Key Takeaways
The short version, if you're skimming.
- One pump, many flow rates. A VFD lets a pump run at different speeds without a gearbox or a different motor.
- How it works. The drive changes the frequency and voltage going to the AC motor.
- Built for changing loads. Slurry goes from thick to thin in seconds, and a drive can adjust as it happens.
- Lower running costs. Slowing the pump down can cut power use and the cost of every gallon pumped.
- Pick the motor carefully. Use one that can handle VFD operation, especially at 460V.
- Less babysitting. EDDY Smart Pump Technology can adjust the drive for you as the material changes.
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. A variable speed drive (VSD), another term for a VFD, plays a crucial role in motor control technology by adjusting both frequency and voltage, ensuring your pump operates at peak performance.
What is a Variable Frequency Drive?
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, VFDs change the frequency 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. The output voltage is generated from a PWM waveform created by the switching of IGBTs, and it consists of pulses rather than a smooth sine wave. 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.
How Do VFDs Work?
A Variable Frequency Drive (VFD) operates by converting the incoming AC power into a DC signal, which is then transformed back into an AC signal with variable frequency and voltage using a voltage source inverter. This conversion process allows the VFD to precisely control the motor speed by adjusting the frequency and voltage of the output signal.
The heart of this operation lies in the use of Insulated Gate Bipolar Transistors (IGBTs). These transistors switch the output signal on and off rapidly, creating a Pulse Width Modulation (PWM) signal that effectively simulates a sine wave. This PWM signal is then sent to the motor, enabling it to adjust its speed according to the requirements.
Additionally, VFDs incorporate a sophisticated control system that regulates the output signal to ensure the motor operates at the desired speed. This control system can be programmed to respond to various changes in the system, such as load, temperature, pressure, or flow rate, making it highly adaptable to different operational conditions.
Types of VFDs
Variable Frequency Drives (VFDs) come in several types, each designed for specific applications and voltage levels:
Low-Voltage VFDs (LV VFDs)
Ideal for residential and commercial buildings, these VFDs handle low-voltage applications efficiently.
Medium-Voltage VFDs (MV VFDs)
Commonly used in industrial and commercial settings, MV VFDs are suited for medium-voltage applications.
High-Voltage VFDs (HV VFDs)
These VFDs are designed for high-voltage applications found in heavy industry and power generation.
DC VFDs
Specifically for DC motor applications, such as conveyor belts and hoists, these VFDs provide precise control over DC motors.
AC VFDs
Used in AC motor applications like pumps, fans, and compressors, AC VFDs offer versatile control over a wide range of equipment.
Each type of VFD is tailored to meet the specific needs of different environments and applications, ensuring optimal performance and efficiency.
Why Do I Need a VFD for Energy Savings?
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. A variable speed drive (VSD), another term for a VFD, adjusts both frequency and voltage to control motor speed, ensuring efficient operation. 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.
Benefits of Using VFDs
Implementing Variable Frequency Drives (VFDs) in your systems can yield numerous benefits
-
Energy Savings: By adjusting the motor speed to match load requirements, VFDs significantly reduce energy consumption.
-
Save Energy: VFDs eliminate the need for energy-wasting devices like throttling valves, leading to substantial energy savings.
-
Improved Power System Efficiency: VFDs enhance the overall efficiency of the power system by minimizing energy loss as heat.
-
Increased Motor Windings Life: By reducing the stress caused by sudden speed changes, VFDs extend the life of motor windings.
-
Improved Motor Controllers: VFDs offer advanced motor control capabilities, including precise speed and torque control.
-
Induction Motor Efficiency: VFDs optimize the efficiency of induction motors by adjusting their speed to match load requirements.
-
Pulse Width Modulation: VFDs use PWM signals to simulate a sine wave, reducing stress on the motor and other components.
-
Load Torque Control: VFDs maintain constant torque even when the load changes, ensuring stable operation.
-
Voltage Source Inverter: VFDs provide a VSI output signal, allowing motors to operate at variable speeds.
-
Electric Motor Efficiency: By matching motor speed to load requirements, VFDs improve the efficiency of electric motors.
-
Motor Controller: VFDs offer sophisticated motor control features, enhancing overall system performance.
-
Synchronous Speed Control: VFDs enable motors to maintain a constant speed regardless of load changes, ensuring consistent performance.
These benefits highlight the versatility and efficiency of VFDs, making them an invaluable component in modern industrial and commercial applications.
