Efficiency and power consumption calculation of all-digital AC servo drives

Abstract : Due to their high switching frequency and large rated current characteristics, IGBTs have become the main power devices in mainstream all-digital AC servo drives. However, due to the switching characteristics of IGBTs, their power loss is the main source of heat generation in the drive. To improve the reliability of all-digital AC servo drives and accurately estimate their power loss and efficiency, power consumption and efficiency calculation formulas were established based on IGBT datasheets in the design. Prototype experimental data shows that the proposed method can accurately calculate the power consumption and efficiency of the IGBT module, demonstrating good practicality.

Keywords: IGBT; all-digital AC servo driver; power consumption; efficiency

1. Preface

In recent years, with the rapid development of computer technology, semiconductor technology and power electronics technology, and the continuous global attention to energy issues, permanent magnet servo motors have become increasingly widely used in the transmission field due to their small size, high efficiency, large output power and torque, and flexible control. At the same time, the continuous development of permanent magnet servo motors has put forward higher control requirements and reliability requirements for servo drives. AC servo technology has also been mastered by more and more manufacturers. In addition, the continuous innovation and intelligence of upstream chips and various power modules of AC servo systems have enabled domestic servo drive manufacturers to achieve a development trend from the start-up to full expansion in less than ten years. While servo technology has developed in an all-digital way, the power consumption calculation and efficiency analysis that directly affect the thermal stability of servo drives have become increasingly important [1]. This article introduces how to use the IGBT data table to query the design parameters of power devices when designing a fully digital servo drive using the SPWM algorithm, and how to use these parameters to calculate the power consumption and analyze the efficiency of the servo drive.

2. IGBT Parameter Analysis and Power Consumption Estimation

2.1 IGBT turn-on/turn-off losses

In order to facilitate the analysis and calculation of IGBT losses, IGBT losses are divided into conduction losses and turn-off losses according to the working process of IGBT. These two parameters are key data in the data sheets provided by all IGBT manufacturers, directly indicating the power loss value of IGBT. In the datasheets provided by the manufacturers, the parameters of IGBT conduction loss (Eon) and turn-off loss (Eoff) are obtained by testing under specified gate drive resistors Ron and Roff and rated gate drive voltage VGE. As can be seen from Figure 1, under certain conditions, the conduction and turn-off losses of IGBT also increase with the increase of current Ic. Therefore, the conduction and turn-off losses are expressed as Eon (Ic_igbt) and Eoff (Ic_igbt), respectively. Figure 1 shows the curve of IGBT power consumption changing with IC [2].

Figure 1 Characteristic curves of IGBT versus Ic variation

2.2 Parameter Analysis of Freewheeling Diode

From the IGBT schematic and conduction characteristics, it can be seen that each time the IGBT switches from turn-on to turn-off, due to the influence of the IGBT's parasitic parameters, a very large reverse recovery current Irr is generated. To enable the IGBT to turn off quickly, an ultra-fast recovery freewheeling diode is connected in parallel between the IGBT's collector and emitter terminals to ensure that the reverse recovery current can be released through the freewheeling diode each time it is turned off. Compared with the collector current IC, the reverse recovery current Irr is also a very large value, so this part of the loss generated during the turn-on and turn-off process of the freewheeling diode must also be included in the calculation of system losses. Consulting the device datasheets of well-known IGBT manufacturers, it is found that the current characteristic curve of this freewheeling diode is represented by two function graphs, and the reverse recovery current value IF of the freewheeling diode is the same as IC. Because the AC servo motor is an inductive load, the collector current IC of the IGBT will not change abruptly during reverse recovery. Based on the above analysis, the forward voltage VF of the freewheeling diode can be regarded as a function of Ic, VF(Ic_igbt), and the reverse recovery loss is Eoff_diode(Ic_igbt) [2].

2.3 Parameter Analysis of Three-Phase Rectifier Bridge

At present, the main circuit of the all-digital AC servo drive still adopts the AC-DC-AC conversion scheme. The AC input power supply is converted into a constant DC bus voltage by the three-phase uncontrolled bridge rectifier module. The DC bus voltage is converted into a sinusoidal output voltage with adjustable voltage and frequency by PWM chopping technology to power the AC servo motor [3]. Now, many IGBT manufacturers, in order to increase the integration of devices and improve the reliability of modules, usually integrate a three-phase uncontrolled rectifier bridge in the IGBT module. In the parameter table of the device, only the forward voltage value VF of the rectifier bridge diode is listed, and this parameter is a fixed value in the IGBT datasheet. Since the output power and loss power of the three-phase rectifier bridge depend on the working state of the IGBT, we first need to determine the total power consumption Ploss_inverter of the IGBT and the freewheeling diode, and then calculate the output power Poutput based on the working current. Through this decomposition, the power output calculation formula of the three-phase rectifier bridge can be obtained as follows:

(Formula 1)

Based on the above analysis, the power consumption calculation formula for a three-phase rectifier bridge can be derived as follows:

(Formula 2)

