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On-Board Inverter (DC/AC converter) for Auxiliary Motors

Auxiliary motor inverters (DC/AC converters) convert high-voltage dc from power batteries into alternates, mostly for commercial vehicles. The DC/AC converter supplies power to and effectively controls auxiliary motors such as steering pump boosters or brake pump boosters. To ensure reliability during vehicle travel, the DC/AC converter for steering pump booster motors is always working throughout the service, while the DC/AC converters for brake pump boosters need to determine whether they are working according to the pressure conditions in the vehicle's brake air path.





1 Basic functions of the DC/AC converter



On-board DC/AC converters are mostly for a single load, i.e. a DC/AC converter controls an auxiliary motor that reverses the bus DC voltage into a three-phase AC voltage, and the drive motor converts electrical energy into mechanical energy. Its main function is to provide power supply for the steering power pump and brake system gas pump motor, and has the functions of control, monitoring, fault alarm and information communication with the vehicle controller. The controller is the core to ensure the normal operation of the steering system and braking system of the vehicle, and also affects the energy consumption, which has a great impact on the safety, reliability and economy of the vehicle. Common motors meet the load for AC asynchronous motors and permanent magnet synchronous motors, but the structure of the corresponding DC/AC converters is not different, generally including the following three modules.


1)Electronic control module

The electronic control module mainly includes the hardware circuit and the corresponding control software. Hardware circuits mainly include microprocessors and their minimum systems, detection circuits for auxiliary drive motor current, voltage, temperature and other conditions, various hardware protection circuits, and communication circuits that interact with external control unit data such as vehicle controllers. The control software implements the corresponding control algorithm according to the characteristics of different types of motor control objects.

2)Drive module

The main function of the driver module is to convert the microcontroller's control signal to the auxiliary drive motor into a drive signal to the IGBT power module and to isolate the power signal and control signal.

3) Power transformation module

The main function of the power transformation module is to control the motor current and turn the high-voltage DC reversal of the power battery into three-phase ALTER to drive the AC motor to operate. The power device for the DC/AC converter used to assist the drive motor is IGBT.


It is worth noting that the low voltage power supply for the DC/AC converter needs to be provided by the DC/DC converter, as shown the following figure.






In the DC/AC converter low-voltage normal working state, for its access to high-voltage electricity, the converter began to automatically pre-charge, pre-charged, you can accept the vehicle controller and harness switch control for the corresponding control, and through the CAN message to feedback the status.


The on-board DC/AC converter is similar to the working state of the industrial vehicle, and is based on the speed control target, so that the accused motor can run at a given speed. However, due to the limitations of actual operating conditions, there are some differences in demand with industrial inverters, including:

1.)The working state of the on-board auxiliary motor and controller is relatively fixed, and there is generally no need to start, slow down or reverse frequently.

2)The load change of the steering pump power motor is frequent and drastic, the transient response speed of the controller is high, and the transient impact of peak load can be sustained.

3)There is still air pressure in the brake air path when parking, so the brake pump power-assisted electrical opportunity has a load-driven starting condition, extreme conditions need to be close to the motor's full-load power start.

4)The on-board DC/AC converter has a harsh working environment and needs to be able to maintain normal operation in high and low temperature, humidity and large electromagnetic radiation environments.


Key technology for DC/AC converters

1.Dual closed-loop control system

The classic servo motor control system is generally a dual-closed-loop control system with a control system architecture as shown in the following Figure. Shown is a control system for permanent magnetic synchronous motors, but the control idea of the dual press ring is suitable for all types of motors:

1.)The outer ring is a speed change, using proportional and integral adjustment(Proportional Integral, PI controllers (or other controllers) to generate the reference current of the inter-axis (the cross-axis reference current is related to the output torque) based on the error of the actual speed relative to the target speed.

2)The inner ring is a current ring, which uses the PI controller to generate the reference voltage under the two-phase rotation coordinate system according to the error of the actual current relative to the target current, thus realizing the control of the motor speed of the whole system.



2.PWM inverter technology

In the motor control system, when the motor is operating at different speeds and load conditions, the voltage requirements of the motor stator end are different, which requires the voltage inverter that supplies the motor with voltage regulation function. When the three-phase voltage power inverter is operating in square wave mode, the base wave voltage value of its output cannot be changed. To solve this problem, various pulse-width modulation output techniques have been proposed, commonly including sine-width modulation output(SPWM),spatial vector-wide modulation output (SVPWM),and current hysteresis-loop wide modulation output (CHBPWM).


2.Motor control technology without position/speed sensors

In order to obtain accurate electrical angle and angular speed values, most Sinovation motors are equipped with high-precision mechanical sensors, such as photoelectric encoders, rotary transformers, etc., to provide speed and position information of the motor. However, electric vehicle auxiliary drive motors to be installed with steering power pumps or brake pumps, the installation size requirements are often more stringent, it is difficult to leave enough space to install motors with mechanical sensors. At this time, the control technology of the DC/AC converter needs to use the position and speed estimation algorithm to estimate the position and speed of the motor rotor in order to control the positionless/speed sensor of the motor.


3.Sinovation DC/AC related technical requirements

As an on-board power converter module, Sinovation DC/AC converters and DC/DC converters are similar in many technical requirements, including environmental adaptability, vibration resistance, protection, high voltage safety, noise, output capacity, electromagnetic compatibility, durability and so on.

1)Efficiency testing

Using the test bench, under the conditions of minimum voltage, rated voltage and maximum voltage power supply, the motor operates at the rated speed, adjusts the bench load, records the operating current value under different load conditions, DC/AC converter input/output power, motor mechanical power, etc., and calculates the relevant efficiency parameters.

2)Speed control accuracy test

Under the specified environmental conditions, rated voltage and continuous operation, the actual allowable speed of the motor is measured by using the speed measurement device of the bench, and the deviation from the rated speed is not more than 0.5%.

3)Operating current test (overload test)

Test the continuous operating time of the DC/AC converter at rated currents, short operating currents and maximum operating currents under specified environmental conditions and rated voltages in accordance with the requirements of GB/T 18488.1-2015.

4)Response time test

In accordance with the requirements of GB/T 18488.1-2015, the test motor is accelerated from standstill to the desired value of the specified tolerance range under specified environmental conditions and at rated voltages.

5)reliability

Reliability tests are tested and evaluated in accordance with the relevant regulations of GB/T 29037-2012.














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