Design of Energy-saving Controller for Motor

introduction

三相异步电机是广泛使用的一种动力机械，每年的耗电量占我国总耗电量的50%以上。在满负荷工况条件下，电机的效率一般较高，通常在80%左右；然而，一旦负荷下降，电机的效率便随之显著下降。因为电机选型时是按最大可能负荷和最坏工况所需的功率而定的，多数电机在大部分运行时间的负荷率都在50%～60%，所以实际运行时的效率都是比较低的。因此，提高这部分电机的运行效率，有着巨大经济效益和社会效益。

1 Energy saving principle

The efficiency of a motor is the percentage of the ratio of the motor output power to the input power. Therefore, the electrical energy of the power supply machine, i.e. the input power, is not only used to drive the motor, i.e. the output power, but also part of it will become the inherent loss of the motor. The main losses of the motor are copper loss and iron loss. The copper loss is caused by the current flowing through the motor winding and is proportional to the square of the current; the iron loss is caused by the magnetizing current in the stator and rotor cores and is proportional to the supply voltage. Other losses are very small and can be ignored.

The principle of voltage regulation and power saving is that when the load decreases, the power supply voltage can be appropriately reduced to reduce iron loss. At the same time, the current decreases, which also reduces copper loss and unnecessary waste. At this time, the efficiency of the motor will be improved. The detection of motor load is usually carried out using the power factor method: if the motor load is large, its power factor is large; if the motor load is small, its power factor is small.

2 Technical Difficulties and Solutions

① Detection of power factor angle. Under normal circumstances, the current waveform is complete, and the phase difference obtained by detecting the zero-crossing point of voltage and current is the power factor angle. However, since this controller uses thyristor AC voltage regulation, when the conduction angle is small, the current waveform becomes intermittent. The current continues to make the current zero-crossing detection invalid. For this reason, we compare the current with the micro-level to obtain the part of its positive half-cycle continuous waveform, and then obtain an approximate phase difference.

② Detection of effective values ​​of voltage and current. Generally, the circuit for detection according to the definition of effective value needs to use analog multipliers, so the circuit is relatively complex and the cost is high. Since there is a certain correspondence between the effective value and the absolute average value, and the detection accuracy requirement here is not high, we first detect the absolute average value and then convert it into the effective value.

③ System reinforcement under strong interference. This power saver works in the harsh environment of the factory. Strong electromagnetic interference will seriously affect the normal operation of the microcomputer system. For this reason, we have taken a variety of protection measures: the digital circuit part is installed separately in a metal casing to shield the electromagnetic interference in space; high-quality switching power supplies and sensors are selected to reduce the interference from the line; serial interface chips are widely used in the microcomputer peripheral circuit to simplify the circuit board wiring; the widely used WDT circuit is used to improve the software's anti-interference ability.

④ Phase-shift trigger circuit of thyristor. In the three-phase AC voltage regulation circuit, a very important indicator is the three-phase balance problem. In the past, three-phase AC voltage regulation often used three single-phase phase-shift trigger chip designs (such as TA785), and careful debugging was required to achieve three-phase balance. We use the latest three-phase phase-shift trigger chip AT787 to simplify the circuit design and save the circuit from complicated debugging; at the same time, we also use the strong trigger technology of thyristor to make it trigger more accurately.

3 Hardware Design

This controller is mainly composed of three parts: the thyristor and phase-shift trigger circuit part, which receives the control signal from the control board and implements the adjustment of the AC voltage; the signal detection board part, which receives the signal from the sensor and processes it to obtain the effective value of the standard voltage and current and the power factor and sends it to the control board; the single-chip control board part, which receives the signal from the signal detection board, sends the control signal to the phase-shift trigger circuit through control calculation, implements the best power factor control, and at the same time, the control board also modifies the controller parameters through the keyboard display panel and displays the controller operation status.

