How to choose the Right AC Inverter Drive

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Having more choice is not always a good thing. Take AC drives for example: in the past only a select number of companies manufactured AC drives and they were primarily used for basic motor speed control. Now that there are hundreds of brands, multiple types and a wealth of communication and control options, it can all get a bit confusing. So if you're struggling knowing your Allen Bradley from your Baumuller, here are some important points to ensure you select the right variable speed drive for a new project.

Know your motor

Checking the nameplate on your motor is the first step to specifying the correct inverter drive. Most AC motors will have a nameplate which will specify the supply voltage, kW size or horsepower, and amperage. To convert HP to kW simply multiply the horsepower by 0.75. Write this information down as this is extremely important to specifying the correct drive. Please note that you will need a DC drive for a DC motor.

There can be a lot of information on your motor nameplate so if you aren’t sure what you are looking for then take a picture of the plate and email it to us.

Understanding your requirements

It might seem an obvious question but understanding your requirements is key to ensuring you don’t pay too much for your inverter drive. Most modern variable speed drives have heavy duty and normal duty ratings. These are often referred to as variable torque (ND) or constant torque (HD) applications. In basic terms, constant torque is like driving on a motorway where your speed is steady for a number of hours. Variable torque is like driving in a city where you are constantly stopping and starting. Applications such as fans and pumps are usually normal duty as they slowly ramp up to speed then run for hours. Things like conveyors and compressors are heavy duty as they can start and stop and the loads change many times an hour.

Consider the environment

Take some time to think about where the variable speed drive will be installed. If the environment is corrosive or very wet then it might be wise to mount the inverter drive in a panel or purchase a higher IP rated drive. IP stands for Ingress Protection and is usually a two digit number. The higher the rating the higher the protection. Variable speed drives which are subject to wash down procedures or extreme conditions would need to be sealed to at least IP65 or IP67.

Controlling your drive

Consider how you are controlling the speed of your inverter drive. Are you planning to use a keypad, pot or PLC to control the speed of your AC drive? If the variable speed drive cannot be easily accessed (i.e. inside a panel) then you might want to consider using a remote keypad or PLC.

Determine your inverter I/O requirements

Your new inverter drive will need to interact with other devices in your plant. It is therefore important to know how many inputs and outputs you will require and the type of input and outputs.

Digital inputs are used to interface the inverter drive with devices such as PLCs, switches and push buttons. These signals can be used for functions such as start/stop, forward/reverse, external fault, preset speed selection.

Analogue inputs on the other hand allow the drive to be controlled by a simple 0-10VDC or 4-20 mA signal. This can be something as simple as a pot or feedback from a PID Controller.

PTK inputs can be used to monitor a simple temperature sensor within the motor windings to avoid damage in the case of an overload.

Digital and relay outputs are useful when you want to notify external devices of your inverters status, for example if there is a fault or when it has reached its set speed.

Communication requirements

Profibus, Modbus, Ethernet, CanBus... the list of communication options available is overwhelming. Thankfully, most modern inverter drives, like the Yaskawa A1000 and Control Techniques Unidrive SP, have plugin expansion modules which offer access to most communication protocols.


Programming an VSD can be a daunting task. Unfortunately too many manufacturers focus on high end applications and make programming simple applications much more difficult than it needs to be. It is estimated that 60% of operators only ever use the first 7 parameters. For simple applications we recommend the Control Techniques M200 or Danfoss Micro Drive ranges which are extremely easy to setup up and have preloaded parameters for standard tasks. For more complex tasks we recommend the Control Techniques M700 or Danfoss VLT AutomationDrive which are also easy to setup but offer more functionality than the simpler M200 and Micro Drive AC drives.

To filter or not to filter

All variable speed drives require a filter to avoid sending electrical noise back into the mains supply. Some inverter drives feature an integrated industrial filter as standard whilst others have no internal filter at all. Choosing which is better for you depends on how you are installing the drives, for example drives with no filter could offer a cost saving if you are installing multiple drives in one panel as you can use one filter to supply all of the drives. If you are installing the drive in a residential or commercial environment, for example an escalator in a shopping centre, you will need a commercial filter.

Selecting the right brand

As we have already mentioned there are now hundreds of inverter drive manufacturers. We prefer Danfoss and Control Techniques but Siemens, Allen Bradley, Mitsubishi and Lenze all offer good value products. Each use their own parameter structure or software for setting up but Danfoss and Control Techniques offer a good balance of simplicity and functionality. As we set these drives up every day for customers we can offer the best technical support for these brands if necessary.

If in doubt

If in doubt why not drop us an email or phone call. We can help you specify the right inverter drive at the right price for your applications.

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