A method of inverter with inductive load

Abstract

The application provides a method for inverter with inductive load, specifically, the voltage shape is changed through rapid detection of overload current to avoid overload, and the inductive load is put into load through slow start. The application detects the load current in real time, and if the impact current is detected, the modulation ratio of the inverter is changed, so that the voltage is reduced and the current is reduced. The detection and comparison are performed every sampling period (switching period), so that the voltage is reduced in real time and quickly.

Description

Technical Field

[0001] This invention belongs to the field of power supply technology, and specifically relates to a method for driving an inverter under load. Background Technology

[0002] Inductive loads typically refer to motors and transformers. These loads generate a large inrush current when energized, which is generally 3 to 10 times the rated current compared to purely resistive loads. If an inductive load starts successfully and operates normally, the current will drop to the rated current.

[0003] An inverter is a power electronic product that uses power semiconductors as its core conversion device. Semiconductor devices have very fast conversion times, but they also fail quickly under overload, and their overload capacity is limited. Driving inductive loads easily leads to overload protection failures or even damage. Currently, there are two types of inverters that can drive inductive loads: one is a dedicated inverter for driving motors, which has proprietary algorithms to adapt to motor starting shocks and can only drive one specific motor; the other is a general-purpose inverter, used as a general AC power source, and can drive a wide variety of load types and numbers. General-purpose inverters typically only output fixed AC power, such as UPS (Uninterruptible Power Supply).

[0004] Existing technology cannot use proprietary algorithms to start one or more of the loads. Overload protection or even damage may occur when dealing with inductive loads.

[0005] Under current technology, when choosing an inverter to handle inductive loads, the capacity needs to be greatly increased. For example, to power a 100W refrigerator (the refrigerator compressor is a motor, which is an inductive load), a 1000W inverter is usually required, which is large in size and expensive. Summary of the Invention

[0006] The technical problem to be solved by the present invention is: the present invention provides an inverter load-driving method that solves the problem of weak inductive load-driving capability of ordinary inverters.

[0007] The objective of this invention is achieved through the following technical solution: a method for an inverter to drive an inductive load, comprising:

[0008] S1. Set the inverter switching frequency;

[0009] S2. Use the PWM output method with triangular wave comparison, and set it to sample twice in one switching cycle; or set a second timer, the timing period is 1 / 2, 1 / 4 or 1 / 8 of the PWM, the time is greater than the sampling period of the sampling device, and sample once in each timing cycle;

[0010] S3. Set the timer interrupt, and run the inverter control algorithm in the interrupt routine;

[0011] S4, Real-time sampling of load current iL ;

[0012] S5, Real-time load current i L With rated current I N Comparison, when i L >1.2I N When i changes the modulation ratio M; when i L ≤1.2I N At the same time, maintain the normal modulation ratio Mo;

[0013] The modulation ratio M is changed according to the following formula:

[0014] Difference Err = (i L -I N ) / I N ,

[0015] M=1-(0.2+k1〃Err+k2〃(Err-Errlast)),

[0016] Errlast = Err;

[0017] Errlast is the difference from the previous value; k1 and k2 are both parameters;

[0018] S6. After the inductive load is running normally, when i L <1.0I N When this happens, M returns to its normal value Mo.

[0019] A system with an inverter and an inductive load, comprising:

[0020] The first module is used to set the inverter switching frequency; it uses a triangular wave comparison PWM output method to set the sampling to twice per switching cycle; it sets a timer interrupt to run the inverter control algorithm in the interrupt routine.

[0021] The second module is used for real-time sampling of the load current i L Real-time load current i L With rated current I N Comparison, when i L >1.2I N At that time, change the modulation ratio M; wait for the inductive load to operate normally, when i L <1.0I N At that time, the modulation ratio M returns to its normal value Mo.

[0022] The modulation ratio M is changed according to the following formula:

[0023] Difference Err = (i L -I N ) / I N ,

[0024] M=1-(0.2+k1〃Err+k2〃(Err-Errlast)),

[0025] Errlast = Err;

[0026] Errlast is the difference from the previous value; k1 and k2 are parameters.

[0027] The second module also includes: when i L ≤1.2I N At the same time, maintain the normal modulation ratio Mo.

[0028] The advantages of this invention compared to the prior art are:

[0029] (1) This invention solves the problem of large inrush current when a general inverter is connected to an inductive load. When the inductive load is connected, a specific method is used to quickly reduce the voltage and limit the inrush current to a controllable range to start the inductive load. At the same time, it will not seriously affect the normal power supply of other loads.

