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buzzp (Electrical) (OP)
29 Sep 04 11:40
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 (Industrial)
29 Sep 04 12:16
no
Skogsgurra (Electrical)
29 Sep 04 13:55
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 (Electrical) (OP)
29 Sep 04 16:18
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 (Electrical)
29 Sep 04 16:32
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 (Electrical)
29 Sep 04 16:37
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 (Electrical)
29 Sep 04 20:19
There can be short high current spikes due to interwinding capcitance.
Skogsgurra (Electrical)
30 Sep 04 3:57
Yes, and cable capacitance...
busbar (Electrical)
30 Sep 04 10:54

Now, when the contacts that switch the coil open, it gets interesting.
  
buzzp (Electrical) (OP)
30 Sep 04 11:18
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 (Electrical)
30 Sep 04 12:28
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 (Electrical)
30 Sep 04 13:00
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 (Electrical) (OP)
30 Sep 04 16:00
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 (Electrical)
1 Oct 04 12:59
You're correct buzzp. Long time since school. And V=C dv/dt
sreid (Electrical)
1 Oct 04 15:07
Or better yet,

I=C dv/dt
buzzp (Electrical) (OP)
1 Oct 04 16:19
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 (Electrical)
1 Oct 04 17:10
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 (Electrical) (OP)
1 Oct 04 19:09
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 (Electrical)
1 Oct 04 20:07
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 (Electrical)
2 Oct 04 4:40
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.
ScottyUK (Electrical)
2 Oct 04 4:52
Hi logbook,

Are you using one of the low-cost Hall-effect transducers? I have seen this problem before:

Some of the range of current probes and transducers manufactured by LEM-HEME have excellent bandwidth specs, but are prone to dV/dt pickup from the conductor under test. High dV/dt shows up in the output as apparent transients in the current. I first encountered this effect when making measurements on a current-source inverter, and at initially I believed I was seeing very high dI/dt in the circuit in spite of the presence of the huge inductance inherent in the design of a CSI. This defied any kind of reasonable explanation, and was ultimately disproven using one of the excellent (and expensive!) Tektronix AM-503 current probe amplifers and a P6304 (perhaps) probe. The Tektronix probe is virtually immune to dV/dt pickup, and showed the Hall-effect probe's shortcomings very clearly. The link between dV/dt on the conductor under test and the transients in output of the Hall-effect current probe was clear when conductor voltage was displayed alongside the output from both types of current probe on a multi-channel 'scope.

The Hall-effect probes can be made to perform better in terms of dV/dt immunity by a couple of tricks: monitor current on the 'earthy' conductor if possible - whether it is a DC return to ground, or an AC neutral conductor. This reduces the problem at source by keeping dV/dt small. The other trick is to create an earthed electrostatic screen between the conductor and the probe. I use the self-adhesive copper foil sold for EMC purposes, with a drain wire down to chassis ground. The foil can be applied to either the conductor if it is insulated, or to the bore and end faces of the probe. Do not totally cover the probe in this foil, or you will create a shorted turn through the ferrite core of the probe. With high current, high frequency  measurements this can get interesting, as a colleague found out: the probe casing melted!

Hope this helps anyone using these otherwise good low-cost devices.

As an aside, anyone who uses the Tektronix current probe amplifier in an environment where high external magnetic fields are present, such as an inverter with large air-cored inductors, a word of caution:

The AM503 amplifier used with the current probe has an earthed chassis, as has the 'scope. The two are interconnected by a 50 ohm coaxial cable. A classical ground loop is formed by the protective earthing conductors in the mains power leads and the wiring of the building. Sketch out the ground conductors and you will understand. The output of the AM-503 amplifier is very low, and there can be enough current induced in the ground loop which adds (or subtracts) from the real signal to give some wierd results on the 'scope. I used an isolation transformer for the amplifier and removed the power ground connection, thus breaking the loop. The problem went away. Make sure you re-instate the ground connection when you are finished, and confine this sort of thing to controlled test environments.


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

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

logbook (Electrical)
2 Oct 04 5:05
ScottyUK,

Thanks for the info. Of course I knew all that, and have done that sort of thing before. I just didn’t bother doing the screening on the probe because I know which signals are real and just ignore the measurement artefacts. The probe is a battery powered LEM PR30 (I think).

