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.
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DC coil in rush current?
- Thread starter buzzp
- Start date
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!
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!
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
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
If you have a 7000 series scope fitted with a 7a13 dif comparator that will eliminate all the noise from your hall effect probe.
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!
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
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.
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!
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
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.
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!
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
- Thread starter
- #30
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.
Gotta go, post more later.
- Thread starter
- #31
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.
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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