How To choose a Correct VFD or AC drive/ Frequency convertor?

How to do Proper Selection of VFD/ VSD?

  1. First and very important step to select a Drive is to check Current of Motor which you can easily find on name plate of Motor. Along with this you note down the Horsepower/Kilowatt rating and Motor Input Voltage. Most of the motors will come with multiple Ratings of Input Voltage. When connected as Star and Delta. W. r. t. this Motors will have different Current Ratings too.

Let’s see a Motor Name Plate Of a very popular Brand Siemens


In above motor Name plate you can see various details Such as Voltage. Current, Frequency, power factor, Etc. Here you will find dual ratings in Voltage and Current whereas all other parameters are steady. In this case you may get confused about selection. So Let us clear this with above example.

Motor Name plate states that when Motor is connected in Delta its specs will be 380V  and 41.3A and when Motor is connected in Star its specs will be 660V 23.8A. So if you are using drive as star connected and 440V than your Drive rating will be as follow

Input/Output Voltage of VFD                    –  380V

Kw / Hp Ratings                                               – 22Kw   

Current ratings                                                 – 23.8 Amps or above 


  1. Check the basic features required to run your application .
    1. Control Method
      1. V/F Control
      2. Sensorless Vector Control
  • Close loop Control

Choose one of the above mentioned control method required as per your application. If you want to run your application in close loop make sure your VFD has Encoder card in built or available as option for that particular model.

  1. Braking Unit.

If you are using drive for application such as cranes, Elevators, etc where braking is mandatory, your drive should have Braking Transistor/Braking chopper in your drive. When you go for higher ratings of drives you will get braking chopper as optional. So make sure about this before you move further.


  1. Constant Torque or Variable torque

You should analyze whether your application is Constant torque or Variable torque. Applications Such as cranes, Agitator, Elevators requires High Starting torque (150%) and these are generally Constant torque applications. Whereas applications such as Fan, Pump. Etc are Variable torque applications and they will require comparatively less starting torque (120%). We will discuss separately on Variable Torque and Constant torque. While selecting VFD be sure to check Overload Capacity in terms of 60secs/1 min.

  1. If you wish to locate your keypad on panel than check whether drive has removable keypad. Also check the cable availability to connect keypad.
  2. PID (Proportional Integral Derivative Controller)

If your application such as Dancer Winder, Compressor etc which demands PID loop than make sure you have PID function as in-built in your VFD

  1. Check the no of control Inputs and outputs required. There are limited no of digital inputs and digital outputs (transistor and Relay). These inputs and outputs are given in enough to meet most of the applications. But for any reasons or any specific applications who requires more Digital inputs or Digital Output check for available option cards for that particular model. Same is in case for analog Inputs (0-10 V or 4-20 ma) and analog outputs (0-10 V or 4-20 ma)
  2. Don’t forget to check ambient temperature before you buy a AC drive. Mostly Drives comes in 2 ambient temperatures (40 ˚C and 50 ˚C). If your working conditions have more temperatures (either environment conditions or high working temp. conditions like forging and steel mills.) you have to derate the VFD rating i.e. you have to select one size (next kw/hp) higher VFD.
  3. If you are going to use your inverter drive with a system you may need to put your VFD on Communication. Communication protocols such as RS-485 /422 are standard with most of the drives. But if you need other communication protocols you may need to check whether options cards are available for particular drive model or not.
  4. If you require input EMI Filter to reduce electromagnetic interference or noise on the input side of the inverter. Please check whether it is available as inbuilt or Optional.


If still you have any doubt regarding selection of AC Drive/ VFD/ Inverter you can contact us.  We will be glad to help you in selecting VFD.