dc

series motor, after that researcher explained the system he is going to use to reduce the

harmonics in the electrical circuit caused by switching action of thyristor, after which he focused

on RC snubber network which can be used to minimize the over voltage to the dc series motor,

after that ways to improve motor efficiency were discussed along with causes for energy losses,

at last the energy consumption by the LRT system was calculated and the traction drive rating

required for moving the locomotive was calculated.

For future improvements researcher would focus on using AC Motors (induction) as, the

power grids around the world supply AC power, this AC power can be feed directly to the AC

drives without much change. As such now days usage ofAC motors (induction) have increased

rapidly, maintenance and repair cost are low, plus the energy losses in these motors are far less


30/31

30

compared to series dc motor. In all AC motors (induction) are more efficient than DC series

motors.

References

Books

y Rashid, M.H., 2007, PowerElectronics: Circuits, Devices, and Applications, 3rd edition,Prentice Hall of India Private Limited, New Delhi, India

y Sen, P.C., 2009, PowerElectronics, Tata McGraw-Hill Education Private Limited, NewDelhi, India

Online

y DC Motors: High Efficiency Designs. 2011. DC Motors: High Efficiency Designs.[ONLINE] Available at: http://www.ohioelectricmotors.com/dc-motors-high-efficiency-

designs-782 [Accessed, 28th

September 2011]

y DC Series Motors [Online] http://nptel.iitm.ac.in/courses/IIT-MADRAS/Electrical_Machines_I/pdfs/2_8.pdf[Accessed, 28

thSeptember 2011]

y DC motor starting and braking (Theory): Power Electronics and Electric DrivesLaboratory: Electrical Engineering: IIT DELHI Virtual Lab. [ONLINE] Available at:

http://iitd.vlab.co.in/?sub=67&brch=185&sim=470&cnt=1 [Accessed, 28th

September

2011]

y Direct Current Motors, Series Wound DC Motors, Compound Wound Motors, SquirrelCage Induction Motors, Wound Rotor Induction Motors, Single Phase Induction Motor,

Mumbai, India. [ONLINE] Available at: http://www.micromotcontrols.com [Accessed,

28th

September 2011]


31/31

31

y Drive Characteristics 2011 [ONLINE] Available at: http://kaliasgoldmedal.yolasite.com[Accessed, 01 October 2011]

y Electronic Power for Trains. 2011. Electronic Power for Trains. [ONLINE] Available at:http://www.railway-technical.com/tract-02.shtml [Accessed, 28

thSeptember 2011]

y How manufacturers improve electric motor efficiency and why they cost more | PlantServices. [ONLINE] Available at: http://www.plantservices.com/articles [Accessed, 28

th

September 2011]

y Speed Controllers. 2011. Speed Controllers. [ONLINE] Available at:http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

[Accessed 28th

September 2011]

y Understanding Motor Characteristics. [ONLINE] Available at:http://lancet.mit.edu/motors/motors3.html [Accessed, 28

thSeptember 2011]

y What is harmonic. Definition from WhatIs.com. [ONLINE] Available at:http://whatis.techtarget.com/definition [Accessed, 28

thSeptember 2011]

y Xu, H. et al, 2007, High Performance DC Chopper Speed and Current Control ofUniversal Motors Using a Microcontroller [O NLINE], Renesas Technology America,

Inc., Available at: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=04347860

[Accessed, 28th

September 2011]

y Severns, R., 2011, Design of Snubbers for PowerCircuits [ONLINE], Cornell Dubilier,Available at: http://www.cde.com/tech/design.pdf[Accessed, 28

thSeptember 2011]

Documents

Development of Electric Drives in Light Rail Transit (LRT) System