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If you’ve ever had to specify an AC drive for a project, you’ll have encountered all the different brands, ranges and models there are to choose from. You'll also need to consider whether you need communication, if so, which network? How much I/O do you require? Do you need positive or negative logic? A lot of the answers will come from your application and the motor you wish to drive, so how do you determine what you should look for in your drive?

First things first - know your motor

The very first step in choosing your drive is to check the nameplate of the motor. This is where you will find all the important information such as supply voltage, kW/HP size and amp rating. If you need to convert HP to kW, simply multiply the horsepower by 0.75 and for kW to HP, divide the kW by 0.75. Nameplates often have a lot of information on them so if you aren’t sure what is important and what isn’t, take a photo of the nameplate and email it to us.

Where are you installing the drive?

Considering where your inverter drive will be installed is the next step in selecting the right AC drive. Many drives feature an IP rating which stands for Ingress Protection. The IP rating is usually a two digit number with higher numbers offering higher protection. Users in wet or corrosive environments may want to mount the variable speed drive in a panel or choose a higher IP rating. We suggest that inverter drives subject to wash down procedures or extreme conditions are sealed to at least IP65 or IP67. You can see a full breakdown of IP ratings and their meanings in this article.

Determine your I/O requirements

Your new inverter drive will need to interact with other devices in your plant, so you need to know how many inputs and outputs you require and the type of inputs and outputs.

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

Analogue inputs enable 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 drive’s status, for example if there is a fault or when it has reached its set speed.

Programming the variable speed drive

Programming your AC inverter drive can appear daunting. Despite it being estimated that 60% of operators only ever use the first seven parameters, many manufacturers focus on high-end applications. This makes programming more simple applications a lot more difficult than necessary. We recommend the Commander C200 range for simple applications as it's so easy to set up, with 10 preloaded parameters for standard tasks. If your application is more complex, we recommend the Control Techniques M700 or Danfoss VLT AutomationDrive which offers more functionality than the Control Techniques Commander but is also easy to set up.

What are your requirements?

Understanding the requirements of your application is key to making sure you don’t pay over the odds for your variable speed drive. Nowadays, modern drives tend to have both normal duty and heavy duty ratings which can also be referred to as variable torque (ND) or constant torque (HD). In basic terms, constant torque can be compared to driving on a motorway where speed is steady for a number of hours whilst variable torque is more like city driving where you are constantly stopping and starting.

​​​​​​​Applications including fans and pumps are frequently normal duty as they ramp up speed gradually and then run for hours, whereas other applications like conveyors and compressors are heavy duty as they start and stop with the loads changing frequently every hour. A normal duty application may be suited to a cheaper drive without many complex features, whereas the heavy duty applications may require a more expensive, sophisticated solution.

Controlling your AC Drive

Take some time to think about how you want to control the speed of your variable speed drive. You have a few options when it comes to controlling the drive including keypads, a potentiometer or a PLC. If you are mounting your AC drive in a panel or somewhere else where it is not easily accessed, a remote keypad or PLC could be beneficial.

Communication requirements

There is an overwhelming amount of communication options available such as Profibus, CanBus, Ethernet and Modbus. Thankfully most new drives, such as the M700 by Control Techniques and the VLT AutomationDrive by Danfoss, have expansion modules that you can plug into the unit which provide access to most communication protocols.

To filter or not to filter?

Without a filter inverter drives send electrical noise back into the mains supply. Some variable speed drives come with an integrated industrial filter, such as the Control Techniques Commander C200 and Unidrive M700, whereas others offer no internal filter at all. If you are using multiple drives in one panel a drive without an internal filter may offer you a cost saving as one filter can be used to supply all of the drives. Residential or commercial drive installations, such as an escalator in a shopping centre, will require a commercial filter.

If in doubt

Still not sure about which is the best inverter drive for your application and environment? Drop us an email or give us a call and we can help you specify the right variable speed drive at the right price for your applications. You can also filter through the options on nicontrols.com when choosing a drive, to see only relevant products for your specific requirements.