coil inrush

Status
Not open for further replies.

jtb

Senior Member
Location
Pennsylvania
OK, I am gonna give this a try, even though I don't know who is who now....I had certain prefered 'mentors' on this site, so I could quickly get a reliable answer by looking for their comments. Perhaps that is what the "Rate this member" is for, to weed out the "I think it is done this way..." crowd from the "Code Article ### says this, and this is what is meant" crowd.

OK, whew.

Question is....I recieved some prints for a lighting control system. Among other obvious errors, only 1 30A 120V circuit was alotted for control power. The inrush current of 9 coils adds to 45A. Granted this is seen only for a short while during switching, and never for more than 2 seconds (momentary pulse control), but the maximum wire size for the devices/control relays/timers feeding them is 10 AWG. Is this allowed? If so, is it wise? Could it possibly result in the contactors not transfering? And I will not even mention the 16AWG photocell that was in series with the contacts. I have redesigned the circuit, but is the control power sufficient? I may have trouble locating a breaker for a second control circuit, and this will also require another redraw. Thanx for any info. ~Jim, formerly J B, _thunder__@yahoo.com.
P.S.-Rate me, I still haven't integrated the 1/2 wave RMS yet! :roll:
 

templdl

Senior Member
Location
Wisconsin
Re: coil inrush

Check with the contactor manufacturer who should be able to provide you which the minimum pull in voltage value. You will probably find it to be around 75% of the contactor coil voltage rating. Off hand a 45a inrush draw on a 30a circuit wouldn't appear to be a problem. But, you may want to assure that the source of the 30a is stiff enough, which most likely won't be a problem, and consider the length of run for the #10 wire which may cause a voltage drop problem.
Regaring the photo cell, is it being used to contol the contactors, the 45a going through the #16 wire? If so you may want to consider an interposing scheme where the photo cell only needs to provice current to a contactor or relay coil (acting as an interposing relay interposing relay) with contacts that are rated for the that current which then controls the other contactors. If you can find a relay that has contacts that are rated for the current you can use that.
 

jtb

Senior Member
Location
Pennsylvania
Re: coil inrush

The original drawing I had showed each lead from the photocell going to opposite sides of a dry contact. I have never seen THAT work before..:0

I have an interposing relay in my new layout.
Thanx for the info, but the vendor didn't seem knowledgable about the contactors when I talked to him, and I am pinched for time. The specs on the contactor say 5A inrush per coil, 9 coils. Pull in time was not on spec, but minimum pulse for transfer was specified as 2 seconds. I don't think pull in will take anywhere near that long.

The contactors are mechanically latching. I do not have the supporting drawings from which to determine circuit distance. I can stagger the 'on' pulses from a timer control by reprogramming a second channel, but the photocell will kick them on together, and I don't have the luxury/panel space for a time delay relay.

I guess I will run the circuit as shown, and eat the cost if a change is necessary. Thanx again, I wasn't sure I was going to get a reply to this one.
 

luke warmwater

Senior Member
Re: coil inrush

Jim, I would agree that this may not be the best design. I don't think that there would be a problem with the contactors not transferring, but the photocell will probably not last long with this load, as they are normally rated in watts, an 1800w photo-cell would only be rated 15amp at 120v. You shouldn't need a second control circut to hook-up the photocell to a contactor or relay, just take it off of the line side of the coil circut. And it would probably be less expensive to have the photocell operate a 2-pole contactor coil than a timer relay to stagger the on times.
Are the plans engineered, or are they design as you go, and how was the job bid? You might not have the authorithy to change engineered plans, and if so, why should you then have to eat the cost? Can't you do a change order? Todd
 

jtb

Senior Member
Location
Pennsylvania
Re: coil inrush

The photocell works the coil of a relay, so it won't see much current now. I have authority to change engineered plans with change order. Concern is time and equipment already purchased.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Engineer
Re: coil inrush

jtb,

Usually you do not need to consider contactor/relay coil inrush currents when chosing standard time-delay overcurrent protective devices such as circuit breakers. Size the conductors and OCPD using the coil "sealed" ratings

The coil inrush current should be used when calculating "inrush" voltage drop, most AC coils require 85% voltage to "pick up" and only 65% voltage to "hold in".
 
Status
Not open for further replies.
Top