Transformer sizing vs. AC drive inrush current

[nmi]

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.

 

[bigamp]

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

 

[gcaudill]

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.

 

[jbartos]

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.