Coil Inrush Current 3

[rockman7892]

I understand that with a motor during inrush and starting the current is proportional to the voltage, and therefore any decrease in voltage results in an decrease in current. Obviously the opposite is true during running.

What is true for the current vs voltage relationship when energizing a coil such as a starter coil or transformer? Will the current be proportinal with the voltage when energizing a coil? In other words with decreased voltage will there be more current when energizing a coil or transformer? When a coil is in operation I understand that the current is then inversly proportionally to the voltage.

 

[Skogsgurra]

It depends on AC or DC. It also depends on what kind of coil you have. Not possible to answer without more facts.

But one thing is for sure, the coil current very seldom (never) goes down as voltage goes up. Why do you think that would be so?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rockman7892]

The types of coils I'm referencing are AC coils. To use and example I am talking about a starter coil for example on a size 4 starter. Somebody here at the plant was trying to tell me that a coil burned-up/failed becasue the rated 120V dropped below 120V and therefore caused more current to be drawn onto the coil thus burning it up.

Like I mentioned I know when motors are running voltage and current are inversely proportional because the motor is a constant machine, and was wondering if the same could be said about coils/transformers.

 

[Skogsgurra]

Yes. But that is because the plunge didn't pull in. Then the inductance of the coil stays at, or close to, initial inductance so that impedance never goes down to design value. Which in turn makes current stay high.

That is not a normal behaviour of a coil. That is bad use of a coil.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

When a coil is first energized there is typically a large air gap in the magnetic circuit. From this follows low inductance, low inductive reactance, low impedance and higher current. When the coil pulls in the air gap is eliminated. Inductance, inductive reactance, and impedance increase and current drops. Under conditions of low voltage there may not be enough amp turns or magnetic force to move the magnet armature. The impedance stays low, the current stays high and the coil fails.

Bill
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"Why not the best?"
Jimmy Carter

 

[roydm]

Skogsgurra & Waross are right on, I worked at one location where the 120V control transformers were center tap to ground. If for any reason a coil was fed 60VAC it would burn out in a matter of minutes.
In the DC equivalent the coil pulls a high current until the armature closes then a contact opens to introduce an "Economy Resistor" in series with the coil.
Roy

 

[rockman7892]

Thanks for the responses.

What about an AC transformer like RoyDMatson mentioned. With no moving or mechnical parts and a fixed impedance how does a control transformer or 3-phase transformer react to the conditions mentioned above.

Why would the 60VAC burn up the control transformer mentioned in the above example.

 

[Skogsgurra]

The current into a transformer - loaded or not - decreases when voltage goes down.

Is it possible that you confuse a switching power supply with a transformer? The current of a switchmode power supply usually goes up as voltage goes down. That is because it tries to deliver constant power.

Motors, single phase transformers, three-phase transformers or any of the examples you have mentioned do no increase current as voltage goes down.

Period.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Mobius44]

Rockman-
As I read RoyDMatson's example using 60VAC the coil failed, not the control transformer.

The voltage-current relationship is not always related as simply as you're trying to make it, sometimes increasing voltage causes the current to increase, and sometimes increasing voltage causes the current to decrease.

The voltage-current relationship depends on the load type and the amount of voltage change relative to the expected voltage. For example increasing an AC induction motor's voltage 5% may decrease the motor current, while increasing the same motor's voltage 20% may increase the motor current.

-Sean

 

[waross]

Small control transformers are often sized quite close to the load demand. When the coil is drawing excess current the transformer will be also. Often the coil fails first. Sometimes the overloaded transformer fails first.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

Just to elaborate, the project used centertapped 120 VAC, just one common transformer per MCC with individual X1, X2 fuses for each starter. We never had a problem with the transformers. Occasionally you would get a short to ground in the field which would blow the X1 fuse and provide a path to the coil and back to X2. Now the coil has 60 VAC instead of 120. Next time it tries to pull in the field is not strong enough but the current is high because the magnetic circuit is open and it burns out.
The centertap system also made it difficult to troubleshoot as you had 60 VAC on both sides of an open contact. Not a problem in the panels where you could use X2 for a reference but in the field you didn't kow if it was X1 or X2 voltage as you typically had to use ground as the reference
Roy

 

[rockman7892]

For a multi-tap transformer I notice that the current goes down at the taps are changed for higher voltage. Is this because the impedance changes when taps are changed.

For a transformer operating under normal conditions (say fully loaded 1500kVA) does the transformer not strive to keep a constant kVA, thus increasing the current with a decrease in voltage?

Skogsgurra

You mentioned that transformers, motors, etc to not exhibit an decreased current with an increase in voltage. I have always understood however that a motor is a machine that strives to keep a certain power output and therefore adjusts current with relation to voltage based on P=IV

 

[Skogsgurra]

Rockie

The reason that current goes down when you move the tap up is simply because the turns ratio is changed. Moving the tap up does not mean that you increase the input voltage.

