Single-phase to three-phase inverter

　　Can single-phase power be converted into three-phase power?

　　Use to convert single-phase to three-phase

　　One of the most common calls we receive asks about phase conversion: Can a variable-frequency drive (VFD) convert my single-phase power supply to run a three-phase motor? Many callers hope to combine phase conversion and speed control in a single device, aiming to save money, hassle, and space. However, as with most issues, there is no simple answer.

　　Single-phase AC is common in many residential and agricultural settings, though it can also be found in some industrial environments. It typically consists of two conductors (L1 and L2) and may include a neutral wire. Single-phase systems rated at 120 V, 240 V, and occasionally 480 VAC are widely used. Three-phase power comprises three phases (L1, L2, and L3). In the United States, three-phase systems are usually configured at 240 V and 480 VAC; in certain applications, systems as high as 600 VAC are also employed.

　　Many people encounter phase‑conversion issues when purchasing new or used motors, finding that three‑phase motors do not operate effectively with single‑phase power supplies.

　　Yes, a variable frequency drive can supply power to a three-phase motor from a single-phase input; however, phase conversion introduces numerous factors that are typically not considered when purchasing a VFD. In this article, we will examine in detail VFDs designed to convert single-phase to three-phase power, explore how to use standard VFDs when custom‑designed units are unavailable, and discuss alternative phase‑conversion options when a VFD is not the optimal choice.

　　Single-phase input inverter

　　Many manufacturers produce variable-frequency drives that accept a single-phase power supply and deliver a three-phase output. For example, the Galt G200 series, as well as Mitsubishi’s D700 and E700 series, all come equipped with factory‑configured drives capable of operating on a single-phase input to generate a three-phase output for driving induction motors.

　　In fact, an inverter designed in this way cannot accept a three-phase power supply at all. This is because the AC input has only two terminals that can be connected to the live wires, and thus cannot accommodate the additional conductors required for a three-phase input.

　　Galt Electric G200 series single-phase VFD, without a third input terminal.

　　For Mitsubishi D700 series single-phase drives, please note that the third terminal (counting from the left) is blocked.

　　If you need a ready‑made VFD to convert single-phase power to three-phase, this is usually your best option. The ratings and dimensions of these VFDs are determined by the motor’s rated three‑phase current, making it easy to select the right size and simplifying installation.

　　A drawback of inverters configured in this manner is that they typically can only drive smaller motors. The Galt and Mitsubishi series mentioned above, when set to single-phase input, are limited to 3 horsepower, which restricts the applications in which they can be used.

　　Another issue arises if the site is upgraded to a three-phase power supply. Although the cost of converting the entire system to three-phase power makes this scenario relatively uncommon, should it occur, these VFDs would be unable to operate on a three-phase system. Such VFDs are often less expensive than most others, yet even so, discarding them when they become obsolete remains a regrettable outcome.

　　Perform phase conversion using the standard.

　　If your motor is too large for a VFD designed specifically for phase conversion, you can use a standard VFD as a single-phase power supply. To do this, connect the two live wires of the single-phase supply to the VFD’s AC input terminals, and leave one input terminal open and unused. This approach does, however, raise certain considerations that you must take into account.

　　Since you are now concentrating the same current into two phases instead of three, the VFD’s input diodes may fail. To address this issue, you must size the VFD larger to accommodate the increased current rating. A conservative rule of thumb is to double the required VFD size.

　　For example, if your motor’s full-load amperage (FLA) is 15, double that value and size the VFD accordingly to accommodate a 30-amp motor. If you encounter this situation, we recommend contacting our experts, who can guide you through the sizing process and help you select the right VFD for your application.

　　This process of derating the VFD’s rated capacity does have some drawbacks. Compared to powering a three-phase motor with a three-phase input, you’ll need to purchase a larger drive, which entails higher costs and greater space requirements. We always recommend, whenever possible, using a three-phase power supply to drive your motor; however, there are times when this isn’t an option.

　　Another factor to consider is how this mode of operation might affect your warranty. There are many VFD brands, and whichever one you choose may have its own warranty policy. If using the VFD in this manner voids your warranty, you may want to explore alternative options.

　　Other phase conversion options

　　In some cases, a VFD is not the best choice for phase conversion. One of the most common issues we encounter with VFD‑based phase conversion is when users attempt to convert single-phase power to three-phase—not just for motors. While VFDs excel at converting the phase of AC motors, they are less effective at handling the power requirements of peripheral devices, which typically include relays, lighting, control‑power transformers, and other electronic equipment.

　　If you don’t need to control the motor’s speed either, you’ll end up paying for many features in the VFD that you won’t use. VFDs are primarily designed to regulate motor speed, so if you want the motor to run at full speed all the time, your system may be over‑engineered.

　　In this situation, you should consider using a phase converter. There are several types of phase converters, each with its own advantages and disadvantages. A static phase converter is a very cost-effective option, but it typically cannot enable the motor to operate at its full rated power. A rotary phase converter performs well in power conversion, but it has moving parts and generates significant noise. Digital phase converters generally deliver the best performance when converting phases, allowing the motor to achieve its maximum rated power, though they are more expensive.