Today's industrial servo drives mostly use AC servo systems that drive permanent magnet synchronous motors (PMSM). Its AC drive unit usually uses a three-phase full-bridge voltage best solar inverter. The frequency conversion control technology of PWM modulation realizes real-time control of the dynamic torque of the AC motor, which greatly improves the control performance of the servo system.
However, for PWM inverters like 2000w inverter or 3000w inverter, a delay time is inserted into the switching signal that drives the power tube to prevent the DC bus from being directly short-circuited. The introduction of the delay time will lead to a dead time effect, causing distortion of the inverter output waveform and fundamental wave. The voltage drop affects the further improvement of servo system performance.
Main content:
1. Compensation method for inverter dead time
To compensate for voltage fluctuations caused by td, researchers have proposed various compensation methods, which can be roughly divided into three categories.
The most common method is to add a pulse train of opposite polarity according to the missing pulse train in the interval with the same current polarity to offset its influence. Since one phase of the three-phase current must have an opposite polarity to the other two phases, a simple method is to implement twice the voltage overcompensation for the phase with the opposite polarity, so that the effects of the three-phase voltage dead time cancel each other out, and the line voltage waveform is sinusoidal.
The second type of method implements dead time compensation based on the principle of ineffective devices. At any time, only one of the two power devices in each arm of the inverter is active. When the upper arm device is turned off, regardless of whether the lower arm device is on or not, the output voltage is the negative terminal voltage of the DC bus.
At this time, the lower arm device is said to be "invalid". The method of dead time compensation is to keep the driving signal of the effective device unchanged and change the driving signal of the invalid device to meet the requirements of setting the dead time.
Since the power-on and power-off of the "invalid" device does not affect the output voltage state, then there is no need for a drive signal. It is enough to only send the drive signal to the effective device.
In this way, there is no need to add a dead time, and there is no need to add a dead time. The issue of dead time compensation is gone. However, this method will cause distortion due to errors at the zero-crossing point of the current. Therefore, when using this method, attention should be paid to the processing of the zero-crossing area of the current.
The third type of method is current predictive control. Establish a more accurate motor system model, analyze the distortion of the current waveform, and realize the correction of the current waveform through current predictive control. The matrix equation of the asynchronous motor model can be established and its space voltage vector can be compensated based on the prediction of the stator phase current in the SVPWM algorithm.
The interference voltage caused by the dead time can also be estimated through time delay control and fed back to the voltage reference to compensate for the effect of the dead time. The calculation of the current prediction method is cumbersome, and the compensation effect is directly related to the accuracy of the motor model and the time-varying parameter values, making it difficult to obtain satisfactory results.
2. Impact of inverter dead time
It can be seen from the basic principle of PWM dead time that the voltage deviation caused by the deviation pulse of the output inverter voltage within the winding current period t1 can be equivalent to a square wave. For the convenience of analysis, it is assumed that the voltage deviation pulses are equal in time.
As the polarity of the current changes, the direction of the error voltage pulse also changes, and as the carrier frequency increases, the number of error voltage pulses also increases, although the dead time is very short, which is only a few microseconds, the accumulation of error voltage within one cycle will have a great impact on the fundamental amplitude of the output voltage. The qualitative relationship between the error voltage, the ideal voltage and the actual output voltage is shown in the figure below.
According to the Fourier analysis on the deviation square wave in the figure above:
Among them, ω1 is current fundamental wave angular frequency; ψ refers to the phase difference between the desired voltage and the motor current. Therefore, ignoring the high-frequency noise caused by the power switch, the output voltage of the inverter is:
Among them, the ma modulation degree is the ratio of the amplitude of the modulated sine wave to the amplitude of the triangular wave carrier.
We can see from the above formula that due to the existence of the inverter dead time, not only the fundamental wave of the inverter output voltage changes, but also the output voltage contains high-order harmonics such as the 3rd, 5th, and 7th harmonics.
The switching dead time causes the inverter output voltage waveform to be distorted, causing the output current waveform to be distorted, that is, current crossover distortion.
