Sizing of Control Transformers with DC Power Supplies

[mvpieper]

In our typical motor control panel, we have 480Vac 3ph power into the panel. We transform 480Vac to 120Vac (1ph) and then convert this to 24Vdc for various instrumentation.
When sizing the control transformer, I know (per literature of several manufacturers) that I need to use the inrush VA as well as the sealed VA. For example on an Allen-Bradley 100-C09 contactor: sealed/inrush = 8/70 VA.

How should the inrush of the DC power supply be handled?
For example, an Allen-Bradley MicroLogix1500 (part# 1764-24AWA) has power consumption of 70VA and a maximum power supply inrush current of 25A for 8 milliseconds. Resultant inrush = 3000VA! 40x greater than steady state current! I know it is common for power supplies to have an inrush factor 20x or so.
Should the mfg's stated inrush be used in sizing the xmfr? Or could a reduced factor be used?

How should expected power-up sequence effect the factor used?
For example, if the power supply is not cycled on/off during the process as a motor contactor would be, then is it considered acceptable to use a reduced inrush factor (e.g., 2-5x instead of 20x)? I presume that the inrush associated with a laptop's dc adaptor should use the mfgr's stated inrush when sizing the xmfr, because it is switched in/out during the process.

In the past I have simply oversized the control transformer, but I prefer to design a leaner panel (or simply know how much power is left in my control xmfr). I appreciate any comments.

 

[dpc]

Since the inrush only occurs when the panel is first powered up (I assume), it doesn't seem too critical. Duration is generally pretty short. If the 120 V drops below about 85% voltage, any contactors energized by this 120V are at risk of dropping out or chattering, but this shouldn't be a problem when the panel is first energized.

If it were me, I might put my 24 vdc PS on its own transformer for a lot of reasons. Big motor contactors can create a lot of transients.

 

[itsmoked]

Yep, usually you just make sure you have 10% or so over on your control transformer. Just realize that it's output voltage will be pretty spongy when pull-ins occur limiting the actual inrushes. Now if you have some sensitive instrument also hooked to that source, then it might reset or hang up during a large brief voltage drop. In that case a separate source would be in order rather than some massive control transformer.

Another words you should give a little thought on the other loads sharing that control transformer.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com