Electrical Section of a Solar Car_peoplecopy

Electrical Section of a Solar Car

KanagaRagav(Ragav).K

Lutron Electronics, 12/28/2012

Agenda• System overview• Choice of subsystem components• Complete system diagram• Brief discussion of the design of an in-house

charge controller• Proposed test plan for motor and motor

controller• Timeline check and risk analysis

Electrical section of a Solar Car, Presented at Lutron Electronics, 12/28/2012.

System overview


PV Array

Charge Controller

Battery Bank

Battery Management

System (BMS)

Motor Controller

Motor

Charger/ Adaptor

Control signals

Power flow

Choice of components -- Battery Lithium Ion – weight is constrained (20kg - R) and highest

energy density required; 72V system decided to start with.

Options: BAK 704878P, Panasonic - NCR-18650A.

Both ≈ specs; Weight: 46g, 3Ah, 3.6V; Whrs/g = 0.235; ≤60mΩ internal resistance ;

Panasonic decided based on stock availability Number of batteries = 20000/46 ≈ 434.


Choice of components – Battery Pack Arrangement


a b

a. Cells in parallel b. Batteries in parallel;

Figure taken from Chapter 6 - Deploying a BMS; Battery Management Systems for Large Lithium-Ion Battery Packs by David Andrea; Artech House publications

• Cells in parallel preferred:1. Lesser BMS slave boards

required2. Has more capacity in the

presence of random less-capacity cells

• For 72V system require (72/3.6 = 20 blocks(cells in parallel))

• With 434 cells available, (434/20 = 22 cells in a block)

Choice of components - Motor

DC motor preferred – alleviates the need for DC-AC conversion and hence saves energy.

Brushless not chosen because of high cost 500lb load; speed 50kmph requires a rated torque of

10Nm*. Starting torque ≈ twice the average torque. Motor should have reasonably good torque constant, i.e.: to

make sure that the battery does not drain off fast. High efficiency brushed DC motor for automotive

applications – Agni 143 series chosen.

* - Calculations based on air drag and frictional force.Electrical section of a Solar Car, Presented at Lutron Electronics, 12/28/2012.

Choice of components - Motor


• The graph shows that a peak current of 100A would be required during the start. This dictates the rating of the controller

• Also at an average torque of 10Nm the motor would consume 50A. The proposed battery pack would supply 66Ah(22*3). So it is expected that the pack would last for at least an hour.

Choice of components – Motor Controller

Should be able to handle 72V/100A peak current Desired options:

1. H-bridge based control for bidirectional motion

2. CAN communication in-built module

3. Regenerative breaking

4. Short circuit protection and limitation

Kelly PM72201 controller was found suitable. Main contactor coil driver, configurable regen current and

5V sensor supply current were additional features.


Choice of components – Solar cell Required strict adherence to regulations and had to choose

from approved cell arrays Chose the cell with highest efficiency, best spectral

response & temperature performance.

Source: Sunpower C60 solar cell datasheet


Choice of components – Charge controller A DC-DC converter that can operate at the peak power point

of the solar cells to deliver maximum power to the battery. AERL RACEMAX 600B had an excellent efficiency among the

options and also had reasonable cost. Based on its current handling capability and the allowed

area of cell arrays it was decided to have three controllers in parallel.


Choice of components - BMS Lithium ion batteries needs to be protected over/under

voltage, excessive charge/discharge & increase in temp. Charge balancing also needs to be done. A distributed monitoring with a centralized data collection

using CAN/Ethernet is preferred. Elithion BMS and slave cell boards chosen based on in-built

features of current sensing and contactor drivers


• 20 cell boards wired in series to the master;

• Balance boosters may be needed for faster balancing action

Choice of components - BMS


1 GND Signal ground

2 V+ Full voltage utility supply

3 V+L Power in from load

4 V+S Power in from source

5 Cont. Req,

Contactor request

6 SRC Source current

7 5V 5 V utility supply

8 CANL CAN Bus low

9 PGND Power common

10 LLIM Low Limit

11 HLIM High Limit

12 FLT Fault

13 SOC State Of

Charge analog out

14 DCL Discharge Current Limit

15 CCL Charge Current Limit

16 CANH CAN Bus high

- Power Signals

- I/P Status

- O/P Status

Complete System diagram


External PS’s to be turned ON(R) Ignition ON, PWR to motor controller; MRLY driven low; Main

contactor gets latched; Power from PV/adaptor gets to battery and motor driven by its

controller; When HLIM goes high the source side is cut off first; If still HLIM

high and current from current sensor is high then motor is cut off(regenerative may be too much)

When LLIM goes high then throttle cut off; Switches and relays rated accordingly Precharge resistor selected according to the motor controller’s

internal resistance

Risks:• After HLIM have to restart again.• No communication; static control.


Complete System diagram

Design of an in-house charge controller


• To get the maximum power from the solar panel, the solar panel must always be operated at its PP(peak power)

• Non-linear characteristics, temperature and irradiance dependence makes the PP point difficult to predict or control

• MPPT’s are used to track the PP point and are used to charge the battery at the PP from the panels

Design of an in-house charge controller A boost converter is used because

the Voc of the panels < Vbatt.

During the initial simulation a linear thevenin model(Vth and Rth chosen based on a sample VI curve) is used for the solar panel to decide on the passive components.


Solar Cells

Boost Convert

er

VI Sensor

Digital Controlle

r

Battery

Load

Charge Sensor

Diagram quoted from Solar Panel Peak Power Tracking System, Eric Anderson et all, Worcester Polytechnic

=

Design of an in-house charge controller MOSFET selection based on power loss:

Due to rds, I2rds Due to charging of gate-source capacitance, QGSVGSf

Due to transient delay, (trise+tfall)VdsIf/2

104

105

0

0.5

1

1.5

2

2.5

3

frequency

Power(Watts)

FDP6030LIRLU7807IRL7833IRL3713


• Four MOSFET’s were compared. Choose either IRL7833 or FDP6030L for a switching frequency as supported by the microprocessor

• Calculations for inductor and diode losses also

Design of an in-house charge controller Inductive current sensing for reading the current through

the inductor – makes use of internal resistance

Design of appropriate filters to reduce HF noise needs to be done


Choose the pole at a lower frequency than the zero to read the average value (i.e): RFCF > L/RL

Design of an in-house charge controller


Schematic_V1.0

Proposed test plan


Timeline check & Risk Analysis


Subsystem Testing Time

Motor and Controller 1 week

BMS and battery 2 week

Cells and charge controller

1 week

Integration Time

Physical enclosure 2 week(p)

Final Assembling 2 week

Off-vehicle & In-vehicle test

2 week

Subsystem tasks Time

Soldering battery pack + BMS

1 week

Assembling the arrays 2 week

Verify & completing the design

1 week(p)

Technical Risks

Probability

Consequence

Strategy

Many push Switches

0.7-0.8 Damage to controller

Change them to signal lines

Arcing at the source side

0.4 Reduces lifetime of switches

Snubber.

Slave cells identification

0.6 Entire battery pack could be turned off

Verify at testing the working of BMS

Non-technical

Probability

Consequence

Strategy

Members 0.8 Slowdown of work

President to address circuits 101 students.

Funds 0.3 Parts purchase might be affected

Better website, brochure and in-person visits to local solar companies

Conclusion Subsystems have been finalized Initial wiring diagram completed Order for motor and motor controller placed Test plan for the motor and controller

prepared Preliminary design of in-house MPPT initialized Timeline check and risks assessed

All set to establish a good beginning for the SolarGators.


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