Research on EMC Simulation of Electric Drive System of Electric Engineering Machinery

Abstract

In order to suppress the electromagnetic interference in electric construction machinery, improve the stability and safety of the vehicle on the influence factors of electric construction machinery EMC system analysis, electric drive system due to the internal power electronic equipment for a long time in the high voltage, high current conditions and become the main influence factors of electric construction machinery EMC. The main methods to reduce electromagnetic interference of electric drive system grounding, shielding and filtering are expounded, which leads to the emulated research of EMC simulation of electric drive system of electric engineering machinery. The equivalent circuit model of battery, electronic control, motor and test system is established, and the low-pass filter composed of inductor and capacitor is designed. And the combined three electric system, test system, filter circuit composed of the model of simulation analysis, with or without filter input current, different frequencies of input and output voltage signals compared, the results show that adding filter can effectively improve the conducted interference, the use of RLC filter composed of four RLC components, can effectively improve the signal low frequency bias and high frequency distortion.

1 Overview

Electric construction machinery has achieved intelligent control through high electrification, but how to achieve electromagnetic compatibility (EMC) with so many electronic components becomes a hot topic. EMC of electric construction machinery is a hot research topic at present. The EMC performance of electric vehicles in the world has a strict standard system, such as GB/T 18,387-2017, GB/T 14,023-2011, ECE R10.05 and so on [1].

EMC refers to the ability of electrical equipment and systems to work acceptably in their electromagnetic environment [2]. EMC mainly includes the following two aspects: 1. Electromagnetic compatibility; The electromagnetic interference (EMI) generated by the equipment during normal operation cannot exceed a certain limit value. 2. The equipment has a certain immunity to electromagnetic interference in the environment, that is, electromagnetic Susceptibility (EMS) [3]. Electromagnetic interference that causes performance degradation or failure of equipment must have three elements at the same time: 1. Interference; 2, sensitive equipment; 3, coupling path [4].

For the electric engineering machinery three electric system, because the battery pack only outputs direct current, so it can be excluded from the interference source [5], the electric drive system (electronic control and motor) contains a large number of high-frequency switches and other electronic components, is the main source of EMI, in the switching process of these semiconductor devices, The rate of change of current and voltage is very large (can be as high as 10 kV/µs or 10 kA/µs speed switch, the frequency can be as high as several hundred kHz), through the circuit or component of the parasitic inductance or parasitic capacitance caused by conduction and radiation electromagnetic interference [6]. According to the three elements of electromagnetic interference, methods to reduce electromagnetic interference generally include grounding, shielding and filtering [7].

1.1 Shielding

For high-voltage equipment, the method of shielding and grounding is generally used. Aluminum foil can be covered in places where there may be electromagnetic leaks, such as cables, to shield interference [8]. Figure 1 shows a dedicated cable with a shielded connector [9].

Fig. 1

Cable with shielded connector

Communication cables generally use shielded twisted pair, the twisted pair mode can make the induced magnetic field cancel each other, and there is a metal shield between the twisted pair and the outer insulating sheath, which can reduce radiation. Figure 2 shows Communication cables use shielded twisted pair.

Fig. 2

Shielded twisted pair cable

1.2 Filtering

In 2021, Tan et al. [10, 11] proposed a simulation model of multi-in-one electric drive system that integrates conducted emission risk prediction and interference suppression, as shown in Fig. 3. Based on this model, they studied the effect of filter circuit Y capacitive parasitic inductance on the system.

Fig. 3

Simulation model of electromagnetic interference in electric drive system

In this study, as shown in Fig. 4 π-type filters are used, where Cy1 is the front-end capacitor near the power supply and Cy2 is the back-end capacitor near the IGBT.

Fig. 4

Equivalent circuit of π-type filter

In order to determine the effect of the parasitic inductance of the filter Y-type capacitor in the model on the conducted interference voltage, the parasitic inductance is set to 5 nH, 50 nH and 500 nH, respectively. The simulation results are as follows Fig. 5.

Fig. 5

Simulation results of different parasitic inductors

It can be concluded from the simulation results that the larger the parasitic inductance, the smaller the conducted noise voltage generated in the high frequency band (5–30 MHz).

2 Three-Power System Modeling

The three-power system simulation model based on LTspice was established, and the effect of the filter was simulated.

2.1 Battery System Simulation Modeling

As shown in Fig. 6, the battery system is simplified into a high voltage source (HV voltage source) V2 and its internal resistance Rbat. The HV voltage source can continuously output 355V DC, and the corresponding R is bat0.15 Ω.

Fig. 6

Simulation model of battery system

2.2 MCU Simulation Modeling

The role of MCU is to convert the DC current of the battery into the AC power supply of the motor. Therefore, the MCU can be thought of as an inverter consisting of six IGBTs, as shown in Fig. 7. The MCU controls the input voltage and current by controlling the on–off of the switching tube, thus controlling the speed and direction of the motor; IGBT parameters are set as follows: internal inductance = 4 nH, internal capacitance = 500 pF, internal resistance = 1 mΩ; Since the motor controller gets the RMS or peak pulse current from the battery pack, it will generate a high pulse voltage on the DC bracket, which the motor controller can not bear, so it is necessary to select a capacitor (C7) to connect the battery and MCU, and the capacitor of C7 is set to 650 uF.