- The longer the dead time, the greater the inverter output fundamental voltage loss and the greater the voltage waveform distortion; the more the load fundamental current amplitude drops, the more serious the current waveform distortion.
- For a certain dead time, when the load power factor becomes smaller, the inverter output fundamental voltage amplitude increases, the voltage waveform distortion rate becomes smaller, the fundamental current amplitude decreases, and the current waveform distortion rate becomes larger.
- When the output voltage is low, the space voltage vector amplitude is very small, the relative conduction time of the three-way bridge arm becomes shorter, and the influence of the dead time becomes greater.
- The dead time not only affects the output voltage amplitude, but also affects its phase; the dead time makes the PWM waveform no longer symmetrical to the center. Therefore, the amplitude of the space voltage vector deviates and the phase also changes.
3. Dynamic dead time compensation method based on position
A common feature of various dead-time compensation methods is to compensate the voltage signal based on the current waveform. Therefore, it is necessary to detect the actual current value, determine the positive and negative of each phase current, and use its zero-crossing point as the switching moment of the compensation voltage signal.
The current detection link consists of a current sensor, a low-pass filter and an A/D conversion. In order to reduce the noise, digital filtering is also required in the program. The detected current has errors caused by device accuracy and interference, and has a phase delay. Therefore, it is difficult to accurately compensate for the dead time effect using the actual detected current signal, and may even cause greater current distortion due to erroneous compensation near the zero-crossing point.
Today, the torque control of PMSM is mostly achieved through vector control. In order to accurately control the motor current, its current loop response frequency is very high, which can reach more than 1khz, and the actual current can accurately track the current command signal.
In high-precision AC servo systems, high-resolution position sensors are required to achieve high-precision position servo control, generally reaching 16 or 17 bits. High-speed and high-precision A/D devices are relatively expensive, and their resolution is generally 10 or 12 bits.
Since the current vector is related to the rotor position, if the position signal is used to determine the positive and negative current and a voltage dead time compensation signal is applied, the compensation accuracy can be higher than the actual current signal used and is not affected by interference signals.
It can be seen from the PMSM vector diagram that the current vector controlled by the magnetic field orientation is 90°(electrical angle) with the rotor magnetic pole and rotates synchronously with the rotor. The position of the rotor magnetic pole can be determined by a high-resolution encoder.
After the rotor magnetic field directional control, the electrical angle of the current changing in time coincides with the spatial rotation angle of the magnetic pole spatial change, and then the spatial position of the current vector can be obtained. According to the spatial position of the current vector, the zero-crossing point of each phase current can be determined.
The phase relationship between the position angle of the magnetic pole and the current is fixed. After analysis, voltage compensation can be processed according to the following position change rules:
- When the angle is 0<θ<π, ia>0, phase a compensates the forward voltage; otherwise, it compensates the reverse voltage.
- When the angle 2π/3<θ<5π/3, ib>0, phase b compensates the forward voltage; otherwise, it compensates the reverse voltage.
- When the angle is -2π/3<θ<π/3, ic>0, phase c compensates the forward voltage; otherwise, it compensates the reverse voltage.
The amplitude calculation formula of the compensation voltage is:
In this formula, Factor is the adjustment coefficient, which is generally taken as 0.7.
The picture above shows the comparison of experimental results without dead time compensation and with dead time compensation. It can be seen from the current waveform that the current without dead time compensation is distorted at the zero-crossing point and has a flat step. After adding the dead time compensation method proposed above, the actual current shown on the right tracks the given current, and a sinusoidal inverter waveform with good effect is obtained.
Conclusion
The switching dead time effect of the inverter has a great impact on the performance of the AC servo system, so it is necessary to correct and compensate for the switching dead time. Based on the analysis of various dead time compensation methods, this article proposes a dynamic compensation method based on position detection signals. This method uses a high-resolution encoder to improve the accuracy of judging the current direction, and experiments have proven that it has a good compensation effect.
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