9 Frequently Asked Questions About AC Drives

Using an AC Drive is one of the most efficient ways to improve your application’s reliability and reduce power consumption. Among the benefits that come with using an AC Drive are extended motor life, low power demand and huge range of control, as we outlined in our Top Ten AC Drive Benefits guide.

To further help you decide whether purchasing an AC Drive is the right solution for you, here are 9 Frequently Asked Questions related to AC Drives and answers provided by automation experts at Danfoss Drives.


danfoss vlt drive

The Danfoss VLT AutomationDrive FC302, available at Galco.


1. Why should I install an AC Drive? Controlling the speed of a motor that runs your equipment is the easiest way to reduce power consumption. Because equipment is sized to handle full loads under worst-case condition, without any degree of control the equipment will operate at maximum load. This is often unnecessary, and consumes wasteful amounts of electricity. Slowing a motor down, for example, when 100% output isn’t need reduces that motor’s power consumption, extending the motor’s lifetime and saving big on energy.

2. How do I fit a drive and how much space is required? Modern AC Drives are compact and can be installed almost anywhere. From the control room to right next to your motor, you should have no trouble finding a drive that suits your needs. Just be mindful of the environment the drive will operate in. Outside? You’ll need a drive with a ingress protection rating that suits the outdoors (IP66, for example). Enclosure? Ingress protection may not be much of a concern.

3. How do I start a drive? Tell the drive how to run, and at what speed. That’s it. Modern drives are designed for simplicity: receive your input and do all of the configuration for you. On many units you’ll have access to a keypad, to input parameters that will dictate the drive’s function for more specific operations. Others will have I/O terminals, or receive input from a Fieldbus. In any case, the flexibility of your drive can be as simple or as complex is you need it to be.

4. How do I control the speed of the motor once the drive has been fitted? Drives will either use a speed reference to control motor speed, or a set point. Speed reference is given be the operator or a PLC, and the drive follows this reference to adjust the motor’s speed accordingly. When a set point is used, the drive will also need a corresponding actual value. When the set point is given to the drive, the drive will use the corresponding value to control the speed. Internal PID regulators maintain the set point.

5. Is an AC Drive the same thing as motor starter? Not exactly, but it can be used this way. Some applications require additional equipment between the drive and the motor to disconnect the motor when it is stopped. But for most purposes, the drive acts as a starter, and does much more. The drive can start the motor at minimum speed and uniformly increase the speed from there (drive = soft starter), and then the drive can adjust speed at any time during the motor’s operation, which is where it really shines.

6. What maintenance is needed for my newly fitted AC Drive? Drives, like any other piece of valuable equipment, require regular maintenance and care. Always adhere to the manufacturer’s recommended guidelines for maintenance, but a bit of common sense can go a long way. Keep the drive clean and dry. Ensure the drive’s fans and heat sink are free from dirt and dust, to prevent overheating. Simple, frequent maintenance checks will guarantee a longer lifetime.

7. How long is the useful life of an AC Drive? A drive should function without problems or need for major repair for up to ten years if installed correctly and operated within the manufacturer’s specified conditions. This is just a minimum, as long as the drive is properly maintained and used responsibly. With regular maintenance and smart operating practices, you can expert your drive to operate for 15 years or longer.

8. What sort of fault protection does my drive give me? Drives are equipped with features that allow them to protect themselves from common faults, like short circuits and ground faults. Drives may also come equipped with functions that can be used to protect external equipment, like a blocked pump, by alerting the operator when there is a fault in the process.

9. What length of cable can I use between the drive and the motor? This would depend on the drive, and is likely found within the drive’s technical documentation provided by the manufacturer. Look for the maximum cable length, defined by the design of the drive, the internal components used and any applicable operational standards. These vary based on the location the drive will be used in. Also, be sure to check the motor’s specification, which will recommend cables for variable speed use.

Equipped with these tips, you should be ready to choose your drive! Browse Danfoss AC Drives here on our website, and hundreds of other options from other top AC Drive manufactures.

This article is derived from a series of frequently asked questions and answers provided by Danfoss Drives, originally published July 26, 2016 on the Danfoss “Focus on Drives” blog.


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