VFD vs. Throttling Valve vs. Fixed Speed
Three common ways to control a slurry pump, and where each one makes sense.
| What matters | Fixed speed | Throttling valve | VFD |
|---|---|---|---|
| How flow is set | The pump runs at one speed, so it has one flow rate. The only other option is off. | A valve adds restriction downstream while the pump keeps running at full speed. | The drive changes motor speed, and flow follows. |
| Energy use at lower flow | Full draw whenever the pump is running. | Stays high, because the pump is still working against the valve. | Drops with speed, and power use falls faster than speed does. |
| Fit for slurry and sludge | Fine for steady, predictable duty. | Valves can wear and clog in abrasive, solids-heavy lines. | Handles thick and thin material without adding parts to the flow path. |
| What you need | A standard motor. | The valve, plus sensors to control it. | A VFD and a motor that can handle VFD operation. At 460V, that means an inverter-duty motor. |
| Automation | Manual start and stop. | Possible, with extra control hardware. | Can run on its own with EDDY Smart Pump Technology. |
How much you save depends on how often your pump runs below full flow, so the real numbers change from site to site.
Who's Behind This VFD Guide
We build the pumps these drives run, so this is where our advice comes from.
Building pumps since 1984
EDDY Pump has designed and built slurry and dredge pumps in El Cajon, California since 1984. Our technicians help customers set up VFDs and motors for their own pumping applications.
Made for tough material
Our patented eddy-current design is built for abrasive, solids-heavy slurry and thick sludge. That's the duty this guide is about, so the advice comes from the same problems our customers bring us.
Trusted in demanding industries
Our pumps work in dredging, mining, wastewater, and pulp and paper, and for the U.S. Navy. They're built in the USA, and EDDY Pump is a BABA (Buy American, Build America) program member.
Straight talk, and someone to call
VFD settings and savings depend on your site, so we size drives and motors with your numbers instead of quoting a generic result. Call 619-404-1916 or email info@eddypump.com.
Written by the EDDY Pump team
FREQUENTLY ASKED QUESTIONS
VFD and slurry pump FAQs
Straight answers on what a VFD does for a slurry pump, how it affects your power bill, and what to check before you add one.
What does a VFD actually do for a slurry pump?
It changes the frequency and voltage going to the motor, which lets the pump speed up or slow down instead of running at one fixed speed. That matters with slurry, since the load never stays put. One minute the pump is pushing thick sludge and the next it hits a thin pocket of liquid, and the drive adjusts to keep up. You'll also hear the term VSD. For a pump with an AC motor, it's the same thing for practical purposes.
Will a VFD lower my energy costs?
In most cases, yes, as long as the pump doesn't need to run flat out all day. Throttling with a valve burns off power you already paid for. Slowing the motor down just draws less to begin with, and power use falls faster than speed does. How much you actually save depends on how often you run below full flow, so the numbers are different for every site.
Can I use any AC motor with a VFD?
Not really. The drive sends the motor a pulsed signal instead of a smooth sine wave, and that can cause harmonics, resonance, and voltage spikes that slowly wear down the insulation. If you're on 230V with a standard NEMA motor, you're usually fine. At 460V, pick a motor that's built for VFD (inverter-duty) operation. Close coupled, single-stage pumps also tend to handle resonance better than framed or multistage ones.
Do I still need control valves if I add a VFD?
Usually fewer, and sometimes none. With a VFD you set flow by changing pump speed, so you don't need a string of control valves, each with its own sensor to install and maintain. That's a real plus on slurry, where valves tend to be a poor fit anyway. You'll probably still keep isolation valves for servicing the line.
Does a VFD need EDDY Smart Pump Technology to work?
No. A VFD runs fine on its own and you can set the speed yourself. EDDY Smart Pump Technology just takes that job off the operator. Its onboard chips adjust the drive as the material changes, so there's little to no hands-on tweaking and the pump stays closer to its best operating point.
Can EDDY Pump help me set up the VFD and motor?
Yes. Our technicians help customers with VFD and motor setup for their specific pumping application. It speeds things up if you have your material, the flow range you want, and your motor voltage on hand when you reach out.
Sources and Further Reading
If you want to check our work or dig deeper, start here.
- Variable Speed Pumping: A Guide to Successful Applications. A guide from Europump and the Hydraulic Institute, available from the U.S. Department of Energy, on when variable speed pays off.
- NEMA MG 1, Part 31: Definite-Purpose Inverter-Fed Polyphase Motors. The scope of the standard behind the inverter-duty motor advice above.
- What Is Inverter Duty Anyway? Webinar slides from Pumps & Systems on what inverter duty means for a motor.
Questions about your own setup? Call 619-404-1916 and we'll look at it with you.
LET’S PUT YOUR PROJECT IN MOTION
Tough material?
Bring it to EDDY.
Tell us what you’re pumping, where it needs to go, and what your
operation needs to achieve.
Talk it through
with our team.
Equipment selection, project planning, or an existing pump—we’ll help you find the right next step.
CALL SALES & ENGINEERING
+1 619-404-1916
EMAIL YOUR PROJECT DETAILS
info@eddypump.com