Now that you mention it, I was measuring on the power rail, the coils being switched to earth, so the dV/dt shouldn’t be there anyway. That just means the probe is even more rubbish than I thought. It could even be picking up in the output lead. As I said, I could hunt it down a bit and reduce the noise spikes, but for the purposes of the tests at hand I didn’t have either the time or the interest, having seen it all before
cbarn24050 (Industrial)
2 Oct 04 11:16
If you have a 7000 series scope fitted with a 7a13 dif comparator that will eliminate all the noise from your hall effect probe.
ScottyUK (Electrical)
2 Oct 04 20:50
And it will do that how....?

The Tektronix probe in question is an integrated unit producing a single-ended output, so the use of a diff. amp will not get rid of anything, unless you break the ground loop by disconnecting the coax at one end or the other and connect the diff. amp between the probe ground and its output.

All the dV/dt-related noise induced into the LEM type probe will still very much be there so far as I can determine, irrespective of whether you use a single-ended or diff. amp at the 'scope. If you know better from practical experience, I'd be delighted to know how it's done, even if I had to buy a different 'scope. Thanks in advance.



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

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

cbarn24050 (Industrial)
3 Oct 04 10:10
Hi Scotty, I must have missed something, I thought we were having problems with hall effect clamp on probes not the Tek probe. Allmost all noise is common mode pickup the diff amp removes it very well.
ScottyUK (Electrical)
3 Oct 04 10:36
Hi cbarn,

The Hall-effect probes give a single-ended output too. High dV/dt causes a problem through capacitive coupling into the circuitry within the probe, which then manifests itself at the output. It isn't pickup in the connecting cable from the probe to the 'scope: I know because I removed the cable and connected a precision diff. amp direct to the transducer, and it still gave lousy results.

I don't think the diff. amp can help with these pre-manufactured probes and transducers because the problem appears to be internal to the probe's signal conditioning circuitry, although it beats me why LEM can't just fix the damned design in the first place. If you had a raw Hall-effect device with no signal conditioning then I guess you could use differential techniques. I've never done any design with raw Hall-effect devices so I'm not sure, and given my current role I'm not likely to in the immediate future. Have you tried playing with them at some point?

Does anyone know if Hall-effect devices are themselves inherently susceptible to capacitive effects i.e. is the high dV/dt directly affecting the sensing element, or is the conditioning circuitry just poorly designed?

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

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

cbarn24050 (Industrial)
3 Oct 04 14:21
Hi Scotty, I use 1 of those clamp ons made by Beckman and it certainly works. I not relly sure quite where the noise comes from, its not cable pickup as it's wide band noise, but it is common mode. There is no problem with the output being single ended, you just connect the shield to the amps negative input, you could do the same thing with the tek probe as well, you could make an adapter with a bnc socket and 2 bits of wire.
ScottyUK (Electrical)
4 Oct 04 8:24
I think we're looking at two different problems, but it is useful to know that the Beckman probe appears to have better immunity to the dV/dt problem than the LEM offering. Which model is it that you are using?

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

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

cbarn24050 (Industrial)
4 Oct 04 10:42
Hi Scotty, it's a CT233
buzzp (Electrical) (OP)
4 Oct 04 12:17
I am sure the hall-effects are susceptible to any field. Hall effects work by applying a fixed, know current across the sensor, say from top to bottom, then from left to right, the field you are measuring will distort the current coming from the other direction causing a voltage difference. This is what is measured.
Gotta go, post more later.
buzzp (Electrical) (OP)
4 Oct 04 13:35
Current probes (not hall effect) tend to dampen out transients so this method would not be the first choice in determining the peak of any inrush current. The ideal method would be to use a calibrated shunt.

Hall-effects, by themselves, have no inherent inductance (although the torroid will, if I can use that term since the torroid is really gapped for the hall effect to be placed) so they are more sensitive to transients than a typical torroid CT and have a wider current range due to the much larger saturation current required with a gapped core. Any field that is not perpendicular to the face of the hall effect will not have much effect on the readings. I believe you seen a large current due to the intial charging of the core of the CT, not due to fields near the current probe. These would not have much effect, assuming a design as I spoke about above.   

It would be interesting to perform the same experiment with a shunt.  

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