An electrical engineer's understanding of electric vehicle's electric power drive and optimization of speed control

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Xiaomi SU7


Introdution:

With the increasing popularity of electric vehicles and having personally used an electric car, I have been pondering over how the electric power drive and speed control are achieved with such smoothness and efficiency. This article aims to delve into the electric power drive modes of electric vehicles and explore methods for optimizing speed control. Please note that my understanding is based solely on my own knowledge base, and the accuracy of my interpretations is not the primary focus here. Rather, the emphasis lies in sharing our comprehension of closed-loop speed control, variable frequency speed regulation, and multi-stage speed control.


1

   

Electric Power Drive Modes of Electric Vehicles

The electric power drive of electric vehicles can be categorized into three primary modes: steady-state operation, acceleration, and deceleration.


1.1

   

Steady-State Operation:

Steady-state operation refers to the state where the motor torque is equal to the load torque. By adjusting the speed and torque of the motor, the torque between the motor and the load is balanced at a given speed, thereby achieving steady-state operation.

To alter the speed, adjustments can be made to the motor's speed and torque to ensure that the motor torque matches the load torque at the new speed. Additionally, electric braking methods can be employed, which convert mechanical energy into electrical energy, generating a torque opposite to the direction of motion. Electric braking offers advantages over traditional mechanical braking, such as simpler maintenance, longer lifespan, and energy recovery.


1.2

   

Acceleration:

Acceleration refers to the operational mode where an increase in speed is required. During acceleration, adjustments need to be made to the motor's speed and torque to ensure that the motor torque exceeds the load torque. The duration of acceleration is determined by the inertia of the motor-load system and the amount by which the motor torque exceeds the load torque.

During acceleration, the motor's current will increase. In applications involving prolonged acceleration, the current must not exceed its rated value. However, in applications with short acceleration periods, it is permissible for the current to exceed its rated value. In high-performance drives, to achieve high acceleration rates, the current can be limited to its maximum value.


1.3

   

Deceleration:

Deceleration refers to the operational mode where a reduction in speed is necessary. Deceleration occurs when the load torque exceeds the motor torque. In some applications, simply reducing the motor torque to zero can achieve deceleration, but in other cases, the use of mechanical brakes or electric braking methods may be required to achieve the desired deceleration magnitude.


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Optimizing Speed Control

The speed control of electric vehicles is crucial for both safety and performance. The following are several potential applications of speed control:

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Closed-Loop Speed Control:

Closed-loop speed control achieves precise speed regulation through the use of a feedback mechanism. By employing encoders or sensors to measure the actual motor speed and comparing it to a set speed, the controller can adjust the motor's input voltage, current, or frequency to maintain a consistent match between the actual and set speeds.


2.2

   

Variable Frequency Speed Control:

Variable frequency speed control is a commonly used method for speed regulation, which involves controlling the motor speed by altering the frequency of the power supplied to it. A frequency converter, an electronic power device, converts the input power's frequency and voltage into adjustable output frequency and voltage to drive the motor. By adjusting the output frequency of the frequency converter, precise speed control of the motor can be achieved.

2.3

   

Multi-Level Speed Control:

Multi-level speed control involves dividing the motor operation into multiple speed ranges for control. By utilizing multiple switches and controllers within the motor's input circuitry, each speed range is associated with specific control parameters and constraints. This approach enables greater control precision and efficiency within different speed ranges.


Conclusion

In summary, the electric propulsion and speed control of electric vehicles (EVs) are crucial for achieving safe and high-performance driving. By understanding the electric drive modes and optimizing speed control applications in EVs, we can enhance the driving experience and efficiency. Methods such as closed-loop speed control, variable frequency speed control, and multi-level speed control all play significant roles in various scenarios. As EV technology continues to evolve, we can anticipate the emergence of more innovative speed control methods, further advancing the performance and reliability of electric vehicles.