Variable Frequency Drive (VFD) working principles
A variable frequency drive (VFD) consists of three main elements: an AC motor(usually a 3-phase induction motor), adrive controller and a user interface.
The AC motor
The AC induction motor often has a fixed speed so that the controller is relatively simple. This supplies voltage in three phases in a sequence into the coils of a stator in the motor, creating a rotating magnetic field. This induces an electric field to drive the rotor.
The AC drive controller
A typical industrial AC drive controller has three half-bridges, each delivering a sine-wave voltage to the stator. For the adjustable frequency, scalar algorithms are used in the controllers to vary the voltage to determine the frequency of the phases, or volts/hertz. The use of more sophisticated algorithms is becoming increasingly popular in high-end motors. Examples include vector control or field-oriented control (FOC) that are used to control the frequency of multiple phases.
Adding in variable speed drives increases the complexity of the control algorithms.
These drives also need the controller to provide a soft-start capability to reduce the load on the motor and the current surge of the motor during start-up. This is essential to reduce the stress on the motor and provide the maximum lifetime for the industrial drive on the factory floor.
This is because the motors can have inrush currents seven to ten times that of the operational current, and the required torque can be three times higher at the start compared to when in operation. A digital soft-starter controller continuously monitors the voltage during start-up, adjusting to the load of the motor to provide smooth acceleration, speed and torque control. This is used to start and stop the AC motor drive system, reducing the stress on the drive. This also puts less load on the power supply, reducing the energy consumption and leading to significant cost savings.
The AC drive interface
The interface to the AC drive can be via a number of different connections. These can vary from a 4 to 20 mA current loop, a 0 to 10 VDC voltage signal or a remote connection via Ethernet or a wireless communication link.