Contactor Rating for Switching Transformer Primary
Contactor Rating for Switching Transformer Primary
(OP)
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.
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.






RE: Contactor Rating for Switching Transformer Primary
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If we learn from our mistakes I'm getting a great education!
RE: Contactor Rating for Switching Transformer Primary
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:
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RE: Contactor Rating for Switching Transformer Primary
Keith Cress
kcress - http://www.flaminsystems.com
RE: Contactor Rating for Switching Transformer Primary
RE: Contactor Rating for Switching Transformer Primary
rasevskii
RE: Contactor Rating for Switching Transformer Primary
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.
RE: Contactor Rating for Switching Transformer Primary
Best to get the opinion from the transformer manufacturer on the intended operation procedure, it could affect the guarantees.
just my humble opinion.
rasevskii