Keywords

1 Introduction

Motor control system is not only the important basic supporting system for electric vehicle, but also the significant link for the process of commercialization and industrialization of electric vehicles. The electromagnetic compatibility of motor control system have direct effects on the reliability of the motor control system as well as the safety of the electrical vehicle, so it is especially important to research on electromagnetic interference restraining for motor control system.

There are many references on the whole electric vehicle or charging system in previous researches. However, there is little research on electromagnetic compatibility of motor control system for electric vehicle [1, 2]. The reason why the motor control system generates electromagnetic interference (EMI), the transmission paths, the hazard of EMI and the corresponding technologies to restraining EMI are presented through analysing the structure and control strategy of motor control system in this paper. The results of electromagnetic emission test of electric vehicle motor control system show that the motor control system designed by electromagnetic compatibility technology has good EMI restraining performance.

2 Working Principle of Motor Control System

2.1 Topological Structure of Motor Control System

The motor control system of electric vehicle consists of two parts, as shown in Fig. 1, which are the power circuit and the control circuit. The main power circuit uses three-phase voltage mode inverter circuit which consists of six IGBT-modules. The master control chip DSPTMS320LF2812 is used to perform real-time digital control in the control circuit which consists of current feedback, voltage feedback, torque feedback, rotate speed feedback, PWM signal output circuit etc.

Fig. 1
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Working principle of motor control system

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2.2 Working Principle

According to the signals analysed by the vehicle control unit, the torque signal and speed signal are sent to motor controller control circuit by CAN bus. According to the designed program and the current feedback signals, the voltage feedback signals, rotate speed feedback signals, the PWM signals are generated by the master control chip TMS320LF2812. The PWM signals are imported to the driving circuit after optoelectronic isolation to control the switch states (on or off) of six IGBTs. The PWM wave signal is a kind of positive and negative half-cycle symmetrical high frequency pulse. Thus the power circuit can complete the DC/AC conversion and provide expected voltage to the motor. Therefore the required rotate speed and torque of the motor are finally generated to satisfy the driver’s command.

3 Electromagnetic Interference Existing in Motor Control System

3.1 Electromagnetic Interference Source

Electric vehicle motor control system contains system interference source and out system interference source. System interference source mainly refers to the EMI generated by voltage and current sudden change in short time during the switching process of the six IGBTs in the power circuit. And in this process, high frequency pulse signal would be generated, which has big du/dt and di/dt. Due to the inductance and capacitance devices existing in circuit, lead inductance existing in IGBT itself and other devices, and routing stray inductance and capacitance, noise voltage and noise current are generated. These noise voltage and noise current would not only affect the power module, but also harm the control performance by flowing into control circuit via the power supply and ground. Also, a current loop with big di/dt is a radiated source. It would radiate electromagnetic wave in space to form the very strong EMI, and affect other parts in vehicle. In addition, besides the useful fundamental wave, there is large amount of high harmonics in PWM wave produced by inverter switches. The radiated interference would be produced to influence other equipment in electric vehicle. Out system interference sources include lightning, electrostatics, charging system and other power switching devices and so on.

3.2 Coupling Path

In electric vehicle motor control system, there are two electromagnetic disturbance coupling paths which include radiated coupling way which transmit in space and conducted coupling way which transmit in circuit. The radiated coupling way mainly refers to the near-field coupling way, otherwise the conducted coupling way is an important path for interference transmission. According to the difference of interfering ways, EMI source include differential mode (DM) interference and common mode (CM) interference [3].

The DM interference as shown in Fig. 2, DM voltage would be generated from the output side of the inverter when IGBT switch on or off, and DM interference current i dm1 and i dm2 are formed on the output side of the inverter, so there are two interference transmission paths. Loop i 1 produced by the DM current, shown in Fig. 2, can be seen as a small loop antenna, which radiate disturbance signal to space.

Fig. 2
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Differential mode interference

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As can be seen in Fig. 3, there is one transmission path of CM interference current. The CM current i CM flows from the output side of the inverter, along the motor shell and the chassis (ground), finally flows back to the inverter. The inverter and the motor shell cannot completely insulate from the reference ground, therefore there is stray capacitance exist between them. The du/dt would be generated large CM interference current on the output side of the inverter.

Fig. 3
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Common mode interference

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3.3 Sensitive Equipment

Sensitive equipment of the electric vehicle motor control system mainly include DSP digital control circuit, the PWM inverter, the signal driving circuit, signal amplifying circuit, current sensors and voltage sensors, CAN bus, ABS, EDS, lightning equipment, audio equipment, in-car entertainment system, GPS, parking sensors, semiconductor device, signal cables and so on.

Interference to any of the above components may affect safety and stability of the entire system. Take charging system for example, the signal distortion of the sensors may lead to overcharge and damage the battery. These sensitive devices also can be interference sources, which may interfere on other on-board equipment with high sensitivity.

4 EMI Suppression Measures

4.1 EMI Suppression of Power Device

In general, such methods, like changing the topology and structure of circuit, improving control strategy and optimizing driving circuit, would be adopted in order to decrease the strength of interference generated by switching of power devices. Increase the turn-on and turn-off time of power devices; Reduce the electric field coupling between IGBTs and radiator [4]; Every IGBT’s gate driving circuit adopt independent insulated power, and should not share current branch with the main circuit’s current; To avoid noise interference, snubber circuit should be adopted between power device’s collector and emitter, as shown in Fig. 4. These measures are taken to decrease the change rate of voltage and current greatly when IGBTs switch on or switch off, thus, the strength of interference source is decreased.

