performance improvement of dc electric traction motors using

8/10/2019 Performance Improvement of DC Electric Traction Motors Using

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PERFORMANCE IMPROVEMENT OF DC

ELECTRIC TRACTION MOTORS USING A

NOVEL SWITCHING TECHNIQUE

M.Sc.Engg. Thesis Presentation

Presented by

S.M.FerdousStd. ID092606


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Overview of the Presentation

Performance improvement of a DC Drive basedtraction system using a novel winding switch-over

technique in Compound Motor.

Modeling and characterization of the motor.

Analysis of Traction Load characteristics.

Design and Simulation of Converter.

Controller Circuits.

Overall System Modeling and Simulation.

Conclusion


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Introduction Electric Traction

LocomotiveMain Line Supply System Based.

Battery Operated.

Electric Vehicle

Battery Operated (Stand Alone).

Hybrid

Electric Traction Drives

AC Drives (Induction Motors, PMSM)

DC Drives (Series Motors, PMDC Motors)

Modern Drives (SRM, LIM, Magnetic Levitation)


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Electric Traction Drives

Electric Vehicles will lead to revolutionaryimprovements on vehicle performance, energy

source and pollutant emissions.

Very high efficiency and optimal performance.

Energy Conservation.

Ideal for traction system.

Advantages of Electric Drives for traction

Disadvantages


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Characterization of Electric Motors for Traction

Application

High speed motors capable of operating in extended constant

speed region are best suited for Electric and Hybrid vehicles

(EV and HEVs).

Vehicle Operating Constraints like- initial acceleration and

gradability can be met with minimum power rating.


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Characterization of Electric Motors for Traction

Application (contd.)

Longer constant power range operation of the motor

effectively reduces the motor power rating.

Reduced Power consumption.

Improved, fast and rapid acceleration. Gradeability of the vehicle is improved.

Single and simple gear transmission.

Reduction in size and capacity of battery. Design of the vehicle is compact, robust, highly

efficient and reliable.


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Tractive effort and power versus vehicle speed with

different speed ratio, x


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Introduction of Compound Motor as

Traction Drive

DC Series motor is widely and conventionally used for tractionpurpose as its characteristics matches the Traction loadcharacteristics the most.

At the same time DC series motor suffers from two significantdisadvantagesField control is not suitable and unstable

during regenerative braking. A Compound motor provided with winding change over

facility should outdo the performance of DC series motor.

This will enable the motor to operate at three differentconfigurationCompound, Series and Shunt.

This would suit the traction characteristics more.

Switching will prolong the constant power range operation ofthe motor.


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Introduction of Compound Motor as

Traction Drive (contd.)

Fig.3. Different Torque-Speed Characteristics of a DC Machine of same power

rating (175W) with three separate configuration.


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Introduction of Compound Motor as Traction Drive

(contd.)


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Introduction of Compound Motor as Traction Drive

(contd.)


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Winding Change over design

This simple arrangement can be designed for winding change overusing a SPST and a SPDT switch.

Change over will take place by sensing the speed of the vehicleusing a tachometer and a F/V converter.

Simple Relay contacts (N/O and N/C) can be used for windingchange over purpose.

Free wheeling path for the winding current must be provided .


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Benefits of winding change over Technique used

for the motor

An Optimum performance would be obtained using a DCCompound motor with winding change over technique

High starting Torque with Low speed

Due to winding change over a high final speed is attained with adrop in Load Torque.

Very smooth regenerative braking is possible as the machine will beconfigured as Shunt Motor during the time of regenerative brakingwhich is very much stable for this kind of operation.

Reduced Power rating of the motor to achieve same performance.

Single gear transmission instead of Multi gear transmission system.

Reduced sizing of the on board energy storing device or converselymileage of the vehicle will be increased with the storage battery ofsame size and capacity.

Saving in energy is increased as the kinetic energy of the vehicle willbe used to charge the battery through regenerative braking whichimplies as almost 30-40% of energy can be saved by the system.


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Modeling and Analysis of the Motor

The General equations of the Motor suggests that, a

compound motor is highly non-linear in nature andhence its analysis would be very difficult.

Torque-Speed, Torque-Current and Speed-Currentequations are highly no linear in nature.


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Specification of the Motor

Ia= Armature Current

IF= Field Current V = Supply Voltage

EB= Back EMF

RT= Total Resistance of the armature circuit = Ra+Rse

Ra

= Resistance of the armature

Rse= Resistance of the series winding

RF= Resistance of the field winding.

