Early electric cars usually used DC motor drive systems. The trolleybuses widely used in cities in our country still use DC drive systems. In electric cars such as electric forklifts, DC drive systems are also used.
1) Working principle of DC motor
A DC motor consists of two parts: a stator (fixed) and a rotor (rotating). There is a gap between the stator and the rotor, which is called an air gap. The stator part includes the frame, main magnetic pole, commutating pole, end cover, brushes and other devices, and the rotor part includes armature core, armature winding, commutator, shaft, fan and other components. The overall structure is shown in Figure 1. Show.
Figure 2 (a) shows a simplified model of a DC motor. If the brushes A and B are connected to the DC power supply, the brush A is connected to the positive pole of the power supply, and the brush B is connected to the negative pole of the power supply, current will flow in the armature coil at this time. The side of the coil ab under the N pole and the side of the coil cd under the S pole are supplied with a direct current i. According to Ampere’s law of electromagnetic force-the principle of electric motors, the magnitude of the electromagnetic force F generated in the conductor should be:
Where l—the length of the coil conductor ab, m;
B－Magnetic induction intensity, T;
θ—The angle between B and i in space.
Due to the physical structure of the commutator and the brush, the DC motor ensures that B and i are perpendicular to each other in space, θ=90°. In the case of Fig. 2(a), the conductor ab located under the N pole receives force from right to left, and the conductor cd located under the S pole receives force from left to right. The product of the electromagnetic force and the radius of the rotor is the electromagnetic torque, and the direction of the torque is counterclockwise.
When the electromagnetic torque is greater than the resistance torque, the coil rotates in a counterclockwise direction. When the armature rotates to the position shown in Figure 2(b), the conductor cd originally located under the S pole turns to the N pole, and its force direction changes from right to left; while the conductor ab originally located under the N pole turns To the S pole, the direction of force on the conductor ab changes from left to right, the direction of the torque is still counterclockwise, and the coil continues to rotate in the counterclockwise direction under the action of this torque. In this way, although the current flowing in the conductor is alternating, the direction of force on the conductor under the N pole and the direction of the force on the conductor under the S pole have not changed, and the motor rotates under the constant torque in this direction. The function of the brush is to turn the direct current into an alternating current in the coil. According to the excitation mode, DC motors can be divided into two types: permanent magnet type and electric excitation type. The permanent magnet type is to provide a magnetic field by magnetic materials; the electric excitation type is to wind a coil on a magnetic pole, and then a direct current is applied to the coil to generate an electromagnetic field.
According to the connection relationship between the excitation coil and the rotor winding, the excitation type DC motor can be subdivided into:
Separately excited motor, the excitation coil is separated from the power supply of the rotor armature.
In a shunt-excited motor, the excitation coil and the rotor armature are connected in parallel to the same power source.
In a series-excited motor, the excitation coil and the rotor armature are connected in series to the same power source, which is also called a series-excited motor.
For compound-excited motors, the excitation coil and the rotor armature are connected in series and parallel, and are connected to the same power source.
Figure 3 shows the four excitation methods for DC motors.
2) Mathematical equation of DC motor
(1) The induced electromotive force in the armature
After the armature is energized, electromagnetic torque is generated, which causes the motor to rotate in the magnetic field. The energized coil rotates in a magnetic field, and an induced electromotive force (denoted by E) is generated in the coil.
The induced electromotive force is:
In the formula, KE—constant related to motor structure;
φ—Magnetic flux per pole of the motor, Wb;
n—Motor speed, r/min.
From equation (2), we can see that E is proportional to n.
(2) Armature voltage equation (Figure 4)
Where U-external power supply voltage, V;
Ia—electric current, A;
Ra—armature resistance, Ω.
From equation (2) and equation (3), the following conclusions can be drawn.
① The size of the armature back EMF is proportional to the magnetic flux and speed. If you want to change E, you can only change the middle φ or n.
②If the resistance Ra in the winding is neglected, when the applied voltage is constant, the motor speed is inversely proportional to the magnetic flux, and the speed can be adjusted by changing φ.
(3) Electromagnetic torque equation
In the formula, T——electromagnetic torque, N·m;
KT—Constant related to motor structure.
It can be known from the torque formula:
The conditions for generating torque must include excitation flux and armature current, and are proportional to the product of the two.
When the magnetic flux is constant, the torque is proportional to the current. As long as the armature current is controlled, the torque can be controlled.
Changing the direction of motor rotation can be achieved by changing the direction of armature current or changing the direction of magnetic flux.
