Transformer Inrush Current

What is the operating voltage of the contactor coils, and is it AC or DC, what is the coil VA rating. How big are the contactors and how many of them are there.

There are no special requirements, you either go with a transformer that is correctly rated for the output load of 125VA or you limit the coil inrush current from the transformer using capacitors, etc so that the 40VA transformer can power the circuit without its output voltage sagging excessively.

operating voltage for contactor coils is 24 Volts AC , primary is 230 Volts AC, coil VA rating is about 20 VA per contactor, there are two contactors to operate compressors each 65 Amp rating .

How can I select value of capacitors and how can I connect to the transformer ??

Only what I need is that the sec. voltage from the transformer can make enough mag. field to pull the moving contact of contactors and keep it energized.

Lots of information being provided here, but none of it is very helpful. As your contactor coils are running on AC (and exibiting an inrush at turn-on due to the impedance drop within the contactor) you can either use a series connected inductor in the coil circuit or just use a bigger transformer for your application (which with 2 coils at 125VA inrush gives 250VA, as a rough guide for safety). What about using 230V AC coils insted of 24V AC. If your application is only for 1 system with 1 transformer one of these may be your best and cheapest bet (remember time is money). What does the documentation on the GE website say, I was under the impression that they also had contactor operation guides available for download on it (among other things).

As for using capacitors across the AC coil or transformer, that would actually increase the inrush current (if it were AC or DC), and increase the actual load continuously drawn by the transformer (for AC - remember, capacitors act like resistors when exposed to AC).

try to put small capacitor in parallel with transformer which may draw equivalent inrush current in leading which may cause less requirement of KVA.

There are a couple of approaches you could try. One is design your transformer so that its core saturates during inrush, giving a "soft" start. Once the inrush tapered off, the core would no longer be in saturation and would function like a normal transformer core. This approach was used back in the days of tube/valve circuits to avoid hitting the filaments too hard on power-up.

Another approach is to use a suitable inductor in series with the transformer to limit the inrush current to some acceptable value.

What is your particular concern with the inrush current? Is it pitting your relay contacts? Blowing breakers? Something else? If pitted contacts are the problem, consider using a mercury-displacement relay to switch the load.

For what it's worth.

-e

I need this design to be applied in mass production of package A/C units, so I need low cost and reliable solution to apply.

GE catalogue for such contactors says that inrush current needed to energize contactor contacts is about 125 VA while normal operation for coils draws only 20 VA per each contactor

Just install some inexpensive capacitors to resolve this issue and make shirr you install them in parallel with the inductive load. Also when you switch the load on and off do it after the capacitors (switch/disconnect at the load) this will allow the capacitors to charge before the load is applied.

I am not sure about yours but some relays need high voltage for energizing period if it is so, my advice use a capacitor and feed it with high voltage with small current and after triggering the relay, replace it to 24 volts, you can do it with same power supply.

"Overkill" is not a bad word. The conservative approach of going with the 125 va or larger control transformer will not be much different in price and size in comparison to possibly replacement when you add another device in the control circuit.

I always take a conservative approach to design for that 10 % that I just don't know about.

The use of a capacitor in this circuit can have a MTBF of 6 to 10 years in comparison to a transformer of 25 years.