Inverter Device Variable Frequency PWM Waveform Distortion
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional inverter devices experience waveform distortion when a power factor lagging load, such as an LR load, is connected, causing the output current to lag behind the output voltage, leading to increased oscillation and distortion in the load voltage due to the addition of dormant periods in pulse-width modulation signals.
Innovation Solution
The inverter device employs a power converter with switching elements and a bidirectional switch, where the frequency of switching is reduced in periods where the output voltage and current polarities differ, allowing for alternate on and off operations at a lower frequency to minimize waveform distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If pulse-width modulation with dormant periods is used to control switching elements, then the inverter can output PWM voltage, but waveform distortion increases when output current lags behind output voltage
Solution Approach 1:
The patent applies dynamics by making the switching frequency variable rather than fixed. The switching frequency is dynamically adjusted based on the polarity relationship between output voltage and output current. When polarities differ (indicating current lag), the frequency is reduced to minimize the impact of dormant periods on waveform accuracy. This dynamic adaptation resolves the contradiction between maintaining PWM output capability and preserving waveform accuracy under lagging power factor conditions.
Solution Approach 2:
The patent changes the parameter of switching frequency based on operating conditions. Specifically, it monitors the polarity relationship between output voltage and current, and adjusts the switching frequency accordingly. When the polarities differ (indicating a lagging current condition), the switching frequency is reduced, which decreases the relative proportion of dormant periods and thereby reduces waveform distortion while maintaining the ability to output PWM voltage.
2Measurement precision
If switching frequency is maintained at high level for PWM output, then voltage control precision is improved, but waveform distortion increases during current lag periods
Solution Approach 1:
The patent implements dynamic frequency adjustment where the switching frequency is not fixed but varies according to the polarity relationship between output voltage and current. During periods when polarities differ (indicating current lag), the frequency is reduced to minimize waveform distortion. During normal periods, high frequency is maintained for precise voltage control. This dynamic approach resolves the contradiction between voltage control precision and waveform accuracy.
Solution Approach 2:
The switching frequency parameter is changed based on the detected polarity relationship. When output current lags behind output voltage (detected by polarity difference), the frequency parameter is reduced. This parameter change allows the system to maintain high voltage control precision when needed while minimizing waveform distortion during lagging current conditions, thus resolving the contradiction between the two precision requirements.
3Reliability
If dormant periods are added to prevent short circuit, then switching element protection is improved, but output voltage waveform distortion increases
Solution Approach 1:
The patent uses dynamic switching frequency adjustment to mitigate the negative impact of fixed dormant periods. By reducing the switching frequency during periods when output current lags behind output voltage, the relative duration of dormant periods is reduced, thereby minimizing waveform distortion while still maintaining the protective function of dormant periods for preventing short circuits.
Solution Approach 2:
The switching frequency parameter is dynamically changed based on the polarity relationship between voltage and current. When polarities differ (indicating current lag and increased impact of dormant periods), the frequency is reduced. This parameter change allows the system to maintain switching element protection through dormant periods while minimizing their adverse effect on output voltage waveform accuracy during critical operating conditions.
Data Source
Figure 1
Figure 2(a)~2(c)
Figure 3
AI summary
When in an operation mode in which an alternating current voltage Vout is output using a positive voltage V1 of a direct current power source 1 input via a switching element Q1, a negative voltage V2 of the direct current power source 1 input via a switching element Q2, and a zero voltage Vz input via a bidirectional switch element BS1 or an alternating current voltage Vs input via a bidirectional switch element BS2, in the period in which the polarity of the output voltage Vout and the polarity of an output current Iout are the same, a control signal of an element to be caused to perform an on/off operation is generated by being pulse-width modulated using a carrier signal S with a first frequency, and in the period in which the polarity of the output voltage Vout and the polarity of the output current Iout are different, a control signal of an element to be caused to perform an on/off operation is generated by being pulse-width modulated using a carrier signal with a second frequency lower than the first frequency.