The Secret Life of Motor Driver
The invention of motors that converts electrical energy opened numerous avenues for the application of electricity in industrial and domestic applications. Motors were invented in the second half of the 19th century and used through the 20th and 21st centuries. In fact, 70% of the current energy consumption worldwide is consumed by motors. Even though several improvements were made to the motors, most of the general working principles have remained relatively the same throughout the period. However, human activities, more so in transportation and motion, have grown tremendously over the past few decades, leading to a significant modification of the motors to achieve various characteristics such as low power consumption, high torque, and high revolutions.
After the invention of motors, there was an apparent limitation that hindered the application of the motors in some fields. Most of the limitations came from the speed and torque of the motors. Some industries needed high torque motors at a lower speed to achieve a specific task, such as clothe weaving in the textile industry. The looming machines had to move slower to avoid damaging the fabric, and most of the machines were also very heavy, thus demanding a high toque. However, most of the motors did not have an inbuilt mechanism to control the speed and torque of the output. Therefore, most industries used gears to control the motors’ speed, direction, and torque. However, some innovators were not satisfied with gears as the only way to reduce motor speed. Most of them went back to the drawing board to come up with a way to reduce the speed and change the direction of the motors by changing the characteristics of the electric input.
Enters Variable Speed Drive
Yaskawa was the first company to develop earlier forms of motor drivers used in an industrial setup. Yaskawa was established in 1915 to serve the coal mining business with a keen consideration of total consumer satisfaction. The earliest electric device produced by Yaskawa was an electric device used in haulage as a speed controller of wound rotor induction motors. Induction motors are motors with a coil on the main body with the current on the body winding generates a magnetic force on the coil around the shaft. The winding on the shaft is forced to move to balance the force generated, leading to movement of the shaft. In 1937, Yaskawa produced a practical constant speed control method for motors used in a cement kiln system. The speed controller was attached to the electrode operation motor of a liquid rheostat. The controller then used the output signal of a speed detection transducer to control the speed of the motor, ensuring that the speed is within the desired bounds. Many industries and manufacturers, such as Yaskawa, realized a need to develop an AC motor speed controller since the DC motors were considered limiting in robust environments (it is still limiting). However, most of the DC motors were easier to control compared to the AC types. Therefore, the dream of every engineer at this time was to create DC-like AC motors.
After the 950s, manufacturing plants were created to move from one stage to another in a production line. The motor driver technology was essential in driving the product in the production lines. Most of the production lines used variable-speed motors since the speed could be changed without altering the gearing of the conveyor belts. The variable-speed motor controllers combined AC induction motors with eddy current coupling to produce economic motor drivers connected to a socket outlet, enabling remote control. The motor controller became very popular in most industries, making variable speed motors the general term for motors with speed control. Yaskawa further developed their motor driver technology to include directional and position control, adopting mechatronic as their trademark in 1972.
Direct Current Motor Drivers
Direct current motor speed is normally affected by the current and voltage of the power source. Lowering the voltage normally reduces the motor speed. However, to maintain the motor torque, most DC motor drivers modify the input power by adjusting the current source to a pulse and varying the pulse duration or frequency. Varying the pulse width normally reduces the cumulative voltage while maintaining the motor torque characteristics. The motor controllers integrate with the motor circuit, supplying the control signal to the motor driver. Most motor controllers are tailored to control the speed and torque for power tools, robot actuators, and electric vehicles. A DC motor controller’s key specifications are the intended application, drive operating mode, motor type, control loop system, power rating, communication interface, output signal type, and voltage rating.
Servo Motor Controllers
Since the invention of motors, the desire to control the motor speed and position has been a major requirement; research and development have significantly captured this area. Servo motor is one of the products of research on motor controls. Earlier servos were controlled using the finger control method since it could produce fast and smooth operation. The finger technology emulated the fingertip control using actuators. One of the first motors to use the control method was the slotless core armature structure DC motor, called Minertia motor. The motor was first produced in 1960 and was designed as a low inertia product. Furthermore, the motor could achieve fast response time due to low electric time constant and low armature inertia.
