VFDs for Single-Phase Applications

 
Achieving efficiency in certain applications can be difficult when held back by single-phase motors. KEB’s single-phase input VFD with a 3-phase motor output works to get precise speed control, increase motor protection, and improve energy savings by 40%. Contact application experts at KEB to find the right VFD for your single-phase motor.

VFDs and Single-Phase AC Motors

My first job out of school was with a motor manufacturer providing technical support. Being in the Midwest, we had a lot of farm and agricultural customers.

Their applications ranged from running fans, pumps, elevators, agitators, augers, conveyors, etc. The farm installations often didn’t have access to three-phase power and had to make do with single-phase 230V. We sold a lot of single-phase Farm Duty/Ag motors into these installations.

People often wanted to run the motors at reduced speeds so they asked, “Is it possible to add a VFD to my single phase motor?” In general, single-phase motors cannot be run with VFDs. However, it is possible to input a single-phase to a VFD and output variable voltage to a three-phase induction motor. This article describes how that works and offers some considerations.

The Single-Phase Motor Challenge

High Starting Current Issues

One of the challenges that comes with operating large single-phase AC motors from the line is the high starting current. A 10HP single phase motor will pull 38A nominal (at 230V).

But that motor (NEMA B design) will pull 6-8 times the nominal current when starting up – or 234 Amps! This high amp draw can cause issues at the distribution panel. Even the utilities might notice.

To be fair, the problems associated with high starting currents will also affect a line-fed three-phase motor. But in the case of a three-phase motor, a person can easily add a VFD. One benefit of VFD operation is that while ramping up the motor speed, it will limit the motor current so these large peaks are avoided.

Centrifugal Switch Limitations

Another challenge comes with the motor itself. There are a few different designs of single-phase motors, but one specifically that I have seen most in industrial applications – ones with a capacitor start and centrifugal switch. This design uses a capacitor network that is in the motor circuit at low speeds. The capacitors help provide torque at zero speed and get the motor started in the correct direction.

Once the motor is spinning and has inertia, a centrifugal switch opens and the capacitor network is disconnected from the primary motor windings. The speed at which the switch opens happens before reaching the motor’s normal operating speed at 60Hz.

Speed Control Limitations

For this reason, it is not a good idea to use a motor designed for 50Hz on a 60Hz main. At least not without swapping out or adjusting the centrifugal switch. It could be possible that the switch never opens when running at 50Hz. This could damage the capacitors or overheat the motor windings.

Related Article: Motor Thermal Protection

A similar concern would be using a VFD to control the speed of a single-phase motor. Lowering the speed would effectively keep the capacitors in the circuit for extended periods of time and could potentially damage the motor.

VFD Solution: Single-Phase Input to Three-Phase Output

How It Works

So if you can’t use a VFD with this design of a single-phase motor, what is the solution? The answer is to input a single-phase to a VFD. The VFD can then act as a phase converter and output a three-phase to a three-phase motor.

Technical Considerations

There are some considerations to keep in mind, particularly with sizing. Some VFDs are designed and rated to input both single and three-phase. It’s important to check with the VFD manufacturer, but you’ll see something similar to this in the manual which denotes both phases.

Sizing and Installation Guide

When selecting VFDs for single-phase input applications, proper sizing becomes critical as most manufacturer ratings assume three-phase input power. While single-phase input operation is technically feasible, it often requires derating considerations that directly impact drive selection and system performance.

VFD Sizing Rules for Single-Phase Input

With larger drives, the ratings tend to only indicate a three-phase input. Single-phase input is possible but a single-phase derating is likely needed.

Let’s look at a VFD application with three-phase input running a 10HP motor. Let’s assume there aren’t any losses and PowerIN = PowerOUT. The input current and the output would be the same.

Now, take that same application running a 10HP motor but with a single-phase input. PowerIN = PowerOUT. Except all the power at the input is now going through one conductor instead of three. Effectively, there is a √(3) factor applied to the single-phase input current compared to the three-phase current.

Again, some drive sizes already have input rectifiers over-dimensioned and can inherently handle the increased single phase input current – this should be reflected in the power stage ratings. For larger HP applications, the net result is the drive might need to be upsized to handle the larger input current.

As a rule of thumb, we suggest rounding up and assume the single-phase input current will be double that of the three-phase input current.

Line Reactor Requirements

It’s also a good idea to use a 5% line reactor when applying single-phase input power to a drive. During power-up, the drive will have an inrush of charging current to the unit. The 5% reactor will help reduce the peak charging current and protect the VFD’s input rectifier stage.

