Abstract: This paper introduces a drive circuit for voltage and speed regulation of a DC motor based on pulse width modulation (PWM) signals and employing an H-bridge. The circuit utilizes PWM to adjust the on-time of the output waveform, thereby achieving voltage and speed regulation. The power electronic design employs a bridge connection consisting of diodes and an insulated-gate transistor (IGMT) to regulate the reversibility of the DC motor and control the timing of the width-adjustable output waveform to achieve speed regulation. Experimental simulations demonstrate that the system satisfies both reversibility and speed regulation functions.
Keywords : speed regulation; reversible; pulse width modulation; H-bridge drive bridge;
0 Introduction
PWM control technology, with its advantages of simple control, flexibility, and good dynamic response, has become the most widely used control method in power electronics technology and a hot research topic. As the boundaries between disciplines have blurred in today's scientific and technological development, combining modern control theory or implementing resonant-free switching technology will become one of the main directions for the development of PWM control technology. PWM control modulates the bias of the transistor base or MOSFET gate according to changes in the corresponding load, thereby changing the conduction time of the transistor or MOSFET and thus changing the output of the switching power supply. This method allows the power supply output voltage to remain constant under changing operating conditions and is a very effective technique for controlling analog circuits using digital signals from a microprocessor.
Pulse width modulation (PWM) is a highly effective technique for controlling analog circuits using the digital output of a microprocessor. It is widely used in many fields, from measurement and communication to power control and conversion.
Since the advent of fully controlled rectifier power electronic devices, high-frequency switching control methods using pulse width modulation (PWM) have emerged, forming a pulse width modulation converter-DC motor speed control system, or DC pulse width speed control system, or DC PWM speed control system for short. PWM systems have significant advantages in many aspects:
The main circuit is simple and requires few power devices;
High switching frequency, easy continuous current, fewer harmonics, and less motor loss and heat generation;
It has good low-speed performance, high speed stability, and a wide speed range, which can reach about 1:10000.
If paired with a fast-response motor, the system has a wide bandwidth, fast dynamic response, and strong dynamic anti-interference capability.
Power switching devices operate in a switching state with low conduction losses. When the switching frequency is appropriate, the switching losses are also not large, resulting in high device efficiency. When the DC power supply uses uncontrolled rectification, the grid efficiency factor is higher than that of phase-controlled rectifiers.
1. Working principle of DC motor
A current-carrying conductor experiences an electromagnetic force in a magnetic field—this is the law of electromagnetism. Electric motors operate based on this law. The diagram below shows the schematic of a DC motor.
Figure 1 Schematic diagram of a DC motor
The armature winding is connected to a DC power supply via brushes, and the rotation axis of the winding is connected to the mechanical load. Current flows into the armature winding from brush A and out from brush B. The armature current Ia interacts with the magnetic field to generate an electromagnetic force F, the direction of which can be determined using the left-hand rule. The electromagnetic torque T formed by this pair of electromagnetic forces causes the motor armature to rotate counterclockwise, as shown in Figure a above.
When the armature rotates to the position shown in Figure b above, due to the action of the commutator, the power supply current Ia still flows into the winding through brush A and out through brush B. The direction of the electromagnetic force and electromagnetic torque still causes the motor armature to rotate counterclockwise.
When the armature rotates, it cuts magnetic lines of force, generating an induced electromotive force (EMF). This EMF (determined by the right-hand rule) always has the opposite direction to both the armature current Ia and the applied voltage U; it is called the back EMF Ea. Its function differs from that of the generator's EMF. The generator's EMF is the power source's EMF, which generates current in the external circuit. Ea, however, is the back EMF; the power source can only supply current to the motor by overcoming this back EMF.
It is evident that while the electric motor outputs mechanical power to the load, the power source inputs electrical power to the electric motor, thus the electric motor plays the role of converting electrical energy into mechanical energy.
The electromagnetic torque T of a generator and a motor have different functions. The electromagnetic torque of a generator is a resistive torque, which is opposite in direction to the driving torque T1 of the prime mover. The electromagnetic torque of a motor is a driving torque, which causes the armature to rotate. The electromagnetic torque T of a motor must be balanced with the mechanical load torque T2 and the no-load loss torque T0, i.e., T = T2 + T0. When the mechanical load on the motor shaft changes, the motor's speed, back electromotive force, current, and electromagnetic torque will automatically adjust to adapt to the load change and maintain a new balance. Therefore, although both a DC motor operating as a generator and as a motor generate electromotive force and electromagnetic torque, their effects are completely opposite.
