Variable Frequency Drives (VFD)

A variable-frequency drive (VFD) (also termed adjustable-frequency drive, variable-speed drive, AC drive, micro drive or inverter drive) is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage.

VFDs are used in applications ranging from small appliances to the largest of mine mill drives and compressors. However, about a third of the world's electrical energy is consumed by electric motors in fixed-speed centrifugal pump, fan and compressor applications and VFDs' global market penetration for all applications is still relatively small. This highlights especially significant energy efficiency improvement opportunities for retrofitted and new VFD installations.

Over the last four decades, power electronics technology has reduced VFD cost and size and improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software.

VFDs are available in a number of different low and medium voltage AC-AC and DC-AC topologies

Rotary Encoder

A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code.

There are two main types: absolute and incremental (relative). The output of absolute encoders indicates the current position of the shaft, making them angle transducers. The output of incremental encoders provides information about the motion of the shaft, which is typically further processed elsewhere into information such as speed, distance, and position.

Load cell

                        A Load cell is a transducer that is used to convert a force into an electrical signal. This conversion is indirect and happens in two stages. Through a mechanical arrangement, the force being sensed deforms a strain gauge. The strain gauge measures the deformation (strain) as an electrical signal, because the strain changes the effective electrical resistance of the wire. A load cell usually consists of four strain gauges in a Wheatstone bridge configuration. Load cells of one strain gauge (quarter bridge) or two strain gauges (half bridge) are also available. The electrical signal output is typically in the order of a few millivolts and requires amplification by an instrumentation amplifier before it can be used. The output of the transducer can be scaled to calculate the force applied to the transducer. The various types of load cells that exist include Hydraulic load cells, Pneumatic load cells and Strain gauge load cells.

Weigh Feeder Operation principle

         


  
           Weigh feeder is a system specially designed for controlled feeding or custom feeding in very accurate measures. The feed control is achieved by controlling the speed of the conveying belt. Pulses from encoder and the weight signal from load cell are the main inputs of PLC. Set point is the value we are entering through HMI or sends through remote station to the PLC system as required TPH. From the inputs, PLC generates an exact TPH which is actually flowing. By comparing this Exact TPH (F/B) and set point TPH, a control value is generated inside the PLC by means of a PID algorithm. A 4-20ma signal is generated in the PLC according to this control value. PID always tries to match the process value and set point by generating a control value accordingly. The control value (4-20ma) decides the motor (conveyor drive) speed via VFD. Thus we get a controlled or custom feed flow by varying or controlled conveyor speed.

Weigh Feeders

Weigh belt feeders are a good choice when feeding relatively free flowing materials not requiring containment. Weigh belt feeders operate by continuously weighing a moving bed of material on its short conveyor, and controlling belt speed to result in the desired flow rate at discharge. Weigh belt feeders can achieve high rates while remaining compact, simply through a combination of manipulating material bed geometry and operating at higher belt speeds.