How do I size motor starter transformers?

Question

I am looking for guidelines to help me size the control transformers for various magnetic starters. Size 00 to size 3 is typically for what I run into these days from Allen Bradley, GE, Square D, Westinghouse, etc. I cant seem to find too much data from the manufacturers. It seems that you need a higher VA rating to pull the mag in but once it is closed the power consumption is tiny. I have always erred on the high side because I have no specific guidelines.

Any insight?

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JT · Accepted Answer

Use this link to Industrial Control Transformers/Introduction.There is a chart for selecting transformer by inrush VA. The starter coil data should include an inrush specification.

Bob · Answer

One rule-of-thumb that I always used when sizing for multiple coils was total sealed+ largest inrush. its a bit of an overkill, but it provides a margin of error that I was comfortable with. If it is just one starter, just size it based on the sealed VA and make sure you buy a Control Power Transformer (as opposed to a signal transformer or a potential transformer). CPTs are specifically designed up front to provide the inrush current you would expect from an AC coil.

How do I size motor starter transformers?

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