Why are VFDs Popular?

The industrial space witnesses many innovations today. This is possible due to easily affordable and available semiconductors of various types, which makes it easier for manufacturers. One of the most popular innovations is the VFD or variable frequency drive.

Earlier, a prime mover had only a fixed speed, and its use was limited to expensive, non-efficient devices. With the advent of VFDs, it was possible to have an easy, efficient, cost-effective, and low-maintenance method of controlling the speed of the prime mover. This addition to the control of a prime mover not only increases the efficiency of the operation of equipment but also improves automation.

OEMs typically use VFDs for small and mobile equipment. They only need to plug it into a commercial single-phase outlet, in the absence of a three-phase power supply. These can be hose crimpers, mobile pumping units, lifts, fans/blowers, actuator-driven devices, or any other application that uses a motor as the prime mover. Using a VFD to vary the motor’s speed could improve the operation of the equipment. Apart from the benefits of variable speed, OEMs also use VFDs because of their ability to use the single-phase power source to output a three-phase supply to run the motor.

Although the above may not seem much, the value addition is tremendous, especially for the production of small-batch items. As VFDs output three-phase power, they can use standard three-phase induction motors, which are both widely available and cost-effective. VFDs also offer current control. This not only improves motor control but also helps in avoiding inrush currents that are typical when starting induction motors.

For instance, a standard duplex 120V 15A power source can safely operate a 0.75 HP motor without tripping. However, a VFD, when operating from the same power source, can comfortably operate a 1.5 HP motor. In such situations, using a VFD for doubling the prime mover power has obvious benefits for the capacity or functionality of the application.

The above benefits make VFDs an ideal method of controlling motors for small OEM applications. VFD manufacturers also recognize these benefits, and they are adding features to augment them. For instance, they are now adding configurable/additional inputs and outputs, basic logic controls, and integrated motion control programming platforms to VFDs. This is making VFDs an ideal platform for operating equipment and controlling the motor speed, thereby eliminating any requirement for onboard microcontrollers.

However, despite several benefits, VFDs also have some limitations. OEMs typically face problems when using GFCI or ground fault circuit interrupter breakers with VFDs. A GFCI typically monitors current flowing through the ground conductor. Leakage currents through the ground conductor can electrocute users.

A VFD consists of an inverter stage that works on high-frequencies. Harmonics from this stage can create ground currents, also known as common-mode noise. The three-phase waveforms generated by the inverter do not always sum to zero (as is the case in a regular three-phase power source), leading to a difference of potential causing capacitive induced currents. When these currents seek a path to the ground, they can trip a GFCI device. However, this can be minimized by lowering the operating frequency.