Final Project: Electric Rickshaw modelling

Aim:

Create a detailed MATLAB model of an electric rickshaw. The following details have been considered:

• Rear wheels driven by PM brushed type motor
• Assume efficiency points of motor controller and motor
• Make an excel sheet of all input and assumed data
• Results: For any three standard driving cycles show energy consumption, the temperature rise of the motor and controller for 100 km constant speed driving at 45 mph.

Theory:

Here Mahindra TREO e-Rickshaw has been modeled.

Vehicle Parameters :

Drive train: Pure Electric Vehicle

Vehicle Type: E-Auto

Battery Pack: Li-ion Battery Pack (7.37 kWh)

Nominal Voltage: 48 V

Motor: 8 kW (Rated Power) DC Motor

Vehicle Kerb Weight: 377 Kg

Payload: 300 Kg

Width : 1.765 m

Height: 1.595 m

Dimensions - Length x Width x Height : 2.769 x 1.35 x 1.75 m

Methodology:

1. First all the input and assumed data are put in excel

2. The propulsion power and traction force required are calculated

3. Best suitable motor and battery are decided.

The Mahindra TREO Power Calculations and the assumed data can be seen in the excel file below:

EV Motor Power Calculations	Mahindra Treo
Rolling Resistance
Curb Weight (Kg)	377
Payload(Kg)	250			Default EV
Coefficient of Rolling Resistance (urr)	0.015
Rolling Resistance Force (N)	92.169			Tyre 10R/20
			Dia	0.20	M
Air Drag Calculation			Radius	0.10	m
Air Density (Kg/m3)	1.25
Width (m)	1.35		Total Traction Force	635.1768125	N
Height (m)	1.75
Frontal Area (m2)	2.3625		Gear	5	(1:5 Gear Ratio)
Drag Coefficient	0.2
Speed(kmph)	30		Wheel Torque (Nm)	63.51768125	(Rated)
Air Drag(N)	20.5078125
			Motor Torque (Nm)	12.70353625	(Rated)
Total Power at given speed (kW) Air Drag + Rolling Resistance	0.9389734375	kW
			Speed of axel	83.3333333333333	rad/s
Hill Climbing
Angle (Degree)	0		Speed of shaft	416.666666666667	rad/s
Speed during hill climbing (kmph)	10
Force(N)	0		Power (Rated)	5.29314010416667	kW
Hill Climbing Power (kW)	0	kW
Acceleration Calculation
Final Speed (0 to x kmph)	45
Acceleration Time (sec)	15
Acceleration (m/s2)	0.833333333333333
Acceleration (N)	522.5
Acceleration Power(kW)

MATLAB / Simulink Model

The MATLAB model has five components :

1. Vehicle Body: It includes the vehicle body, tires, and gear

2. Electrical System: It includes the power converter and dc motor

3. Battery: It includes calculation of State of Charge and battery model

4. Driver: It includes the control of the throttle based on the driving cycle

5. The drive cycle.

Fig 1: SSystem-LevelDesign

1. Vehicle Dynamics

It constitutes the vehicle body, load, gear, and tire. We will look into each in detail.

Fig 2: Vehicle Dynamics

The vehicle body has a total mass of 377 Kg. There is one rear axel connected to the gear with two wheels. The front wheel is free to rotate. The frontal area of the vehicle is 2.365 m^2 for the calculation of drag force. H is the connection or horizontal movement of the vehicle. W is the road incline angle, which we have taken as 0. The outputs are V or the velocity, in m/s. NF and NR are the front wheels and rear wheel forces.

Fig 3: Vehicle Body Parameters

The tires are modeled based on the longitudinal traction force. The peak traction force calculated is 600 N which has been used as a parameter. The nominal force is taken as 450 N. The Tire (Magic Formula) block is used here. The output here is the slip. The radius of wheels is taken as 0.2 m which is the same as for the 10R/20 model used in Mahindra TREO.

Fig 4: Tire Parameters

The transmission gearbox in the electric vehicle has a single gear ratio. The gear ratio chosen is 7. The output and input shaft rotates in the same direction.

Fig 5: Gear Ratio

2. Electrical System

The electrical system consists of the power converter and DC motor. The throttle output is controlled by the driver. This output is the control signal for generating PWM signals for the power converter. The power converter used here is a 4-quadrant H bridge. This converter can work in all four quadrants and can be used for regenerative braking as well. Here a PMDC motor is used. The field is provided by permanent magnets. Here, the solver configuration is set to use the Backward Euler solver, which increases the simulation speed significantly. The number of iterations is also high to get greater accuracy.

Fig 6: Electrical System

The DC motor is using permanent magnets for in the stator. The rated output of the motor is the same as calculated i.e. 6 kW. The rated DC supply is 48 V. The block assumes that no electromagnetic energy is lost, and hence the back-emf and torque constants have the same numerical value when in SI units. When a positive current flows from the electrical + to - ports, a positive torque acts from the mechanical C to R ports. Motor torque direction can be changed by altering the sign of the back-emf or torque constants. The temperature dependance of the motor has been modelled. Thermal mass of the motor has been taken as 20000 J/k. Here the initial temperature is chosen to be 298.15 K and the temperature variation can be seen.

