AC-Drives
What is a variable speed AC Drive and how does it work?
An AC Drive is an electrical device that is used to control the speed of an electrical motor, either an induction motor or a synchronous motor. AC Drives are also known by various other names such as adjustable speed Drives (ASD) or adjustable frequency Drives (AFD) or variable frequency Drives (VFD) or variable speed Drives (VSD) or frequency converters (FC).
The first electrical AC motor was designed in 1899. Electrical motors convert electric energy into mechanical energy by electromagnetic induction. These motors are characterized by:
* fixed speed, determined by the frequency of the power supply
* fixed torque
Obviously, a fixed speed is not suitable for all processes in all circumstances; thus, the need for adjusting the speed according to need.
Industrial machinery is often driven by electrical motors that have provisions for speed adjustment. Such motors are simply larger, more powerful versions of those driving familiar appliances such as food blenders or electric drills. These motors normally operate at a fixed speed. If speed control is required, that controller is called a (variable speed) AC Drive. AC Drives are used in a wide variety of industrial applications. To give an easy example, AC Drives are often used with fans to provide adjustable airflow in large heating and air conditioning systems. The flow of water and chemicals in industrial processes is often controlled by adjusting the speed of pumps.
However, variable speed AC Drives are commonly used in more complex and difficult environments such as water and wastewater processing, paper mills, tunnel boring, oil drilling platforms or mining.
The Technology
The speed is controlled by changing the frequency of the electrical supply to the motor. The 3-phase voltage in the national electrical grid connected to a motor creates a rotating magnetic field in it. The rotor of the electrical motor will follow this rotating magnetic field. An AC drive converts the frequency of the network to anything between 0 to 300 Hz or even higher, and thus controls the speed of motor proportionally to the frequency.
1. Rectifier unit
The AC drive is supplied by the electrical network via a rectifier. The rectifier unit can be uni- or bidirectional. When unidirectional, the AC drive can accelerate and run the motor by taking energy from the network. If bidirectional, the AC drive can also take the mechanical rotation energy from the motor and process and feed it back to the electrical network.
2. DC circuit
The DC circuit will store the electrical energy from the rectifier for the inverter to use. In most cases, the energy is stored in high-power capacitors.
3. Inverter unit
The inverter unit takes the electrical energy from the DC circuit and supplies it to the motor. The inverter uses modulation techniques to create the needed 3-phase AC voltage output for the motor. The frequency can be adjusted to match the need of the process. The higher the frequency of the output voltage is, the higher the speed of the motor, and thus, the output of the process.
The Benefits
The types of motors that AC Drives control are normally operating at constant speed. Enabling the user to control the speed of motor potentially gives various benefits.
Process control benefits include:-
* Smoother operation
* Acceleration control
* Different operating speed for each process
* Compensate for changing process variables
* Allow slow operation for setup purposes
* Adjust the rate of production
* Allow accurate positioning
* Control torque or tension
* System stress
System benefits include:-
* Reducing the start-up current, which reduces peak loads on the electrical network
* Reducing the mechanical shock in start and stop situations
* Saving energy and greenhouse footprint
An AC Drive often uses less energy than an alternative fixed speed mode of operation. Fans and pumps are the most common energy saving applications. In these applications, energy savings are typically 20-50%.
When a fan is driven by a fixed speed motor, the airflow may sometimes be higher than it needs to be. Airflow can be regulated by using a damper to restrict the flow, but it is more efficient to regulate the airflow by regulating the speed of the motor.
A water pump may need to vary it’s output flows to suit fluctuating demand throughout the normal operating cycle. Again is more energy efficient to regulate the water flow by regulating the speed of the AC motor and the pump accordingly.
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