coil overheating in nema 1 starter

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tonyou812

tonyou812

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why would a 24 volt coil on a nema 1 starter powering a 3 phase (480 volt, 12.9 amp connected load) motor keep burning out? The motors are cooling fans for 3 generators. And all three sets of starters burned out withing 1 min. there was a reading of 25.2 vots on the coil.
 
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AC coil receiving DC?

What's the source of the 24V control power, and how far away is it?

Did you read the tag on the coil for yourself, to see what particular coil it is?
 
Since you have three pieces of equipment, is it possible that 3 separate sources of 24V AC power got connected together at some point in the wiring? That might measure 25 on a regular meter, but have a pretty high RMS value. Just some guesses.
 
the AC DC dilemma is always a good one to start with. I had a panel builder build some panels for a water plant once and he used all AC relays (160 of them) instead of DC . Needless to say no one caught it until panel checkout (at the end of a long shutdown weekend)as soon as I energized the panel they went poof . It took a lot of scrambling and phone calls to find that many relays on a Sunday afternoon . Luckily the checkout as 100% after replacing them.
 
tonyou812 said:
why would a 24 volt coil on a nema 1 starter powering a 3 phase (480 volt, 12.9 amp connected load) motor keep burning out? The motors are cooling fans for 3 generators. And all three sets of starters burned out withing 1 min. there was a reading of 25.2 vots on the coil.
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If the starter is not "sealing" correctly it will eventually overheat the coil.
Things that might cause the starter to pull in completly include:
Rust on the magnet faces
Worn/poor lubricated mechanism parts
Dust/or dirt jamming the mechanism
Low voltage to the coil

When did you measure the voltage of 25.2V?
With all of the starters off? When all three starters are turning on? After all three starters are on and sealed?
 
Well that was the question of the day. I asked the representative from asco what the coil was rated for and it only said 24 volts. the first set of starters were a french company for the starters and a czech company for the overloads. the guy said an ac coil can accept dc but a dc cannot accept an ac coil. I personally would think that it would need to be ac current but im not sure.
 
The standard coil voltage used in a lot of EU equipment is 24VDC. In fact, they look at us funny when we say we use 24VAC for controls. If you order a "24V coil" in Europe without specifying AC or DC, you will get DC because that's what they are used to. If you were to ask for a 24VAC coil, you would have to wait for it to be made, or shipped back from the US.
 
ac /dc coil

ac /dc coil

what is the difference in a dc coil from ac coil. and how do the windings differ.
 
tonyou812 said:
... the guy said an ac coil can accept dc but a dc cannot accept an ac coil. ....
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domnic said:
what is the difference in a dc coil from ac coil. and how do the windings differ.
Click to expand...
DC coils have a lot more turns and smaller wire than AC coils. The resistance of a DC coil is all that limits the current. AC coils have inductance to help limit the current.

Tony -
I think the "guy" was goofed up. DC coils will operate on AC, but they buzz a lot. DC coils don't have a shading coil to dampen the AC ripple.

AC coils will burn up on DC. They don't have enough resistance to limit the current.

carl
 
Just a quick thought

Just a quick thought

Is there a pilot light attached to the coil wire that indicates the starter is energized? I have seen shorted pilot light receptacles take out control fuses and control transformers. Edit: This would only be the problem if the light and coil is wired in series.
 
What is the coil fused at?

Its probaly what JD said about not sealing correctly. IOW, the armature is not being pulled all the way into the magnetic field of the coil. If it was not fused correctly the coil very well could destroy itself.

If it is fused correctly then it probaly is a specification issue with the coil.

Let us know what you found.
 
As others suggest, AC coils used on DC will draw a greatly excessive current and be rapidly destroyed, this could certainly happen in one minute.

If the contactor is not fully pulling in due to dirt or rust, then that would also cause excessive current and coil failure, with an AC coil.
However in my experience, the extra current is not so great as to destroy the coil quickly, it seems to be more of a long term problem.

DC coils can not be damaged by the contactor not fully pulling in, the current is limited by the DC resistance which is not influenced by the closure or otherwise of the magnetic circuit.

"universal" contactor coils that work on AC or DC are available here in the UK and presumably elswhere. They are a DC coil with a bridge rectifier built in, therefore the coil only sees DC, even if the supply is AC.

They are used when silent operation is required, and to reduce spares stockholding, since the same item can replace an AC or DC coil.
 
Thanks everyone....Im pretty sure it was due to them using DC on ACcoils. Two different sets of controlers (all AC coils on DC) burnt up after about two-Three minutes after the controlers were turned on. Is it common for there to be DC current in the system?
 
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The common practice here in the UK is to use 24 volts, sometimes AC sometimes DC for small control relays and pilot lamps, large power contactors generally have line voltage (230 volt) coils, controlled by the small 24 volt relays.

Lower voltage contactor coils certainly exist but are not common.
 
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