Applied to photovoltaic pumping systems in Brushless DC Motor Control

In recent years, with the power electronic devices and the rapid development of control theory, permanent magnet brushless DC motor with its high efficiency, good speed, easy maintenance and has been widely applied. Conventional permanent magnet brushless DC motor is often used to determine the rotor position sensor location, which not only increases the installation of the motor size, increase costs and reduce its reliability. Currently, sensorless brushless DC motor usually start with three-step approach, starting at the beginning when starting torque is relatively small, and there is fluctuation, for a starting torque requirements of the system has limitations, and in small and medium sized solar photovoltaic pump system, the load torque is increased as the speed increased, excluding friction, load torque is zero when at rest, so brushless DC motor can be applied to photovoltaic pumping systems, and the entire system is DC without inverter, then the application of photovoltaic pumping systems brushless DC motor, for improving system efficiency, simplified system installation on great significance.

1 Introduction photovoltaic water pump system

PV pump system consists of photovoltaic arrays, controllers, motors, pumps four parts. Photovoltaic solar array composed of many series and parallel form, directly to the solar energy into direct current. At present solar cells are used for the silicon solar cell, including monocrystalline, polycrystalline and amorphous silicon solar cells. Since the output of photovoltaic arrays - voltage curve has a strong non-linear, and, and solar irradiance, ambient temperature, overcast, sunny, rain, fog and other weather conditions are closely related, so that if the solar water pump system work in ideal conditions, you need to use the controller to regulate and control the entire system. The electric motor is used to drive pumps, because the power factor and voltage levels to a large extent by the solar array voltage and power levels of constraints, therefore, on the pump head and flow requirements are reflected in the motor, often In both conditions, the array structure designed specifically. Required flow rate for small, high lift users should use positive displacement pumps; the need for greater flow, but the head has a lower user, generally should adopt self-priming pump.

Applied to photovoltaic pumping systems in Brushless DC Motor Control
2 MCU M68HC908JK3ECP Introduction

This is Motorola's 8-bit microcontroller family, a member of the same high performance and low cost. It is embedded 4k flash memory FLASH, 128 bytes of RAM; with 10 channels of 8-bit precision ADC module, 15 I / O ports; clock module with input capture, output compare and PWM functions, the system can meet requirements.

3 sensorless brushless DC motor control principle

Brushless motor for the three phase stator windings, by way of two-phase power control circuit when a certain sequence according to the two connected to the stator into DC current to generate the stator magnetic potential Fa; rotor for the permanent magnets to produce magnetic force Ff, by the interaction between the two, can produce electromagnetic torque T = FaFf | sinθ |, obviously, when θ = 60 ° ~ 120 °, the average maximum torque. Therefore, detection of rotor position, magnetic force, magnetic force when the rotor angle is 60 °, three-phase windings of a two-phase conduction, turned 60 ° when one phase of the power is turned off, the other phase The power tube conduction. In this way, ensure that the angle between the stator and rotor magnetic potential is 60 ° ~ 120 °, to achieve the purpose of maximum torque. As each turned 60 ° off only one power tube, so each power transistor conduction angle of 120 °, in this way for the 120 ° conduction mode.

Main circuit three-phase full-controlled bridge, shown in Figure 1. Figure 2 shows three-phase 6 shooting mode work with the typical back EMF voltage waveform. From Figure 2 we can clearly noticed in the phase vacant state (Guo zero region before and after the 30 °) under, winding induction Fan Dian Shi varies by sine law, flattened part Raozu inverse transform power excitation voltage clamp with the main Dian Lu caused. Commutation after zero-crossing point in 30 °, using the back EMF method for electronic commutation, that is, zero-crossing detection circuit detects zero-crossing point for 30 ° after the commutation. Three-phase 6 shooting mode, the conduction in the order S1, S2-S2, S3-S3, S4-S4, S5-S5, S6-S6, S1-S1, S2. EMF-based electronic commutation of means, such as "1 / 2 bus voltage comparison method", "terminal voltage comparison" However, these measurement methods have weak anti-interference ability, especially the situation in PWM modulation , the measurement must take special measures to avoid or suppress interference and increase the complexity of the control circuit, and the possible commutation delay. A "virtual midpoint of the Law" can solve the above problems, and in case of PWM modulation, the switching noise does not affect the phase windings of the zero measurement, detection circuit is also relatively simple.
At rest or low speed electric potential value of the anti-0 or small, can not back-EMF method to determine the rotor position, usually start with three-step approach to solve this problem, that press he controlled synchronous motor running status from rest begins to accelerate, as when a certain speed and then switch to the EMF to control the state, including the rotor position, step start-up and free to switch three stages. First of all on-rotor positioning the two power tubes, generally leading through S6 and S1, some time after the completion of the initial rotor positioning. Step starting from the initial position, beginning with the turn in front of the order followed by on-the-power tube, but the conduction time can be regularly reduced in order to achieve the purpose of speed. Start a free after stepping switch, to ensure the accuracy of commutation, while, PWM chopper to gradually added to the DC voltage on the main circuit, so that the speed of brushless DC motor speed according to control requirements, equivalent to a soft start motor speed process, thus avoiding the initial starting motor will produce high-current, reducing the impact on the main circuit to extend the life of the power tube.

4 System Implementation

Hardware circuit from the main circuit, driver circuit, zero crossing detection circuit, sampling circuit, various protection circuit. Zero-crossing detection circuit detects zero signal, and the zero signal to the microcontroller to capture JK3 mouth, JK3 MCU receives zero signal, the delay time calculated by the software, and the delay time to the pulse signal issued after the commutation, The drive circuit is converted to drive signals to drive the power tubes, thus achieving a single chip of the brushless DC motor control. There are over-voltage protection circuit charge protection, low-level protection.

System software is modular in design, including the initialization module, PWM interrupt module, capture interrupt module, sample protection module. PWM module interrupt a brushless stepper motor starting, switching to run free. PWM module interrupt the flow chart shown in Figure 3.

Applied to photovoltaic pumping systems in Brushless DC Motor Control

Complete the process initialization module of the initialization of variables used, PWM interrupt initialization, capture interrupt initialization, made the initial pulse positioning; capture interrupt EMF zero crossing to complete the capture and determine the phase change cycle; sample protection module is mainly used to collect DC voltage and current, as well as determine and deal with failure. Experimental data show that the commutation time phase tracking accuracy and fast right motor control of Xingneng impact Jida, Dian Zi switch the Zhunque commutation Dianmei Ci Du Bu incentive in the phase winding EMF Guo zero after the 30 ° of power angle and position, because the operation is a variable speed motor running, commutation cycle is changing, and therefore it can not accurately determine the delay in 30 ° electrical angle of commutation time, only several months of the previous trend of the commutation cycle, the time commutation time for rational and effective filtering and prediction, with digital filtering and phase-locked tracking in two ways.

Figure 4 shows the system during normal operation the measured line voltage waveform, spikes in part by the PWM and commutation chopper caused. From the figure we can see more close to the ideal voltage waveform, indicating the exact commutation point, which proves the whole system of thought control is correct.

5 Conclusion

The thought has 500W solar photovoltaic pumping systems have been tested. System, the normal starting current when starting, accelerating smoothly after step start.