Power system for high-voltage brushless power tool

Abstract

The present invention relates to the technical field of high-voltage brushless power tool control, and in particular to a power system for a high-voltage brushless power tool. The system comprises a rectifier bridge, a filter unit, and an inverter; a positive electrode output end of the rectifier bridge is configured to be connected to the filter unit, and is then connected to the inverter; the filter unit is used for performing rectifying and filtering on the voltage output by the rectifier bridge, and then sending the voltage to the inverter; the filter unit is a valley-filled passive PFC and a high-frequency filter capacitor C3; and the valley-filled passive PFC and the high-frequency filter capacitor C3 are connected in parallel between the rectifier bridge and the inverter. The use of the system can greatly improve the power factor of the tool while improving and ensuring the output and high efficiency of the tool; additionally, the glitch current and the glitch voltage on the bus after the rectifying are reduced, so that the service life of the filter capacitor and the service life of the inverter power device are prolonged.

Description

A high-voltage brushless power tool power system Technical Field

[0001] This invention relates to the field of high-voltage brushless power tool control technology, specifically a high-voltage brushless power tool power system. Background Technology

[0002] Currently, there are two main drive methods for high-voltage permanent magnet brushless power tool motors. One method uses a three-phase six-step method to generate a square wave current to drive a permanent magnet brushless motor with a square wave back electromotive force, as shown in Figure 1. This scheme has a simple and reliable control algorithm and high voltage utilization. The other method uses SPWM or SVPWM to generate a sinusoidal current to drive a permanent magnet brushless motor with a sinusoidal back electromotive force, as shown in Figure 2. This scheme has advantages such as stable torque, low vibration and noise, and high efficiency, which can improve the working quality of power tools and significantly reduce user fatigue.

[0003] As shown in Figure 3, the power supply systems used in the two schemes above typically utilize a rectifier bridge to rectify and filter AC power into DC power before supplying it to the permanent magnet brushless motor. The rectifier bridge converts the full-wave periodically changing AC power into two half-wave DC power. The filter capacitor at the rear of the rectifier bridge is one of the design challenges of the system. If the capacitance of the filter capacitor at the rear of the rectifier bridge is very large, this capacitor will filter the two half-wave DC power into DC power with nearly equal amplitude. In this case, the motor can operate at full load at any time, with high output and high efficiency. However, this large capacitor scheme results in a large spike current injected into the capacitor by the rectifier bridge output current, as shown in Figure 4. This scheme leads to a very low power factor for the entire power system, which has an adverse effect on the power grid and is not conducive to meeting the electromagnetic compatibility safety requirements of the tool. If the capacitance of the filter capacitor at the rear of the rectifier bridge is very small, the voltage input to the motor is close to two half-waves, with large amplitude oscillations, as shown in Figure 5. When the half-wave voltage is high, the motor operates at full load; but when the half-wave voltage is low, the motor outputs almost no power. Therefore, this small filter capacitor design will result in low motor output and low efficiency. However, the small filter capacitor design has the advantage of a high power factor.

[0004] Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a high-voltage brushless power tool system. The system can significantly improve the power factor of the tool while ensuring the output and efficiency of the tool. At the same time, it reduces the glitch current and glitch voltage on the bus after rectification, thereby improving the life of the filter capacitor and the inverter power device.

[0006] To solve the above problems, the following technical solutions are provided:

[0007] The high-voltage brushless power tool power system of the present invention includes a rectifier bridge, a filter unit, and an inverter. The positive output terminal of the rectifier bridge is adapted to connect to the filter unit and then to the inverter. The filter unit rectifies and filters the voltage output from the rectifier bridge before sending it to the inverter. The inverter converts the rectified and filtered DC voltage into an AC voltage with a frequency related to the motor speed of the high-voltage brushless power tool, driving the motor of the high-voltage brushless power tool to rotate, thereby converting electrical energy into mechanical energy. The filter unit is characterized by being a valley-filled passive PFC and a high-frequency filter capacitor C3, which are connected in parallel between the rectifier bridge and the inverter.

