# Coil I Draw

Discussion in 'General Electronics Chat' started by qitara, Sep 19, 2013.

1. ### qitara Thread Starter Member

Jan 18, 2013
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Hi Guys

While measuring the current on a contactor I noticed that when the plunger is hold in the open position while the coil is energized, the current draw increases and when the plunger is realised (drawn to the closed position) the current draw is lowered ?

What am I missing ?

does the plunger cause a CEMF that lowers the current draw ? How

2. ### DerStrom8 Well-Known Member

Feb 20, 2011
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This question should not have been asked in the Programmer's Corner forum. You should have posted it in the General Electronics Chat.

Anyway, when you have the plunger inside of the coil, it creates a higher inductance which reduces the amount of current drawn by the coil. When you remove it, the inductance is reduced, allowing more current to flow. It's like connecting a resistor to a battery. A higher resistance will allow less current to flow, a lower one will allow more current to flow. The same goes for inductance.

Jul 18, 2013
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As noted regarding the inductance, you can burn the coil out by doing this.
The coil just presents just above its DC resistance when the armature is not present, the resistance of which is very low.
Max.

4. ### qitara Thread Starter Member

Jan 18, 2013
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I am getting a bit of the idea (Sorry I am not a PRO), could you please explain in a more detailed way, would be very thank full

P.S

I don't know how this thread got here, sorry for posting it in the wrong section.

Jul 18, 2013
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When an AC contactor coil or solenoid is energized, the current is very high due to the high inrush current caused by the low DC resistance of the coil, and the fact the armature has not yet pulled in.
When the armature is in, the inductive reactance (in Ohms) effectively lowers the current.
If you push the armature away from the coil in a contactor or solenoid when it is energized the inductive reactance drops drastically causing the high current which is now basically a result of the coil DC resistance value.
This does not occur in a DC contactor or solenoid as the higher coil resistance is in effect the whole time.
Max.

qitara likes this.

Jan 18, 2013
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7. ### DerStrom8 Well-Known Member

Feb 20, 2011
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Hi qitara--

Sorry, I just re-read my post to you and realized how rude it sounded. That was not my intention, so please accept my apologies. I'm sure if you PM one of the moderators they would be happy to move this thread for you

Best wishes,
Matt

8. ### qitara Thread Starter Member

Jan 18, 2013
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Nothing to worry about Matt, Thanks for the explanation to my question, I appreciate it.

Jul 18, 2013
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No, any AC coil or solenoid where the presence of the armature affects or relies on the Inductive reactance.
I always spec in DC solenoids on any new project, these devices are the most prone to someone pushing the actuator over when the solenoid is powered.
Due to the constant resistance, burn out does not occur as it tends to in AC type.
Max.

Last edited: Sep 24, 2013
10. ### #12 Expert

Nov 30, 2010
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It's about an inductor having a core, or not. Think about a transformer primary coil wound in the air. Then add an iron core. See the inductance go up? That is what reduces the AC current.

One hundred turns around 5 cubic inches of air will burn up when you apply 120 VAC, but when you add 3 or 4 pounds of transformer iron, the current goes down and the coil does not burn up.

Sep 30, 2009
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12. ### Brownout Well-Known Member

Jan 10, 2012
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You can try this expeiment. Remove the plunger and insert a screwdriver into the space. Now, you should see a lower current draw similar to when you had the plunger present. The screwdriver is now acting as the coil's core, and increasing the inductance. In fact, if you ever need to have the coil energized without the plunger present, this is a good way to prevent burning up the coil.

Jul 18, 2013
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In the early eighties, I was charged with the job of converting a factory assembly line from relay logic to PLC control, the Company was also trying to reduce maintenance down time costs.
A check of the parts inventory revealed whole shelves of replacement solenoid coils, (the assembly line relied heavily on Hydraulics).
It came to light that the mechanical maintenance had a habit of pushing the solenoid armatures over on activated coils when trouble shooting, it is then they experienced coil Burn out.
At retrofit time all coils were replaced with 24vdc type.
A check since then has revealed no more coils burn outs and the coil inventory is no longer required.

Moral: Always fit DC coils where possible.
Max.

14. ### qitara Thread Starter Member

Jan 18, 2013
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Have you seen such designs ?.

I have seen DC coils used in switchgear panels to trip the circuit that can't take a trip signal longer then 1,1 sec before getting burned, they usually got burned when some one kept the trip button pressed longer then 1 sec. why are these designed this way ?

any solution around such designs to prevent them from burning up ?

15. ### qitara Thread Starter Member

Jan 18, 2013
93
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Can induction heating be an example of this ?.

correct me if I am wrong, when there is no plunger present in the energized coil, the wound copper wire is not enough large to create a good/large magnetic field that will collapse and create resistance to current flow, but when the plunger is present the plunger gets induced current by the flow of current in the coil and creates its own magnetic field that will help resist current flow.