[0030] (2) The present invention greatly improves the inverter’s ability to carry inductive loads through the design of the inverter load-carrying method. Attached Figure Description

[0031] Figure 1 This is a flowchart of the method of the present invention;

[0032] Figure 2 This is a voltage waveform effect diagram of the present invention. Detailed Implementation

[0033] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection claimed by the present invention.

[0034] like Figure 1 As shown, this invention provides a method for an inverter to drive an inductive load. Specifically, it changes the voltage profile by rapidly detecting overload current to avoid overload, and then uses a soft start to enable the inductive load. The core idea of ​​driving an inductive load is to reduce the inverter's output voltage. As the output voltage decreases, the load current also decreases. Once the inductive load is engaged, the voltage is restored. This invention detects the load current in real time. If an inrush current is detected, the modulation ratio of the inverter's output waveform is adjusted, thus reducing the voltage and current. Detection and comparison are performed in each sampling cycle (switching cycle) to ensure real-time voltage reduction and rapid voltage decrease.

[0035] The inverter samples the load current in real time, twice per switching cycle. One power frequency cycle (50Hz) has 10,000 switching cycles, ensuring real-time load current sampling. When an inductive load is connected, the load current increases rapidly. When the load current exceeds the rated current, the voltage begins to decrease. The magnitude of the voltage decrease is positively correlated with the portion exceeding the rated current, achieving the effect of reducing inrush current while still providing a larger load current. After the inductive load is successfully connected, the load current decreases again, and the inverter restores its output voltage accordingly. The inverter's voltage reduction and restoration are achieved by changing the modulation ratio M in the inverter algorithm, where 0 ≤ M ≤ 1. When M = 1, the inverter outputs its maximum voltage, typically 10% higher than the rated voltage (220VAC). When M = 0, the inverter can output 0VAC. By changing M in software, the inverter's output voltage can be easily controlled. When no inductive load is connected, the modulation ratio is denoted as Mo, approximately 0.9. Because alternating current is an alternating voltage, current will only flow near the peak of the alternating current (where the instantaneous value of the alternating voltage is large), which will reduce the output voltage.

[0036] Example:

[0037] A method for driving an inverter with an inductive load includes the following steps:

[0038] S1. The inverter switching frequency is typically set to 10kHz, meaning the PWM timer's timing period is 100μs. In this real-time example, the normal modulation ratio Mo = 0.9.

[0039] S2. Using the PWM output method with triangular wave comparison, the sampling rate can be further improved by setting the sampling to twice per switching cycle.

[0040] S3. Set the timer interrupt. The inverter control algorithm is placed in the interrupt routine and runs once every 50μs.

[0041] S4, Real-time sampling of load current i L Real-time means that each interrupt will run once, with a time of 50μs.

[0042] S5, Real-time sampling of load current i L With rated current I N Compare:

[0043] when i L >1.2I N At that time, change the modulation ratio M;

[0044] The modulation ratio is changed according to the following rules:

[0045] Difference Err = (i L -I N ) / I N ,

[0046] M = 1 - (0.2 + k1"Err + k2"(Err - Errlast)), where k1 = 0.98, k2 = 1.25, and Errlast = Err; Errlast is the difference from the previous value.

[0047] when i L ≤1.2I N At the same time, maintain the normal modulation ratio Mo;

[0048] S6. After the inductive load is running normally, when i L <1.0I N When M returns to its normal value Mo = 0.9.

[0049] A system with an inverter and an inductive load, comprising:

[0050] The first module is used to set the inverter switching frequency; it uses a triangular wave comparison PWM output method to set the sampling to twice per switching cycle; it sets a timer interrupt to run the inverter control algorithm in the interrupt routine.

[0051] The second module is used for real-time sampling of the load current i L Real-time load current i L With rated current I N Comparison, when i L >1.2I N At that time, change the modulation ratio M; wait for the inductive load to operate normally, when i L <1.0I N At that time, the modulation ratio M returns to its normal value Mo.

[0052] The modulation ratio M is changed according to the following formula:

[0053] Difference Err = (i L -I N ) / I N ,

[0054] M=1-(0.2+k1〃Err+k2〃(Err-Errlast)),

[0055] Errlast = Err;

[0056] Errlast is the difference from the previous value; k1 and k2 are parameters.

[0057] The second module also includes: when i L ≤1.2I N At the same time, maintain the normal modulation ratio Mo.

[0058] like Figure 2The output voltage is a clipped sine wave with a small reduction in effective value. Although there are harmonics, they only occur momentarily when the inductive load starts up and have a very small impact.

