Variable Frequency Drives: Improving Efficiency and Control in Electric Motors

At the heart of a variable frequency drive (VFD) lies its ability to convert incoming AC power from the main supply into DC power. This process, known as rectification, involves the use of semiconductor devices, such as diodes, to convert the sinusoidal AC waveform into a unidirectional DC waveform. Once rectified, the DC power is smoothed and stored in a DC bus, acting as a reservoir to ensure a consistent supply of power.

The next step in the operation of a VFD is inversion, where the DC power is converted back into AC power with adjustable frequency and voltage. This is achieved through the use of semiconductor devices, like insulated gate bipolar transistors (IGBTs), which rapidly switch the DC power on and off at high frequencies.

Central to the operation of a VFD is its control system, which continuously monitors various parameters such as motor speed, torque, and external commands. Based on this real-time data, the control system adjusts the frequency and voltage output of the inverter to meet the specific requirements of the application.

One of the key benefits of VFDs is their ability to deliver significant energy savings by eliminating the need for energy-wasting mechanical devices such as throttling valves or dampers. By allowing motors to operate at variable speeds, VFDs ensure that energy is only consumed as needed, reducing overall energy consumption and lowering utility costs.

VFDs operate on a cubic curve, which is based on the affinity law:

Let’s look at a quick example. Let’s assumed a 10 HP pump is able to reduce its speed from 1,800 RPM to 900 RPM. What is the power consumption?

This cubic relationship can also be visualized using the following chart (green shading representing savings over the traditional throttling devices): 

Moreover, VFDs offer unparalleled precision in controlling motor speed and torque, leading to enhanced process control, product quality, and efficiency. Additionally, VFDs provide soft-start and soft-stop capabilities, which reduce mechanical stress on motors and equipment during startup and shutdown. This extends the lifespan of equipment, minimizes maintenance costs, and enhances overall reliability.