The transformer does not "strive to keep a constant kVA". Why should it?

Motors do not normally (I would say never) run at higher current when voltage is decreased. What happens (three-phase induction motor) is that the slip increases somewhat and that the power factor increases when you reduce the voltage. Better power factor means lower motor current.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rockman7892]

So P=IV with P as a constant horsepower does not mean that I and V adjust proportionally to keep a steady P?

I understand what you are saying about slip however.

I also realize that when the motor is running and voltage is reduced, the torque will decrease proportionally with the square of the voltage. Less torque with then equate to less current. Is this correct?

I could have sworn that I even saw a table in the IEEE red book however that showed the current vs voltage relationship for a running motor being inversely propotional, and during starting being proportioinal. I dont have the book handy to site particulars.

 

[jraef]

Motors are moving dynamic machines. Things change in them based on load, speed, voltage, power factor etc. There is no "one-size-fits-all" relationship that covers all issues at all times.

Coils however are comparatively simple. P=I*E works fairly well AFTER it has pulled in. The only variable work load is in the pulling in process, in which case everything is dynamic for a brief moment.

So back to your original issue:
If a coil is already pulled in and the voltage drops, the coil current will indeed increase. But most coils in things such as motor starters are specifically designed to be able to pull in with 80% of rated voltage, and hold in with down to 70% of rated voltage without suffering significant damage. These values are described in the motor starter data as "Pull-In" and "Drop-Out" voltages.

So if your motor starter coil burned up while the motor was running (coil energized), then there is likely some other thermal issue going on because it SHOULD have held in until the voltage dropped to 70% without damage, below witch t would have just dropped out.

If on the other hand, the voltage was already low when you tried to energize the coil, it may very well have burned up in the attempt to pull in. What typically happens on large motor starters is that the motor trying to start with an already low voltage condition, causes additional voltage drop. The coil voltage then drops below the operating threshold, the coil lets go. The motor contactor opens, the load goes away, the voltage recovers, the coil trys to pull in again, the motor current causes another voltage drop, the coil lets go again, etc. etc. etc. This is called "coil chatter" and the coil current spikes extremely high, overheating it rapidly.

One main cause of this is a control circuit that does not in some what involve a "3-wire" control design. In a 3-wire circuit, the motor stater seals it's own control circuit in after a momentary Start command. If the voltage is too low and the coil drops out, the only way for it to re-energize is if someone gives it another Start command. If on the other hand the Start command is continuous (2-wire control), then the low voltage condition allows a continuous Start command and sets up the possibility of chatter.

 

[Mobius44]

A star for Jraef's fine answer.

I can only think of one thing to add; I suggest those looking for information about motors read a publication from Baldor called the Cowern papers, PR2525.

Here is a link to download it, on page 60 of the papers (page 66 of the PDF) there's a figure showing motor voltage variation's effect on FLA, efficiency, PF, etc.

http://www.baldor.com/support/literature_load.asp?LitNumber=PR2525

-Sean

 

[rockman7892]

Jaref

Great Explanation! Just to clarify about the coil, when it is operating below 100% but above 70% then it will indeed draw more current however the coil is designed to handle this current, and should drop out at 70% before it ever burns up? So when pulling in although dymanic can the same be said that current will be higher down to 80% voltage at which point the coil will not be able to pull in?

I'm assuming the same explanations you posted apply to transformers as well.

Mobius44

Thanks for the reference paper. I looked at that chart and you can see that when voltage decreases the FLA tends to increase however on the other side where voltage increases the current only briefly decreases before it starts to increase. I'm assuming this increase on the high side is due to the motor going into saturation? I also see that there are a number of other factors effected by voltage variation, and a blanket statement cannot cover them all like Jaref mentioned.

 

[jraef]

... when it is operating below 100% but above 70% then it will indeed draw more current however the coil is designed to handle this current, and should drop out at 70% before it ever burns up? So when pulling in although dynamic can the same be said that current will be higher down to 80% voltage at which point the coil will may not be able to pull in?

Yes, although I put emphasis on SHOULD and MAY in there. At any time you can have a defective device of course, and every manufacturer's product tolerances are a little different. These values are in NEMA and other industry standards to which products are designed.

I'm assuming the same explanations you posted apply to transformers as well.

Not exactly. There is no moving armature in a transformer (hopefully) so there is no dynamic reaction in that aspect. But certainly load changes, line changes and temperature have a lot to do with how transformers react and interact. But remember, a transformer is not supposed to be a LOAD. Its only real job is conversion, so EVERYTHING a transformer does (or doesn't do) is dependent upon the downstream load. For example, if the line voltage drops 20% but there is nothing hooked up on the load side, did the load current really change?