DC motor is a motor that converts DC electric energy into mechanical energy. The stator of the motor provides magnetic field, the DC power supply provides current to the winding of the rotor, and the commutator keeps the direction of the rotor current and the torque generated by the magnetic field unchanged. Its features are as follows:
(1) Good speed regulation performance means that the motor changes the motor speed artificially according to the needs under certain load conditions. DC motor can realize uniform and smooth stepless speed regulation under heavy load, and the speed regulation range is wide.
(2) The starting torque is large, and the speed regulation can be realized evenly and economically. Therefore, all machines that start under heavy load or require uniform speed adjustment, such as large reversible rolling mill, winch, electric locomotive, tram, etc., are driven by DC motor.
Therefore, DC motor drive has been widely used in applications requiring adjustable speed, good speed regulation performance, frequent start, braking and reverse rotation. Due to its mature technology and simple control, various DC motor drives have been widely used in different electric traction application systems.
- Working principle and performance
The working principle of DC motor is simple and clear. When a current carrying wire is placed in a magnetic field, a magnetic field force acting on the wire will be generated. The force is perpendicular to the conductor and magnetic field, which is directly proportional to the length of the conductor, the size of the current and the magnetic induction intensity, i.e. f = bil
When the wire forms a coil, the magnetic field force acting on both sides of the coil generates a torque, which can be expressed as t = bildcos α
Where: α Is the angle between the coil plane and the magnetic field, which can be generated by a set of windings or permanent magnets. The former is called wire wound excitation DC motor, and the latter is called permanent magnet DC motor. The current carrying coil is called armature. In fact, the armature is made up of many coils. In order to obtain continuous maximum torque, slip rings and brushes are used to turn on each position α= The coil at 0 has the maximum torque at this time, that is, the DC motor has the characteristics of large torque.
In fact, the performance of DC motor can be described by armature voltage, back electromotive force (EMF) and magnetic flux.
According to the connection between the excitation winding and armature winding, there are four typical forms of wire wound excitation DC motor. They are separately excited, parallel excited, series excited and compound excited DC motors.
The structural feature of DC separately excited motor is that the excitation winding and armature are independent of each other, and the excitation circuit is supplied by another DC power supply. Therefore, the excitation current is not affected by the armature terminal voltage or armature current. The structural feature of DC shunt motor is that the voltage at both ends of shunt winding is the voltage at both ends of armature. However, the excitation winding is wound with thin wires and has a large number of turns. Therefore, it has large resistance, making the excitation current passing through it small. The structural feature of DC series excitation motor is that the excitation winding is connected with the armature, so the magnetic field in this motor changes significantly with the change of armature current. In order to prevent large loss and voltage drop in the excitation winding, the smaller the resistance of the excitation winding, the better. DC series excitation motor is usually wound with thicker wires with less turns. The structural feature of DC compound excitation motor is that the magnetic flux of the motor is generated by the excitation current in the two system groups. For separately excited DC motors, the excitation voltage and armature voltage can be controlled independently of each other. In parallel excited DC motor, the excitation winding and armature winding are connected in parallel to the same power supply. Therefore, by inserting a resistor in series in the corresponding circuit, the independent control of excitation current and armature current or armature voltage can be obtained, but this is a low efficiency control method. An efficient control method is to replace the above resistance with a DC-DC converter based on power electronics in the corresponding circuit. The DC-DC converter can be actively controlled to generate specific armature voltage and excitation voltage. For series excited DC motors, the excitation current is the same as the armature current, so the magnetic flux changes with the armature current. In the product compound excitation DC motor, the magnetomotive force (MMF) of the series excitation winding changes with the armature current, and the orientation of MMF is consistent with that of the parallel excitation winding.
For separately excited and parallel excited DC motors, RA is equal to the resistance of armature winding; For series excitation and compound excitation DC motors, RA is the sum of the resistance of armature winding and applied excitation winding. It is applicable to all DC motors, i.e. separately excited (or parallel excited) DC motors, series excited and compound excited DC motors. For separately excited DC motors, if the excitation voltage remains unchanged, when the torque changes, it can be considered that the magnetic flux is actually unchanged. In this case, the speed torque characteristic of separately excited DC motor is a straight line, and the no-load speed WM is determined by armature voltage and excitation. When the torque increases, the speed decreases, and the speed adjustment depends on the resistance of the armature circuit. Separately excited DC motor is used in occasions where good speed regulation performance and special and common adjustable speed are required.
