|
What is a variable speed AC drive?
- Variable speed AC drives are AC motor speed control systems.
AC drives control the speed of either an induction motor or a synchronous motor by adjusting the frequency of the power supplied to the motor. AC drives are also known 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 AC motor was designed in 1899. AC motors convert electric energy into mechanical energy by electromagnetic induction. AC motors are characterized by:
Obviously, 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 electric 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 paper mills, tunnel boring, oil drilling platforms or mining.
Process control and energy conservation are the two primary reasons for using an AC drive. Historically, AC drives were developed for process control, but energy conservation has emerged as an equally important objective.
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. 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.
See also: Calculate how much energy Vacon AC drives can save in pump and fan applications
The following are process control benefits that might be provided by an AC drive:
-
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
An AC drive can often provide smoother operation compared to an alternative fixed speed mode of operation. For example, in a sewage lift station sewage usually flows through sewer pipes under the force of gravity to a wet well location. From there it is pumped up to a treatment process.
When fixed speed pumps are used, the pumps are set to start when the level of the liquid in the wet well reaches some high point and stop when the level has been reduced to a low point. Cycling the pumps on and off results in frequent high surges of electrical current to start the motors resulting in electromagnetic and thermal stresses in the motors and power control equipment. The pumps and pipes are subjected to mechanical and hydraulic stresses. The sewage treatment process is forced to accommodate surges in the flow of sewage through the process.
When AC drives are used, the pumps operate continuously at a speed that increases as the wet well level increases. This matches the outflow to the average inflow and provides a much smoother operation of the process.
See also: Vacon solutions for water and wastewater treatment
|