Cost Analysis and ROI

Before defaulting to single-phase motors for higher HP applications, conduct a thorough total cost of ownership analysis including the VFD option. The combination of three-phase motor cost savings and operational benefits often makes VFD systems the more economical choice, even before considering the long-term energy and maintenance advantages.

Single-Phase Motor Premium Cost Analysis

When evaluating motor options, the cost differential between single-phase and three-phase systems becomes increasingly significant at higher horsepower ratings. Single-phase motors carry a substantial price premium, particularly in larger HP configurations, driven by both technical complexity and market dynamics.

Taking a 10HP motor as our reference point, the single-phase variant commands approximately a +60% cost premium over its three-phase equivalent. It’s likely that some of the added cost is due to the added parts of the capacitor network and switch. The other part of the cost is because larger single-phase induction motors are more of a specialty compared to the three-phase types.

Make sure to add in the additional cost of a VFD/reactor but also subtract out the premium for the single-phase motor.

ROI Calculation Framework
Total VFD System Cost = (3-Phase Motor + VFD + Reactors + Installation) – Single-Phase Motor Premium

In many cases, this calculation reveals that the incremental cost of adding VFD capability is significantly lower than the initial sticker price suggests, particularly when the single-phase premium is substantial.

VFD vs. Rotary Converter Comparison

Cost can be a big factor in this decision, but it’s also good to compare VFDs with other alternatives. While a phase converter is certainly an option, it will only convert single-phase power to three-phase power. It does not offer the many advantages that a VFD will offer. This includes the ability to control the motor speed, better performance at low speeds, protection functions, and motor temperature monitoring.

There is also a similar argument to be made on the cost of a phase converter. The phase converter will likely not save much, if any money, compared to a drive.

Advanced Features and Protection

Built-in Motor Protection

One advantage of VFDs that is often overlooked is all the protective functions they have that detect abnormal situations and provide long-term value.

• Over/Undervoltage – Automatically shutdowns when there is a brownout or power surge.
• Motor Overheat – This option requires a thermistor or motor temp sensor. It protects the motor investment and is a good idea for expensive motors, difficult-to-service motors, and high ambient temperature applications.
• Overcurrent protection – This could detect an abnormal fault like a shorted motor winding and shutdown.

There are many more protective features for sure, but you get the idea.

PID Controller Integration

A user will also benefit from the energy savings due to the VFD, especially quadratic load applications like fans and pumps. The higher duty of the application, the more savings there will be. Add some basic feedback to the application like a temp or humidity sensor and the VFD can be wired to regulate a process. KEB’s F6 even has a built-in PID controller so the entire process can be regulated inside the drive – removing the need for an external PLC or control.

Process Automation Capabilities

Users will also benefit from going from a line-fed motor to a VFD-controlled motor. They will be able to optimize the motor speed for the process. Maybe this means slowing the conveyor down during loading instead of completely shutting off the motor. Lightly loaded motors can also be oversped to speed up processes – like a conveyor or auger for example.

Frequently Asked Questions

Can I use a VFD directly with a single-phase motor?
No, traditional single-phase motors with capacitor-start designs cannot be controlled by VFDs due to the centrifugal switch mechanism. However, you can input single-phase power to a VFD and output to a 3-phase motor.

How much larger should I size a single-phase input VFD?
As a rule of thumb, size the VFD approximately 2x larger than the motor nameplate for single-phase input applications due to the √3-factor in current distribution.

What’s the cost difference between the VFD solution vs. the rotary converter?
A VFD solution typically costs similar to a rotary converter but provides additional benefits like speed control, energy savings (40%+), and motor protection features.

Do I need a line reactor with single-phase VFD input?
Yes, a 5% line reactor is recommended to reduce inrush current and protect the VFD’s input rectifier during startup.

What applications benefit most from single-phase VFDs?
Agricultural equipment (fans, pumps, conveyors), HVAC systems, water treatment, and any application requiring variable speed control where only single-phase power is available.

How much energy can I save with a VFD on single-phase applications?
Energy savings of 20-50% are common, especially with quadratic load applications like fans and pumps. Higher duty cycles provide greater savings.

Can KEB F6 drives handle single-phase input?
Yes, KEB F6 drives are designed for single-phase input applications. Contact our engineers for specific sizing and configuration assistance.

What motor protection features are included?
VFDs provide over/undervoltage protection, motor overheat protection (with a thermistor), overcurrent protection, and many other protective functions not available with line-fed motors.

KEB Single-Phase VFDs

KEB’s F6 drive can be used with single-phase installations. Download a manual on the F6 VFD product page or contact a KEB America engineer to discuss and decide which VFD is right for your application.