[0008] The valley-filling passive PFC includes diodes D1, D2, and D3, capacitors C1 and C2. The cathode of diode D2 is connected to the anode of capacitor C1. The anode of diode D2 is connected to the cathode of diode D3 and the anode of capacitor C2. The cathode of capacitor C1 is connected to the anode of diode D3 and the cathode of diode D1. The cathode of capacitor C2 and the anode of diode D3 are grounded.

[0009] The capacitance values ​​of capacitors C1 and C2 are close, and both are less than 180 microfarads.

[0010] The capacitance of the high-frequency filter capacitor C3 is between 0.5 microfarads and 40 microfarads.

[0011] The high-frequency filter capacitor C3 can be a single capacitor or a combination of multiple capacitors connected in parallel.

[0012] The positive output terminal of the rectifier bridge is connected in series with a filter inductor L1 and then connected to a valley-fill passive PFC.

[0013] The inductance value of the filter inductor L1 is between 0.01 millihenries and 3 millihenries.

[0014] The back electromotive force amplitude of the motor at the operating speed of the high-voltage brushless power tool is less than 95% of the rated valley voltage of the valley-filling passive PFC.

[0015] The above approach has the following advantages:

[0016] Because the filtering unit of the high-voltage brushless power tool power system of this invention contains a valley-filling passive PFC and a high-frequency filter capacitor C3, the power factor, output, and efficiency of the high-voltage brushless power tool are all improved. When the valley-filling passive PFC is applied in the drive of a high-voltage permanent magnet brushless motor, the frequent switching of the three-phase inverter bridge power transistors causes the motor line current to have a current reverse charging effect on this PFC circuit, resulting in significant spike fluctuations in the rectified bus voltage. The high-frequency filter capacitor C3 connected in parallel with the PFC module reduces the spikes in the rectified bus voltage, making the motor current more stable and reducing the risk of breakdown of the inverter power devices. When the bus current spikes are too large, a filter inductor can be connected in series at the positive output terminal of the rectifier bridge. This filter inductor can reduce current spikes, further improve the power factor, and improve the electromagnetic compatibility performance of the system. When the back electromotive force amplitude at the operating speed of the high-voltage brushless power tool motor is less than 95% of the rated valley voltage of the valley-filling passive PFC, the output capacity of the motor can be improved. Attached Figure Description

[0017] Figure 1 is a waveform diagram of a permanent magnet brushless motor with square wave back electromotive force driven by a square wave current generated by the three-phase six-step method.

[0018] Figure 2 is a waveform diagram of a permanent magnet brushless motor with sinusoidal back electromotive force driven by a sinusoidal current generated by SPWM or SVPWM.

[0019] Figure 3 is a circuit diagram of a high-voltage brushless tool power module that uses a capacitor as a filter unit in the background art.

[0020] Figure 4 shows the current and voltage waveforms of the circuit in Figure 3 using a large capacitor scheme.

[0021] Figure 5 shows the current and voltage waveforms of the circuit in Figure 3 using the small capacitor scheme.

[0022] Figure 6 is a circuit diagram of a power supply system that uses only a valley-fill passive PFC as the filter unit;

[0023] Figure 7 shows the waveforms using the circuit in Figure 6;

[0024] Figure 8 is a circuit diagram of the high-voltage brushless electric tool power system of the present invention (Embodiment 1);

[0025] Figure 9 is a schematic diagram of the 4-pole 6-slot permanent magnet brushless motor in the embodiment;

[0026] Figure 10 shows the bus voltage waveform using the circuit in Figure 8;

[0027] Figure 11 shows the bus voltage waveform using the circuit in Figure 6;

[0028] Figure 12 is a schematic diagram of the high-voltage brushless electric tool power system of the present invention in the structure of Embodiment 2;

[0029] Figure 13 shows the bus voltage waveform using the circuit in Figure 12.

