Variable Frequency Drives 101 - All You Need to Know About VFDs

Variable Frequency Drives 101

High-performance slurry and sludge pumps based on EDDY Pump’s fluid dynamic technology are an ideal solution for dealing with the thickest and most difficult materials such as mud, sludge, slurry. However, they can still be limited in operational speed by the alternating current (AC) motor you choose to power them, and AC motors are further limited by their design. When your slurry pumping application calls for changing speeds on the fly, you’ll need to add a variable frequency drive (VFD) to your equipment to ensure optimal pumping efficiency.

What is a VFD?

VFD technology was developed in response to the limitations of AC power. AC motors, such as the ones powering industrial pumps, rotate at a rate set by the incoming power and the number of poles inside the motor. Since AC power in the US is supplied at a standard 60 Hz frequency, this means that standard single phase two-pole motors spin 60 times per second.

Adding more sets of poles reduces the speed without any need to alter the incoming electrical frequency. However, you can’t just swap out an electric motor with more or fewer poles every time you need to change the operating speed of a pump. Transistor systems that allow you to turn specific motor poles on and off have been available for decades, but these systems are complex and often lack the fine control needed for industrial pumping. Instead, <strong>VFD’s change the frequency</strong> of the power supply, allowing for exact and immediate adjustments to the pump operation.  

The VFD works by taking in AC power at the 60 Hz frequency, converts it into direct current (DC) power through a rectifier circuit, and sends it through a DC bus to filter the voltage further. Then, power reaches the inverter which creates pulses of DC energy that function like AC current. The pulsing nature of the output mimics AC power enough to create the correct induction processes needed to spin the rotor of the motor. Since DC is easier to control in voltage and frequency, using it instead of true AC power allows the VFD to adjust the electrical supply on the fly. A series of transistors, especially the Insulated Gate, Bipolar Transistor (IGBT), give manual or automatic control over the power output and the resulting EDDY pump performance. Power is easily increased to a sludge pump under heavy load and then dropped again after a blockage passes or the texture of the slurry or sludge being pumped changes.

Why Do I Need a VFD?

VFDs are used in many different manufacturing and industrial settings, but they’re particularly valuable for controlling slurry and sludge pumps. Pumping water is a relatively static process since water doesn’t change density or weight during the process. In contrast, a slurry is often unevenly mixed and what’s traveling through the pump may change from a high viscosity to a thin liquid within seconds which can quickly lead to problems when the pump is deployed out in the field. A pump that is laboring to move a very thick mixture with many solids requires a different amount of power than one that suddenly hits a pocket of thin liquid. Since a VFD can adjust the operation of the pump on the fly, these sudden changes in operational load are easily compensated for with no interruption in use. Efficiency drops dramatically when a pump isn’t running at its designed maximum flow, but by altering the amount of power being supplied to the motor, you can compensate for this and still achieve high efficiency.

Benefits of Utilizing a VFD When Using a Slurry Pump

• Increase or decrease flow rate to meet specific application flow requirements.
• Reduce pump maintenance by keeping the equipment running at its maximum efficiency point and reducing total system pressure.
• Increased energy efficiency to power the pump decreases the cost of every gallon pumped.
• Enable the addition of programmable pump controllers like EDDY Smart Pump Technology that can be used to expand the use of the pump.  
• Eliminates or reduces the number of control valves that all require individual sensor installation and maintenance and often aren’t appropriate for slurry or sludge processing.
• Gain control over motor speed without switching to costly DC motors.

Pumping tough, abrasive slurries such as paper pulp or sewage is a challenging process due to high viscosity, the presence of large solids, and changes in viscosity. All of these issues can damage pumps that are only designed to run at a fixed speed and load. Sludges or other heavy material that need to be pumped when dredging or mining also cause similar issues. VFDs allow slurry pumps and their motors to adjust on the fly to changes in the material being pumped, preventing damage and ensuring the system is always running at maximum energy efficiency. Gaining variable speed control without costly programmable logic controllers is especially helpful during dewatering processes that require very slow processing speeds.

By utilizing the EDDY Smart Pump Technology, the VFD can be controlled on the fly using on-board smart chips and computers, requiring little to no input from pump operators, thus keeping the pump running optimally with maximum uptime and more profits.

How to Choose the Right Motor For a VFD

A VFD alone can’t power a pump. Yet not all AC pump motors work well with a VFD either. Since the conversion going on in variable frequency drives can create harmonics and resonant frequencies, you’ll need a motor built to withstand them. Close coupled and single-stage pumps tend to experience less resonance than framed and multistage units. Pumps connected to VFDs have higher grade insulation since operating the pump at a slower than usual speed will interrupt the heat loss pattern. Higher quality insulation might benefit from having the pump withstand the voltage spikes that occur during VFD operation that can destroy the insulation over time, although this is rarely a problem if you stick with NEMA standard AC motors and 230V operation. If you need 460V operation instead, look for a pump motor specifically designed to handle the voltage spikes created by VFD operation.

At EDDY Pump, we have experienced technicians on hand who can provide assistance with VFD and motor setup for your particular pumping application.

Best Applications – Industries Most Served

The EDDY Pump Corporation is a premier manufacturer of pumping and dredging equipment.  If you are pumping or dredging slurry, high solids, extremely viscous material, paste, high abrasives (sand & gravel), and material filled with solids, then you found the best-suited product for the job. 

Mining, Fly Ash, Coal Ash, Oil, Fracking, Gas, Wastewater, Pulp and Paper, Chemical, Energy, Water Municipalities, Irrigation, and Dredging Companies. For Access to Complete Product Line Go to: https://eddypump.com/products/