DC coil in rush current?

[buzzp]

My question: do DC coils (for solenoids - not necessarily relays) have an inrush current associated with them (due to the core)? I know, it seems like an elementary question but if my memory serves me right, I recall some manufacturers literature that claimed they do, as much as 30 times the continuous current rating.

 

[cbarn24050]

no

 

[Skogsgurra]

Well, no - and yes. But never anywhere near 30 times. No way.

But, in older DC motors with massive poles and massive frame, there was a certain amount of eddy current flowing when you switched a DC source onto the field winding. It seldom amounted to more than ten or twenty percent of rated DC current.

Another possibility is that your source was talking about an "accelerated" DC coil. They were sometimes used to be able to pull in large DC operated contactors. Their function was simply a series resistor thet was shorted by a "late opening" N.C. contact. The current could be quite high since it tokk some time to accelerate the armature so that the N.C. contact opened to reduce the current to low holding current. Sometimes, PTC resistors could be used. But they were not as reliable as the shorted series resistor.

Any other situations where an "inrush" DC current exists is hard to imagine.

 

[buzzp]

Skogs,
My concern is over DC coils used in old ITE breakers (trip coil and closing coil). What you describe in your second paragraph may be an issue with the 'closing coil' on this breaker. As there is also a 'holding coil', which is energized right after the closing coil picks up. Although, they are seperate coils.

What your talking about is one coil your simply reducing the current via the resistor. That makes sense and I would agree with that.

Would you agree that a DC coil with DC voltage applied would not see any current above V/Rcoil upon energization? If the current can rise above this level, is it the magnetizing of the core (or armature in this case) that would cause the excessive current?

 

[OperaHouse]

I've actually loked at this when I was designing a high speed solenoid driver. The current ramps up slow enough that you can easily see it on a scope. This technique also allows you to see when the core pulls in because of the change in slope. The DC coil contactors I have seen have two coils and one is switched out when the core pulls in. This played havock with our power supply and required putting an enormous cap on it to supply the surge current.

 

[Skogsgurra]

buzzp,

Your last sentence: Yes. Agree. Curent rises like

I*(1 - exp(-t/T))

where I is U/R and T is L/R

The current starts with slope I/T and approaches I without any overshoot. The magnetization requires no extra current, the energy in the field is built up during the first four or five T's - the rest is purely resistive loss.

 

[sreid]

There can be short high current spikes due to interwinding capcitance.

 

[Skogsgurra]

Yes, and cable capacitance...

 

[busbar]

Now, when the contacts that switch the coil open, it gets interesting.

 

[buzzp]

I understand the events when the coil is switched open.

Does anyone believe that one of these old, heavy duty coils, would have enough interwinding capacitance between the windings to cause a current well above the normal continuous current of the coil? I guess I could see cases where this would happen, especially due to the top layers of the coil since the resistance (current limiting) is reduced on these top layers. Umm.

 

[ScottyUK]

The interwinding capacitance involved must be tiny.

If you ever made a capacitor in physics class at school out of two pieces of foot-square aluminium foil with a bit of cling film between them and measured the resulting pitiful few nF, you will understand. The inrush would be over in microseconds or less; I doubt you would see it without a fast storage scope. The energy transfer would be very small, so I don't see it causing any thermal damage problems for contacts. Cable parasitic inductance must act to limit any inrush too, if we are intent on chasing down all the possible stray effects.





----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[Comcokid]

The old formula L=di/di says it all. Inductors resist instantaneous changes in their current. When you first switch on the voltage, current will build up slowly. When you open the circuit, the inductor will try to keep the same current flowing, which makes the voltage spike.

Current will decrease after a few seconds due to the self-heating of the copper wire in the coil which results in a change in the coil resistance (for copper 0.00393 / degree C). You might see the current slowly rise to a max value and then even more slowly drop. Additionally relay armature closure or solenoid plunger travel represents a change in inductance, which will affect current a finite time after the coil is energized.

 

[buzzp]

I think you mean V=L di/dt. This is easy enough to understand for a simple series circuit. Start adding in parallel capacitances and such then the current out of the source will be affected but not the current through the inductor. Ever apply DC current to a cap and watch the current? It appears like a short circuit until the cap starts charging. This is why current limiting resistors should be used on caps in circuits where the current is not limited already. Without more info on the coils (wire size, number of layers, armature) it is impossible to say. I believe it is not enough to worry about and intend on testing these to find out when we take the unit out of service.

 

[Comcokid]

You're correct buzzp. Long time since school. And V=C dv/dt

 

[sreid]

Or better yet,

I=C dv/dt

 

[buzzp]

Hehe, we know what you mean. Its amazing how much you forget when you dont use it.
I thank all of you for your input.

 

[logbook]

I have been looking at solenoid coils recently and you do see a massive inrush current when the voltage is first applied. This transient rapidly dies and you then get the ramping up of current required by E= L*di/dt. After that the current may start increasing faster than a ramp due to saturation of the core, or it may start slowing down due to the current limiting effect of the winding resistance.

However, please note that this splurge at the start of the waveform is not really there. It is a measurement problem with the current probe. The large voltage spike created when the coil is energised gets through to the current probe output and causes the problem. A better current probe that is shielded against the electric field is needed.

 

[buzzp]

Logbook,
How did you 'see' this transient? It must have been with a scope. Did you use a clamp-on probe or did you use a shunt and measure the voltage across it?

"The large voltage spike created when the coil is energised
gets through to the current probe output and causes the problem. A better current probe that is shielded against the electric field is needed."

Was the probe located right next to the coil or something? I can't imagine this being an issue with your measurement unless it was right next to the coil. Also, what kind of voltage spike are you seeing in relation to that voltage applied?
What is the voltage of the coil? What is its normal function (contactor, actuator, valve, etc)?

"I have been looking at solenoid coils recently and you do see a massive inrush current when the voltage is first applied. This transient rapidly dies and you then get the ramping up of current required by E= L*di/dt. After that the current may start increasing faster than a ramp due to saturation of the core, or it may start slowing down due to the current limiting effect of the winding resistance."
Why is there a voltage spike? If there is, then by the resistive properties you could argue there is a corresponding current spike. However, the inductive properties would limit the current rise. And on and on.
The inductance will limit the current rise no matter what, we all agree on that. However, the current amplitude is what is being debated. It will be interesting to hear how you made your measurement for sure.

 

[sreid]

Solid iron coil cores don't have much AC permiability due to eddy current generation (the cores either need to be laminates or powders). This means for a step change in voltage the coil has less initial inductance. Solenoid coils are usually wound with enough turns that the change is not significant (big inductance to really big inductance).

 

[logbook]

Buzzp,

The current was measured using a LEM clamp meter into a scope. Sensitivity 100mV/amp. The coil is only 1 inch long and 1mm in diameter and the current probe was a good 6 inches away. The coils take an amp when energised but direct magnetic interaction is unlikely. The energising voltage is a fast 30V edge. The current splurge at the beginning is huge (at least an amp). Now the coil driver was powered from a bench supply, isolated from earth. When the driver circuit was earthed to the scope to get a trigger signal the splurge got worse, as you would expect from the capacitive coupling model.

Like I said, the scope is calibrated to read current. It displays a large current transient when the coil is first energised. This current is fictitious being due to the electric field being applied to the current probe. You get these spikes a lot and they just have to be ignored as they are not really there in the circuit.