In the case of series excitation, the increase of torque is accompanied by the increase of armature current, and therefore the magnetic flux also increases. Since the magnetic flux increases with the increase of torque, the speed decreases in order to maintain the balance between the induced voltage and the power supply voltage. Therefore, the speed torque characteristic presents a sharp decline curve. Under the rated torque, the standard series excitation DC motor works at the knee of the magnetization curve. Under the overload operation of high torque (high current), the magnetic circuit is saturated, and the speed torque characteristic is close to a straight line.
Series excited DC motor is suitable for applications requiring high starting torque and large torque overload, such as traction. Before the era of power electronics and micro control, it was only used for electric traction. However, there are some disadvantages of series excited DC motor used in electric traction. Such motors are not allowed to operate without load under full supply voltage. Otherwise, the motor speed will rise rapidly to a very high speed value; Another disadvantage is that regenerative braking is difficult.
- Combined armature voltage and excitation control
Compared with other types of DC motors, the independence of armature voltage and excitation provides more flexible speed and torque control. In the application of electric vehicle, the most desirable speed torque characteristic is that the constant torque is below a certain speed (base speed); In the range above the base speed, the torque decreases in a parabola shape (constant power) with the increase of speed. In the range below the base speed, the armature current and excitation current are set to their rated values to produce the rated torque. The armature voltage must increase in direct proportion to the increase in speed. At base speed, the armature voltage reaches the rated value (equal to the supply voltage) and cannot be further increased. In order to further improve the speed, the magnetic field must increase with the increase of speed, decrease in a parabolic shape, and its output power remains unchanged.
- DC motor chopper control
DC chopper is a converter device that converts a constant DC voltage into another fixed voltage or adjustable DC voltage to meet the DC voltage required by the load, also known as DC / DC converter. It cuts the constant DC voltage into a series of pulse voltages through periodic fast on and off, and the average value of output voltage can be adjusted by changing the pulse width or frequency of this pulse train. In addition to adjusting the DC voltage, the DC chopper can also be used to adjust the resistance and magnetic field. DC drive and switching power supply are two important fields of chopper circuit application. The former is the traditional field of chopper circuit application, and the latter is the new field of chopper circuit application. There are many kinds of DC choppers, including six basic choppers: Buck chopper, boost chopper, boost buck chopper, Cuk chopper, SEPIC chopper and zeta chopper. The first two are the most basic types. There are two working modes of chopper: one is pulse width modulation mode, TS (cycle) remains unchanged, and ton (general, ton is the time when the switch is turned on each time); The second is the frequency modulation mode. Ton remains unchanged and TS is changed (prone to interference).
Using DC chopper instead of rheostat can save electric energy by 20% ~ 30%. The DC chopper can not only regulate the voltage (switching power supply), but also effectively suppress the harmonic current noise at the power grid side. Today’s soft switching technology has made a qualitative leap in DC / DC. A variety of ECI soft switching DC / DC converters designed and manufactured by VICOR company in the United States have a maximum output power of 300W, 600W and 800W. The corresponding power densities are 6W / cm3, 10W / cm3 and 17w / cm3, and the efficiency is 80% ~ 90%.
These control technologies can be divided into time ratio control (TRC) and current limiting control (CLC).
3.1. Time ratio control (TRC)
In time ratio control (also known as pulse width control), the ratio of on time to chopping period is controlled. TRC can be further classified as follows.
(1) With the fixed frequency TRC, the chopper period T remains unchanged, and the on time of the switch is changed to control the duty cycle.
(2) Variable frequency TRC. In this control mode, the change of frequency can be realized by changing t while keeping t unchanged, or by changing T and t at the same time.
In the frequency conversion control with constant conduction time, the low output voltage can be obtained when the chopping frequency is very low. The operation of low frequency chopper in turn affects the performance of DC motor. In addition, the operation of frequency conversion chopper makes the design of input filter very difficult. Therefore, frequency conversion control is rarely used.
3.2. Current limiting control (CLC)
In current limiting control (also known as point-to-point control), the load current is indirectly controlled by setting the load current between a specific maximum and minimum. When the load current reaches the set maximum value, the switch cuts off the connection between the load and the power supply, and when the load current reaches the set minimum value, the switch reconnects the load and the power supply. For DC motor load, this kind of control is actually variable frequency variable pulse width control.
