Top 10 frequently asked questions about AC drives


Maximizing the uptime of processes while minimizing the amount of electricity used is a key responsibility for the people involved in designing, running and managing industrial activities. For the great majority of electricity-thirsty applications, installing an AC drive is the ideal way to improve process reliability while reducing electricity consumption.

To help you, if you’re thinking about purchasing an AC drive, here are some of the most frequently asked questions by customers and the answers provided by our experts.

1. Why should I install an AC drive?

Normally, equipment is sized to handle maximum load under worst-case conditions. This kind of sizing is applied to, for example, pumps, fans and motors. However, maximum load is only needed occasionally. If equipment constantly runs at full power, it uses wasteful amounts of electricity.

The speed of a motor that runs the equipment can easily be controlled with an AC drive. Slowing it down when full output is not needed significantly reduces energy consumption. For example, on a centrifugal pump or fan application a speed reduction of 20% will almost halve power consumption, and this saving goes straight to the bottom line.

2. How do I fit a drive and how much space is required?

Today’s AC drives are compact and can be mounted in electrical control rooms or, as a decentral unit, close to the motors and driven equipment. They can also be retrofitted onto most of the equipment driven by AC motors.

An important thing to consider when choosing a drive is where it will be installed. This has an impact on the type of ingress protection required. For instance, if the drive is to be mounted outdoors or in a harsh environment, an IP66/UL Type 4X or better ingress protection rating will be needed. If it is going to be mounted in an enclosure, an IP20/UL Type Open rating may well be enough. A wide range of suitable protection classes is available for AC drives to meet all installations.

3. How do I start the drive?

Starting a modern drive is almost as easy as flicking on a light switch. You just have to tell it how to run and at what speed. Modern drives will then do all the hard work of configuration for you.

In typical cases, a drive can be quickly and simply started and the speed reference can be given by using the keypad, the I/O terminal or a Fieldbus. PC tools are available for fast commissioning of the drive. If training is needed training courses are available for operation and maintenance personnel.

4. How do I control the speed of the motor once the drive has been fitted?

The two main methods are speed reference and set point control.

When speed reference is used, the drive follows the speed reference and adjusts the motor and equipment speed accordingly. The speed reference can be given by the operator or from the programmable logic controller (PLC).

When set point is used, the drive needs both the set point reference and the corresponding actual value. The reference and actual value typically represent flow, pressure or temperature. The set point is given to the drive, and the drive controls the speed based on the actual value. An internal PID regulator ensures that the set point is maintained.

5. Is an AC drive the same thing as a motor starter?

Many customers buy a drive on the basis that it will act as a motor starter for their application – and, in many ways, it will. Although, in some applications, a contactor is required before or after the drive to positively disconnect the motor when it is stopped.

By using an AC drive, a motor can be set to start rotating at a minimum speed and the speed can be gently increased as needed later on. In this way, the drive acts as a soft start. This is useful in many applications, such as packaging and bottling lines where fragile objects like bottles and jars are transported via a conveyor belt.

The key benefit of the drive is, however, that it is possible to control the motor speed at any time, not just during starting. Additionally, the starting current of the drive is significantly smaller compared to direct online or star delta starting which takes away starting peak currents from the supply network.

6. Is there any legislation I should be aware of when purchasing an AC drive for my motor?

In Europe, the Ecodesign Directive 2009/125/EC is the cornerstone of the EU’s sustainable industrial policy. It is about improving the environmental performance of energy-related products. The EU electrical motor regulation defines minimum efficiency requirements.

Since January 2015, machinery and system manufacturers using new motors with a rated output power between 7.5 and 375 kW have been required to achieve at least efficiency class IE3. IE2 motors may be used as an alternative, but only in combination with an AC drive. From 1 January 2017, this requirement will also apply to motors with a rated output power of 0.75 kW or more.

In general, please contact your drives supplier for information about any local requirements.

7. What maintenance is needed for my newly fitted AC drive?

Like any piece of electrical equipment, AC drives need to be treated with care. Simple maintenance activity will guarantee a longer lifetime. For example, make sure the drive is kept clean and dry. Ensuring that the drive’s cooling fans and heat sink are free from dirt and dust prevents blockages and overheating, so it is important to check on this every couple of months.

8. How long is the useful life of an AC drive?

A drive should function without any problems and without the need for replacement of any major parts for up to ten years if installed and operated within the ambient conditions specified by the manufacturer. But, with good maintenance and proper use, its lifetime can be extended to 15 years or more. Best practices in engineering and modern technology ensure the long lifetime of the drive. For example, film capacitors used in the DC bus of some drives, will increase its lifetime.

9. What sort of fault protection does my drive give me?

Drives protect themselves, for example, against external short circuits and ground faults.

There are also inbuilt features and functions that can be used to protect process equipment, for example detection of dry or blocked pumps. These protections trigger an alarm or fault that alerts maintenance to take care of the matter.

10. What length of cable can I use between the drive and the motor?

The answer depends very much on the design of the drive and can typically be found in the technical documentation of the manufacturer.  Very often there will be a maximum cable length defined which is determined by the drive’s design and internal components used and also another maximum cable length with which the drive complies with EMC standards. Drives are installed into public, commercial and industrial networks. There are different electromagnetic compatibility (EMC) requirements based on the installation locations. The suitable cable types and maximum lengths are described in the documentation of the drives. It is also recommended to check the motor specification suitability for variable speed use.

AC drives are well hidden from the general public, but nevertheless they play a major and very necessary role in modern, everyday life by making the world and our way of living more sustainable. You can read more about Danfoss Drives range of VACON® and VLT® drives here. And if you have any other questions related to drives, don’t hesitate to ask them in the Comments box below. We’ll get back to you as soon as possible.

Author: Hans Carlsson, Product Marketing Director, Danfoss Drives


  1. Thanks for pointing out that Simple maintenance activity will guarantee a longer lifetime. regular maintenance can expand its efficiency and life of Air Conditioning System.

  2. It’s great that this article talks about how AC drives need to be the right size so that the equipment runs easily and has the right output. This would be important to ensure you get the best results and help your system last longer. When choosing an AC drive, it would probably be a good idea to research the various brands and models to figure out which one works best and is the right size for your system.

  3. I appreciate you explaining how an AC drive works and how it helps in to provide a maximum load. I wasn’t aware that when using an equipment that runs in full power, it would just waste the electricity. If ever have a company that would need AC drives to help reach a maximum load for motors, I’ll be sure to buy AC drives to help me in the business.

  4. AC drives are really important for some technical setups, so you really do want to make sure that you know how they work and how they are set up. I also like that the article goes over the proper way to test drive the AC drive. After all, you will want to make sure that the new piece of equipment is working properly before you make it a permanent addition.

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  6. Hello Hans
    Your 10 Questions publication is and excellent reference for getting the most out of a VSD application.

    In line with your publication my methodology is to make the effort to understand exactly what the the client wants to achieve first and then recommend the correct VSD for the application.

    With installation and commissioning no effort should be spared to ensure the performance is optimized to achieve the maximum all round benefits of using a VSD.

    • Rosalind Laaksonen

      Hi and thanks for your comment. I totally agree, it’s vital to really get to grips with what the client wants to achieve and then find the best solution for their application.

      Keep following Focus on Drives and keep commenting. We really appreciate the input.

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