2PWM control speed regulation principle
The basic principle diagram of DC motor PWM speed control is shown in Figure 2. The controllable switch S repeatedly turns on and off at a fixed cycle. When switch S is on, the DC power supply U is applied to the DC motor through switch S, causing the motor to rotate under the power supply's influence, while the motor armature inductance stores energy. When switch S is off, the power supply stops providing energy to the motor, but the energy stored in the armature inductance continues to maintain the motor armature current through the freewheeling diode VD. The armature current still generates electromagnetic torque, causing the motor to continue rotating. When switch S repeatedly operates, a series of voltage pulse waveforms are formed across the motor armature, as shown in Figure 3.
The theoretical formula for calculating the average armature voltage Uav is:
Where α is the duty cycle, which is the ratio of the conduction time to the pulse period.
As can be seen from equation (1), the average voltage is determined by the duty cycle and the power supply voltage. By keeping the switching frequency constant and changing the duty cycle, the average voltage can be changed accordingly, thereby realizing the voltage regulation and speed regulation of the DC motor.
Figure 2 Simple DC PWM control circuit
Figure 3 Voltage and current waveforms
3-pulse width modulation converter
In electric traction equipment such as mainline railway electric locomotives, industrial and mining electric locomotives, urban trams, and subway locomotives, DC series or compound-wound motors are commonly used, powered by a constant-voltage DC power grid. In the past, starting, braking, and speed regulation of the motor were controlled by switching the armature circuit resistance, which consumed a significant amount of power in the resistor. To save energy and implement contactless control, power electronic switching devices, such as fast thyristors, GTOs, and IGBTs, are now widely used. When using simple single-transistor control, it is called a DC chopper; later, it gradually evolved into circuits using various pulse width modulation switches, collectively known as pulse width modulation converters.
The principle of the DC chopper-motor system is shown in Figure 4(a), where VT represents any power electronic device using a switch symbol, and VD represents a freewheeling diode. When VT is on, the DC power supply voltage Us is applied to the motor; when VT is off, the DC power supply is disconnected from the motor, and the motor armature freewheels through VD, with the voltage across its terminals approaching zero. This process repeats, resulting in the armature terminal voltage waveform u=f(t), as shown in Figure 4(b). It appears as if the power supply voltage Us is applied within a time interval ton and then cut off within (T-ton), hence the term "chopping". Thus, the average voltage obtained by the motor is:
Ud=(ton/T)*Us=ρ*Us
In the formula, T is the switching period of the power switching device.
ton---Opening time
ρ --- Duty cycle, ρ=ton/T=ton*f, where f is the switching frequency.
a) Schematic diagram b) Voltage waveform diagram
Figure 4. Schematic diagram and voltage waveform of the pulse width modulation converter-motor system.
4. System Simulation and Debugging
The DC PWM main circuit, serving as the power platform for energy conversion, is pre-built. Simply place the control board above the main circuit and connect J3, J6, and J7 on the main circuit board to their corresponding J3, J6, and J7 on the control board using ribbon cables. Debugging can then proceed. The voltage of all accessible parts in both the main and control circuits is below 30V, preventing electric shock. For ease of debugging and to ensure circuit safety, a debugging box is added. The overall system connection is shown in Figure 5.
Figure 5 Composition of the debugging system
The debugging box houses two switches, two indicator lights, and one ammeter. Its system debugging panel is shown in Figure 6. S1 switches the main power supply, and L1 indicates its power-on state. S2 connects or disconnects the DC power supply to the H-bridge (which can be used for motor start/stop control), and L2 also indicates its power-on state. Ammeter M1 is used to detect the load current. Resistors R1 and R2, each with a resistance of 5 ohms, are connected in series before the H-bridge (i.e., the IPM module) in the main circuit to limit short-circuit current and protect the IPM. Ra is the current sampling resistor for the motor armature circuit (with a resistance of 1 ohm), used to observe the waveform of the armature current.
Figure 6. Panel of the system debugging box
Throughout the entire debugging process, all parts of the system functioned normally, and the current reading on the ammeter remained within 0.5A.
5. Conclusion
Pulse Width Modulation (PWM) is a highly effective technique for controlling analog circuits using the digital output of a microprocessor. It is widely used in fields ranging from measurement and communication to power control and conversion. This paper's power electronic design employs a bridge connection using diodes and an insulated-gate transistor (IGMT) to regulate the reversibility of a DC motor and control the timing of the pulse width modulation output waveform to achieve speed regulation. Experimental simulations demonstrate that this control system is suitable for both motor reversibility and speed regulation functions.
