A variable frequency drive (VFD) is simply a power supply to your AC pump motor. To be very specific, it is a variable frequency inverter. Its job is to control the motor input frequency and voltage and, therefore, control your pump.
Terminology becomes important here, as different people use different names. VFDs are also known as “adjustable speed AC drives”, “AC drives”, “adjustable speed drives”, “variable speed drives”, “micro drives”, and finally “inverter drives”. Clear as mud, right?!
Benefits of VFDs are several. Most prominently, a variable frequency drive reduces power consumption and wear-and-tear on your pump motor. Without the VFD, the pump will run in fixed-speed operation – meaning it is either on or off. There is no variable speed.
Fixed-speed pumps use a lot of electricity when the demand for the pump is not at its peak. In other words, without a VFD, your pump will always use the maximum amount of energy. These pump motors also take a lot of abuse. The high starting current required can be six to seven times the motor’s full load amperage rating. This is not only expensive, but adds severe stress to your pump and to your flow control system in general (see water hammer).
When pumps fail, it can cost more than downtime. Municipalities are painfully aware of the costs of pump and level control failure. When pumping stations stop working, it usually results in either a wastewater or stormwater overflow. Either brings with it flood conditions, property damage, enormous EPA fines, and health and safety risks for the general public.
In the Petroleum and Petrochemical worlds, tank overfill is a dirty word. Any spills or overflows result in damage to the environment, potential spoiling of lands and water sources, and the high risk of fires and explosions.
Keeping pumps up and running as efficiently as possible is a top priority. Regulating the speed of the pump motor with a VFD is the best approach for pump longevity and cost reduction. Of course, to use a VFD successfully, you need a good continuous level sensor or pressure transmitter.
Ultrasonic sensors like our LPU-2428 and submersible pressure transducers like the PT-500 are both good options for tank level applications. For line pressure control, you’ll want a rugged pressure sensor such as our PT-400, or even a digital pressure gauge with an output signal like our PG7.
Of course, you should always consult an expert whenever you consider changes to your pumping system. Selection guides and savings calculators are available through many pump suppliers.