introduction
Currently, in the domestic market, small and medium power AC servo drives generally adopt an "AC-DC-AC" transformer-frequency conversion topology. The AC power supply is rectified into DC, and the DC is inverted into the three-phase power required by the motor through controllable power components [1]. Among them, the input of the AC power supply is generally 3-phase 220V (phase-to-phase voltage is 220V). When the motor power is small, the input of the AC power supply is generally single-phase 220V. During inversion, the conduction time of each switch is calculated based on the DC bus voltage. In many cases, the DC bus voltage VDC is a constant in the program calculation. In practice, the amplitude of the DC bus voltage fluctuates. Since PI control is often used in the current loop control, the fluctuation of the bus voltage cannot affect the final adjustment result, but it does have a certain impact on the adjustment time and dynamic tracking accuracy. Especially when the DC side capacitor value is small and the load is heavy, the impact is more obvious.
This paper verifies the above viewpoint under a single-phase 220V power supply and a large load. Furthermore, comparative experiments demonstrate that the bus voltage compensation method can improve the driver's performance to a certain extent when there is a step change in speed.
The root cause of bus voltage problems
The main power circuit of the AC servo control system for permanent magnet synchronous motors consists of a rectifier circuit, an intermediate DC circuit, and an inverter power transistor. A schematic diagram is shown in Figure 1.
If the energy storage and filtering effects of the DC circuit are ignored, the bridge rectifier waveform of a single-phase 220V AC circuit is shown in Figure 2 [2]. The waveform frequency is 100Hz, which is twice the power frequency. The bridge rectifier waveform of a three-phase 220V AC circuit (with an inter-phase amplitude of 220V) is shown in Figure 3, with a waveform frequency of 300Hz.
In practice, due to the energy storage and filtering functions of the capacitors on the DC side, the DC bus voltage remains stable at around 311V when the motor does not consume high power. When the motor requires high power consumption, as shown in the diagram, the performance of a single-phase 220V power supply is far inferior to that of a three-phase 220V power supply. However, in some situations, due to limitations, the power supply is only single-phase 220V. This leads to fluctuations in the DC bus voltage.
The essence of bus voltage compensation
In general, the DC bus voltage Vdc is a constant in the program calculation, and the conduction time of the switching transistor is [3]:
During motor control, if the DC bus voltage is less than the constant Vdc in the program, the switching transistor's on-time will be shorter than expected, further causing the motor current to be less than the given value. Through current loop PI control, the switching transistor's on-time is automatically increased, ensuring the actual current value tracks the given current value.
If voltage compensation is used, the integral term of the pi controller in the current loop will be reduced, which will reduce the overshoot of the driver and improve the performance of the driver to a certain extent.
In general, the controller used in motor control is a fixed-point controller. This cannot guarantee the time occupied by division during interrupt calculations. Therefore, a lookup table method can be used. During the program initialization phase, the reference values corresponding to different bus voltages are calculated. In other words, this ensures that the real-time DC bus voltage Vdc is used in the program calculations.
Experiments and Results
The experiment used the SWAI-SC series driver board, matching motor, and a self-developed test system from Mianyang Shengwei Company. The experiment used a single-phase 220V AC input. The experimental motor had a rated torque of 4 Nm and a rated speed of 4000 r/min, and was operated under load. The motor's rated power was:
At the time of testing, the mains voltage was slightly higher at 235V. Therefore, the initial amplitude of the bus voltage in the figure is approximately 333V. The uncompensated bus voltage test curve is shown in Figure 4 (with a step speed change of 0 to 5000 r/min for the driver), and the compensated test curve is shown in Figure 5. The left axis in the figure represents the rotational speed in r/min (revolutions per minute). The right axis represents the amplitude of the bus voltage in volts (V), and the horizontal axis represents time in 120 microseconds (the communication cycle of the test software). It can be seen that when the rotational speed changes stepwise, the DC bus voltage exhibits a periodic (100 Hz) decrease.
Comparing Figures 4 and 5, we can see that the rise time from 0 to 4000 rpm is 322.6 ms in Figure 4 and 320.3 ms in Figure 5. Experiments show that this scheme offers very limited improvement to servo performance under step input. This is because, under step input, the current loop input within the driver rapidly increases to and maintains its maximum value. The PI control of the current loop ensures that the actual current tracks the given current. Therefore, the slopes of the speed rise curves in Figures 4 and 5 are almost identical.
However, when higher demands are placed on the driver, such as fast dynamic response, minimal speed overshoot, and strong rigidity, this method, combined with other improved control methods, can improve the driver's performance to some extent.
Conclusion
Experiments show that this scheme is reasonable and feasible. Although the improvement in driver performance is limited, this method is still feasible when higher requirements are placed on servo drivers. Furthermore, designers must fully consider the motor output power, power supply power, and power supply conditions when selecting the DC bus capacitor value to ensure that the bus voltage has strong anti-disturbance capability.
