A variable frequency drive (VFD, in short) is a device that is used in the electro-mechanical drive system so as to control torque and AC motor speed by changing the frequency input of motor along with the voltage. There are various other names for variable frequency drive: variable speed drive, micro drive, adjustable frequency drive, AC drive and inverter drive.
Vacker Kuwait supplies a wide range of variable frequency drives (VFDs) all over the Kingdom of Kuwait (Kuwait). We supply a variety of VFDs to most of the major cities in Kuwait and they are Al Ahmadi, Hawalli, As Salimiyah, Sabah as Salim, Al Farwaniyah, Al Fahahil, etc. The range that we supply covers from small appliances to large mine mill drives. We all know that almost 25% of the world’s electrical energy is consumed in industrial applications by electric motors that are actually helpful in saving energy in centrifugal load service by using variable frequency drive. Variable data loggers are not been applied fully on every field. The cost and size has been decreased by improving performance through advanced semiconductor switch devices.
Sub-systems of Variable Frequency Drives
Variable frequency drive is available in different low and medium voltage topologies. A variable frequency drive is used in y system that consists of 3 sub-systems. They are: AC motor, main drive controller and drive operator.
Main drive controller
The Variable frequency drive controller is a power electronics conversion system that is made up of three sub-systems: rectifier bridge converter, direct current link and an inverter. The most common and widely used drive is voltage-source inverter. Drives are organized as DC-AC drives in places like buses or solar applications. In a Voltage source inverter, the DC link has the capacitor that helps to smooth the DC output of the converter. This DC voltage is converted to AC voltage which is done by using inverter’s active switching element.
The controller is also arranged or organised as a phase converter that consists of single-phase converter for input and three-phase inverter for the output. The advancement of controller has spoiled the increase in the voltage and ratings of the current. The main drive controller is also used in switching occurrence of solid-state power devices.
The means that provides an operator to start and stop the motor and amend the working speed is operator interface. This operator interface controlling systems. These operator interface functions include reversing, amending between manual speed and help in automatic control. The operator interface consists of alphanumerical display which provides the information and data about the operation of the drive. A series of portable communications is available to allow the Variable Frequency Drive to configure, adjust and control by the use of the computer.
The drive operation works on the electric motor speed chart. The drive applications can be categorized as single-quadrant, two-quadrant, and four-quadrants are defined as follows:
It refers to driving and motoring, and forward accelerating quadrant with positive speed.
These refer to driving and motoring and reverse accelerating with negative speed.
It is connected with generating or braking and reverses braking with negative speed and positive torque.
Most of the application generally involves the single-quadrant load operating such as centrifugal pumps and constant load torque is used in extruders. The two-quadrant loads are applied in certain places where the operating speed is positive inn both the directions. The places where speed can be in any direction such as in elevators, high performance applications are required need four-quadrant hoists.
In Variable frequency drive, the starting and stopping procedure is totally opposite. The motor is shut off when the frequency approaches zero. The voltage and the frequency rate are inclined to a controlled rate. The Variable frequency drive is able to brake the load by applying a reverse torque with a four-quadrant rectifier.
The ac motor is a three –phase induction motor that is used in Variable frequency drive. The three-phase induction motors are generally used as the most cost effective motors rather than the single phase motors that are only useful in some situations. The Variable frequency drive generally prefers to use motors that are created for fixed-speed operations.
Benefits of Variable Frequency Drives
Below listed are few of the benefits of a VFD:
The speed motor load helps in saving energy by supplying power directly from the AC line. Such type of cost efficiency can be done usually in pump applications where the power varies with the square and cube of the speed. This type of change gives a large benefit of reduction in the power.
The AC drives provide improvement in quality and enhancement in the process of industrial and commercial application. The motor is subjected to high current that is generally up to eight times more than the full-load current in fixed-speed loads. The use of DC and AC is determined by the respective performance factors that include frequent acceleration, operation at low speed and regeneration by the four-quadrant operation.
Types of Variable Frequency Drives
There are basically six types of Variable frequency drives. They are as follows:
- Voltage-source inverter (VSI) drive topologies.
- Current-source inverter (CSI) drive topologies.
- Six-step inverter drive topologies.
- Load commutated inverter (LCI) drive topologies.
- Cycloconverter or matrix converter (MC) topologies.
- Doubly fed slip recovery system topologies.