(1) The power supply voltage is discontinuous and in the form of pulse. The pulse current makes the input power peak demand high and may lead to power supply voltage fluctuation. The power supply voltage waveform can be decomposed into DC component and AC harmonic component. The AC fundamental frequency is the same as the chopper frequency. AC harmonics are undesirable because they interfere with other loads connected to the DC power supply and cause RF interference through conduction and electromagnetic radiation. Therefore, an LC filter is usually included between the chopper and the DC power supply. At higher chopping frequencies, harmonics can be reduced to an acceptable level through a low-cost filter. It can be seen that the chopper should work at the highest possible frequency.
(2) The load terminal voltage is not an ideal DC voltage. In addition to the DC component, the load terminal voltage also has various harmonic components related to the chopping frequency. The load current also has AC pulsation.
Chopper is called type a chopper. It is one of many chopper circuits used to control DC motor. This chopper can only provide positive voltage and current, so it is called single quadrant chopper. It can control the separately excited DC motor under the condition of positive speed and positive torque in the first quadrant. Because this chopper can change the output voltage from V to 0, it is also called step-down chopper, or DC-DC step-down converter. The basic principles contained in it can also be used to realize boost chopper or DC-DC boost converter.
Boost chopper this chopper is called type B chopper.
If the switch is biased forward, the presence of the control signal IC means the duration of the switch on. In a chopping period T, 0 ≤ t ≤ α During T, the switch remains closed while α The switch remains off during t ≤ t ≤ t. During conduction, is increases from is1 to is2, thus increasing the energy stored in the inductor L. When the switch is disconnected, the current flows through the parallel load and capacitor C.
Since the current is forced from low potential to high potential, the current change rate is negative. During switch off, the current decreases from is2 to is1. The energy stored in the inductor L and the energy provided by the low voltage source are transmitted to the load. Capacitor C has two uses. At the moment when the switch S is disconnected, the power supply current is is not equal to the load current IA. When the capacitor C does not exist, the switching off of the switch s will force the two current values to be equal, which will cause a large induced voltage in the inductance L and the load inductance. Another reason for using capacitor C is to reduce load voltage fluctuations. The purpose of diode VD is to prevent current from flowing from the load to switch s or power supply v.
theoretically α Through 0 to 1 control, the output voltage Va can change from V to ∞. In fact, VA can be adjusted from V to a high voltage, which depends on the capacitance C, as well as the parameters of the load and chopper.
The main advantage of boost chopper is that the fluctuation of power supply current is small. Although most applications require step-down choppers, step-up choppers are suitable for low-power battery driven vehicles. The working principle of boost chopper can also be used for regenerative braking driven by DC motor.
- Multi quadrant control of chopper fed DC motor
The application of DC motor in electric vehicle requires multi quadrant operation of motor, including forward rotation, forward rotation braking, reverse rotation and reverse rotation braking. For vehicles with reverse gear, it is required to operate in two quadrants (forward rotation and forward rotation braking, i.e. the first quadrant and the fourth quadrant). However, for vehicles without reverse gear, four quadrant operation is required. The multi quadrant operation of separately excited DC motor is realized by controlling the polarity and amplitude of voltage through a chopper based on power electronics.
4.1. Two quadrant control of forward rotation and forward rotation regenerative braking
The two quadrant operation composed of forward rotation and forward rotation regenerative braking requires a chopper, which can give forward voltage and current in either direction. The two quadrant operation can be realized by the following two circuits.
1) Single chopper with reversing switch
Chopper circuit for forward rotation and forward rotation regenerative braking, s is an automatic commutation semiconductor switch, which works periodically, that is, the duration of keeping closed is ar; Keep disconnected for (1)- α） T。 C is a manual switch. When C is closed and S is in working state, forward rotation operation is allowed. Under these conditions, endpoint A is positive and endpoint B is negative.
