VFD is Variable Frequency Drive used to vary the frequency of input power to the motor, thereby controlling motor speed. The VFDs are now commonly applied to air handlers, pumps, chillers and tower fans.
There are several applications where VFD can be used which are listed below:
Cooling tower:
The cooling tower is set up used to chill water thus to increase the efficiency of the condenser.
When the condenser water is flowing fully over the tower the fan or fans are cycled on and off to control the water temperature. Fitting a VFD to the fan system can control the speed of the fans, the VFD system compares the temperature of the water with the outdoor temperature.
This makes the system more efficient when the water temperature changes the fan adjusts its speed.
Boilers and Forced Draught Fan:
On large boilers modulating control is used to regulate the boiler output to match the building load. The boiler is burnt by combustion which needs fuel and air mixture in a definite proportion.
To maintain efficient combustion, the volume of air supplied to the burner is regulated, in relation to the amount of fuel. The installation of a VFD in the forced draft fan motor allows varying the speed of the fan in relation to the volume of air required, which provides more precise control and ensures efficient combustion.
Constant Air Volume (CAV) System:
A constant air volume supplies a constant air to a typical space to control the temperature in the space.
The system design provides sufficient capacity to deliver supply air to the space for design load conditions. If the VFD is fitted to the air blowing system the air flow will be controlled with VFD by controlling the speed of blowers related to the load condition. The motor will only run at full speed when full heating or full cooling is called for.
Pump control:
At all times a fixed speed circulating pump supplies the design water volume required to satisfy maximum load conditions.
In all other conditions where full load supply is not needed, the energy is being wasted. A more economical system would be to install a VFD to control the circulation speed of the pump against the return temperature measured in T2. The setpoint of the controller would be the return temperature (design at full load).
Screw Press
A screw press is used to reduce the moisture content of a wet product.
Conventional control uses a set of mechanically variable pitch pulleys. The operator fixes the speed of the screw in order to meet the process requirements. However, differences in the wetness of the mixture means that the end product suffers from inconsistency. The mechanical speed variation is also unreliable and wear can take place thus changing the speed with respect to the adjuster position.
Elevators:
In the small conventional elevator system, single or two-speed motors are connected directly to the elevator car. Which will cut off the power supply to the motor when the car reaches the desired level, which results a sudden stop to the car. Starting is also coarse as the motor is generally switched directly on to the power supply causing harsh acceleration.
By using a VFD to the existing control, smoother starting is achieved and stopping is more precise. This is a totally controlled stop, so it is a repeatable process. It is not easy to give a design for a control interface since there are many different possibilities, but the VFD application engineer must be familiar with VFD capabilities to ensure that the correct operation mode is used.