Control Strategies On Electric Compressor Used In Electric Cars

By Author: Guchen EAC
Total Articles: 19
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1.Introduction
With the rise of electric cars, people have put forward higher requirements for the comfort, reliability, energy conservation and environmental protection of EVs in recent years. When it comes to comfortability, we all know that the performance of electric car air conditioning system directly affects the comfort of passengers.
The main difference between electric air conditioning system and the traditional one is the change of power source, so the working principle of the compressor has also changed, from mechanical compressor to electrically-driven compressor. The common structures of both of them include piston type and scroll type, but the main difference lies in the electric drive part of electric compressor.
In this article, Guchen EAC analyzes the electric compressor control principle, software protection strategy and interaction with the vehicle. It helps global readers understand the control logic of electric compressors to a great extent, provides certain help for automotive ...
... engineers in the development and design of electric compressors, and has certain guiding significance for after-sales maintenance of electric compressors.

2.Design of Electric Compressor Control System
The hardware part of electric compressor controller can be divided into high voltage (HV) power supply, low-voltage control part and a high-voltage interlock loop (HVIL). When the compressor receives low-voltage control signals, the IGBT turns on the HV power supply, and the electric motor drives the compressor to work. The control principle of a typical electric compressor controller is shown in Figure 1.

Figure 1. Schematic diagram of electric compressor control principle

2.1HV Power Supply
The high-voltage power supply part includes high-voltage connectors, high-voltage filter circuits and IGBTs.
◆ The high-voltage connector is connected to the wiring harness of the vehicle. It is connected to the compressor housing through bolts.
◆ The high-voltage filter circuit is generally composed of multiple filter capacitors in parallel. There are two main purposes: one is to filter out the abnormal fluctuation of the bus voltage, so that the voltage input to the compressor is stable, and avoid large voltage fluctuations that cause damage to the compressor; The second is to absorb the ripple voltage generated by the compressor itself due to power changes, to prevent the ripple generated by the compressor from affecting the entire vehicle and other HV components.
◆ IGBT is a very important component in the air-conditioning controller. Generally, the electric compressor consists of 6 IGBTs. Its function is to turn on the high-voltage circuit through the on/off of the IGBT, so as to drive the electric car AC compressor to start working. Because the IGBT will be connected to high voltage and will frequently pull up/pull down, its working environment is the worst in the entire controller. Common IGBT damage modes include overcurrent, overvoltage and overheating. In order to ensure that the IGBT works within a suitable temperature range, the electric compressor controller will add a lot of protection logics.

2.2LV Control Part
The low-voltage control part includes LV connector, filter circuits, DC-DC converters, LIN/CAN communication loops, sampling circuits and ECUs.
● The low-voltage connector is connected to the vehicle wiring harness to provide low-voltage power and LIN/CAN signal communication for the electric compressor controller.
● The filter circuit is mainly to ensure the stability of the low voltage input voltage.
● DC-DC converter is to convert 12V into different voltages, to provide power for different electrical appliances. For example, IGBT drive circuits generally require 15V regulated power supply, sampling circuits generally require 3.5V power supply, LIN/ CAN isolation and sleep require a 5V supply.
● LIN/CAN communication is the communication channel between e-compressor and the electric car. The vehicle sends various commands to the compressor via LIN/CAN communication. At the same time, the compressor feeds back its actual status to the vehicle through LIN/CAN to judge its operation status.
● The sampling circuit is used to detect the voltage, current temperature and hardware protection circuit of the electric car compressor.
● ECU is the core of signal interaction and transceiver of electric compressor controller. ECU receives vehicle LIN/CAN signals, and simultaneously collects its own voltage, current, temperature and the feedback status of each sampling circuit to determine whether the compressor meets the starting state. If the starting conditions are met, ECU sends a start signal to the IGBT driver chip to drive the IGBT on off to start the electric motor. At the same time, it has been monitoring the working status of each component. If there is any abnormality, the electric compressor will be stopped immediately and the failure mode will be fed back to the vehicle.

2.3High Voltage Interlock Circuit
HVIL can prevent the electric compressor connector from being unconnected or poorly connected to cause leakage and personal safety. It uses a low voltage loop to ensure that all the high-voltage and low-voltage electrical circuits of the vehicle work normally. Only when all the connectors are inserted in place, the high-voltage components can communicate normally. When it is detected that the current of the interlock circuit exceeds the normal value, the electric vehicle will report an interlock circuit fault, and the vehicle cannot be powered on by high voltage.

3.Software Protection Strategy
All protection strategies is to protect the compressor to reduce the operating power or shut down in time when the operation is abnormal, and to protect the compressor from more serious damage. The protection strategies of the electric compressor mainly include LIN/CAN communication protection, over temperature protection, overcurrent protection, overvoltage protection and low temperature preheating.