= Total Flux = se+sh

se= Flux produced from Series field (Wb) =

sh= Flux produced from Shunt field (Wb) = = Angular velocity (rad/sec) = ; N = R.P.M of the motor

KB= Back EMF Constant


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Specification of the Motor The specification of motor is as follows :

Voltage, V = 60V, Ia(rated) = 40A , IF= 5A, Total Current, ITotal= 40+5=45A

Total armature Resistance, Rtotal= Rse+Ra= 0.15;

Field Resistance, RF= 12

Rated power, P

No load Speed of the motor, NNL = 1800 RPM No load angular velocity, NL= 188.4 rad/sec

To overcome the maximum torque offered by the load (i.e. the vehicle

itself) the motor must be capable of developing a torque of 65Nm at rated

condition. So, the rated torque of the motor should be 65N.m and must be

developed at rated power. rated=38.1 rad/sec


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Characteristics of the Motor

Speed-Current characteristics is given by-

Torque-Current Characteristics is given by-

Speed-Torque characteristic is given by-

where,


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Characteristics of the Motor


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Non-Linear Model of the Motor

N Li M d li f th M t


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Non-Linear Modeling of the Motor

(contd.)

Assuming the field Current is constant the model can beconverted into-


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Linearization of the Model

The linearized system model equations can be written as(neglecting all the small terms with values very close to zero)


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Linearized Block Diagram


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Linearized Transfer Function

The Linearized transfer Function obtained as-

where,


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Dynamics of Traction Load Modeling of traction load means is to develop an equation for the

tractive effort required for the propulsion of the vehicle. Total force required for the propulsion of the vehicle is given by-

Fte

= Frr

+ Fad

+ Fhc

+ Fla

+ Fa

Where,

Frris the rolling resistance force, F

adis the aerodynamic drag,

Fhc

is the hill climbing force,

Fla

is the force required to give linear acceleration

Fa is the force required to give angular acceleration to therotating motor

We should note that Fla

and Fawill be negative if the vehicle is

slowing down, and that Fhc

will be negative if it is going downhill.


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Dynamics of Traction Load (contd.)

Putting all the vehicle parameters the final equation for

tractive load can be obtained as-

Fte

=

The system arrangement for vehicle propulsion can be shown

with the following diagram-


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Dynamics of Traction Load (contd.)

Load Torque referred to motor shaft can be written as-


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Specification of the Vehicle

The electric vehicle has a mass of 380 kg, with a typical passenger ofmass 180 kg (for 3 passengers with average mass of 60kg) so totalmass m = (180+200) = 380 kg.

To incorporate the angular acceleration of different rotating parts ofthe vehicle along with motor, m is increased by 5% in the linearacceleration term only. A value of 400 kg will thus be used for total

mass of the vehicle. The drag coefficient Cdis estimated as 0.3, a reasonable value for a

small electric vehicle whose shape of the body is aerodynamicallydesigned and optimized.

The frontal area of vehicle and rider = 1.2 m2.

The tires and wheel bearings give a coefficient of rolling resistance,

rr

= 0.005 which is a typical value for specially designed tires forelectric vehicle.

The motor is connected to the rear wheel using a 2:1 ratio beltsystem, and the wheel diameter is 60 cm. Thus G = 2 and r = 0.3 m.


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Equation of Dynamics of the vehicle

Putting all the values in the obtained equation of thetorque-

This equation defines the load torque in terms of

motor speed. Where as a torque equation in terms

of vehicle speed can be obtained by simple

manipulation as-


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Simulation of vehicle acceleration and other

parameters

Two equations are obtained to simulate the vehicleperformance parameters. The 1stequation is applicable when

motor speed is less than Base speed and the 2ndequation will

be applicable when the motor speed is greater than Base

speed.

(


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Simulation Results (without winding

Change over)

Figure 17: The initial acceleration and final

velocity of the vehicle. From the figure it is

clearly evident that the vehicle takes just

over 5 seconds to reach its maximum speed

of 22.5kmph.

Figure 18: The torque-velocity curve of

the motor and vehicle respectively.


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Simulation Results (with winding Change over)

Fig. 19. Simulated Speed, acceleration and Torque characteristic of the vehicle with

the feature of winding change over facility.


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A comparative diagram showing the speed without and with the winding

change over facility would be more helpful to justify the improvement in

the performances of vehicle. A diagram of such kind is shown in Figure 20in the following-

Figure 3.11: Comparative analysis showing the differences in terms of final speed

between the two types of motor.


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There is a sharp rise in current to a extremely high value

which is sufficient to damage the motor. So, Power electroniccurrent controller along with converter must be provided to

limit the current with in a permitted range.

