Top Ten AC Drive Benefits

Adjustable Speed Drives, more commonly known as Variable Frequency Drives (VFD), are among the most critical components of mechanical equipment applications that are powered by electric motors.


ABB's ACS355 AC Drive

ABB ACS355 AC Drive

Drives provide precise electrical motor control – motor speeds can be ramped up and down, and the exact frequency required for a specific application can be maintained without excess energy requirements. Through the use of a motor controller, end users of motors can achieve anywhere from 25 to 75 percent energy savings when running their application.


Apart from energy savings and precision control, VFDs offer a bevy of advantages that end users would otherwise miss if their motor application operates without a drive. The following list will outline 10 benefits realized with the use of an AC Drive.

1. Controlled Starting Current – It takes as much as seven to eight times the motor full-load current to start an AC Motor across the line. This means the motor draws a massive amount of inrush current upon startup, generating excess heat that will reduce the longevity of the motor. With an AC Drive, end users can start the motor at zero frequency and voltage, and ramp up the speed from there. As frequency and voltage build, the motor windings are magnetized over time, drawing just 50 to 70 percent of the motor full-load current. What does this mean? Less wear and tear on the motor, and extended motor life.

2. Reduced Power Line Disturbances – As stated above, starting a motor cold requires a huge current draw – seven to eight times the motor full-load current – which places enormous pressure on the power distribution system connected to the motor. Supply voltage sags, and the sag gets worse as the motor gets bigger. Voltage sags effect the entire distribution system, to the point where sensitive equipment connected to the same system can trip offline due to the low voltage. Using a Variable Frequency AC Drive eliminates this voltage sag, since the motor starts at zero voltage and increases from there.

3. Low Power Demand on Start – Because power is a product of current and voltage, the power needed to start an AC Motor across the line is much higher than it would be with an AC Drive. The power surge at startup without an AC Drive is taxing on the distribution system, which can lead to increased costs (power distributors often charge industrial customers for power surges that jostle the network).

4. Controlled Acceleration – When a motor starts with an AC Drive, it accelerates smoothly to the end user’s desired frequency. Cold starts, on the other hand, are a tremendous shock to the motor and connected load. Over time, this shock will increase the wear and tear on the motor, and may even result in immediate motor failure.

5. Adjustable Operating Speed – Using a Variable Frequency AC Drive allows the user to optimize their application. Changes can be made to operating process in real time, and adjustments can be made remotely with programmable controllers or process controllers.

6. Adjustable Torque Limit – With an AC Drive, the amount of torque being applied to the motor and load can be controlled accurately, protecting machinery from damage, the operating process, and the product built or refined. For example, if a machine jams and its motor is not equipped with an AC Drive, it will continue trying to rotate until its overload mechanism trips. An AC Drive can limit the torque on the load so the motor never exceeds that limit.

7. Controlled Stopping – Much like controlled acceleration, an AC Drive can decelerate a motor at a smooth, controlled rate to reduce wear and tear.

8. Energy Savings – Many mechanical applications can see energy saving benefits when operated with an AC Drive. To illustrate, consider centrifugal fan and pump loads, which follow variable torque load profiles. If the speed of the fan is cut in half, the horsepower then needed to run the fan is cut by a factor of eight. Using a fixed speed motor would need some other component to achieve the throttling necessary to maintain that speed, and still the motor would run at full load and speed. An AC Drive controlling the motors would simplify the system, reduce the power and load and ultimately save energy.

9. Reverse Operation – A Variable Frequency AC Drive eliminates the need for a reversing starter, since the output phases sent to the motor can be changed without an mechanical devices. This eliminates maintenance costs and saves panel space.

10. Elimination of Mechanical Drive Components – An AC Drive can operate with an infinite variable speed, meaning it can deliver whichever low or high speed required by the end user’s application directly to the motor from the load. This eliminates the need for additional mechanical devices like gearboxes, jackshafts, reducers and encoders, all of which are expensive and require individualized maintenance.


This article is derived from a white paper issued by the Low Voltage Drives Group of ABB Automation Technologies.
ABB is a leader in power and automation technologies that enable utility and industry customers to improve performance while lowering environmental impact. The ABB Group of companies operates in around 100 countries worldwide and employs approximately 135,000. ABB’s North American operations are headquartered in Cary, North Carolina, and employ approximately 30,000 people in multiple manufacturing, service and other major facilities.


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