The basic connection of the TC787 chip for the thyristor and shift trigger circuit is shown in Figure 1.

The three-phase zero-crossing signal from the synchronous transformer is coupled to the 6V DC signal through the capacitors C1, C2, and C3 and sent to pins 18, 2, and 1. TC787 performs zero-crossing detection, and forms a triangle wave with the zero-crossing point as the starting point through the integrating capacitors C4, C5, and C6, which is compared with the trigger control signal introduced by VR, and then modulated into a trigger pulse through C7, which is output from pins 12, 9, 10, 7, 8, and 11, and the thyristor is driven by the pulse transformer.

The effective value conversion circuit of the standard voltage and current of the signal detection board is shown in Figure 2.

This circuit is based on the basic absolute value circuit. The filter capacitor C1 is added to change the absolute value of the AC signal into an average value. The resistance value of R5 is reasonably designed to change the average value into an effective value.

The phase detection circuit is shown in Figure 3.

The voltage signal VA and the current signal IA are compared with the micro-level signal REF to obtain the positive half cycle of the voltage and current signals; after RC filtering, the signal "OR" circuit forms a signal containing the power factor angle; the single-chip microcomputer removes the voltage half cycle to obtain the power factor angle.

The basic circuit of the microcontroller control board is shown in Figure 4.

TLC0834 is a 4-channel 8-bit A/D converter that collects 1 voltage and 3 current signals; TLC5615 is a 10-bit serial D/A that converts the control quantity into an analog voltage signal to control the thyristor AC voltage regulation; X25045 is a multifunctional circuit containing WDT and EEPROM, which is responsible for the safety monitoring of the microcontroller system and the protection of important parameters; SN75176 is an RS485 interface to realize network monitoring.

4 Software Design

The microcontroller software is programmed in C51 language, which has the advantages of high programming efficiency and easy code maintenance compared with assembly language. The program mainly consists of keyboard and display monitoring part, serial interface chip driver part and signal acquisition and real-time control part.

The serial interface chip driver mainly simulates the serial port with the I/O port of the microcontroller according to the chip manufacturer's timing diagram to realize the read and write operations of the serial chip. In this project, since the microcontroller has many I/Os, each chip uses a separate I/O signal.

The signal acquisition and real-time control part uses the real-time clock as a reference to collect voltage and current signals to monitor the safety of the system. The collected power factor signal is compared with the optimal value, and the PI control algorithm is used for calculation. The control command is issued in a timely manner to adjust the voltage of the motor so that it runs at a high efficiency state.

5 System Debugging

During the system debugging process, we discovered and dealt with the following problems.

① The stability of the motor thyristor AC voltage regulation. Since the motor is a large inductive load, it is best to use a half-controlled form when the outer triangle connection is used. The data tube plays a role in absorbing harmonics. To use the full control form, it is best to use the inner triangle type. In this connection method, each winding is powered separately, and there will be no mutual interference between the windings.

② The device selection problem of the three-phase voltage regulation phase shift trigger board. The values ​​of the three integral capacitors must be consistent with each other, with an error within 1%. The value of the modulation capacitor C7 cannot be too large, and the coupling capacitors C1, C2, and C3 cannot be too large, otherwise the circuit will not be able to operate for a long time or there will be an imbalance in the three phases.

③ The optimal power factor setting of the power saving controller. The optimal value is generally around 0.85, and the fan can be set around 0.9, which is slightly different for different motors. If it exceeds this range, it is an abnormal phenomenon. Because the motor theoretically has a maximum efficiency point around 75% to 80% load rate, if the motor ages and does not have this characteristic, energy saving cannot be achieved. This principle must be paid attention to in the application.

6 Conclusion

In addition to the power factor control and energy saving function, this motor energy-saving controller also realizes functions such as soft start, phase failure protection, overcurrent protection, overheat protection, etc. User tests show that this motor energy-saving controller has reasonable design, reliable operation and obvious energy-saving effect.