Inrush current with resistance??
- Thread starter mull982
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zog
Senior Member
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- Charlotte, NC
mull982 said:
No such thing as pure resistance. The wire has both Xl and Xc which will contribute to inrush. However, it is hardly noticable in a light bulb, all depends if you are talking theory or being realistic.
zog said:
Zog
I see your point about theory. I guess I am talking more realistic. Realistically would a light bulb circuit show any kind of inrush that could trip a breaker?
I understand the cable may have Xl and Xc characteristics, however the light bulb itself is purely a resistive load correct?
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No. The phenomenon of "inrush" takes place because of the energy that gets stored in the magnetic field of an inductive load or in the electric field of a capacitive load. A load that has neither inductance nor capacitance has no means of storing energy.mull982 said:
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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Actually, a light bulb happens to not be a linearly-resistive load; not due to reactance, but due to the fact that filaments have a much lower resistance when cold.
Because of that, incandescent bulbs do have a relatively high inrush current, and that's also why they usually blow upon turn-on. A bulb can be used as a current limiter.
Because of that, incandescent bulbs do have a relatively high inrush current, and that's also why they usually blow upon turn-on. A bulb can be used as a current limiter.
winnie
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The term 'inrush current' gets applied to many different situations, often with different root physical causes.
The 'cold resistance' of an incandescent lamp is much lower than the operating resistance once the filament has warmed up. An incandescent lamp will draw approximately 10x normal current at the instant of start, quickly falling to normal operating current when the lamp reaches full operating temperature. This has nothing to do with inductance, but instead the fact that the resistance of the lamp changes, quite drastically, in a few AC cycles.
Lamps can also draw much more than expected current when the 'blow'. Sometimes an arc will form when the filament blows, and this arc can have much lower resistance than the filament itself, with some halogen lamps causing enough current flow to trip the breaker.
-Jon
The 'cold resistance' of an incandescent lamp is much lower than the operating resistance once the filament has warmed up. An incandescent lamp will draw approximately 10x normal current at the instant of start, quickly falling to normal operating current when the lamp reaches full operating temperature. This has nothing to do with inductance, but instead the fact that the resistance of the lamp changes, quite drastically, in a few AC cycles.
Lamps can also draw much more than expected current when the 'blow'. Sometimes an arc will form when the filament blows, and this arc can have much lower resistance than the filament itself, with some halogen lamps causing enough current flow to trip the breaker.
-Jon
electricalperson
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when i was in high school we had light bulbs in series with the test stations. when we had a ground fault or something like that the lamp would turn on. we would never trip the breaker. even when we used the right angle drills the lamp would turn onLarryFine said:
zog
Senior Member
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- Charlotte, NC
mull982 said:
Would it trip a breaker? No, we are splitting hairs here. But,even a lihgt buld is not purly resistive, look at the filliment, it is a little coil.
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electricalperson said:
In the old homes in Brooklyn we often had to find shorts. My father would use a 100 watt bulb and screw it into the old fuse panels where the problem was and we would work from there.
jrclen
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Dennis Alwon said:
My dad did the same thing and taught me.
gar
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080731-2040 EST
The comments on tungsten filament lamps are correct with respect to the temperature coefficient of resistance. Check the resistance of a 100 W bulb when cold with an ohmeter. It is about 10 ohms. Also while doing this cold test shine a 250 W heat lamp on the test lamp filament and you will see the resistance rise.
The peak of a 120 V sine wave is 169.7 V. If you close the switch to the bulb at that peak time, then the inrush current is 17 A. If you close the switch at a zero crossing the peak will be much smaller because the filament is starting to heat at lower voltages. Once steady state is reached the 100 W lamp will have a resistance of about 144 ohms, and the current will be about about 0.83 A. If you rapidly switch the lamp current on and off you will get an intermeadiate resistance.
An Edison carbon filament lamp shows much less change with temperature. At room temperature one lamp has a resistance of 297 ohms. At 120 V I = 0.62 A or 193 ohms. The wattage rating would be about 75 W. Note this is a negative temperature coefficient in contrast to the positive coefficient for tungsten.
Pure inductors do not not cause high inrush current rather they actually impede the flow of current. What confuses this issue is that a transformer or other magnetic core inductor may have a high inrush current. I will not explain why at this time. Many of you have enough to absorb without getting into a transformer inrush discussion.
A capacitor with no initial charge looks like a short circuit when a voltage is applied. Thus, it can cause a high inrush current. Actually a difference between the source voltage and the voltage on a capacitor will cause a high inrush current.
The coiling of the tungsten filament has no significant inductive effect in relationship to the lamp resistance at power line frequencies.
.
The comments on tungsten filament lamps are correct with respect to the temperature coefficient of resistance. Check the resistance of a 100 W bulb when cold with an ohmeter. It is about 10 ohms. Also while doing this cold test shine a 250 W heat lamp on the test lamp filament and you will see the resistance rise.
The peak of a 120 V sine wave is 169.7 V. If you close the switch to the bulb at that peak time, then the inrush current is 17 A. If you close the switch at a zero crossing the peak will be much smaller because the filament is starting to heat at lower voltages. Once steady state is reached the 100 W lamp will have a resistance of about 144 ohms, and the current will be about about 0.83 A. If you rapidly switch the lamp current on and off you will get an intermeadiate resistance.
An Edison carbon filament lamp shows much less change with temperature. At room temperature one lamp has a resistance of 297 ohms. At 120 V I = 0.62 A or 193 ohms. The wattage rating would be about 75 W. Note this is a negative temperature coefficient in contrast to the positive coefficient for tungsten.
