The Design of DC Motor Driver for Solar Tracking Applications

Zi-Yi, Lam, Member, IEEE, Sew-Kin, Wong, Member, IEEE, Wai-Leong, Pang, Chee-Pun, Ooi

Faculty of Engineering (FOE) Multimedia University, Persiaran Multimedia

63100 Cyberjaya, Selangor Darul Ehsan, Malaysia Email: zylam@mmu.edu.my (corresponding author), skwong@mmu.edu.my, wlpang@mmu.edu.my, cpooi@mmu.edu.my

Abstract - Solar trackers rely on a direct-current (DC) motor

driver circuit to control the movement of the solar panel. However, conventional DC motor drivers used in solar tracking system do not provide any options for speed and torque control. Hence, the fixed speed of the DC motor leads to either too fast or too slow tracking movement. Usually, the output torque is set to the maximum. If the load (solar panels’ weight) is small, the maximum torque is not fully utilized and therefore energy is wasted. So, a fully adjustable DC motor driver for solar tracking system shows great potential in commercialization because highly energy efficient DC motor driver circuit can improves the total efficiency of the solar tracker. Fewer solar panels are needed when they are attached to high efficiency solar tracking system, which will be reflected to lower system cost. Therefore, this enhanced motor driver will bring significant impact to the solar energy industry. The proposed DC motor driver is fully adjustable in term of speed and torque. The speed and torque of the motor is directly proportional to the output voltage and output current of the proposed DC motor driver, respectively. Adaptive controlling of the output voltage and current are possible by installing algorithm in the microcontroller of the DC motor driver and it can be reprogrammed according to the requirement. The speed control makes the solar tracking system to track the Sun more accurately and the torque control saves energy.

I. INTRODUCTION

Solar tracking system is used to track the position of sun in order to get the maximum energy by aligned the solar panel perpendicular to the incidence sunlight [1, 2, 3, 12, 13]. Solar panels need to move along with the direction of sunlight during the tracking process. A DC motor driver is needed to control the directions as well as the turning speed of the solar panel. The maximum torque of the DC motor decides the total amount of solar panels can be mounted on the tracking system. The common limitation of the DC motor driver is the speed is fixed and the default torque is set to maximum. Logically, it is not preferable to set the motor in high turning speed since Sun tracking is a slow process. While setting the motor to operate at maximum torque causes the system to consume the highest amount of power constantly, regardless of the amount of load attached. It will bring significant impact on the efficiency of the system, since the energy consumed by the system is generated by the solar panel itself. Research has been done that DC-DC buck

converter is used to step down the voltage in order to reduce the turning speed of solar panel [4].

Fig.1. Block diagram of motor driver

However, the system reported has single speed only

without torque control. System proposed by V. Gupta managed to control the torque by changing the duty cycle of Pulse Width Modulation (PWM) signal that feed into the low-side switching elements of the H-bridge circuit [5]. The proposed DC motor driver shows that it is possible to make both speed and torque adjustable. It brings great advantage to the solar tracking system as the tracking speed can be optimized according to the tracking condition. Moreover, power consumption of the tracking system can be reduced greatly since the DC motor driver supplies the amount of torque just sufficient to move solar panels attached. An addition current limiting circuit has been added in the motor driver in order to limit the maximum amount of current that can flow through the DC motor to avoid motor stall. Fig. 1 shows the block diagram of the proposed DC motor driver.