Servo motors were designed to work within a closed-loop control system. The servo controller provided a precise position control using a sensor to determine the current position of the motor. The motor position was then matched with the desired position, moving the motor to the desired position. Servo motors are also used to control the linear position of motor-driven elements such as sliding doors. The sophistication of servo motors has also led to use in robotic manufacturing and computer numerical control.
The first AC servo motors emulated the brush-type DC servo motors. The motor comprised a three-phase permanent magnet and sinusoidal back EMF created by overlapped windings and skewed magnets. An encoder and a tachometer were added to monitor the velocity. The brushless designs significantly improved the durability of the motors. The motors use a split ring commutator with two sections wired to the opposite poles of the motor. Electric servo motors have a significantly higher efficiency than traditional motors since they do not have rotor current.
Furthermore, the motors can produce 100% torque instantly. The instant torque production makes them very popular in electric cars. The modern servo motors have a slightly different design since they incorporate the latest magnet technology and controlled using variable speed drivers, significantly improving their performance.
Stepper Motor Controller
Stepper motors are designed to move in steps instead of continuous motion once it is powered. Stepper motors normally have different steps per revolution, but the common steps are 200 and 400 steps per revolution. Stepper motors cannot be powered directly since they must be powered through the stepper motor controllers for them to work. The inventor of the stepper motor is currently not very clear since the original versions were not considered stepper motor. However, engineers normally attribute the invention to Frank W. Woods, who patented a motor based on five stator coils that could be charged in different stages to move the motor in steps. Stepper motors were first used in commercial applications by the British Royal Navy in the 1930s. The system was developed to control gun turrets and cannons in large ships. In the 1960s, the basic stepper motors were replaced by large-angle permanent magnet stepper motors. The motors had some significant problems that limited their application; the position accuracy was limited by the lack of advanced stepper motor controllers, and resonance within the motor casing led to stalling. However, from the 1970s, better controllers were developed to reduce the resonance and control position. The stepper motors were still very expensive and only used in defense and aerospace applications. By early 2000, the stepper motor technology significantly improved, reducing the cost of the motor. Therefore, the motors could be used in applications where they could not be used previously. With the ability to control the motors in steps, it is also possible to achieve a precise motor position. Therefore, stepper motors were quickly adopted in applications where precise positions should be achieved at the shortest time possible.
The stepper motors are brushless motors with a magnet directly attached to the shaft of the motor. The magnet has a set of teeth around it, with the north and south poles alternating. The coils controlled by the stepper motors to move the shaft magnets are located on the shell of the motor. Most stepper motors have two sets of coils arranged opposite to each other. The motor is turned on and off to control the motors. For a forward movement, ten coils are energized, one positive and the other negative, creating a dual push/pull force that moves the motor one step forward. After the step, the stepper motor driver energizes the coil for the next step. The stepper motor driver can control the coils quickly, achieving up to 100 revolutions per minute. Micro-stepping can also increase possible steps by changing the energizing time to achieve a fraction of the desired step. The standard stepper motors, such as NEMA 23 stepper motors, have 200 steps per complete revolution. Therefore, every step covers 1.8 degrees when operating on full-step mode. However, some stepper motor controllers, such as Zikodrive ZD2, can operate at up to 128 micro-steps per step. Therefore, one can achieve a higher position accuracy using the micro-stepper controls. Therefore, stepper motors are widely used in areas where high position accuracy is desired, such as 3D printers and process controls.
Motors are a key element of every machine as they are the major converters of electric energy to mechanical energy. However, many applications require controlled speed, position, and torque. There is where the motor drivers come in since they can alter the motor operation to achieve various speed and torque without relying on mechanical parts of the machine. Therefore, with a motor driver, we can produce various speeds with minimal moving parts. However, when choosing the best motor driver, one should look for the target motor and application.
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