[0030] Figure 14 shows the bus voltage waveform when the circuit in Figure 12 is used with an added filter capacitor.

[0031] Figure 15 is a current waveform diagram of the power supply circuit of a permanent magnet synchronous motor driven by a sinusoidal current using the structures of Embodiment 1 and Embodiment 2.

[0032] Figure 16 is a comparison of the power factor / torque of the power supply circuits of permanent magnet synchronous motors driven by sinusoidal current using the structures of Embodiment 1 and Embodiment 2.

[0033] Figure 17 is a comparison diagram of the back electromotive force of motor A and motor B in Example 3;

[0034] Figure 18 is a diagram showing the bus voltage and line current of motor A in Example 3;

[0035] Figure 19 is a diagram showing the bus voltage and line current of motor B in Example 3;

[0036] Figure 20 is a comparison of the output characteristics of motor A and motor B in Example 3. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0038] Example 1

[0039] As shown in Figure 8, the high-voltage brushless tool power system of the present invention includes a rectifier bridge 2, which consists of four diodes connected end-to-end. The specific structure of the rectifier bridge 2 is prior art and will not be described in detail here. The positive and negative input terminals of the rectifier bridge 2 are connected to municipal power 1, i.e., 220V AC power, and the negative output terminal of the rectifier bridge 2 is grounded. The rectifier bridge 2 performs full-wave rectification of the AC sinusoidal municipal power into two half-wave pulsating DC power. The positive output terminal of the rectifier bridge 2 is grounded through a filter unit 3, which is responsible for filtering the two half-wave pulsating DC power after rectification.

[0040] The filtering unit consists of a valley-fill passive PFC3 and a high-frequency filter capacitor C3. The valley-fill passive PFC3 and the high-frequency filter capacitor C3 are connected in parallel between rectifier bridge 2 and inverter 4. The valley-fill passive PFC3 contains diodes D1, D2, and D3, and capacitors C1 and C2. The cathode of diode D2 and the anode of capacitor C1 are both connected to the positive output terminal of rectifier bridge 2. The anode of diode D2 is connected to the cathode of diode D3 and the anode of capacitor C2. The cathode of capacitor C1 is connected to the anodes of diodes D3 and D1. The cathode of capacitor C2 and the anode of diode D3 are grounded. One end of the high-frequency filter capacitor C3 is connected to the positive output terminal of rectifier bridge 2, and the other end is grounded. The positive output terminal of rectifier bridge 2 is connected to inverter 4.

[0041] The working principle of the valley-filling passive PFC3 is as follows: When the voltage amplitude of the two half-wave pulsating DC currents output from rectifier bridge 2 rises to a certain level, single-phase conducting diodes D1 and D2 turn off, while D3 turns on. At this time, capacitors C1 and C2 are connected in series, and the rectifier circuit charges both capacitors, with each capacitor bearing half of the rectified voltage. Once C1 and C2 are fully charged, diode D3 turns off, providing isolation. As the voltage drops from the peak to the valley of the pulsating DC voltage, the output voltage will decrease accordingly. When the output voltage is lower than the voltage of a single capacitor, capacitors C1 and C2 begin to discharge through their respective diodes D1 and D2, respectively, filling the output voltage gap. In this case, diode D3 provides isolation. Capacitors C1 and C2 are charged in series, resulting in a small charging capacity; however, they discharge in parallel, resulting in a large discharging capacity. The charging and discharging method of this filter capacitor makes the voltage output after rectifier bridge 2 more stable, transforming the original pulse peak waveform of the current into a current waveform close to a sine wave, thereby achieving the purpose of correcting the power factor. The high-frequency filter capacitor C3 filters out voltage spikes on the rectified bus.

[0042] The high-frequency filter capacitor C3 can be a single capacitor or a combination of capacitors. If the capacitance value of this filter capacitor is too small, its filtering effect on bus voltage spikes will be poor; if the capacitance value of this filter capacitor is too large, it will reduce the power factor of the system. Therefore, considering all factors, the capacitance value of this filter capacitor is chosen to be between 0.5 microfarads and 40 microfarads.

