Speed Control Of Dc Motor Using Pwm Ppt Free Download
Malvina Mago
Choosing a speed controller is an important part of building an electric motor drive application and greatly impacts the project's performance, cost, efficiency, and longevity. While there are many different types of motors, speed controls can be broadly categorized as AC and DC, which operate on different fundamental principles.
Because an AC motor's speed is effectively determined by the frequency of the AC power supply, speed control is achieved by modifying this frequency. A device that does this is known as a Variable Frequency Drive or VFD. VFDs first convert the AC power supply to DC using a rectifier, and then back to AC at the desired frequency, using an inverter.
Controlling a DC motor's speed is achieved simply by controlling the voltage of the supply power (within the safe operating range for the motor) using a potentiometer. DC motors maintain consistent torque across the entire speed range without the need for additional components. This makes controlling their speed considerably easier than AC motors, and they are well suited to applications requiring precise control at any speed.
However, further considerations are depending on the requirements of the speed controller. DC controllers operating on AC power require conversion of the supply using a rectifier. Unlike AC motors, braking or reversing a DC motor requires additional components, typically a power resistor for braking and a relay for switching the polarity of the supply power to reverse the motor. It is also necessary to ensure that the motor has stopped before reversing the polarity of the supply, which requires a means of sensing when the motor is at a standstill. This can add up to a significant additional cost, especially for larger applications.
Traditionally, for applications requiring a high level of speed control, choosing a DC drive was the only real option. Today, however, technological advancements have enabled AC drives to catch up in terms of capability. Modern AC vector drives can provide the range and precision of speed control required in even the most exacting applications, such as servo motors. In some cases, AC drives even provide an advantage, especially when frequent braking and reversing is required.
Generally, AC speed controllers are more expensive than DC controllers due to their greater complexity. However, because AC motors are typically cheaper, the controller/motor combination cost may be less than an equivalent DC drive, especially for applications over 2 HP horsepower. The cost of AC speed controllers is also declining, as rising demand drives improved manufacturing techniques and technological innovation. Therefore, it is important to compare cost in terms of the full scope of the application over its entire life cycle.
Because AC speed controllers are more complex, they typically require configuration and tuning during installation, whereas DC drives are relatively simple to connect and use. However, this enables them to offer a wider range of programmable failsafe protections, and modern software is improving the ease of installation of AC drives, such as enabling the transfer of configuration data between units to make replacement faster and easier. For automated control systems applications, AC speed controllers may be the better choice because they often come with the hardware and software capabilities required for integration into a monitoring and control network.
For high precision speed control, both AC and DC applications require a speed sensor such as a tachometer or encoder to operate in a closed-loop configuration. This enables them to achieve extremely precise control in varying torque applications.
AC and DC motor speed controllers operate on different design principles, each with its own advantages and disadvantages. When selecting a motor speed controller, it is important to consider all your application requirements to make the best choice. eMotors Direct provides a wide range of electric motors and speed control solutions to suit every project, as well as online tools to help you select exactly the motor/drive combination you need.
The speed of a DC motor can be controlled by adjusting the voltage applied. This is because the speed and load torque of a DC motor is inversely proportional, and this translates with changes in drive voltage. Linear or PWM control can be used to vary the motor drive voltage, but PWM control has come to predominate in recent years due to its high efficiency.
DC motors are further divided into brushed DC motors and brushless DC motors. Brushed DC motors have coils in their rotor, and alters the way current flows through the coils based on a mechanism using commutators and brushes. Brushed DC motors generate electrical and acoustic noise, and require frequent maintenance because their brushes and commutator are both consumable parts. But, they also feature a simple design and can operate without an electronic drive circuit if speed control is not needed.
Unlike AC motors, DC motors are very easy to use because of the ease with which their speed can be changed. So, how is this achieved in practice? The following explanation starts with looking at DC motor characteristics.
The characteristics of a DC motor are represented by a torque-speed curve that slopes downward to the right, with torque as the horizontal axis and speed as the vertical axis. The speed is highest when there is no load, falling away to the right until maximum torque is reached at zero speed.
A look at the relationship between torque and current shows that these are proportional to one another. The ratio between the two is constant for a motor, with the relationship remaining the same regardless of changes in motor speed or drive voltage. This means that measuring the motor current on its own is enough to determine the motor torque.
So, what happens to the torque-speed curve when the voltage used to drive the DC motor is changed? The graph below shows the torque-speed curves for different voltages. Doubling the drive voltage doubles both the no-load motor speed and the starting torque (torque when motor is locked in position). In other words, increasing the voltage shifts the torque-speed curve upward, parallelly. The torque-speed curve for a DC motor can be adjusted at will, by changing the voltage applied to the motor.
The torque-speed curve of a DC motor translates with changes in drive voltage. This means that the objective above can be achieved by simply adjusting the drive voltage. When looking at the graph below, if the requirement is to rotate at speed ω1 when the load torque is T0, for example, a drive voltage of V4 is too low, resulting in a speed of ω2. A drive voltage of V0 is too high, resulting in a speed of ω0. Driving the motor at the intermediate voltage of V3, however, is just right to achieve the desired speed of ω1.
Linear control works by placing a variable resistor in series with the motor, and adjusting the resistance to vary the voltage across the motor. While a transistor or other semiconductor device can be used as the serial-connected variable resistor, this approach has poor efficiency due to the large amount of heat generated by the resistance (semiconductor), and therefore it is rarely used these days.
An alternative technique is the PWM control. The voltage applied to the motor can be varied by turning a semiconductor switch (such as a transistor or an FET) on and off at high speed, with the voltage being determined by the on and off pulse widths. The high efficiency of this technique makes it most commonly used nowadays.
Maintaining a constant speed despite a variable load requires that the drive voltage be continually adjusted in response to these changes in load. The graph below shows an example where the load torque for a motor operating at a speed of ω0 reduces from T1 to T0, in which case reducing the drive voltage to V0 keeps the motor speed at ω0. If the torque instead increases to T2, maintaining a constant motor speed of ω0 requires that the drive voltage increase to V2.
The speed is measured by a sensor attached to the motor. The difference between the measured and desired motor speed (speed error) is calculated, and the drive voltage is controlled in such a way that it is increased if the speed is too slow and reduced if the speed is too high. Doing this maintains a constant motor speed. While past practice was to use op amps or other analog circuits to control the drive voltage, the use of microcomputers has become the norm in recent years.
A DC motor can achieve steady operation by controlling its speed to remain constant regardless of changes in load. These motors are also suitable for a wide variety of control practices that can be implemented using a microcomputer. DC motors find uses in many different applications that take advantage of their ease of control.
DC motors are powered by direct current, and, unlike AC motors, their speed is easy to adjust. The characteristics of a DC motor are represented by its torque-speed curve, in which speed and load torque are inversely proportional. This torque-speed curve translates with changes in drive voltage. Accordingly, by adjusting the voltage applied to a DC motor, it can be made to run at any speed regardless of load torque.
Linear or PWM control can be used to vary the motor drive voltage. PWM control has come to predominate in recent years due to its superior efficiency. PWM control varies the voltage by turning a semiconductor switch on and off at high speed in such a way that changing the on and off pulse widths changes the voltage.
ASPINA supplies not only standalone brushless DC motors, but also system products that incorporate drive and control systems as well as mechanical design. These are backed by comprehensive support that extends from prototyping to commercial production and after-sales service.
ASPINA can offer solutions that are tailored to suit the functions and performance demanded by a diverse range of industries, applications, and customer products, as well as your particular production arrangements.