II. CIRCUIT DESIGN AND DESCRIPTION

A. Characteristic of DC motor The characteristic of a DC motor is govern by the equation

(1): EA = VA- IARA (1)

Where VA is the applied voltage across the DC motor, IA is the armature current, RA is the armature resistance and EA is the internal generated voltage. In normal operating condition, the armature current will increase with the applied voltage and the internal generated voltage remains constant. Turning speed of motor is directly proportional with internal generated voltage, showing in the equation (2) [6, 14, 15]:

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EA = Køw (2) Where ø is magnetic flux, w is the turning speed of DC

motor and K is a constant. The proposed DC motor driver limits the amount of

current flowing into the motor. In other words, it can maintains the armature current at a constant value although applied voltage increased. When the armature current remains constant, the internal generated voltage and therefore the turning speed will increase propotional to the voltage applied. Torque is directly proportional with the armature current, so the ability in controlling the armature current made torque control possible, as shown in the equation (3) [7]:

τinduced = KøIA (3) Where τinduced is the induced torque, ø is magnetic flux, IA

is armature current and K is a constant. Whereas changing the flow direction of current across the DC motor will change the turning direction of the DC motor [8].

B. Speed Control of DC Motor

The voltage across the DC motor must be step down by using the DC-DC buck converter in order to reduce the turning speed of the solar panel to get accurate sun tracking. Fig. 2 shows the circuit of DC-DC buck converter. M1 is P12PF06 P-channel MOSFET as the switching element and M2 is IRF630 N-channel MOSFET as a driver to drive the P-channel MOSFET. Since PIC microcontroller cannot produce a negative voltage to turn on the P-channel MOSFET directly. Therefore, the N-channel MOSFET is use to turn on the P-channel MOSFET. The relationship between the input voltage and the output voltage of the DC-DC buck converter are given by [9]:

Vo = DVin (4) Where Vo is the output voltage, Vin is input voltage and D

is the duty cycle of PWM signal. The output voltage can be step down by vary the duty cycle of the PWM signal that feed into M2. C. Torque and Direction Control of DC Motor

Fig. 3 shows the H-bridge circuit. M1 and M2 are P12PF06 P-channel MOSFET as the high-side switching elements. M3 and M4 are IRF630 N-channel MOSFET as the low-side switching elements. Another two IRF630 N-channel MOSFET, M5 and M6 are needed as the driver to turn on the M1 and M2. The torque of the motor can be controlled by control the current flow through the motor as the torque is directly proportional to the current across it. This can be adjust by vary the duty cycle of PWM signal which feed into M5 or M6. Base on [10], increase the drain to source voltage (Vds), will increase the drain current (Id) of the MOSFET as well. Changing the duty cycle of PWM signal will change the average output voltage which is known as Vgs, when there are changes in Vgs, Id which is the current that flow through the motor will change as well. This is how the torque of the motor being controlled.

Fig. 2. DC-DC buck converter circuit

D. Current Limiting Circuit

Current limiter circuit is used to limit the current flowing through. This circuit is added right after the power supply to the H-bridge circuit to prevent the motor being stalled and protect the MOSFET. As shown in the data sheet of the DC motor, the stalled current is 3A. So the design of this current limiting circuit is to limit the current that above 3A as shown in Fig. 4. Basically, this circuit consists of two op-amps. U1 as differential amplifier which is use to amplified the voltage drop across the sense resistor (Rs) since the Rs is very small and the voltage across it is also very small. U2 used as a comparator to compare the input voltage with the reference voltage. If the current flow across is too high, which means that the output voltage of U1 is higher than the reference voltage, there will be output coming out from U2 and turn on the transistor Q1. As Q1 being turn on, the PWM signal will be grounded instead of feed into the MOSFET of H-bridge. So there will be no current flowing through the MOSFET as well as the motor. It prevents the motor being stalled, protect the MOSFET and saves the current consumption.

Fig. 3. H-bridge circuit using power MOSFET

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Fig. 4. Current limiting circuit.

III. RESULTS AND DISCUSSION

The revolution per minutes (rpm) of the DC motor is

measured by using the digital stroboscope tachometer every time there are changes in output voltage. Fig. 5 shows the speed of DC motor changed with the output voltage of the driver. The results show that the driver manged to control the motor speed in a linear manner. Fig 6 shows the changes of the driver output power corresponding to various speed of the motor. As the speed of motor increase, the output power will remain closed to 5W. Which means that the driver output power has the ability to remain constant throughout the speed range. This characteristic enables the motor to turn slowly without lossing power, which is very important in promoting higher efficiency of solar tracking.