Fig. 7

MCU simulation model

2.3 Motor Simulation Modeling

Ac after MCU inverter, before supplying power to the motor, it is necessary to reduce the terminal voltage of the motor through the inductor (L1, L2, L3), so as to reduce the current flowing through the motor; Capacitors (C1, C2, C3) are the starting capacitors of the motor, which can make the current of the secondary winding 90 degrees ahead of the current phase Angle of the main winding, thus generating magnetic field torque and starting the rotor; The inductor in the motor (L4, L5, L6) is affected by the rotating magnetic field, causing the current flow lag, generating torque and generating power; The internal resistance of the motor is R1, R2, R3; The parameters of the motor are as follows: inductance of L1, L2, L3 = 200 uH, inductance of L4, L5, L6 = 10 nH, capacitance of C1, C2, C3 = 1 nF, resistance of R1, R2, R3 = 1 mΩ.

The simulation model of Motor is shown in Fig. 8.

Fig. 8

Motor simulation model

2.4 LISN Simulation Modeling

Line Impedance stabilization network (LISN) is an important auxiliary equipment for EMC test of power system. When the equipment is in normal operation, it is directly powered by the battery. Therefore, it is possible to test the EMC performance of the device by connecting the test instrument, however, if EMI is detected during this test, it is impossible to distinguish whether it is the cause of the device under test (DUT) itself, or the influence of the environment or the battery. As shown in Fig. 9 EMC measurement method.

Fig. 9

EMC measurement method

As shown in Fig. 10 LISN test topology. LISN can isolate conducted emissions outside the DUT and only measure conducted emissions from the DUT. At the same time, it can provide a stable impedance to the product's power cord over the entire frequency range of the conducted emission measurement.

Fig. 10

LISN test topology

Figure 11 shows the internal structure of LISN. The battery and DUT are connected in series on either side of the LISN. In the LISN, the functions and parameters of the electronic components are set as follows:

Fig. 11

Internal structure of LISN

Capacitor (C1) 1 uF, inductor (L1) 5 uH, used to filter grid-side interference to provide consumers with maximum undisturbed power;

Resistors (Rmess, 50 Ω) are located inside the receiver and their role is to convert the noise current into noise voltage;

The resistor R31 kΩ is used for 0.1 uF capacitor discharge; Capacitor (C2) 0.1 uF series capacitor, Rmess and R3 parallel branches, used to block DC and prevent DC components from damaging the receiver. Figure 12 shows LISN symbol in LTspice.

Fig. 12

LISN symbol in LTspice

2.5 Three-Power System Simulation Model

To sum up, the three-power system simulation model is shown in Fig. 13.

Fig. 13

Three-power system simulation model

3 Design of Filter

In EMC design, filter is an important method. The filter usually refers to a low-pass filter composed of inductors and capacitors, and the effectiveness of the filter is not only related to the structure, but also to the impedance [12] of the connected network. Capacitors are commonly used for decoupling, filtering, bypass, and voltage regulation of power busbars. Inductors can increase the impedance of the loop to reduce the interference current in the loop, so as to achieve the purpose of suppressing interference.

The structural design of the filter complies with the “maximum mismatch principle”, that is, in any filter, there is a high impedance at both ends of the capacitor and a low impedance at both ends of the inductor. In order to suppress differential mode interference, a filter capacitor is connected between the two power lines, which has a low impedance to high-frequency interference, so the high-frequency interference between the two power lines can pass through it, and has a high impedance to the power frequency signal, so there is no effect [13] on the transmission of the power frequency signal.

3.1 The Self-Resonant Frequency of the Decoupled Capacitor

Actual capacitors all have parasitic inductors. The size of the lead basically depends on the length of the lead, for the round wire type of lead, the typical value is 10 nH/cm [14] the typical ceramic capacitor's lead is about 6 mm, will introduce about 15 nH of inductance.

The inductance of the lead is estimated as follows [15]:

$$ L_{s} = \frac{{u_{0} }}{2\pi }l\left\{ {\left[ {\frac{l}{r} + \sqrt {\left( \frac{l}{r} \right)^{2} + 1} + \frac{r}{l} - \sqrt {\left( \frac{r}{l} \right)^{2} + 1} } \right]} \right\} $$

(1)

In this equation,

$l$ Represents the length of the cable,

$r$ Represents the radius of the cable.

