×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Transformer sizing vs. AC drive inrush current

Transformer sizing vs. AC drive inrush current

Transformer sizing vs. AC drive inrush current

(OP)
We are currently designing a system where we will be using 10 AC drives connected to one 50KVA Delta/Wye transformer (460/400).  The inrush current for each drive is I peak=250A, this reduces to 37% in 0.75ms.  This gives us a peak current of 2500A which reduces down to 925A in 0.75ms.  

When the system is running the primary leg current is 55A,
Therefore: 55*460*1.732=43.8KVA.

The design of the system is such that the primary of the transformer will be powered through a main breaker.  The drives will each have seperate breakers and contactors.  Upon resetting the system the contactors would pull in and at this point we would see the peak inrush on the secondary side of the transformer.  

Ignoring the obvious facts that we could stagger the contactors or use drives with built in softstarts or use external softstarts.  

We are looking for some technical documentation that will show transformer ratings and how to incorporate inrush current.

RE: Transformer sizing vs. AC drive inrush current

I don't know of any standards that apply to your situation.  Normal transformer overloading standards do not really apply for motor re-starting because the motor re-starting load is of such short duration.

Some companies that have in house engineering standards will specify their transformers to be suitable for short term overload of say 180% during the motor re-start period.

It is important to make the transformer vendor aware of any short term overload requirements, including motor starting.

In your case, the problem will be reactive voltage drop and if you wish to re-start all motors at the same time you may well require a larger transformer.  Better to do some voltage dip calculations and see what is the largest number of motors that can be started at one time and then stagger the re-starts to suit?

Regards

RE: Transformer sizing vs. AC drive inrush current

I assume the drives you speak of are soft starters and not VFD's, as VFD's would not experience such inrush current.  The peak current you are seeing is approximately 450% which is not uncommon for an induction motor.  The peak current demanded by the load wil result in a kVA demand of nearly 15 times the transformer rating.  This will surely cause the transformer to saturate most likely resulting in reduced secondary voltage and reduced motor starting torque.  Because the transformer will probably saturate at 2x the nameplate rating the conductor voltage drop on the primary can be predicted and accomodated.  You should probably increase the transformer size, or investigate different transformer types to accomodate the transient loads.  Maybe a buck-boost transformer would be adequate.  That's a question for a good transformer person.    

RE: Transformer sizing vs. AC drive inrush current

Suggestion: It is necessary to design the transformer for the inrush current to have permissible voltage drop across the transformer impedance to be able to start motors and keep them running when there is voltage dip or voltage sag. It is known that the motors may stall when the voltage reaches certain percentage of the motor rated terminal voltage shown on the motor nameplate, e.g. 85% of the motor rated terminal voltage.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources