Contactor Rating for Switching Transformer Primary

[X49]

I'm in the process of designing a system where there will be several 75kVA 600V-120/208V three phase transformers. I wish to switch the primary of each transformer with a contactor. The load on the secondary of the transformer will be almost entirely resistive. I'd like the contactor to be rated for at least 100,000 operations.

Question is, how do you size a contactor for this application? I assume transformer inrush current must be taken into account, but I can't seem to find any references.

 

[ScottyUK]

In the IEC world you would look for the AC-6a rating to IEC 60947-4.


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If we learn from our mistakes I'm getting a great education!

 

[X49]

Thanks Scotty. I've been looking for the something for NEMA contactors and I found a rating for "Maximum Circuit Closing Inrush Current [A] Peak Including Offset" for all NEMA contactor sizes.

I'm taking this value, converting to RMS and comparing to the transformer manufacturer's rated inrush current. For a Size 3 contactor, 947A peak = 670A rms. 670A/72A = 9.3

So I would have to choose a transformer that has an inrush current no higher than 9.3 times its rated current in order to use a Size 3 contactor, which is do-able.

Manufacturer's contactor rating chart for reference:

http://www.ab.com/en/epub/catalogs/12768/229240/229248/10521726/10525386/10539953/tab8.html

 

[itsmoked]

How about some Soft Starters? Or are a bunch of deep voltage dips alright in your project? You might then be able to use smaller contactors not rated for the in-rush.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[WoodrowJWeen]

The rating chart you posted shows that a size 5 contactor would work for up to a 117kVA xfmr with inrush 20X or less.

 

[rasevskii]

Why not leave the transformers energized all the time (the usual practice) and switch the load on the secondary side? Possibly it is asking for trouble by switching the primaries, namely voltage dips and possibly blown fuses now and then. It depends how often the switching operations will occur, the more often the more risk of a fault. Transformers like to be energized all the time, if they are dry type, it will help keep moisture out as well.

rasevskii

 

[X49]

"The rating chart you posted shows that a size 5 contactor would work for up to a 117kVA xfmr with inrush 20X or less."

Yes, using my method above with an inrush of 20x yields the same result, but 75kVA transformers are available with inrushes as low as 6x rated current. This would allow a size 3 contactor instead of a size 5, and would save a good $40k in cost on the project.

I could switch the secondary, but I would also need a size 5 contactor to do that.

We will likely have about 8 of these transformers on the same bus and they won't be switched simultaneously, so I'm not too worried about voltage dip, but maybe more investigation is required.

The application is a large parking lot with receptacles for engine block heaters. A controller will be used to cycle power to the receptacles with a duty cycle that depends on outdoor temperature. The contactors will normally switch about four times per hour, a few months per year. Any further feedback is appreciated.

 

[rasevskii]

All the more reason to use contactors on the secondaries, and leave the transformers energized all the time. The frequent switching will not be beneficial to the life of the transformers. I am assuming that you will use dry-type units with attendant issues regarding moisture and open circuit air cooling (under freezing conditions?).

Best to get the opinion from the transformer manufacturer on the intended operation procedure, it could affect the guarantees.

just my humble opinion.

rasevskii