Magnetic starter control transformer

[RichT]

Is there any rule of thumb or table that lists the size of a control voltage transformer for 120 volt motor starter coils from size 0 thru size 6, assuming that the starter coil is the only load on the transformer?

 

[DanDel]

The manufacturer should list the inrush current and holding current for the coils for all sizes of contactors. At minimum, size the CPT for the inrush of the largest starter coil while all the other coils are already pulled in.

 

[busbar]

Here’s a few pages on the subject…

http://www.cutler-hammer.eaton.com/fred/tmgweb/00020106.pdf

Seems like above size 4, interposing relays with 480V magnetic-starter coils may be appropriate. CPT sizing is somewhat starter-manufacturer dependent.

 

[jbartos]

Suggestion: Visit

http://www.ramusa.com/Data/TechDocs/TechDoc_981505753/DBS INLINE.pdf

for Table 7 on Page 7 giving sizes of control transformers.

 

[BigRed2]

Best way to calc transformer capacity (assuming it is a MACHING TOOL transformer) is VA= SQRT((VA inrush * VA inrush) +(VA sealed *VA sealed))

 

[jbartos]

Suggestion to the previous posting: The inrush VAs should suffice. However, your formula appears to be "super-safe."