Fig. 4
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Inverter EMI suppression circuit

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4.2 PCB EMC Design

Electromagnetic compatibility design for DSP digital control PCB of electric vehicle motor control system control circuit includes PCB layout and routing.

Grouping layout of components is used here. Space on PCB is divided according to the groups, and those components, which are in the same group, are placed together so that interference in space would be avoided among groups. Classification of group layout includes high-speed and low-speed category, high current and low current category, analog circuit and digital circuit category. In this main control board, the classification is done according to high-speed and low-speed, shown in Fig. 5. The high-speed CAN bus transceiver circuit is arranged on one side of the circuit board, and the low-speed circuits including digital circuits and analog circuits (PWM inversion circuit) are arranged on the other side of the board [5].

Fig. 5
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Layout with reduced interference

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The width and clearance of PCB routing are according to density of routing and high frequency characteristics of wires. During the distribution of the power line, the decoupling capacitors should be properly used at the power input side to reduce the coupling noise that gets into the printed circuit board and ground noise voltage between power and earth wire. To avoid the common ground wire impedance coupling interference, the incompatible ground wire, such as large current and small current, high frequency and low frequency, analog and digital, different supply voltage ground wire, should be set separately. As showed in the Fig. 6, By divide the ground plane, the ground connections of the master control board can be divided into 4 parts, which is AGND, DGND, 5VGND, 15VGND and CAN5VGND. Therefore the grounding wire inductance is minimum to reduce the grounding impedance, then the coupling of CM impedance is restrained. When comes to double panel, the ground network should be designed at first in order to effectively reduce the area of signal loop. The DM radiation and CM radiation are proportional to the area, so the DM radiation and CM radiation are decreased.

Fig. 6
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Multi-point grounding

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4.3 Grounding Design

The EMI is restrained by the method of filter isolation for the electric vehicle motor control system. The ground loop generates ground loop interference which is a usual EMI. Filter isolation technology is applied to restrain ground loop interference. The isolation measurements are used to isolate the motor control system circuit and common ground or common conductor which can generate loop current. The isolation measurements usually adopt transformer or optocoupler. Take CAN bus interface circuit for example, as shown in Fig. 7. There are two optocouplers 6N137, and they isolate the process of sending and receiving data respectively.

Fig. 7
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The principle drawing for CAN bus interface circuit

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4.4 Shielding Design

Electromagnetic shielding technology is used to restrain the transmission of disturbance in the space, which cuts off the transmission channel of the radiated coupling. Shielding is one of the most basic methods of EMI restrain control. Electromagnetic radiated disturbance transmits in a way of electromagnetic wave by monopole antenna and small loop antenna in near-field and far-field. Use high conductivity materials and grounding to cut off the electric field disturbance in near-field, and use high conductivity materials and grounding to cut off the electric field disturbance in far-field.

The electromagnetic shielding design of the electric vehicle motor control system includes the shielding design of system chassis and cable. The overall shielding effectiveness of shielding chassis is determined by the shielding body’s weakest link which refers to various gaps and holes. In fact, there are many leaking sources on the chassis, such as gaps of the junction of different parts, vents, display windows, buttons, indicator lights, cables, power lines. Usually electromagnetic sealing gasket is used at apertures, cut off waveguide filters is used at ventilated places, and conductive glass is used at the display apparatus, etc., to improve the shielding function.

4.5 Filtering Design

Conducted disturbance can transmit through wires such as power lines, signal lines, interconnect lines, and conductors like shielding bodies, grounding conductors and so on. Usually signal line filters and power line filters are used to cut off cable coupling paths [6]. The motor driving control circuit of the motor control system in Electric Vehicle is low source impedance and high load impedance, so LC type of filter structure is chose, shown in Fig. 8. Because there is DM interference at the input port of power module, a CM choke should be added, seen it in Fig. 8.

Fig. 8
figure 8

Filtering design for power source module

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4.6 Transient Pulse Suppression Design

The transient disturbance pulse and surge voltage that generated by lightning, electrostatic, switches, motor drive system, and other power switching devices, etc., would interfere the motor control system by near-field coupling through CAN bus and other cables. When design the CAN bus circuit, Transient Voltage Suppressors (TVS) should be chose to absorb the surge voltage (shown in Fig. 7) and protect components, then the reliability of system would be enhanced greatly.

5 Test Results

Firstly, the motor control system power on to preheat, and achieve stable working state, make sure the input signal, output signal of motor control system and the communication signal of CAN bus perform normally; According to the band width and minimum measuring time provided by national standard, the measuring receiver can scan at a applicable frequency from 10 kHz to 10 MHz. The test results of power line in control module of charging system is shown in Fig. 9, it can be seen that the conducted disturbance generated on the power source port in control module of charging system meet the limits requirements provided by standard.

Fig. 9
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EMI test result

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6 Conclusion

The electromagnetic radiated interference and conducted interference are generated by DM current and CM current through analysing the structure and working principles of motor control system. In order to restrain the EMI, power module EMC design, PCB EMC design, grounding design, shielding design, filtering design and transient pulse suppression design are adopted. The results of electromagnetic emission test of electrical vehicle motor control system shows that the motor control system designed by EMC technology has good Emi restraining performance.