3) Analysis of mechanical characteristics of DC motors
(1) Mechanical characteristics of series-excited DC motors
The mathematical model of the series motor is as follows:
Where Rf——series winding resistance, Ω;
From equation (5), we can get:
The corresponding mechanical characteristics are shown in Figure 5.
The characteristics of this excitation method are: when the voltage U is constant, with the increase of the load torque, n decreases quickly, and the motor will not be overloaded due to the increase of the load. This characteristic is very suitable for low-speed traction electric cars. But when the load torque tends to 0, the speed n tends to infinity, so the series motor can not run without load.
(2) Mechanical characteristics of separately excited motor
From equation (7), we can get:
The corresponding mechanical characteristics are shown in Figure 6.
The characteristics of this excitation method are: when the armature voltage is constant, the armature resistance Ra of the separately excited motor is very small, so when the load torque changes, the speed n does not change much, and the mechanical characteristics are also harder. Permanent magnet DC motors and shunt motors also have similar mechanical characteristics.
(3) Compound Excitation DC Motor
The mechanical characteristics of separately-excited DC motors are very hard, the mechanical characteristics of series-excited DC motors are very soft, and compound-excited DC motors can combine the characteristics of the two. If the direction of the magnetic potential of the series winding is the same as that of the shunt winding, it is called a product compound-excited DC motor; if the direction is opposite, it is called a differential compound-excited DC motor, as shown in Figure 7.
The mechanical characteristics of the compound-excited DC motor can be used for reference from the separately-excited mechanical characteristic equation, and the shape of the curve depends on the strength of the series excitation potential.
Suppose the total excitation flux of the motor is φ
Among them, the parallel excitation flux is φB, and its magnitude has nothing to do with the armature current ia; the series excitation flux is φC, and its magnitude is related to i1. The start-up torque of the product compound excitation motor is large, which is suitable for the occasions where the load torque is constant, and is generally used in the fields of elevators and hoisting machinery.
4) Principle of DC motor controller
The main circuit of the DC motor controller is shown in Figure 8. It is mainly composed of two IGBT power tubes VT1 and VT2, two anti-parallel power diodes VD1 and VD2, filter capacitor C, DC motor M and battery Ud, which can realize two motors. Run within the quadrant. The control circuit realizes the control of the motor by controlling the gate switching signals of VT1 and VT2, that is, pulse width modulation (Pulse Width Modulation, PWM chopper).
Figure 9 (a) is the gate switching signal of the IGBT, and Figure 9 (b) is the IGBT’s collecting and shooting voltage waveform. The duty cycle of the control modulation signal (τ=Tp/T) can be adjusted from 0 to 100%, so the average voltage of the DC motor can be adjusted arbitrarily between 0 and Ud.
When the motor is electric, it is realized by turning off the VT1 tube and applying a PWM pulse signal on the grid of the VT2 tube. When the VT2 tube is turned on, the power supply voltage Ud is applied to the DC motor through VT2, and the current in the armature winding forms a loop through VT2 and the power supply. At this time, the terminal voltage of the motor is the power supply voltage Ud; when the VT2 tube is turned off, the armature The current is freewheeling through VD1. During freewheeling, the voltage across the armature is zero, and the power supply no longer delivers energy to the motor.
When the motor needs regenerative braking, it can be realized by turning off VT2 and performing PWM control on the VT1 tube. Regenerative braking converts the mechanical energy of the car into electrical energy to charge the battery. At this time, the motor is equivalent to a generator. Assuming that the electromotive force of the motor armature before the feedback braking is E, through PWM modulation, the sum of the induced electromotive force in the leakage inductance and the armature electromotive force E of the motor is greater than the battery voltage Ud, and the current in the armature circuit is opposite to that of the motor. This current interacts with the magnetic field of the motor to generate braking torque to decelerate the car. When the VT1 tube is turned on, the electrical energy is stored in the armature leakage inductance. When the VT1 tube is turned off, the magnetic field stored in the armature leakage inductance can be fed back through VD2 The battery realizes regenerative braking.
The accelerator and brake pedals of electric cars are given torque signals, and the motor controller implements torque closed-loop control. By adjusting the duty cycle τ to control the armature voltage, thereby controlling the electromagnetic torque, the controller uses a PI regulator, and the system block diagram is shown in Figure 10.
5) Features of DC motor drive system
The DC motor drive system has the advantages of the lowest cost, easy to smooth speed regulation, simple controller, mature technology, etc. However, the efficiency of the motor itself is lower than that of the AC induction motor because the DC motor needs to be commutated by the brush and the commutator during operation. At the same time, the brush needs regular maintenance, causing inconvenience to use. In addition, the size and weight of the motor itself, the commutator and brushes restrict the speed of the DC motor, these factors limit its application in electric cars.