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17 comments
Active Contributor

Join Date: Oct 2006
Posts: 16

Contactor Inrush Current

04/04/2007 10:13 AM

I've a critical field problem repeated in many packaged air conditionning units; the problem is that the compressor's contactor receives the 24 V signal from the thermostat correctly on its coil terminals but it isn't able to pull close the contactor contacts ( eighter chattering or not moving at all ); This happened because of the inrush current caused by the contactor coil while energizing is making higher voltage drop on its contacts ( actual measured 15 V ); what are the practical solutions which could be offered to solve these problems

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Power-User

Join Date: Jan 2007
Location: Los Angeles, CA
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#1

Re: Contactor Inrush Current

04/04/2007 11:42 AM

Clean and make new all electrical connections, you may have a build up of oxidation that is preventing a proper working voltage

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Guru
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#2

Re: Contactor Inrush Current

04/04/2007 3:49 PM

What size and length is your cabling?

Typically you use the small size cable (telephone wire) that does the job quite well for short lengths but show these kind of problems as soon as you cross a certain length.

Did you measure at the level of the supply? Does this remain stable?

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Guru

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#3

Re: Contactor Inrush Current

04/04/2007 5:22 PM

You have to check the amp draw on the low voltage circuit...Typically this is under 1 amp....If it is well over this at say 6 amps, you have a shorted coil...But most likely you would know this because you would be cooking transformers.... If you are saying that the current is dropping to 15v from 24v when energized, then you don't have enough current available, would check the connections from the transformer...It sounds to me like you have a batch of bad contactors...Try a different brand, if they work fine, then it's the contactors...

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#4

Re: Contactor Inrush Current

04/04/2007 7:13 PM

Where are you obtaining you 24V feed from? Is it ac or dc.? You need to find a feed that is divorced from the load supply. If it is an ac contactor ( I mean the coil connection) Is it run off a separate transformer? If dc can you tweak the regulator? If none of this is within your remit try asking the manufacturer for their advice.

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#5

Re: Contactor Inrush Current

04/05/2007 12:38 AM

Most thermostats have anticipators, which are open wound variable resistances (resistance wire on a shaped phenolic strip, which are to be set to the thermostat current, which depends upon the relay or contactor used for the A/C or boiler). The anticipator is wired in series with the operating contacts, and is used to heat the bimetal strip before the room reaches the desired temperature. That is done to compensate for the lag in heating or cooling the room. Without the anticipator, the temperature will overshoot and/or undershoot.

Make sure the anticipator is set to the lowest resistance possible consistent with decent temperature control. If it needs to be too low in resistance, one option is to use a smaller relay as a pilot device to operate the contactor.

Hope this helps.

BernieK

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#6

Re: Contactor Inrush Current

04/05/2007 1:20 AM

I BELIEVE THE ANSWER TO YOUR PROBLEM IS: Here is what is happening. A contactor with an AC coil has a much lower reactance when the magnetic circuit is open (when the contactor is not energized). Once the armature of the contactor closes the reactance goes up due to the completion of the magnetic circuit that occurs when the armature moves to be in contact with the frame. So the solution to your problem is to use a shorter run of wire or a larger gage of wire to handle the inrush current. I had the same problem with a site a long time ago and burned up a couple of contactors (yes the coil can overheat since the "inrush" current will never drop if the armature does not close) before I figured out what the problem was. The resistance of the wire and the capacity of the transformer must be adequate to present at least the minimum required voltage to the terminals at the moment it is energized. Hope this solves your problem.

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#8
In reply to #6

Re: Contactor Inrush Current

04/05/2007 3:11 AM

You are 100% right; it seems that using lower gauge wires from thermostat to the contactor coils terminals is the main reason of this problem; today I'll check the status after using bigger size wires same length.

Also the transformer capacity maybe critical and needs to be some higher to overcome inrush current and primary voltage variation ( usually 210 instead of 220 V )

thanks

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#9
In reply to #8

Re: Contactor Inrush Current

04/05/2007 4:37 AM

Great! Let me know if that's the fix. It seems counter intuitive until you realize what is happening because it may actually draw two or three time the rated current but never operate which is of course bad for everything.

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#12
In reply to #8

Re: Contactor Inrush Current

04/05/2007 5:44 AM

The too long/to small wire are a common problem, often not recognised.

Attention: the coil could burn due this as the impedance keeps on being to low and the current to high.