Fig. 22: Speed-current characteristic of the

motor. After winding change over, the value of

the current remains very high during the

entire period of its acceleration.

Fig.21 : Current profile of the motor during

its entire period of operation.


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Design of Converter and Controller

For any traction application a two quadrant converterwith a pair of reversing switch is necessary. Otherwise itis not possible for the motor to operate at all fourquadrants as it is mandatory for any motor to becapable of operating in all the four quadrants employedfor traction application.

In this design a Two Quadrant Class C DC-DC converteralong with a pair of reversing switch are used. Theconverter has a novel integrated feature of both PWM

and Hysteresis controller, where the PWM controller isused for variable voltage operation of the motor (to runthe vehicle at different speed) and hysteresis controlleris used for the purpose of current control


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Design of Converter and Controller

The designed system is shown in brief in the following blockdiagram-

Fig.24 : Block Diagram Representation of the Motor Controller


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2-quadrant Class C DC-DC Converter

A typical class C converter is made of one pair of diode and one pair of switch.

Generally, it is made from one buck and one boost converter. For normal motoringmode the circuit operates as buck controller. During braking of the motor which is

also known as regenerative braking, the converter operates as a boost converter to

feed back the stored kinetic energy of the motor to the source and thus reducing

its speed.

Fig. 25 : Class C DC-DC converter


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Buck Operation (Motoring Mode)

Transistor T1and Diode D1will be operating in

Motoring Mode and hence the converter will

act like a buck converter.

Fig. 26 : Buck Converter


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Hysteresis

Controller

Current SensingCircuit

PWM Controller


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Simulation Result

Fig. 27 : Motor Current, Output Voltage and PWM signal of the converter.


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Simulation Result

Fig. 28 : Motor Current with out controller.


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Simulation Result

Fig. 29 : Hysteresis current controller action to limit the starting motor current

within its maximum limit. If the motor current exceeds twice the value of the rated

current the controller turns off the power supply and when the current falls to

value sufficiently low enough the controller again turns on the power supply.


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Simulation Result

Fig. 30 : Output voltage of the converter at a Duty cycle of 90%. The variable output

voltage can be obtained by varying the duty cycle of the converter. Variation of duty

cycle is possible by varying the reference voltage of the PWM comparator


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Boost Converter (Regenerative Braking)

Transistor T2 and Diode D2 will be operating

together in this mode and the circuit behavelike a Boost converter.

Fig. 31 : Boost Converter for regenerative braking


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Simulation Result (Boost Converter)

Fig 31 : Simulation of Boost Converter for Regenerative Braking


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Variable Duty cycle

PWM signal generating

Circuit for the Boost

Converter


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Fig. 32 : Output Voltage and Current of the Boost converter during Braking


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Fig. 33 : Generation of Reference signal to vary the duty cycle of the converter


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Fig. 34 : Boost Converter Input Power due to the kinetic energy stored in the vehicle .


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Fig. 35 : Boost Converter Output Power. This amount of energy which is equal to the

area under the curve, is feed back to the source


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Overall System Simulation

Fig. 36 : Simulation of the entire Electromechanical System Using SIMULINK


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Simulated

Converter

Block


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Simulated

Traction Load


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Switch to

Simulate the

Winding

Change over

of the motor


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Fig. 37 : Speed response of the vehicle with simulated in SIMULINK

Instant of Winding

Change over

O ll S Si l i


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Fig.37 : Current output of the motor. The motor current is being regulated by

the Hysteresis controller, always remains in the permissible limit of operation

O ll S t Si l ti


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When Ever the

motor current

exceeds the upper

limit, the hysteresis

controller limits the

current within thepre-defined range

Instant of Winding

change over and thereis a large spike in

current, eventually

limited by hysteresis

controller

O ll S t Si l ti


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Small value

of current as

motor speed

is very low

Motor Currentincreases to its rated

value after winding

change over (40A)



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Fig.38 : Load Torque for the motor, i.e. Torque offered by the vehicle towards

the motor.

O ll S Si l i


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Fig. 40 : Output Power of the motor. Operated at rated power at the design

Speed (2400W)

Overall System Simulation for Series


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y

Motor

Fig. 41 : Final Speed of a DC Motor with series configuration. The final Speed of

the vehicle is 57 kmph.



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y

Motor

Fig. 41 : Armature current of the series Motor which is around 22A.



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y

Motor

Fig. 41 : Output Power of the Series Motor (1375W)

performance improvement of dc electric traction motors using

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