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Take you to know the motor controller of an electric car

- Aug 21, 2018 -

The motor controller, as one of the core components of an electric vehicle, is a decisive factor in the dynamic performance of the car. It obtains the demand of the whole vehicle from the vehicle controller, obtains the electric energy from the power battery pack, and obtains the current and voltage required for controlling the motor through the modulation of the self-inverter, and supplies it to the motor so that the rotation speed and torque of the motor satisfy the whole vehicle. Requirements.

1. Motor controller position and function in electric vehicles

1.1 Location

From the outside, the general motor controller has at least two pairs of high voltage interfaces. A pair of input interfaces are used to connect the high voltage interface of the power battery pack; the other pair is a high voltage output interface, which is connected to the motor and provides control power.

At least one low-voltage connector, all communication, sensors, low-voltage power supply, etc. are led out through this low-voltage connector, connected to the vehicle controller and power battery management system.

The figure below is an electrical diagram of a typical pure electric vehicle power system, in which the blue line is the low voltage communication line and the red line is the high voltage power line. The second column on the far right is the motor controller. The components that have a strong electrical connection with the motor controller are the motor and the power battery pack; the motor controller is connected to the CAN bus of the vehicle, and can communicate with the vehicle controller, the digital instrument panel, the power battery management system, and exchange data. Accept the order.

1.2 Working process

1.2.1 Commands and responses

The motor controller, the trigger signal of the speed control command, and the command from the vehicle controller. The vehicle controller reflects the driver's intention on the one hand, and evaluates whether the response to the driver is reasonable from the safety and the operating state of the vehicle electrical system, and finally executes or discounts execution. The driver's intention is expressed by the accelerator pedal and the brake pedal and transmitted to the vehicle controller.

The vehicle controller gives specific instructions to the motor controller. The following are related to the power system: acceleration, deceleration, braking, and parking. The response of the motor controller is to change the parameters such as the current, voltage, frequency, etc., so that the running state of the motor meets the requirements of the vehicle controller.

1.2.2 Closed loop

The motor controller itself is a closed-loop control system that adjusts the target parameters and detects whether the value of the controlled function reaches the expected value. If it does not match, feedback to the controller and adjust the target parameter again. After repeated closed-loop feedback, high-accuracy control is achieved.

The vehicle controller collects the vehicle speed sensor, the important state parameters such as temperature and voltage of each electrical component, and judges whether the overall situation of the vehicle meets the requirements put forward by the driver and does not hinder the health of the entire system. This process is closed-loop control at the vehicle level.

1.2.3 Direction of improvement

On the one hand, a good control strategy will have an important impact on control accuracy and response speed, and is therefore an important area for R&D personnel to invest their energy.

On the other hand, with the improvement of the computing power of various components, the driving experience of electric vehicles will become more and more "free".

2. The basic composition of the motor controller

The motor controller system consists of a central control module, a power module, a drive control module, and various sensors.

2.1 Central Control Module

Including, PWM wave generation circuit, reset circuit, sensor signal processing circuit, and interactive circuit. The central control module, externally, obtains command and status information of other components on the vehicle through the external interface. Internally, the translated instructions are passed to the inverter drive circuit and the control effect is detected.

2.2 Power Module

The subject of the motor controller is an inverter that controls the motor current and voltage. Power devices that are often used are mainly MOSFETs, GTOs, IGBTs, etc.

2.3 Drive Control Module

The command of the central control module is converted into the on/off command of the thyristor in the inverter, and serves as a protection device with monitoring and protection functions for faults such as overvoltage and overcurrent.

2.4 sensor

The sensors applied to the system include current sensors, voltage sensors, temperature sensors, motor shaft angular position sensors, etc., which are increased or decreased according to design requirements.

3. What should a good motor controller look like?

3.1 Good motor controller features

The difference in the working principle of the motor directly affects the complexity and accuracy of the control process.

According to the control from easy to difficult to arrange, are DC brushless motor, permanent magnet synchronous motor, switched reluctance motor, asynchronous motor.

The difficulty of electronic control includes not only the size and cost of hardware system design, but also the control precision achieved by software algorithms and the robustness of the strategies and methods used to achieve this accuracy.

What people expect is a control system with simple hardware structure, simple software algorithm, high control precision and good system stability.

3.2 Motor controller has national standard

Electric vehicle motor and controller standards, there are national standards:

GB / T 18488.1—2015 "Motors for electric vehicles and their controllers - Part 1: Technical conditions";

GB / T 18488.2 - 2015 "Motors for electric vehicles and their controllers - Part 2: Test methods".

