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Electrical drives are an integral part of industrial and automation processes, particularly where precise control in speed of motor is the prime requirement. In addition, all modern electric trains or locomotive systems have been powered by electrical drives. Robotics is another major area where adjustable speed drives offer precise speed and position control.
AC drives- AC drives are used to control the operating speed of an electrical system specially AC or DC motors. By using AC drives, one can change the rotating speed of the motor torque, or the direction of rotation. An AC drive has a special place in modern electrical and electronic equipment. They have a better interfacing menu,which helps a person in operating the AC drive easily. AC drives can minimise power supply requirement, as the input power to the AC motor is properly utilised to generate maximum power through it. there is a requirement to maintain airflow to cool heavy machines. The speed of an electric motor cannot vary without any drive. By using a damper (on- off valve), airflow can be regulated but electricity consumption will remain the same. if your are using a motor drive, motor speed can vary easily. An AC drive acts as an electrical and electronic control unit between the power supply and motor. By using this electric energy can be saved. If AC drives are not used, it will be hard to adjust the frequency or speed of motor. To adjust the speed of the motor or frequency, bulky pulleys need to be installed. Using AC drive can b e used to control the speed of the motor and the frequency over which it is operating.
It is used in a large number of industrial and domestic applications like transportation systems, rolling mills, paper machines, textile mills, machine tools, fans, pumps, robots and washing machine. A variable speed drive used to control DC motors are known as DC drives and a variable speed drive used to control AC motors are called AC drives. Adjustable speed drives are necessary for precise and continuous control of speed, position, or torque due to different loads. There are many other reasons too to use adjustable speed drives. Some of these are:
Electrical Drive parts- The main parts of an electrical drive are power modulator, motor, controlling unit and sensing units. Power modulator regulates the output power of the source. It controls the power from the source to the motor. The power modulator converts energy according to the requirement of the motor. For example, if the source is DC and an induction motor is used then power modulator converts DC into AC. Control unit controls the power modulator, which operates at small voltage and power levels. The control unit also operates the power modulator as desired. It also generates the commands for the protection of the power modulator and motor. Sensing unit senses certain drive parameters like the motor current and speed. It is mainly required either for protection or for closed loop operation.
AC drives are used to drive the AC motor especially three phase induction motors because these are predominant over other motors in most of the industries. In industrial terms, an AC drive is also called variable frequency drive (VFD), variable speed drive (VSD) or adjustable speed drive (ASD). Though there are different types of VFDs (or AC drives), all of them work on same principle, which is converting fixed incoming voltage and frequency into variable voltage and frequency output. The frequency of the drive determines how fast the motor can run. Supply The combination of voltage and frequency decides the amount of torque that the motor can generate. A VFD is made of power electronic converters, filter, a central control unit, a microprocessor or microcontroller and other sensing devices. The process of converting AC power into DC power is called rectification. It can be achieved by using a single diode or a group of diodes.
Full wave rectifier- A full wave rectifier converts both half cycles of the AC signal into a pulsating DC signal. It converts both positive and negative half cycles of the input AC signal into output pulsating DC signal.
Centre Tapped Transformer- When an additional wire is connected across the middle of the secondary winiding of a transformer, it is known as a centre tapped transformer. The secondary winding of the centre tapped transformer divides the input AC power or AC signal (VP) into two parts.
parts of Centre Tapped Transformer- The centre tapped full wave rectifier is made of an AC source, a centre tapped transformer, two diodes and a load resistor The AC source is connected to the primary winding of the centre tapped transformer. A centre tap (additional wire) connected in the middle of the secondary winding divides the input voltage into two parts.
How a Centre Tapped Full Wave Rectifier Works- When input AC voltage is applied to the primary winding, it is then step down by the secondary winding of the centre tapped transformer. This secondary voltage is divided into two parts: positive and negative. The output of a centre tapped full wave re rectifier is pulsating DC Pulsating means that the output is not constant or smooth. To smoothen the pulsating output, use filters. Filter is made of capacitor and resistor his filter is known as resistor capacitor (RC) filter. the charging and discharging time of the capacitor is different. The charging time of the capacitor is less compared to the discharging time of the capacitor. The output of the rectifier is pulsating DC voltage. The rising edge of the pulsating DC output will charge the capacitor. In addition, the falling edge of the pulsating DC will discharge the capacitor. Due to the low charging time and the high discharging time of the capacitor, the output voltage will be smooth. The charging and discharging part in the DC output waveform. Central controller is like the brain of the VFD. It controls and monitors the operation of the VFD. Inside the central controller, a microcontroller or microprocessor is used for controlling the operation.
There are different speed control techniques implemented for variable frequency drives. The major classification of control techniques are following.
The speed of a conveyor can be controlled by using AC drives, for instance, a machine, which operates at different speeds using an AC drive. For example, a conveyor may have bottles bunched close together for filling and then spread out for labelling. For this, two motors and two drives would be required. One motor to run the filling section at a given speed and a second motor to run the labelling section slightly faster, spreading the bottles out. This action can be precisely executed by an AC drive.
a soft starter is any device that reduces the torque applied to an electricmotor. It generally consists of solid statedevices like thyristors to control the application of supply voltage to the motor. The starter works on the premise that the torque is proportional to the square of the starting current, which in turn is proportional to the applied voltage. Thus, the torque and current can be adjusted by reducing the voltage at the time of starting the motor.
SCR or silicon controlled rectifier is a three-pin device, with three basic terminals – anode, cathode and gate. SCR has four layers. The gate terminal is the control terminal when external voltage is applied across the anode-cathode voltage.
SCR Firing Concept-In the case of SCR, the word ‘firing’ defines the sudden discharge of electrons. This discharge can be controlled by applying voltage at the gate terminal. As SCR is used as a switch, applying appropriate voltage at the gate terminal will turn ‘ON’ the SCR.
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