Whats going on with Drive over current protection sizing?

[jmarshall]

I've noticed the overrcurent protection sizing for VFD's and Soft Starts is a little odd.  According my understanding, (at least how it used to be), when you add a drive to a system you throw out the motor tag and size according the drive specs.  (This assumes you installed the correct drive to begin with).  However, if you look at these recommendations they are essentially calling for motor protection as I would have installed it without the drive.

Example: 

   I'm looking a a 50hp motor starter (ATS22D32S6U)

    My motor is a 50hp 480V 3~ 58FLA motor

   Without a soft start I would rate:

      * 58 FLA x 2.5 = 145A 

      * Round up to 150A over current

   With the soft start listed above, I would look at the Nominal VFD amps and rate it like any other device:

       * 65A x 1.2 = 78A

       * Round up to 80A

    However, if you look at the recommendations for circuit protection for the linked soft start in the linked in the document it shows a 150A protection device?  

    That doesn't make any sense to me.  Can someone explain why you would install a drive but still rate the circuit the same? 

[BJacqz · Accepted solution]

In your example, you mention Soft Starter as well as VFD (Drive). As soon as you give example on ATS22, I will consider you want to select a Soft Starter accoding to your motor. I would recomend ATS480.

Our Soft Starter catalogue would help you to select the right type. In your example, motor 50Hp, 480V and 58 FLA, the catalogue will give you the following reference :  ATS480D75Y (see on p 20 on the catalogue joined). This soft Starter allows a 75 A (nominal), which is more than the 58A required.

Another way of seletion is to use the tool we have developped for selection, the selection tool: Motor control configurator : https://www.se.com/fr/fr/product-range/63492-ecostruxure-motor-control-configurator/#

Using this tool, the result will be as following :