Current flow through the motor is measured under two conditions, i.e. with load and without load. The shaft of the DC motor is attached with weight of 130 grams as a load. Fig. 7 shows the relationship between the driver output current and duty cycle when there is a load attached. The results showed that the driver managed to control the output current linearly, and therefore the torque by using PWM control signal. This enables the solar tracking system to work properly later on when there is a need to mount more solar panels on it. When there is no load mounted on the motor, the driver only consume current maximum at 0.05A throughout the duty cycle range as shown in Fig. 8. Most of the time, solar tracking system is installed in a remote rural area where power grid is nowhere to be found. This will be a very

important power saving feature that helps in conserving the energy stored in the battery bank and allows the system to work in longer hours per full-charge cycle. To summuries, the performance differences between conventional and the proposed motor driver for solar tracking applications is listed in Table 1.

Fig. 5. Speed of motor when output voltage varies

Fig. 6. Output power when speed of motor varies

Fig. 7. Ouput current changes with the duty cycle when load is attached

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Fig. 8. Ouput current changes with the duty cycle when no load is attached

Table 1. Performance comparison between conventional and the proposed

motor driver for solar tracking applications [16]. Conventional motor driver Adaptive motor driver

Single speed control Variable speed control

Either too fast or too slow (tracking speed)

Tracking speed can be programed to the needs (PID control is possible)

Full-torque (no matter how much load)

Adjustable torque according to the load attached

Operating at full torque, consumes maximum amount of energy

Giving only adequate amount of torque depending on the load attached, saves energy

IV. CONCLUSION

With the combination of the carefully designed torque,

speed and direction control circuit, it can form a better motor driver for solar tracking system. This motor driver circuit is builded based on the idea to control the cost as low as possible. Therefore, the list of components used is simple and easy to be found in the market. Although the cost is low, the resultant driver delivers excellent capability of controling the speed and torque in a linear manner. The driver managed to control the current and the voltage of the output at the same time and therefore it can controls the resultant power transfer

to the motor. The motor is only consumes maximum of 0.05A when it is idle. This stable and easy control definitely will shorten the development period for engineers to design the solar tracking system.

REFERENCES

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[5] Vibhor Gupta, ‘‘Working and Analysis of the H --- Bridge Motor Driver Circuit Designed for,’’ University Institute of Engineering and Technology, pp. 441-444 (2010).

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[7] Ayetül Gelen and Saffet Ayasun, “Effects of PWM Chopper Drive on the Torque-Speed Characteristic of DC Motor,” Universities Power Engineering Conference, pp. 1-4 (2008).

[8] Ibrahim Sefa, et al., “Application of one-axis sun tracking system,” Energy Conversion and Management, pp. 2709-2718 (2009).

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[10] STMicroelectronics datasheet. [11] Cytron technologies datasheet. [12] Ahmed, M.M. and Sulaiman, M. “Design and proper sizing of

solar energy schemes for electricity production in Malaysia” Power Engineering Conference, pp. 268-271, (2005).

[13] J. Rizk and Y. Chaiko. “Solar Tracking System: More Efficient Use of Solar Panels,” Proceedings of World Academy of Science, Engineering and Technology, pp. 2-3, (2008).

[14] Wai Phyo Aung, “Analysis on Modeling and Simulink of DC Motor and its Driving System Used for Wheeled Mobile Robot”, World Academy of Science, Engineering and Technology, pp. 32 (2007).

[15] R. Sahu and G. A. Rincon-Mora, “A High-Efficiency, Dual-Mode, Dynamic, Buck-Boost Power Supply IC for Portable Applications” , 2005 18th International Conference on VLSI Design, pp.858-861, 3-7 January, (2005).

[16] R. Valentine, “Motor Control Electronics Handbook”, McGraw-Hill Handbooks, United States, 1998, pp. 59-83.

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