When your facility only has single-phase power but your motor requires three-phase, efficiency drops, costs rise, and production can stop. The solution? A Variable Frequency Drive (VFD).
Yes, a VFD can convert single-phase input into three-phase output for running motors efficiently. However, selecting the right inverter VFD model and sizing it correctly are critical to ensure stable performance, prevent overheating, and protect your equipment.
Let’s explore how VFDs make phase conversion possible, when to use them, and what to avoid in your system design.
Using VFDs To Convert Single-Phase To Three-Phase
A Variable Frequency Drive (VFD) — also known as a frequency inverter or Variable Speed Drive (VSD) — is designed to control the speed and torque of an electric motor by adjusting the frequency and voltage of the power it delivers.
In a typical setup, a VFD first converts AC power to DC using a rectifier, then converts it back to AC through an inverter stage. When supplied with single-phase AC input, a properly designed inverter VFD can output three-phase AC power, effectively enabling phase conversion.
This means a VFD can run a three-phase motor even where only single-phase power is available — common in farms, workshops, and rural installations.
For industrial users, this feature combines two key benefits in one compact device: phase conversion and motor speed control. You save on equipment costs, space, and wiring complexity while achieving better energy efficiency.
Single-Phase Input VFDs
Many VFD manufacturers in China and worldwide, such as USFULL, design dedicated single-phase input VFDs for small and medium-sized motor applications.
These single-phase input, three-phase output frequency inverters come with built-in rectifiers optimized for single-phase power. Because they are purpose-built, these drives cannot accept three-phase input — they are engineered specifically for single-phase supply lines (L1 and L2).
Advantages:
Easy installation and configuration
Built-in protection for input imbalance and current surge
Cost-effective solution for small motors (usually up to 3 HP or 2.2 kW)
Compact and efficient, ideal for solar pumping, HVAC, and small machinery
Limitations:
Restricted power range; not suitable for large industrial motors
Cannot be reused if your site upgrades to a three-phase grid supply
USFULL, a leading VFD manufacturer in China, produces robust and efficient single-phase input inverter VFDs with customizable logos, ensuring quality and flexibility for global OEM clients.
Using Standard VFDs for Phase Conversion
If your motor power exceeds the rating of dedicated single-phase input models, you can still use a standard three-phase VFD — but with some adjustments.
To connect single-phase power to a standard VFD, use only two of the three input terminals (L1 and L2) and leave the third terminal open. This setup works, but it creates unbalanced current across the VFD’s input diodes, increasing the risk of failure if the drive is not properly sized.
The solution: Oversize the drive.
A common rule of thumb is to double the drive’s current capacity compared to your motor’s rated Full Load Amps (FLA). For instance, if your motor’s FLA is 15 A, select a VFD rated for at least 30 A.
This approach ensures stable performance, longer service life, and fewer overheating issues. However, note that some VFD brands may void the warranty if used this way — always consult your VFD supplier or manufacturer for confirmation.
At USFULL, our technical team provides detailed sizing guidance for both Variable Frequency Drives (VFDs) and Variable Speed Drives (VSDs) to ensure compatibility and reliability in real-world applications.
What About Voltage?
While VFDs are excellent for converting phases, they are not voltage transformers.
A frequency inverter can change frequency and phase, but not increase input voltage.
For example:
If your supply is 220V single-phase and your motor requires 380V three-phase, a standard VFD cannot step up the voltage.
You will need an external transformer to adjust voltage before or after the drive.
In certain cases, motors with dual-voltage ratings (e.g., 230V/460V) can be rewired to match your system voltage, allowing your variable frequency inverter to run efficiently without additional hardware.
Still, as a best practice, always match the VFD input and output voltages to the motor’s rated values. Voltage mismatch leads to inefficiency, excess heat, and premature failure of both the motor and the drive.
USFULL’s frequency inverter drives are designed with built-in voltage protection and advanced algorithms to prevent overvoltage, undervoltage, and phase loss — ensuring reliable performance in harsh industrial conditions.
Other Phase Conversion Options
Sometimes, using a VFD is not the most efficient or cost-effective way to convert single-phase to three-phase power — especially if your system does not require speed control.
Here are three common alternatives:
Static Phase Converters
Low-cost and simple to install
Suitable for light-duty applications
Cannot deliver full torque or efficiency
Rotary Phase Converters
Provide balanced three-phase power
Reliable for larger loads
Mechanically complex and noisy
Digital Phase Converters
Offer clean, precise phase conversion
Ideal for sensitive electronic equipment
More expensive but highly efficient
If your system includes relays, sensors, or lighting circuits, a VFD may not be suitable, since it is optimized for motor control rather than powering auxiliary devices.
However, if you want motor speed regulation, soft start, and energy savings, a Variable Frequency Drive remains the best all-in-one solution.