Pure inductors do not not cause high inrush current rather they actually impede the flow of current. What confuses this issue is that a transformer or other magnetic core inductor may have a high inrush current. I will not explain why at this time. Many of you have enough to absorb without getting into a transformer inrush discussion.
A capacitor with no initial charge looks like a short circuit when a voltage is applied. Thus, it can cause a high inrush current. Actually a difference between the source voltage and the voltage on a capacitor will cause a high inrush current.
The coiling of the tungsten filament has no significant inductive effect in relationship to the lamp resistance at power line frequencies.
.
gar
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mull982:
You need to get some books on basic electrical components and DC circuits and study these. Then progress to AC circuits.
People with information on recent books may help you with suggestions. The Radio Amateur's Handbook might be a starting point.
.
mull982:
You need to get some books on basic electrical components and DC circuits and study these. Then progress to AC circuits.
People with information on recent books may help you with suggestions. The Radio Amateur's Handbook might be a starting point.
.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
If you ever get the chance to look at a filament under a microscope, you'd see that it's not just a coil, and it's not even a coiled coil. It's a coiled coiled coil.zog said:
gar
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mull982:
The answers may seem confusing to you.
If you had an ideal resistor, then its resistance would be invariant. In this case v = R*i on an instantaneous basis, transient or steady-state.
If inrush current means that there is larger current for a short time and the current settles down to some different steady value, then an ideal resistor does not exhibit an inrush current characteristic when a fixed steady voltage is applied to the resistor.
An old book on resistance is "Resistance and Resistors", by Charles L. Welland, McGraw-Hill, 1960. On p14 and 15 are tables of alloys and their temperature coefficients. Three alloys with very low coefficients are Karma, Advance, and Manganin. Nichrome is about 10 times worse, but still fairly good. Tungsten is 0.0045, copper is 0.0039, Nichrome 0.00015, and Advance +/-0.00002 . The units are percent per deg C. If Rt2 is resistance at temperature t2 and Rt1 at t1, then
Rt2 = Rt1 ( 1 + A (t2-t1) ) where
R is in ohms, t in deg C, and A is the temperature coefficient.
Most heating elements use Nichrome, power resistors may use different materials depending upon application requirements.
Tungsten is used in modern incandescent lamps and the operating temperature of the filament is very high. See
http://hypertextbook.com/facts/1999/AlexanderEng.shtml
Assume 10 ohms for my bulb of an earlier post and a 2500 deg rise, then from the above equation we predict a hot resistance of 10*2500*0.0045 + 10 = 122 ohms. I had calculated 144 ohm from 100 W and 120 V. I did not actually measure the hot resistance. To be more accurate I have just measured the current at 0.83 A at 122 V. Thus, I will still use the 144 ohms.
If the temperature coefficient is actually 0.0045 over this whole range, then I would predict the filament temperature rise as t2-t1 = (14.4 - 1)/0.0045 = 2977 deg rise. This may be somewhat high so to actually know the filament temperature we would need to determine its color temperature.
A normal device called a resistor operated within a limited temperature range would not generate an appreciable inrush current, but an incandescent lamp, which is a resistor, would exhibit inrush current when a fixed voltage was applied.
Let us know if you are understanding the comments.
.
mull982:
The answers may seem confusing to you.
If you had an ideal resistor, then its resistance would be invariant. In this case v = R*i on an instantaneous basis, transient or steady-state.
If inrush current means that there is larger current for a short time and the current settles down to some different steady value, then an ideal resistor does not exhibit an inrush current characteristic when a fixed steady voltage is applied to the resistor.
An old book on resistance is "Resistance and Resistors", by Charles L. Welland, McGraw-Hill, 1960. On p14 and 15 are tables of alloys and their temperature coefficients. Three alloys with very low coefficients are Karma, Advance, and Manganin. Nichrome is about 10 times worse, but still fairly good. Tungsten is 0.0045, copper is 0.0039, Nichrome 0.00015, and Advance +/-0.00002 . The units are percent per deg C. If Rt2 is resistance at temperature t2 and Rt1 at t1, then
Rt2 = Rt1 ( 1 + A (t2-t1) ) where
R is in ohms, t in deg C, and A is the temperature coefficient.
Most heating elements use Nichrome, power resistors may use different materials depending upon application requirements.
Tungsten is used in modern incandescent lamps and the operating temperature of the filament is very high. See
http://hypertextbook.com/facts/1999/AlexanderEng.shtml
Assume 10 ohms for my bulb of an earlier post and a 2500 deg rise, then from the above equation we predict a hot resistance of 10*2500*0.0045 + 10 = 122 ohms. I had calculated 144 ohm from 100 W and 120 V. I did not actually measure the hot resistance. To be more accurate I have just measured the current at 0.83 A at 122 V. Thus, I will still use the 144 ohms.
If the temperature coefficient is actually 0.0045 over this whole range, then I would predict the filament temperature rise as t2-t1 = (14.4 - 1)/0.0045 = 2977 deg rise. This may be somewhat high so to actually know the filament temperature we would need to determine its color temperature.
A normal device called a resistor operated within a limited temperature range would not generate an appreciable inrush current, but an incandescent lamp, which is a resistor, would exhibit inrush current when a fixed voltage was applied.
Let us know if you are understanding the comments.
.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
No, and don't call me Shirley! :grin:mivey said:
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