[0059] Using this invention, a three-phase inverter with a rated power of 10KW (3.3KW per phase) can only handle a maximum motor power of 0.3KW without this method, causing it to trip when a large refrigerator starts. With this method, the inductive load capacity can be increased to 2.4KW, allowing simultaneous startup of a large refrigerator and a water pump.

[0060] Through the design of the inverter load-carrying method, the ability of ordinary inverters to carry inductive loads of ≤10% of rated capacity has been increased to ≤70% of rated capacity, which has outstanding substantial features and significant progress.

[0061] Because the real-time current-based voltage reduction algorithm is used, the voltage reduction only occurs near the peak of the AC voltage, resulting in a smaller impact on the effective value compared to the overall voltage reduction. This maximizes the reliability of power supply to other loads. The voltage waveform after using the algorithm of this invention is as follows: Figure 2 As shown, although the waveform is slightly distorted, it has no impact on household electrical loads, which are more sensitive to the RMS value than the voltage waveform. Moreover, the waveform distortion only occurs at the moment the inductive load is connected, lasting only 30ms to 300ms, which is a very short time and has little impact on power quality.

[0062] The parts of this invention not described in detail are well-known to those skilled in the art.

Claims

1. A method for an inverter to drive an inductive load, characterized in that, include: S1. Set the inverter switching frequency; S2. Using the PWM output method with triangular wave comparison, sampling is set to twice per switching cycle; S3. Set a timer interrupt to run the inverter control algorithm in the interrupt routine; S4, Real-time sampling of load current i L ; S5, Real-time load current i L With rated current I N Comparison, when i L >1.2I N At that time, reduce the modulation ratio M; S6. After the inductive load is running normally, when i L <1.0I N At that time, the modulation ratio M returns to its normal value Mo.

2. The method for an inverter to drive an inductive load according to claim 1, characterized in that, The modulation ratio M is changed according to the following formula: The difference Err = (i L - I N ) / I N , M= 1-(0.2+k1·Err+k2·(Err-Errlast)), Errlast is the difference from the previous value; k1 and k2 are parameters.

3. The method for an inverter to drive an inductive load according to claim 1, characterized in that, Also includes: when i L ≤1.2I N At the same time, maintain the normal modulation ratio Mo.

4. A method for an inverter to drive an inductive load, characterized in that, include: S1. Set the inverter switching frequency; S2. Set a second timer with a duration longer than the sampling period of the sampling device, and sample once per timing period; S3. Set a timer interrupt to run the inverter control algorithm in the interrupt routine; S4, Real-time sampling of load current i L ; S5, Real-time load current i L With rated current I N Comparison, when i L >1.2I N At that time, reduce the modulation ratio M; S6. After the inductive load is running normally, when i L <1.0I N At that time, the modulation ratio M returns to its normal value Mo.

5. A method for an inverter to drive an inductive load according to claim 4, characterized in that, The timing period of the second timer is 1 / 2, 1 / 4, or 1 / 8 of the PWM duration.

6. The method for an inverter to drive an inductive load according to claim 4, characterized in that, The modulation ratio M is changed according to the following formula: Difference Err=(i L -I N ) / I N , M = 1 - (0.2 + k1·Err + k2·(Err - Errlast)), where Errlast is the difference from the previous time; k1 and k2 are parameters.

7. A method for an inverter to drive an inductive load according to claim 4, characterized in that, Also includes: when i L ≤1.2I N At the same time, maintain the normal modulation ratio Mo.

8. A system with an inverter and an inductive load, characterized in that, include: The first module is used to set the inverter switching frequency; it uses a triangular wave comparison PWM output method and sets it to sample twice per switching cycle. Set a timer interrupt to run the inverter control algorithm within the interrupt routine. The second module is used for real-time sampling of the load current i L Real-time load current i L With rated current I N Comparison, when i L >1.2I N At that time, reduce the modulation ratio M; wait for the inductive load to operate normally, and then when i L <1.0I N At that time, the modulation ratio M returns to its normal value Mo.

9. A system for an inverter with an inductive load according to claim 8, characterized in that, The modulation ratio M is changed according to the following formula: Difference Err=(i L -I N ) / I N , M = 1 - (0.2 + k1·Err + k2·(Err - Errlast)), where Errlast is the difference from the previous time; k1 and k2 are parameters.

10. A system for an inverter with an inductive load according to claim 8, characterized in that, The second module also includes: when i L ≤1.2I N At the same time, maintain the normal modulation ratio Mo.

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