Variable Frequency Drive (VFD) FAQ



Q: What is the difference between a phase converter and a variable frequency drive (VFD)?

A:Phase converters provide three-phase, 60 Hz power like what a utility would provide and can be used to power machines that have internal electronics that control motor speed. Variable frequency drives are used to vary the speed of electric motors.


Q: Why are my lights flickering when the equipment is running and how do I remedy it?

A: This phenomena is called electromagnetic interference (EMI) and is typically caused by poor grounding or wire routing. A poor electrical ground doesn’t allow high frequency noise to easily travel to ground, causing interference on other circuits. Additionally, if power lines are routed near adjacent wiring, high frequency noise can be radiated onto nearby wires, causing interference. An EMI filter can sometimes eliminate EMI but the ideal method is to address grounding and wiring issues. See Phase Technologies’ recommendations to mitigate EMI here:



Variable Frequency Drives


Q: What is the advantage of a voltage-doubling variable frequency drive?

A: Voltage-doubling VFDs is a technology unique to Phase Technologies. The main advantage comes when dealing with deep set well pumps. 460 V power require smaller wires than 240 V power, meaning substantial savings if your application requires several hundred feet of wire.


Q: Will my VFD run a pivot?

A: No, pivots contain their own electronics to control speed and would require utility three-phase power or a phase converter. Phase Technologies offers a unique solution allowing you to run a pump and a pivot. 1LH-C and -X models have a standard VFD output and an additional 480 V three-phase output that will run a pivot.


Q: Will my VFD run a CNC machine?

A: No, CNC machines have their own electronics to regulate motor speed so a phase converter is the correct option to run CNC equipment.


Q: Can I use a phase converter to power my VFD?

A: You can, but Phase Technologies offers phase-converting VFDs, eliminating the need for a phase converter. See our SD and 1LH product lines. If you decide to use a phase converter to power a VFD, see product manual for enabling VFD mode.


Q: What if my load needs 208 V but I only have a 240 V source?

A: All Phase Technologies drives have a parameter called Output Voltage, which can be reduced below the incoming voltage to meet the motor specifications. It is recommended to increase the drive by one model size to handle the additional current.


Q: In Constant Pressure, why is pressure dropping below the Drawdown psi (aka Wake Up psi)?

A: When a valve opens and pressure drops below the value set for “Drawdown psi,” the drive will start pumping until the pressure setpoint is reached, or until it reaches the value set by Boost Amount. If the pressure drops very quickly, the system may need more time to respond. Adding or increasing the size of a pressure tank is one way to allow the system more time to respond.


Use the keypad to navigate to Read Measured Values to ensure that the 4-20 mA pressure transducer is reading correctly. See Transducer Troubleshooting Guide for more information:


                Other potential solutions include moving the transducer out of turbulent water flow, increasing the Integral Gain and/or Boost parameters, or using a larger pressure tank.


Q: My utility company requires drives to comply with IEEE519 harmonics standards. Does Phase Technologies offer products that will comply?

A: Yes, our 1LH and 3LH products utilize an active front-end and a harmonic filter to limit total harmonic distortion (THD). Phase Technologies guarantees that these drives will meet the strictest IEEE519 standards as long as the drive is operating at 50% of its rated load or higher.


Q: What are the benefits of an active front-end (AFE) VFD, such as 1LH and 3LH products?

A: Because AFE VFDs have active switches (IGBTs) on the input instead of passive diodes like six-pulse drives, they offer several benefits without the need for an additional harmonic filter:


LH products are guaranteed to comply with the strictest IEEE519 requirements as long as the drive is operating at 50% of its rated load or higher. This means less distortion on the grid that can cause problems for nearby equipment and eliminates the need for an additional harmonic filter.


AFE drives also allow the user to boost the output voltage, so if your source voltage is lower than needed, the output voltage of the VFD can be increased to meet the demands of the load.


AFE drives power condition the incoming power, allowing it to feed clean, balanced power to the load even in areas with poor power quality.


Because LH products are regenerative, they will not work with generators and a standard six-pulse drive, such as the DX and SD products, would be a better fit.