Are Your Motors Running on Overdrive?

Motors operate at a constant speed. However, the devices they operate, such as pumps and fans, frequently run at a lower rate. Allowing a motor to run at a higher rate than needed wastes energy.

Variable frequency drives (VFDs) adjust motor speed automatically, reducing energy costs. In addition to speed control, VFDs provide soft starts, which help to maximize equipment life by reducing mechanical stress. VFDs also improve the power factor of motors.

How VFDs Work

A VFD is an electronic controller that adjusts motor speed by modulating the power being delivered. It does this by converting alternating current (AC) to direct current (DC) and then converting the DC back to AC at a lower frequency and voltage level. This reduces the speed of the motor to the desired level with little reduction in torque or efficiency. VFDs provide continuous control, matching motor speed to the specific demands of the work being performed.

VFDs are ideal for facilities with long operating hours and frequently changing load requirements on key equipment. Variable torque pumps and fans are the most common applications for VFD installations across a variety of industries. VFDs offer significant energy-saving opportunities for some processing equipment in manufacturing facilities, as well as HVAC systems in commercial buildings.

Benefits and Limitations

VFDs offer a number of benefits. Some are easy to quantify, while others are less tangible. They also come with some limitations that you should know about.

Benefits:

• Energy savings. VFDs reduce energy use by matching motor speed to equipment needs. On pumps, for example, they can reduce energy use by up to 50% or more.
• Increased equipment life. Single-speed drives start motors abruptly, resulting in high torque and current surges. VFDs gradually ramp motors up to operating speed (soft-start), reducing mechanical stress. This can lower maintenance and repair costs, and extend motor life.
• Improved control. Flow or speed variations are achieved more rapidly and more accurately when the motor matches process or operational requirements.
• Low maintenance. VFDs are easy to install and have few moving parts, resulting in little wear and tear compared to mechanical dampening and other control strategies.

Issues to Consider:

• Capital cost. VFDs can be expensive to purchase and install. However, reduced energy and maintenance costs can lead to a rapid payback and long-term savings.
• Power quality issues. VFDs may produce harmonic distortion, which can cause power quality issues. Harmonic filters may help to reduce distortion.
• Constant torque applications. VFDs provide limited energy savings in constant torque applications, such as conveyor belts and machine tooling.

When considering a VFD installation, take into account your operating schedule, as well as the type, size and age of equipment in your facility.

Retailer Sold on VFD Energy Savings

Retailer JCPenney was looking for ways to decrease energy consumption and maintain or increase occupant comfort and environmental conditions in their stores. To help achieve those goals, the company retrofitted more than 1,300 rooftop units with VFDs at 131 locations.

The upgrade was very successful. The company achieved energy savings of 22% for annual cost savings of $5 million. The simple payback on the project was less than one year.