[0043] The circuit between the full-wave rectifier bridge 2 and the valley-fill passive PFC 3 module contains parasitic inductance. As the power devices in rectifier bridge 2 switch on and off, the bus current fluctuates synchronously. When the bus current decreases, this inductance induces voltage spikes, the amplitude of which may be much higher than the amplitude of the rectified half-wave voltage, increasing the risk of breakdown of the inverter circuit's power devices. Adding a filter capacitor can significantly reduce the amplitude of these voltage spikes, improving the circuit's operational safety.

[0044] As shown in Figure 6, when only a valley-fill passive PFC is used as the filtering unit, the motor line current has a reverse charging effect on this PFC circuit because the power transistors of the three-phase inverter bridge are frequently switching. During reverse charging, the two capacitors in this passive PFC circuit are charged in series, increasing the bus voltage to the sum of the voltages of the two capacitors. When there is no reverse charging, the bus voltage returns to the value after mains rectification and filtering. Due to the parasitic inductance between the full-wave rectifier bridge and the valley-fill passive PFC module, the bus current will fluctuate synchronously with the switching of the rectifier bridge power devices. When the bus current decreases, this inductance will induce voltage spikes, as shown in Figure 7. The amplitude of these voltage spikes may be much higher than the amplitude of the rectified half-wave voltage, increasing the risk of breakdown of the inverter circuit power devices.

[0045] The selected permanent magnet synchronous motor is a 4-pole, 6-slot structure with a stator outer diameter of 48mm and a stator stack length of 40mm, as shown in Figure 9. Its back EMF coefficient is 7V / krpm, phase resistance is 1.14Ω, d-axis inductance is 2.39mH, q-axis inductance is 3.89mH, rated voltage is AC 220V, rated speed is 27krpm, rated power is 1.3kW, and overload power is 2.6kW. The valley-fill PFC capacitor has a capacitance of 120µF, and the high-frequency filter capacitor C3 has a capacitance of 10µF. Figures 10 and 11 show a comparison of the bus voltage with and without the high-frequency filter capacitor C3. It is evident that the high-frequency filter capacitor C3 can significantly reduce bus voltage spikes.

[0046] Example 2

[0047] As shown in Figure 12, compared with Embodiment 1, this embodiment adds a series filter inductor L1 between the rectifier bridge 2 and the valley-fill passive PFC 3 circuit. Figures 13 and 14 show the comparison of the bus voltage with and without the filter capacitor after adding the filter inductor L1. Therefore, the added filter capacitor can significantly reduce the amplitude of this voltage spike and improve the operational safety performance of the circuit.

[0048] If the inductance value of this filter inductor L1 is too small, the current filtering effect will be poor; if the inductance value of this filter inductor L1 is too large, the inductor size will be too large, and the cost will be too high. Therefore, considering all factors, the value of this filter inductor is chosen to be between 0.01 millihenries and 3 millihenries. This scheme is suitable for permanent magnet synchronous motors driven by sinusoidal current.

[0049] The permanent magnet synchronous motor adopted is a 4-pole, 6-slot structure with a stator outer diameter of 48mm and a stator stack length of 40mm, as shown in Figure 9. Its back EMF coefficient is 7V / krpm, phase resistance is 1.14Ω, d-axis inductance is 2.39mH, q-axis inductance is 3.89mH, rated voltage is AC 220V, rated speed is 27krpm, rated power is 1.3kW, and overload power is 2.6kW. A valley-fill PFC capacitor with a capacitance of 120uF and a filter inductor with an inductance of 1mH are used. Figure 15 shows a comparison of the rectified bus current after rectification with and without a filter inductor, demonstrating that the filter inductor significantly reduces current spikes. Figure 16 shows a comparison of the system power factor after rectification with and without a filter inductor, showing that the power factor of the filter inductor scheme is significantly higher than that of the unfiltered scheme.

