What's your method for sizing control transformer?

[david90]

I read this article
http://www.acmepowerdist.com/pdf/Page_64-65-68.pdf

and did a sample control transformer sizing.

Given: a contactor with 192 inrush VA and 29 Sealed VA (steady state)

According to the table in link above, correct transformer size would be 75 VA if I select the inrush VA 211 from the 95% "secondary voltage" and 40% "power factor" column.

Do you think the sizing is right? 75VA more than 2 times the steady state VA requirement of the contactor.

What would happen to the contactor if the control transformer secondary voltage drops down to 50VAC (120VAC nominal) during inrush?

 

[Lancie1]

Do you think the sizing is right? 75VA more than 2 times the steady state VA requirement of the contactor.

Inrush currents for motor starters are always a shock to new designers, but yes, it sounds about right for a NEMA Size 2 motor starter. For larger starters, the inrush current would be even higher and would need a larger control transformer.

If you have auxiliary devices running off the control transformer (relays, lights, current transducers) then you also have to make it even larger.

What would happen to the contactor if the control transformer secondary voltage drops down to 50VAC (120VAC nominal) during inrush?

The secondary voltage will drop off during the inrush period of 3 or 4 cycles. But if the voltage is low for 1 or 2 seconds, then the same thing always happens: the contactor will chatter, vibrate, and get hot, and will not seal in fully.