You could replace the EM relay by a SSR type.

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#7

Re: Contactor Inrush Current

04/05/2007 1:45 AM

15V, drop? that is more than 50% percent. (you have indicated a 24V supply.)

It looks to me as if your relay power supply is the culprit. If this is a DC coil, you might have a rectifier problem If this is an AC coil, than check your step down transformer. .

Hope it helps.

Wangito.

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#10

Re: Contactor Inrush Current

04/05/2007 5:23 AM

There are also mechanical solution possibilities to consider:

The solenoid may not be strong enough. The springs on the contact switch may be too resistant. You may have permeability issues with the contactors.

You might try contacting a contact switch manufacturer, detailing your requirements, and allowing them to submit a sample or supply for your application. Or perhaps there is another type of thermostatic contactor array that can be used (eprom/robotic, bi-metal, mercury, etc.)

Mark

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#11

Re: Contactor Inrush Current

04/05/2007 5:33 AM

Could be a shorted turn on your contactor coil, Check the current draw by the coil, it maybe pulling the volts down.

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#13

Re: Contactor Inrush Current

04/05/2007 5:50 AM

This is an exact description of the control circuit unable to supply enough current to hold the coil energized. Check the contactor's coil power consumption against what the thermostat's output circuit can supply. I think you'll find it or the power supply it uses is not sized large enough. Funny, I was faced with this exact problem yesterday.

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Associate

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#14

Re: Contactor Inrush Current

04/05/2007 8:29 AM

Many times the compressor motor will pull down the voltage and the supply transformer sees a lower input voltage so its output voltage falls accordingly. What I have done in the past is to go to a 28 v transformer instead to boost the control voltage when the input voltage is low during start up. Most contactor coils can handle the extra voltage when the compressor motor is running with out causing any real problems. The extra voltage will help the contactor close faster and reduce arcing that also reduces the contactor life.

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#15

Re: Contactor Inrush Current

04/05/2007 8:37 AM

If this is a new system, most likely the voltage drop is due to undersized wires for the control circuit length. If it is an existing system, most likely will be loose connections.

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#16

Re: Contactor Inrush Current

04/05/2007 10:43 PM

Bernie Katz is correct!

Since this is a package unit, that tells me that the 24 vac control transformer resides within the unit. The 24 volt signal leaves the unit, travels to the thermostat, then returns to the unit, and the 24 volts is applied directly to the coil of the contactor. If the thermostat cable is less than 100 ft. long and the wire size is a 16 ga. or larger, then you should not have a voltage drop.

So the next logical cause of the voltage drop will be the anticipator (if it is a mechanical / analog thermostat). If you have a digital thermostat then the anticipator is not the problem.

Most residential or commercial HVAC units are of a size that would not require a very large contactor to start the compressor, the condensor fan and the evaporator blower. Most likely, a NEMA size 0 or size 1 for each device. The inrush current for each of these would not be enough to cause a voltage drop, unless the coil was grounded or shorted.

You might want to check the capacity of the 24 vac control transformer to insure that it is large enough to handle the current of all of the devices that are starting at once or in a sequence. The following sizes will give the current listed below, so take your pick:

50 VA = 3.2 amps

75 VA = 5 amps

100 VA = 6.25 amps

All values are for a 24 volt secondary.

Realistically, you should not need anything larger than the 75 VA, unless there is some other detail that has been left out of your description.

Good luck with this project!

Jeff

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Posts: 16
#17

Re: Contactor Inrush Current

04/07/2007 4:24 AM

Many thanks for all whom suggest solutions for this problem; Shortly I found many items are making this problem so complicated; 1) higher inrush current needed by the contactor coil to be energized; 2) using lower sized control wires to connect from thermostat to contactor's coil; 3) Many of field locations are having lower supply voltage problem ( 10 to 15 % lower rating ); and 4) in addition we are using a very fast response compresssor lockout electronic circuit which is locking out compressor if it doesn't energized one second after receiving its thermostat on signal.

- The solution was adding some modifications to our control circuit ( putting intermediate relay to be energized then energize contactor coil which helped me so much to neglect the effect of external low cross section wires.
- Also to overcome lower field power supplys we'll increase the power of control transformer.
In addition we'll instruct our customers to use reasonable control wires to connect between the thermostat and our unit's terminal block.
- Also we may use bigger size wires for internal control circuit to additionally decrease voltage drop due to inrush currents.
Thanks for you all

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