Variable frequency drives, commonly known as VFDs, are highly effective and efficient options for controlling motors that were designed to operate at single speeds. Many people forget that the job of the VFD is not only to provide speed control but to offer better overall motor protection. Protection against over current, over voltage, under voltage, under current, stall protection, phase reversal, short to ground, phase-to-phase short, and more. Systems that use VFD for three-phase power can be significantly more efficient, thus making the use of VFDs a cost-effective solution for a wide range of industries.

Advantages of Using VFDs

VFDs can save a considerable amount of energy depending on their application. Common applications include centrifugal fans and pumps, compressors, and any application using a fixed-speed motor. Using a VFD can also enhance the efficiency of an AC motor. In turn, these benefits also result in a longer operational lifespan and less maintenance. Finally, enhanced speed control means fewer operational errors and less waste.

VFDs by nature provide energy savings, so the cost-benefit for your investment can pay for itself in a few months, to a few years of operation.

Additional Considerations When Installing VFDs

It is not enough to purchase a VFD and connect it to your motor. It is possible you will need to purchase additional components for your system. You may need to install electromagnetic interference filters, radio frequency filters, larger enclosures, new cables, or line/load reactors.

You may also want to further customize your VFD with the addition of pilot lights, a bypass, harmonic mitigation, TVSS, or a dv/dt filter. It is important not to overlook these additions as these elements can make all the difference between a smooth and efficient operation, and longevity in your motor and mechanical system, vs. premature failure of your motor.

When determining which VFD is right for your three-phase power applications, always consider:

• Full Load Amps which is critical for choosing the correctly sized VFD.
• Voltage. Specifically, whether it's 208 VAC, 230/240 VAC, or 460/480 VAC.
• Voltage/frequency variation
• Load type
• Speed range, in particular Turn down Ratio of motor for proper cooling
• Required torque
• Overload current capacity
• Control method
• Inverter rating in terms of temperature, altitude, and other environmental conditions

VFD Drive Operations: Single-Phase Input

The electronics within the VFD are designed to step up and convert the single-phase current. It does this by converting the DC current through the use of diodes, followed by the use of capacitors to generate the desired output. This process creates a pulsed power that mimics AC current.

There are four drive applications based on four basic operations. This includes forward driving or motoring with positive speed and positive torque, reverse driving or motoring with negative speed and negative torque, forward generating or braking with positive speed and negative torque, and reverse generating or braking with negative speed and positive torque.

Considerations When Purchasing a Single-Phase VFD

VFDs that are factory-built to input single-phase power can create three-phase output power to run an induction motor. However, these are unable to input three-phase power because the AC power input only offers two available terminals for hot wires and cannot take the additional wire necessary for a three-phase input. Implementing a VFD that was built for a single to three-phase conversion will ensure easier sizing and installation. Keep in mind, though, these types of VFDs can typically only run smaller motors when setup for single-phase input, limiting their applications.

Another consideration is if your location decides to update to three-phase power, these pre-built VFDs will not be able to function on a three-phase system, quickly making them obsolete. Larger motors will also be unable to utilize the VFDs that are built for phase conversions.

Auto Start & Stop

The majority of VFDs are designed to enable auto-starting. Most start with low-frequency and voltage which prevents the inrush and mechanical issues that are most often associated with direct-on-line starting. Customer typically closes a Digital Input on the VFD ( Dry Contact type) and the VFD will start. When Started, the VFD will start to rotate the motor and generate torque.

When the dry contact is open, the VFD will begin to stop the motor, the process reverses itself as the frequency and voltage are gradually decreased until it approaches zero at which point the motor is shut off, if and only if, the stop function is programmed to Ramp. If The Stop Function is programmed to Coast, the VFD output is immediately shut off, and the motor will coast to a stop.

Phase Conversion: What it Is and Why it’s Needed

Phase converters are devices that produce three-phase electrical power from a single-phase source. This allows three-phase equipment to operate at sites that may only have single-phase electrical service. Although applications and equipment using three-phase power were generally considered to be “industrial,” they are increasing in use in places like farms, wood and metal shops, and more. However, bringing in new service and adding more transformers and wire for three-phase service can be extremely expensive, making phase conversion a more desirable option.

For more information, check out this guide to understanding single- to three-phase conversion from Phase Technologies.

Phase Conversion with Standard VFDs

For those looking for phase conversion options, it is possible to utilize a standard VFD for a single-phase power supply. Essentially, the two hot wires for a single-phase power supply are placed on the VFD’s AC input, leaving one input terminal open. However, this type of conversion may cause some issues.

Concentrating the same amperage on two phases rather than three can result in a failure of the VFD’s input diodes. Resolving this requires using an oversized VFD to account for the larger ampacities, typically double the size of what you’d typically require. For example, a motor’s Full Load Amps (FLA) listed at 15 would require a VFD designed to power a 30-amp motor. This is also referred to as de-rating. Choosing to de-rate a standard VFD requires you to purchase a bigger drive that will end up costing more money and taking up more space, so it’s recommended to utilize a three-phase power source instead where possible.

Also, before you decide to utilize a standard VFD, be sure to look closely at the warranty. Some brands may include policies that will void the warranty if you are using a VFD in this manner.

More Phase Conversion Options

In some cases, VFDs may not be the best option for phase conversion. When using a VFD for converting a single-phase to three-phase for an AC motor, it will do well, but attempting to convert power for peripheral devices, like relays, lights, and control power transformers, it will not work right. Using a VFD may mean you are over-engineering your system if you’re not concerned with controlling the speed of a motor.

Instead of a VFD, look to use a phase converter. There are a few different types to choose from:

• Static phase converter: Budget-friendly but unable to run motors at full rating
• Rotary phase converter: Great at converting power but comes with many moving parts and can be quite noisy
• Digital phase converter: More expensive, but perform best at getting a motor’s full rating while converting phase

When to Choose a VFD vs. Digital Phase Converter

Certain situations and needs will better benefit from a phase converter than a VFD – and vice versa. Understanding when to utilize each will help save time and ensure the best solution for your specific use.

Best Use Cases for a VFD

• Motor control: VFDs can start, stop, and control motor speed as well as communicate with other systems like sensors, floats, and programmable logic controllers (PLC).
• For soft starting: VFDs ensure motors won’t exceed their rated run current during start up for a “true soft start.”
• Constant pressure control: A VFD will allow you to maintain constant system pressure by automatically varying the pump speed based on changes in system pressure.
• When electronics are NOT present: VFDs are only able to run motor loads, so if your device does not contain any electronics, a VFD is a good option. Electronics can include things like transformers, contactors, circuit boards, lights, heaters, and more.

Best Use Cases for a Digital Phase Converter

• Need to power multiple loads: A digital phase converter will provide the flexibility to start, stop, and run equipment at different times. A VFD can run multiple loads at once, but they must all be started and stopped together.
• Using in equipment designed with internal VFDs: Any equipment with an internal VFD should not also be controlled by an external VFD as it is not meant to be started and stopped by the source power. A VFD powering another VFD will end up damaging the system.

Contact Wistex at 1-800-726-7307 for more information about the VFD preprogramming solutions we offer. If you reach out to us outside of our business hours, rest assured we will respond to you as soon as possible. We’re always ready to answer your questions and help you choose the most efficient, effective solution for your operational requirements.