Figure 2. Schematic diagram of the main circuit of the servo driver

3. Calculation of total power consumption and efficiency of servo driver

Currently, domestically produced all-digital AC servo drives generally adopt the SPWM algorithm based on PWM modulation technology for control. This algorithm switches only one device at a time within the small interval between the on and off of the six IGBTs, resulting in low switching losses. However, due to limitations in current IGBT switching technology, the carrier frequency Fcarrier of the servo drive is generally set to 10kHz. Test data from IGBT manufacturers shows that IGBT switching losses increase proportionally with the carrier frequency. Therefore, to improve the drive's control characteristics, reduce overall system power consumption, and enhance thermal stability, the carrier frequency is generally no greater than 16kHz under current IGBT switching technology. In practical applications, the carrier frequency affects the output current waveform of the servo drive. Therefore, for user convenience, servo drive manufacturers set the carrier frequency as a system parameter through software. This allows users to adjust the carrier frequency appropriately within the required range based on the application conditions of the servo motor, achieving better control of the entire motion control system. Based on the SPWM control algorithm of the all-digital AC servo drive and combined with the IGBT parameter table, this paper proposes a method for calculating the drive's power consumption and efficiency to meet practical application requirements.

3.1 IGBT Loss Calculation

According to the AC servo PWM control theory, the sine wave equation is Sin(ωt), and the modulation index is A=(1+AcosΩt)*sin(ωt). Then the duty cycle[3]:

(Formula 3)

When the IGBT operates at the rated carrier frequency Fcarrier, the switching losses of each IGBT module are as follows:

(Formula 4)

VDC/VDCnom is a compensation factor used to represent the ratio of the test voltage to the actual voltage.

Regardless of whether unipolar SPWM or bipolar SPWM control theory is used, within one cycle, the IGBTs of the upper and lower bridge arms alternately turn on and off, that is, each IGBT only works for half a cycle [4]. Therefore, the power consumption calculation formula is:

(Formula 5)

3.2 Calculation of power consumption of freewheeling diode

When the servo driver is working normally, the power consumption of the freewheeling diode consists of two parts: conduction loss Pcond_diode and reverse recovery loss Pcomm_diode. The calculation formulas for these two parts are as follows:

(Formula 6)

(Formula 7)

Therefore, the total power consumption of each freewheeling diode during normal operation is:

(Formula 8) According to SPWM theory, these diodes are only in operation for half a cycle in each SPWM cycle, so their average power consumption is calculated as follows:

(Formula 9)

3.3 Calculation of power consumption and efficiency of servo driver [5]

According to SPWM control theory, when the servo driver is working, three IGBTs are conducting, and three freewheeling diodes are simultaneously in reverse recovery mode. Therefore, the power consumption calculation formula for the inverter section of the servo driver is:

(Formula 10)

Based on Formula 1 and Formula 2, the power consumption calculation formulas for a three-phase rectifier bridge can be derived as follows:

(Formula 11)

3.4 Efficiency Calculation of All-Digital AC Servo Drive

The power devices used in the main circuit of the servo driver all have power losses and cannot achieve 100% energy conversion. Therefore, the following formula can be used to calculate its output power:

(Formula 12)

Vin: Effective value of three-phase AC input voltage;

IC_IGBT: Rated output current of the IGBT;

After calculating the rated output power of the servo driver using the formula, the efficiency calculation formula for the servo driver can be obtained, where Fcarrier = 10kHz:

(Formula 13)

In this way, when designing AC servo drives, we can use the power consumption formula above and estimate the efficiency and power consumption of the servo drive based on the technical manuals of IGBT devices or other power devices provided by the manufacturer, thus providing a theoretical basis for designing a reasonable heat dissipation system.

4. Examples

To thoroughly verify the power consumption and power estimation methods of IGBTs, we used the FP150R07N3E4 IGBT module with a three-phase uncontrolled bridge rectifier circuit as an example for calculation. This IGBT is manufactured by Infineon and is currently widely used in all-digital AC servo drives, making its calculation results more universally convincing. The final calculation results are shown in the table.

5. Conclusion

The method for estimating the power consumption and efficiency of all-digital AC servo drives proposed in this paper is very intuitive and simple. This method can be used at the initial design stage of the drive to design the power consumption and efficiency of the entire device, thereby completing the thermal design of the servo drive. Based on the calculation results, structural engineers can design the heat dissipation system of the servo drive, and the quality of the heat dissipation system design directly affects the reliability and lifespan of the product. Currently, the calculation method mentioned in this paper has been experimentally verified on various servo products, and comparisons show that the error between the calculated values ​​and the test values ​​is within 15% under various conditions. Therefore, this calculation method is effective and practical for servo drive design.

References

[1] Wang Shuiping et al. PWM Control and Driver User Guide and Application Circuits: SPWM PFC and IGBT Control and Driver Section. Xi'an University of Electronic Science and Technology Press.

[2] Qu Xueji. Application of IGBT and its integrated controller in power electronic devices. Electronic Industry Press.

[3] Zhang Xing, Zhang Chongwei. PWM Rectifier and Its Control. Machinery Industry Press.

[4] Zhang Yidong. Research on SPWM Inverter Modulation Method. Science and Technology Innovation and Productivity. May 2011.

[5] Deng Jialing, Zhu Jin. Estimation of power consumption and efficiency of general-purpose AC servo drives. Science and Technology Innovation Herald, 2009, No. 35

Author : Ding Yunfei, born in 1978, is a senior engineer with a Bachelor of Engineering degree. He has been engaged in the research and development project management and quality management of high-end CNC machine tools, all-digital bus CNC systems and servo drives.