Since the late 1980s, Variable Frequency Drives (VFDs), also known as Variable Speed Drives (VSDs), have been the de facto standard for controlling the speed of induction or synchronous motors. Variable frequency drives are used to control motor speed in a variety of applications, including fans and conveyor systems, however, one particularly application of VFD in industries is industrial pumps.
Most industrial pumps are designed to operate at maximum efficiency at maximum flow. However, for many pumping applications, there is rarely a maximum flow, if required. This is because in its traditional form, the only way to control the water level in the tank, in its simplest form, is to install float switches near the top and bottom of the tank. The pump motor is usually controlled by a DOL starter, which means the pump can only be turned on or off. When the pump starts, it will operate at the maximum design flow, the water level will rise as fast as the maximum capacity of the pump and associated piping system can manage, and also depends on the demand level of the process plant.
This will continue until the water level reaches the high float switch, activating it and cutting power to the pump motor. With demand and the water level in the tank now falling, the motor will remain off until the low water level float switch is activated, at which point the motor will restart and the pump will try again to fill the tank at maximum flow. The main disadvantage of this approach is that supply can never keep up with demand.
By replacing the centrifugal pump motor starter with a suitable variable frequency drive, the high and low limit float switches can be removed from the tank and replaced with a suitable level sensing sensor system. If the analog output of this sensor is inversely proportional to the amount of water in the tank, the VFD can receive such a signal and smoothly and variably control the speed of the motor (and flow from the pump) under most process conditions under PID control.
Crucially, this will prevent frequent short-term overloads of the power supply or mechanical shocks (such as water hammer) from damaging pipes, fittings or seals each time the pump is started, and can greatly reduce energy consumption.
Variable frequency drives can also reduce pump system maintenance costs - another major factor in a pump's total life cycle cost. In addition to the significant energy savings, one of the main benefits of using a VFD to control a pump is the inherent soft start and stop capability, which means that pipes and valves are less likely to be damaged by rapid changes in flow.
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