[0050] Example 3

[0051] Compared to Embodiment 1, the back electromotive force amplitude at the operating speed of the brushless motor in this embodiment is less than 95% of the rated valley voltage of the valley-filled passive PFC, thereby improving the motor's output capability. The rated valley voltage refers to half the voltage amplitude after rectification; for example, for 220V AC mains, the rated valley voltage is 155.5V. The motor back electromotive force (EMF) is the induced electromotive force generated by the rotation of the rotor magnetic field at the motor's rated speed, and is one of the most important parameters in motor design. When a motor has a fixed lamination configuration, its back EMF at rated speed is affected by factors such as the rotor magnet grade, rotor magnet length, rotor lamination length, stator lamination length, winding coil method (e.g., delta or star connection), and the number of turns in the winding coil.

[0052] As shown in Figure 17, the back electromotive force (EMF) amplitude of motor A at an overload of 7500 RPM is 136V, which is 95% lower than the rated valley voltage of 155.5V. The back EMF amplitude of motor B at the same overload is 170V, which is higher than the rated valley voltage of 155.5V. At the voltage valley, current still flows through motor A, resulting in torque output, as shown in Figure 18. However, at the voltage valley, the current in motor B is very small, so it provides virtually no power, as shown in Figure 19. The output characteristic comparison shows that the constant speed range of motor A is more than 25% greater than that of motor B, and the output power of motor A is significantly stronger than that of motor B, as shown in Figure 20. The system design in this embodiment improves both the power factor of the power system and the output capability of the high-voltage brushless power tool.

Claims

1. A high-voltage brushless power tool power system, comprising a rectifier bridge, a filter unit, and an inverter; the positive output terminal of the rectifier bridge is adapted to be connected to the filter unit and then connected to the inverter; the filter unit is used to rectify and filter the voltage output from the rectifier bridge and send it to the inverter; the inverter is used to invert the rectified and filtered DC voltage into an AC voltage with a frequency related to the motor speed of the high-voltage brushless power tool, driving the motor of the high-voltage brushless power tool to rotate, so as to convert electrical energy into mechanical energy; characterized in that, The filtering unit consists of a valley-fill passive PFC and a high-frequency filter capacitor C3, which are connected in parallel between the rectifier bridge and the inverter.

2. The high-voltage brushless power tool power system as described in claim 1, characterized in that, The valley-filling passive PFC includes diodes D1, D2, and D3, capacitors C1 and C2. The cathode of diode D2 is connected to the anode of capacitor C1. The anode of diode D2 is connected to the cathode of diode D3 and the anode of capacitor C2. The cathode of capacitor C1 is connected to the anode of diode D3 and the cathode of diode D1. The cathode of capacitor C2 and the anode of diode D3 are grounded.

3. The high-voltage brushless power tool power system as described in claim 2, characterized in that, The capacitance values ​​of capacitors C1 and C2 are close, and both are less than 180 microfarads.

4. The high-voltage brushless power tool power system as described in claim 1, characterized in that, The capacitance of the high-frequency filter capacitor C3 is between 0.5 microfarads and 40 microfarads.

5. The high-voltage brushless power tool power system as described in claim 1, characterized in that, The high-frequency filter capacitor C3 can be a single capacitor or a combination of multiple capacitors connected in parallel.

6. The high-voltage brushless power tool power system as described in claim 1, characterized in that, The positive output terminal of the rectifier bridge is connected in series with a filter inductor L1 and then connected to a valley-fill passive PFC.

7. The high-voltage brushless power tool power system as described in claim 6, characterized in that, The inductance value of the filter inductor L1 is between 0.01 millihenries and 3 millihenries.

8. The high-voltage brushless power tool power system as described in claim 1, characterized in that, The back electromotive force amplitude of the motor at the operating speed of the high-voltage brushless power tool is less than 95% of the rated valley voltage of the valley-filling passive PFC.

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