Ten Reasons to Use Adjustable-Speed AC Drives

Modern AC adjustable-speed drives (ASDs) have come a long way in their own right and as an alternative solution to dc drives. There are many reasons for using ASDs - here's a summary of their benefits:

1. Energy savings

Since more than 65% of industrial electric energy is consumed by electric motors, it makes sense to adjust motor operating speed to demands of the load. Varying load applications like centrifugal pumps and fans in particular benefit from ASDs. For example, when pump speed can be cut in half, resulting power consumption is reduced by a factor of eight! However, you need to know the load's duty cycle to get most out of energy savings.

2. Controlled starting current

High starting currents of ac motors (6-10 times full-load amps) stress windings, generate heat, and shorten motor life. ASDs start at zero frequency and voltage, extending motor life.

3. Reduced power line disturbances

Adjustable-speed ac drives virtually eliminate voltage sags caused by the staring of large or numerous motors. This minimizes tripping of voltage-sensitive equipment, reducing down time.

4. Lower power demand at start

Reducing 'demand charges'—or highest average demand recorded during any one time period within a billing period—also can reduce energy costs. Less power used during motor start means lower demand charges.

5. Controlled acceleration

This ASD feature reduces stress on the motor, as well as on upstream power system components (transformers, switchgear, cabling, etc.). Customer equipment and sensitive products also are protected.

6. Infinitely adjustable operating speed

Adjustable-speed ac drives provide the right speed for the 'job,' allowing a production process to be optimized. Ability to easily make process changes widens applications.

7. Adjustable torque limit

An ASD can limit torque (current) supplied to the ac motor to protect against machinery damage or jamming. It also protects the product being manufactured, which can be fragile.

8. Reverse operation

By changing the firing order of its output devices, the ac drive can electronically swap two output phases to reverse an ac motor's rotation. Eliminating separate contactors decreases panel space needed and lowers maintenance costs.

9. Elimination of components

ASDs can eliminate external components—mechanical (belts, transmissions, gear motors), electrical (PLCs, contactors, motor starters), and other process controllers.

10. Controlled stopping

Control of motor deceleration is as important as acceleration. Stopping time must suit the application, which ASDs provide either internal or external to the drive, without the need for a mechanical brake. Benefits are improved productivity and less scrap produced.

- by Mark Kenyon, Product Manager, ABB Inc.