The parasitic inductors and capacitors will create a series resonance, or self-resonance [16]. Below the self-resonant frequency, the capacitor remains capacitive; Above the self-resonant frequency, the capacitor is inductive, and the impedance becomes larger as the frequency increases, so that the decoupling or bypass function of the capacitor is greatly reduced, therefore, at the self-resonant frequency, the impedance presented by the decoupling capacitor is the smallest, and the decoupling effect is the best. The self-resonant frequency of the decoupling capacitor $f_{0}$ should be selected according to the highest frequency of noise $f_{\max }$, and the best value

$$ f_{\max } = f_{0} $$

(2)

Commonly used power filter, when the electrical sensing resistance rL, the self-resonant frequency are respectively:

$$ f_{0} = \frac{1}{{2\pi \sqrt {LC} }} $$

(3)

3.2 Choice of Capacitance Capacity

The capacity of the decoupling capacitor is usually estimated as follows:

$$ C = \frac{\Delta I}{{\Delta V/\Delta t}} $$

(4)

In this equation,

$\Delta I$ Is a transient current,

$\Delta V$ Is the allowable change in the power supply voltage of the logic device,

$\Delta t$ Is the switching time.

The choice of decoupling capacitor is based on:

• (1) C = 1/f; f is the circuit frequency,

• (2) The voltage difference between the chip and the decoupling capacitor $\Delta V_{0}$ must be less than the noise tolerance difference $V_{NI}$

  $$ \Delta V_{0} = \frac{L\Delta I}{{\Delta t}} \le V_{NI} $$

  (5)

• (3) From the perspective of decoupling capacitors to provide the required current for the chip, its capacity should meet:

  $$ C \ge \Delta I\Delta t{/}{\Delta V} $$

  (6)

• (4) The discharge speed of the chip switching current $i_{c}$ must be less than the maximum discharge speed of the decoupling capacitor current:

  $$ \frac{{di_{c} }}{dt} \le \frac{\Delta V}{L} $$

  (7)

In addition, when the power lead is relatively long, the transient current will cause a large voltage drop, at this time, it is necessary to add a holding capacitor to maintain the voltage required by the device.

Figure 14 shows the RLC filter used in this paper. In order to verify the effect of the filter, two sets of parameters are set for the filter in this paper. The first set of parameters is inductance C1 of 390 nF, capacitance C2 of 1.5 uF, inductance L1 of 4 uH and resistance of 0.17 Ω in Fig. 14. Inductance L1 in parameter group 2 is changed to 8 uH, and other parameters remain unchanged with parameter group 1.

Fig. 14

RLC filter1

Figure 15 shows a tri-electric system with an RLC filter.

Fig. 15

Three-electric system with RLC filter2

4 Simulation and Conclusion

First of all, it is necessary to measure the input current of the motor to confirm whether the input of the entire three-power system simulation model is correct, and then measure the output voltage and input signal of the LISN in the loop, and compare the waveform to analyze the influence of electromagnetic interference on the signal in the three-power system.

The simulation results of no filter are as follows.

As shown in Fig. 16, the DC output of the battery is converted into AC through the motor controller, which conforms to the working principle of the system.

Fig. 16

Input current of the motor without filter3

Figure 17 shows the input and output signals of LISN at different frequencies. It can be seen that when the frequency is low, the offset between the input voltage and the output signal is small, and the same waveform is basically maintained; When the frequency is greater than 8 kHz, the waveform of the input signal begins to have a large deviation; When the frequency is greater than 100 kHz, the input signal is completely distorted.

Fig. 17

Input and output voltage signals of LISN at different frequencies4

Add filter parameter: C(390 nF)-L(4 uH)-R(0.17 Ω), simulation results are as follows:

As shown in Fig. 18, after filtering, the distortion degree of the input signal is improved. When the frequency is greater than 500 kHz, the input signal is offset by about 15 dB, but the waveform is basically the same as the output signal.

Fig. 18

LISN input and output voltage signals at different frequencies5

Add filter parameter: C(390 nF)-L(8 uH)-R(0.17 Ω), simulation results are as follows:

It can be seen from Fig. 19 that the waveforms of the input signal and the output signal are basically the same except for a few peak signals.

Fig. 19

LISN input and output voltage signals at different frequencies6

To sum up, adding filters can effectively improve conducted interference. This paper uses an RLC filter composed of four RLC components, which can effectively improve the low frequency bias and high frequency distortion of the signal.

Electromagnetic interference EMI research methods for motor drive systems and vehicle chargers can be classified as follows from the perspective of suppression technology:

① Add EMI filter device.

By adding an EMI filter containing capacitors, inductors and other energy storage components between the interference source and the output end, the conducted interference current is suppressed or attenuated, and the propagation path of electromagnetic interference is blocked.

② Improve the PWM control strategy or change the internal winding connection mode.

Common-mode voltage suppression from the perspective of optimal control strategy, to reduce common-mode interference from the source, has become the focus of optimization control research. The main method is to reduce the amplitude or pulse number of the common mode voltage by changing the switching sequence of the DC/AC inverter or the operation law of the switching device.

③ Active cancellation suppression method.

Active cancellation means to reduce or eliminate interference by superimposing or injecting reverse common mode current or voltage to the part that needs to be compensated in the power converter. The cost of this suppression method is small, the space volume is relatively small, and the suppression effect can be obtained.

The EMC compatibility design for the integrated controller of the electric excavator can refer to the EMI suppression technology of the electric vehicle drive system and charging system mentioned above, that is, the internal suppression method is adopted: the corresponding filter or common mode choke is designed according to the resonance point inside the controller to adjust the resonance frequency; Or external suppression method: Design effective active or passive EMI filters.