The speed regulation of the electric vehicle motor can be achieved by adjusting the armature voltage and excitation current. Compared with other types of DC motors, separately excited DC motor armature voltage and excitation can be independently controlled, which provides greater flexibility for torque and speed control. In electric vehicle applications, the most desirable torque-speed characteristic is constant torque below a certain speed (base speed); in the range exceeding the base speed, as the speed increases, the torque decreases parabolically (constant power), and the torque and speed characteristics are shown in Figure 1. In the speed range lower than the base speed, keep the excitation current at the rated value, and adjust the armature voltage to adjust the motor speed: the reduction of the armature voltage causes the reduction of the armature current, which in turn causes the reduction of the motor torque, thereby reducing the motor speed; on the contrary, the armature voltage increases and the armature current increases, causing the motor torque to increase, thereby increasing the motor speed. The armature voltage increases or decreases proportionally with the increase or decrease of the rotational speed. Below the base speed, when the armature current reaches the rated value, the motor outputs the rated torque; at the base speed, the armature voltage reaches the rated value and cannot be further increased; to further increase the speed, the magnetic field must become weaker as the speed increases, and the back EMF and armature current remain unchanged; at this time, the output torque decreases parabolically as the speed increases, and the output power remains unchanged, as shown in Figure 1. In the electric drive of electric vehicles, a combination of two control methods is often used for control. When the vehicle starts, keep the excitation voltage unchanged and adjust the winding voltage until the motor reaches the base speed, then keep the armature voltage unchanged at the rated value, and use the magnetic field adjustment control to obtain a higher speed. Electric vehicle electric drive system consists of electric vehicle drive motor transmission, power converter and other parts.
Figure 1 - Torque-speed characteristic curve
Adjusting the armature voltage and the excitation magnetic flux requires a special controllable DC power supply, which is mostly realized by a DC/DC converter at present. The DC/DC converter is also called a chopper, which has high efficiency and flexible control. Due to high frequency operation, the size and weight of the equipment can be reduced, and the response speed is fast and the output pulsation is small. For motor drives, chopper-driven motors can also achieve regenerative braking at very low speeds.
The application of DC motors in electric vehicles requires multi-quadrant operation, including forward rotation, forward rotation braking, reverse rotation, and reverse rotation braking. The speed and torque characteristics of multi-quadrant operation are shown in Figure 2. If a reverse gear is set, only the motor is required to run in the fourth and fourth quadrants. The most commonly used two-quadrant chopper for motor drives is shown in Figure 3. When the driving motor is working, the switch S1 and the freewheeling diode VD1 constitute a chopper, and in the regenerative braking process, the switch S2 and the freewheeling diode VD2 constitute a chopper. Both choppers are controlled simultaneously, whether for running or regenerative braking. Switches S1 and S2 are closed alternately. In the chopping period T, S1 is turned on for a time of δT, and S2 remains on for a period of time from δT to T. To avoid a direct short circuit on the power supply, care should be taken to ensure that S1 and S2 are not turned on at the same time. The two-quadrant chopper has the following characteristics.
①In this circuit, no current discontinuity occurs regardless of the operating frequency.
②Since there is no current discontinuity, the motor current will always flow.
③ During the period of 0≤t≤δT, the positive armature current passes through S1, while the negative armature current passes through VD2. Only during this period there is a supply current whose value is equal to ia. During δT≤t≤T, the positive armature current passes through VD1 and the negative armature current passes through S2.
④ Motor terminal voltage Vs=δUDC.
The use of commutators and brushes in DC motor drive systems presents a number of problems. The commutator causes torque ripple, the motor speed is limited, and the brushes cause friction and audio interference. In addition, the wear of commutators and brushes, and the need for regular maintenance of DC motors, determine that the DC motor drive system has low reliability and cannot be maintenance-free, which limits its use in modern electric vehicle drive systems.
Figure 2 - Speed-torque characteristics for multi-quadrant operation
Figure 3 - Circuit topology of a two-quadrant neutron commonly used in motor drives