Fig 7 : DC Motor

The H Bridge is 4 quadrant DC-DC converter. It has four switches and can work in all four quadrants, i.e. Forward Motoring, Forward Braking, Reverse Motoring and Reverse Braking. In the drive cycle, only forward motoring and forward braking mode is used. The battery gets depleted in the forward motoring mode as the DC machine absorbs power and acts as a motor. In the forward braking mode, the SOC of battery increase, as the battery gets charged. This is possible since the DC machine acts as a generator in the 2nd quadrant / regenerative braking mode. An internal power source of 48 V is provided. With the duty ratio of 100% the nominal voltage of 48 V is obtained at the output of the H bridge. The temperature dependance of the controller has been modelled. Thermal mass of the controller has been taken as 10000 J/k. Here the initial temperature is chosen to be 298.15 K.

Fig 8: H Bridge Parameters

The gate signals are generated using Pulse Width Modulation technique. The carrier signal has the frequency of 5000 Hz. Acceleration command and Deceleration command vary from 0 to 100%. For a 0% signal, the controlled voltage source has a value of 0 V and for 100% throttle , the output voltage is 1. This throttle signal is the control signal for generating PWM signals. Fig 9 shows the 4-quadrant chopper circuit. Here the acceleration command controls the switching of S1 and S4. The deceleration command controls the switching of S2 and S3. The parameters for generating PWM signals can be seen in Fig 10.

Fig 9 : 4-Quadrant Chopper

Fig 10 : PWM Controller Parameters

3. Battery / SOC

Here a rechargeable Li-ion battery pack is used. The battery SOC is calculated using the current flow through the battery. This is represented by a current source. The nominal voltage of the Li-ion battery as calculated to be 48 V at 90% SOC. The rated capacity of the battery is 153 Ah. The battery parameters can be seen in Fig 12.

Fig 11: Battery System

Fig 12 : Battery Parameters

4. Driver :

The driver presses down on the throttle or applies brakes depending on the desired Driving Cycle and the incline/decline of the road or road conditions. This effect has been modelled using the Longitudnal Driver block. THis block generates the normalised acceleration and braking commands that vary between 0 and 1.

Here PI control is used. The tuned parameters for proportional gain Kp is 10 and that of integral gain Ki is 10.

Fig 13: Longitudinal Driver Parameter

5. Drive Cycle :

Here three driving cycles are considered. A manual switch is used to change the driving cycles. First Wide Open Throttle (WOT) cycle is tested. Next, Modified FTP75 drive cycle is used for simulation. This cycle runs for 2474 sec. The maximum speed is 45.6 Km/h and average speed is 12.9 Km/h. There are 22 stops in the cycle. Final cycle tested is the constant speed driving cycle of 45 Kmph. For this the input is created using a signal builder block.

Fig 14: Driving Cycles

RESULTS

The actual speed and the reference speed from drive cycle are compared here in Kmph. For each driving cycle, the distance covered is measured in Km as shown in Fig 15. The controller is working for most of the driving cycle. The controller needs to be further tuned for better speed control. Moreover, physical brakes have not been implemented in the model, so, the vehicle takes more time to decelerate. Further, foe each driving cycle, the battery SOC% and voltage are also plotted. The energy consumption and the motor and controller temperature variation are also shown for each driving cycle.

Fig 15: Calculation of distance covered (Km)

1. Wide Open Throttle (WOT) Test

Fig : Velocity profile for WOT driving cycle

Fig : Battery SOC% for WOT driving cycle

Fig : Battery voltage for WOT driving cycle

Fig : Energy Consumption (Wh) for WOT driving cycle

Fig : Motor and inverter temperture for WOT driving cycle

2. FTP75 Driving Cycle

Fig : Velocity profile for FTP75 driving cycle

Fig : Battery SOC% for FTP75 driving cycle

Fig : Battery voltage for FTP75 driving cycle

Fig : Energy Consumption (Wh) for FTP75 driving cycle

Fig : Motor and inverter temperture for FTP75 driving cycle

3. Constant Speed Driving Cycle

Fig : Velocity profile for constant speed driving cycle

Fig : Battery SOC% for constant speed driving cycle

Fig : Battery voltage for constant speed driving cycle

Fig : Battery current for constant speed driving cycle

Fig : Energy consumption (Wh) for constant speed driving cycle

Fig : Total Distance of 100 Km at constant speed of 45 Kmph

Fig : Motor and Controller temperature for constant speed driving cycle

Conclusion

The Simulink Model of an electrical vehicle is presented here. Powertrain blockset is used here, which simulates the real world electrical and mechanical systems. This is however, a very simple model. The controller and solver have to be tuned properly to get correct results. First, Wide Open Throttle (WOT) test was conducted for a sanity check on the parameters. This is done using the motor rating and simple parameters obtained using excel sheet. Then the modified FTP drive cycle is evaluated. Finally the constant speed test done, where the vehicle covers 100 Km at a constant speed of 45 Kmph.

It is observed that acceleration is faster than the deceleration. This is true in real world as well since the discharge rate of battery is generally more than the charging rate. Mechanical brakes and current limiter circuit are needed better braking action and faster response. Speed of simulation depends on the solver configuration., thus it should be chosen carefully.

The model is simulated to complete 100 Km drive at a constant 45 Kmph speed. The SOC % of the battery drops from 90% at the stant to 7% at the end of 100 Km. 6500 Wh is used during the complete journey. The motor temperature increases from 293 K to 343 K at the end of cycle. The controller temperature increases even more than that of motor. Here the controller cooling is not applied. However, it should be noted that the flow of air itself will cool the inverter and motor. More detailed thermal analysis is however needed. This model proves that the given motor, power converter and battery pack can be used for this electric vehicle application and the vehicle can infact cover 100 Km at a constant speed of 45 Kmph.