Sizing a control transformer for motor starters

[dudikr2]

Hey guys. I'm a young EE, and I'm trying to teach myself industrial motor controls from a textbook I bought, however there's no answer key and want to see if my answer is correct.

You are building a motor control cabinet that contains six motor starters and six pilot lamps. All control components operate on 120 volts AC. Two of the motor starters have coil currents of 0.1 amps each, and four of the motor starters have coil currents of 0.18 amps each. The six pilot lights are rated at 5 watts each. If the supply room has control transformers with the following ratings in volt-amperes (75, 100, 150, 250, 300, and 500), which transformer would you choose to supply power for all the control components?

So I added up all the currents:

(0.18 x 4) + (0.1 x 2) = 0.92 A

Then I used the formula I=(VA/E) but rearranged it so it was VA=I x E = (0.92 x 120) = 110.4 VA

I rounded this up to the next available size transformer, which is 150 VA.

The only trouble I have with this answer is that I didn't factor in the six pilot lamps at all. I don't see how I can factor them in since I don't have a power factor value to convert the watts to VA. So I'm thinking maybe they're irrelevant for sizing the transformer? Or maybe I just did the entire problem wrong. Any assistance would be appreciated.

 

[retirede]

The lamps will have a unity power factor for all practical purposes. Just add their watts to your calculated VA and choose the next size up transformer.

 

[dudikr2]

Oh okay, so treating the power factor as 1, I'd have to add an additional 30 VA (5 VA x 6 lamps) to the total value I had before from the starters. So 110.4 + 30 = 140.4 VA which rounds up to 150 VA control transformer.

Thank you very much for the assistance.

 

[broadgage]

Yes, 5 watts per panel lamp strongly implies small incandescent lamps which have a power factor of unity.
So six lamps will be 30 watts in total.

Possibly LED lamps with series resistance, but 5 watts is a lot for an LED pilot lamp.
Possibly 6 volt pilot lamps in holders that incorporate a transformer to step line volts down to 6 volts.
Either of those less likely possibilities would still have a power factor of close to unity.

 

[Jraef]

Your coil data for the contactors and relays will provide you with two values: sealed VA is what the coil consumes AFTER it has pulled in, Inrush VA is the current it will draw WHILE it is pulling in. Things like pilot lights are counted as "sealed".

A very simple/fast old formula for sizing CPTs when used in panels with multiple starters / relays etc.:

CPT VA = Total Sealed + largest Inrush.

There is a more accurate formal process involved in determining the regulation factor you have to deal with and knowing the power factor of the coils involved, but that's just to end up with a SMALLER transformer than the above quick method results in. That more accurate / detailed process is outlined here:
http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/1497-wp001_-en-p.pdf

If you are looking at coil data expressed in watts, then you are likely looking at DC coil data in which case you must first determine the size of your DC power supply, then the CPT is just whatever the maximum input current of that power supply is.

 

[drktmplr12]

Oh okay, so treating the power factor as 1, I'd have to add an additional 30 VA (5 VA x 6 lamps) to the total value I had before from the starters. So 110.4 + 30 = 140.4 VA which rounds up to 150 VA control transformer.

Thank you very much for the assistance.

for the purposes of the textbook exercise, do as you have done then add the watts of the lights to the total.

in the real world, as jreaf points out. your CPT will need to supply the highest inrush assuming all other coincidental loads are active.

the impact of inrush becomes more prominent as the starter size becomes larger. this is mostly due to the increase in magnetic force required to actuate the heavier contactor. the increased magnetic force is accomplished by designing a larger coil. the larger coil requires higher inrush to saturate the magnetic field within the coil. Here is a document from Siemens that lists typical contactors and their inrush/sealed VA, for your reference.

https://www.industry.usa.siemens.co...ntrols/products/Documents/AC_DC_Coil_Data.pdf

 

[retirede]

I’ve been out of panel design for 25 years, I never considered inrush back then.

The old engineer who taught me, said that the design of control power transformers is (was?) such that if you size for steady-state, they handle the inrush. I can’t independently verify that this is correct, but we never experienced any problems with our designs.