3.1 LIN/CAN Communication Protection
LIN/CAN communication protection means that the electric a/c compressor stops working immediately once the following situations occur:
◆ Bit errors, PID errors, no response errors, frame errors, and physical bus errors occur in the communication between the compressor and the vehicle;
◆ The ECU detection time of any error is longer than a certain time.
When the communication returns to normal, the compressor resumes normal operation.

3.2 Over Temperature Protection
Over-temperature protection means that when the compressor controller detects that the IGBT temperature is too high or too low (exceeds the set temperature value), it reports the IGBT temperature failure in time, and the compressor stops responding at this time. When the temperature drops below the set-point and the shutdown time is longer than the set time, the electric compressor responds to the vehicle demand again. The purpose of this protection is to avoid damage to the IGBT when it operates exceeding maximum allowed temperature.
To avoid inability to detect IGBT temperature due to temperature sensor failure, two temperature sensors are generally arranged inside the controller at the same time. The ECU determines the temperature based on the higher measured temperature sensor value, and the temperature values of the two temperature sensors will be compared in real time. When the absolute value difference between the two exceeds the set temperature, the compressor will report temperature sensor plausibility failure.

3.3 Overcurrent Protection
Overcurrent protection is to avoid damage to the electrical components inside the compressor due to overcurrent. It includes bus current and phase current protection.
◆ Bus current protection means that the input current is limited within a certain value, and the compressor will stop when it exceeds the set value.
◆ The phase current protection is to avoid damage to the IGBT. For electric compressors, the phase current is directly related to the compressor load. The higher the load, the greater the phase current. Therefore, the phase current can also be used as a parameter for overload protection. At the same time, in order to better protect the IGBT and prevent it from working at high current and high temperature for a long time, the phase current protection will be associated with the temperature protection. When the IGBT temperature is low, the phase current temperature limit remains unchanged. When the IGBT temperature is higher, the phase current limit decreases linearly with IGBT temperature until IGBT overtemperature protection.

3.4 Overvoltage Protection
Overvoltage protection refers to a protection method to limit the compressor rotating speed or shut it down when the voltage exceeds the working range. It is divided into HV protection and LV protection.
● Low-voltage protection is further divided into 12V voltage protection and 15V driving voltage protection.
● High voltage protection is to prevent the internal components of the controller from being broken down due to excessive input voltage. At the same time, when the input voltage is too low, and cannot meet the power requirement of the compressor, the electric a/c compressor will works with limited power.
At present, the charging voltage of the charging stations on the market is not stable, and instantaneous high voltage situations exist. Therefore, the electric compressor overvoltage protection generally includes software protection and hardware protection. Their combination can effectively protect the compressor from IGBT breakdown caused by high voltage.

4.Vehicle Interaction
The whole electric car drives the compressor through LIN/CAN communication. Generally, the vehicle will generate speed demand signal, allowable power signal and enable signal.
When the compressor normally receives the vehicle signal, it will start to work in response to the demand of the vehicle. The controller will monitor its own working status in real time: When the compressor is abnormal, the controller will respond immediately and feedback the fault information to the vehicle.
Figure 2 is a schematic diagram of the interaction between the EV and the electric compressor.

Figure 2. schematic diagram of 10 different state transitions of e-compressor

Conversion 1: When the compressor controller receives work request from the EV, it controls the compressor to operate.
Conversion 2: When ambient temperature is lower than the set-point, the compressor turns on the preheating function. After the preheating is completed, the compressor mode is ON.
Conversion 3: When the compressor is in ON or power-limited state, if it fails, it changes to ERROR state.
Conversion 4: When the compressor stops and the controller detects a fault, it changes to ERROR state.
Conversion 5: When the compressor is in FAULT state, after the controller detects the fault contact, the compressor state changes to STOP.
Conversion 6: When the compressor is in the working or power-limiting state, if the controller receives STOP command from the vehicle, the compressor state changes to STOP.
Conversion 7: When the compressor is in an Error Reporting state, if the error is removed and the controller satisfies starting conditions, it changes to the DRIVE state.
Conversion 8: When the compressor is in SHUTDOWN or ERROR state, if the controller is in sleep mode, it changes to a SLEEP state.
Conversion 9: When the compressor is in DRIVING mode, if it is limited in power, the compressor state changes to a power limited state.
Conversion 10: When the compressor is in the power limit state, if the power limit is released, the compressor state changes to the DRIVE state.

5.Conclusion
This article is written Guchen Team. It details the control principle, software protection strategy and interaction with the whole vehicle. It expounds the design ideas of the electric compressor controller, and describes in detail the LIN/CAN communication protection, over-temperature protection, over-current protection, over-voltage protection and low-temperature preheating. It is hoped that it can provide a reference for the design, development and after-sales maintenance of the control part of HV electric air conditioning compressor.

I am Isabella S Johnna, Working at Guchen EAC. We are the top manufacturer of HVAC systems, including bus air conditioning, 12V air conditioner for van, electric AC compressor etc.Guchen EAC manufactures and exports a wide range of electric AC compressor, both high voltage (DC320V, DC350V, DC380V) and low volt compressors (DC12V, DC72V).