When C is disconnected and the armature is reversely connected through the reversing switch RS, i.e. terminal B is positive and terminal A is negative, regenerative braking in the forward direction can be obtained. During the conduction of switch s, the path through which the motor current flows is the motor armature, switch s and diode VD1, power supply V, diode VD2, and then back to the armature, so as to feed energy into the power supply. When the power supply of the armature 1 and 2 is switched to the proper power supply by turning off the power supply of the armature 1 and 2, and pressing the power supply of the armature 2 to make the current of the armature 1 and 2 return to the proper power supply, and then the power supply of the armature 2 is stopped δ The value activates the switch s, so that the motor will restart and enter the forward running state.
2) Type C two quadrant chopper
In some applications, smooth transition from operation to braking is required, or vice versa. For such applications, a C-type chopper can be used. The automatic commutation semiconductor switch S1 and diode VD1 form one chopper, while the automatic commutation semiconductor switch S2 and diode VD2 form another chopper. Both choppers are controlled at the same time, whether in operation or regenerative braking. Switches S1 and S2 are closed alternately. In chopper cycle T, the duration of S1 conduction is α T. And S2 from α Keep on from t to t. In order to avoid direct short circuit on the power supply, attention should be paid to ensure that S1 and S2 are not connected at the same time. Generally, it is obtained by giving some time delay between the opening of one switch and the closing of another switch.
In different time intervals of a chopping cycle, the waveforms of the control signal, VA, IA and is, and the devices in the on state. When depicting these waveforms, the time delay between the opening of one switch and the closing of another switch is usually very small, so it is ignored. The control signals of switches S1 and S2 are recorded as IC1 and IC2 respectively. It is assumed that the switch is on only when the control signal is present and it is at a forward bias.
The following points are helpful to understand the operation of the two quadrant chopper circuit.
(1) In this circuit, there will be no current interruption regardless of the operating frequency. When the armature current drops to zero and remains zero for a limited time interval, there will be current discontinuity. During freewheeling or energy transfer, the current may be zero.
For this circuit, when S1 is disconnected and the current flows through VD1, there is freewheeling. This will happen in α The period T ≤ t ≤ t is also the period during which S2 receives the control signal. If ia drops to zero during freewheeling, the back electromotive force will immediately drive the current in the opposite direction through S2, thus preventing the armature current from being zero in a limited time interval. Similarly, when S2 is disconnected and VD2 is on, i.e. 0 ≤ t ≤ α During T, energy transfer will occur. If the current drops to zero during this period, S1 will turn on immediately because there is a control signal IC1 and V > E. The armature keeps the current from flowing intermittently.
(2) Since there is no intermittent current, the motor current will always flow. Therefore, when 0 ≤ t ≤ α During T, the motor armature will be connected through S1 or VD2. Therefore, the motor voltage will be V, and since V > e, the change rate of IA will be positive. Similarly, in α During t ≤ t ≤ T, the motor armature will be short circuited through VD1 or S2. Therefore, the motor terminal voltage is zero and the change rate of IA will be negative.
（3）0≤t≤ α During T, the positive armature current passes through S1 and the negative armature current passes through VD2. There is a supply current only during this period and its value is equal to. stay α During t ≤ t ≤ T, the positive armature current passes through VD1 and the negative current passes through S2. Va = α 5. When α> E / V, the motor runs in forward rotation; When α< E / V, motor regenerative braking; When α= At e / V, the motor operates without load.
4.2. Four quadrant operation
Four quadrant operation can be obtained by combining two C-type choppers, which is called E-type chopper.
In the chopper, if S2 is always closed and S1 and S4 are controlled, a two quadrant chopper is obtained. The two quadrant chopper provides positive terminal voltage (positive speed) and electric drive current in two directions (positive or negative torque), and motor control is carried out in the first quadrant and the fourth quadrant. Now, if S3 is always closed and S1 and S4 are controlled, a two quadrant chopper can also be obtained. The two quadrant chopper will provide negative variable terminal voltage (reverse speed), and the armature current can be in any direction (positive or negative torque), so as to control the motor in the second and third quadrants.
This control method has the following characteristics: due to the asymmetry of circuit operation, the utilization rate of switch is low; Switches S3 and S2 shall be kept on for a long time, so that when the switch adopts thyristor, the problem of rectification and commutation will occur; Therefore, especially in the thyristor chopper, there is always a limit on the shortest time for the switch to close, and the minimum output voltage directly depends on the shortest time required for the switch to close, so the minimum available output voltage is limited, that is, the minimum available motor speed is also limited.