The 2015 version is the latest version.

The standards set specific requirements for safety and environmental resistance, such as insulation and pressure resistance of various parts and various environmental resistance. The technical parameters of the motor, as a verification project, can be as long as it conforms to the manufacturer's own statement.

4. Motor controller main circuit selection

4.1 Selection basis

As a special function inverter, the motor controller uses the voltage regulation and frequency modulation technology in power electronics technology to modulate the DC power stored in the power battery into a rectangular wave or a sinusoidal alternating current required to control the motor, and change the output. The voltage and current amplitude or frequency of the electric power, in turn, change the motor speed and torque to achieve the purpose of controlling the speed and acceleration of the whole vehicle.

The design of power electronic circuits, according to different speed control 

4.2 Give an example

For example for the control of DC motors. If a single-tube chopper circuit is used, only one-way speed regulation can be used, and the current cannot be reversed. If a double-tube chopper circuit is used, the energy feedback action can be realized, but the DC motor cannot be commutated; if the H-bridge type is adopted The chopper circuit can be regulated by a DC motor, which can be energy-returned, and the excitation current can be reversed.

But the above three choices, one is more complicated than one, and one is higher than one. Designers need to choose between performance and cost. The most expensive is not necessarily the best, the most suitable.

5. Thermal design

5.1 Source of heat

The power module is the main heat source for the entire controller, and the MOSFET or IGBT used is a heat-generating component.

The heating of the thyristor comes mainly from the following parts: conduction loss, switching loss, leakage current and drive loss, of which the first two account for the bulk.

5.1.1 conduction loss

The loss of the internal resistance of the thyristor in the state of being triggered and flowing normally. It is proportional to the time of the flow, the square of the current and the size of its internal resistance.

5.1.2 Switching Loss

Although the thyristor is turned on and off for a short time, there is a continuation of the objective time. The current and voltage on the device are not conducted at all, but work on itself. Its heat is proportional to the device voltage, proportional to half of the maximum current during turn-on and turn-off, proportional to turn-on and turn-off times, and proportional to the turn-on frequency.

5.1.3 Leakage current loss

When the thyristor is turned off, there is still a small current passing through, and the energy dissipated is called leakage current loss. However, the loss of this part is extremely small, and it is negligible when it is generally designed for thermal design.

5.1.4 Drive loss

The control circuit of the thyristor on-off provides the triggering and sustaining voltage, which belongs to the secondary control loop, and is compared with the strong electrical circuit side by side, and there is a gap in the order of magnitude.

5.2 Basic design process of the radiator

5.2.1 Determining the heat conduction path

First introduce a concept, thermal resistance, that is, the difficulty of heat transfer between the medium and the medium, the unit is °C / W. The size of the thermal resistance affects the heat transfer path and the speed of the transfer.

Heat is a propagation path from the die to the outer casing and from the outer casing to the environment; from the die to the outer casing, the outer casing to the heat sink is another path. Calculating the thermal resistance needs to be done for a specific path.

5.2.2 Drawing an equivalent heat group diagram

According to the path planned in the model, the thermal resistance is abstracted, and the same thermal resistance diagram is drawn as the resistance.

According to the maximum temperature rise allowed by the system and the maximum heat power delivered, the former obtains the overall thermal resistance of the system.

Find the power thyristor parameter table selected in the inverter, and find the thermal resistance of the die to the case, the case to the environment, and the case to the heat sink. Finally, the total thermal resistance can be subtracted from the known thermal resistance on the heat transfer path, and the thermal resistance of the heat sink and the environment is finally obtained.

5.2.3 Radiator selection

The main basis for selecting a heat sink, in addition to the structural form of the heat sink, the most important parameter is its thermal resistance. Through the previous calculations, the required heat resistance of the heat sink is derived. The heat resistance of the final heat sink must be less than this calculated value. In theory, the system will not overheat. Of course, the system needs a certain amount of margin, and can be used as a standard for selecting a heat sink after discounting the heat resistance of the heat sink.

5.3 Device layout points

Preferably, the parallel thyristors are attached to the same heat sink, so that the temperature of the parallel devices is uniform, thereby ensuring the uniformity of the gate resistance. Parallel thyristors, the gap between the internal resistance of the gate is too large, and the device with the smallest internal resistance is easily burned due to excessive current.

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