coils
- Thread starter johnnebr
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- Wisconsin
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- PE (Retired) - Power Systems
Re: coils
You need to provide more information.
For example, a set of specifications on a 60A contactor coil, the inrush amps (cold coil):
.148A at 115V, coil resistance 785 ohms
.076A at 230V, coil reistance 3040 ohms
You need to provide more information.
For example, a set of specifications on a 60A contactor coil, the inrush amps (cold coil):
.148A at 115V, coil resistance 785 ohms
.076A at 230V, coil reistance 3040 ohms
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: coils
You really need to ask the manufacturer of the solenoid.
The final answer depends on how strong of a magnetic field is created by the solenoid. Factors affecting this strength, include the size and shape of the coil, and the size and shape of the magnetic flux "path".
You really need to ask the manufacturer of the solenoid.
The final answer depends on how strong of a magnetic field is created by the solenoid. Factors affecting this strength, include the size and shape of the coil, and the size and shape of the magnetic flux "path".
petersonra
Senior Member
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- Northern illinois
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- Semi-retired engineer
Re: coils
do dc coils have an "inrush"?
do dc coils have an "inrush"?
Re: coils
This is how I ubderstand it:
Please note that there is no inductive reactance as with AC so it's the resistance of the coil and as jim dungar stated, hole or cold coil which determines the resistance. That's as simple of a "rule of thumb" that you can get. Inrush? the current will rise to a point as determined be the simple resistance of the wire that makes up the coil which can vary depending upon the temperature of the coil. You see an AC coil has 2 things that are basically different from the DC coil. The first is the iron core that must be laminated to reduce the eddy currents that are caused by the alternating magnetic field which causes the core to heat. You see the iron core has resistance to the eddy currents that are induced which results in heat.
The second thing is the counter EFM generated by the alternating magnetic field opposes the AC current flowing through the coil. That's where petersonra's question is addressed. Inrush is that first 1/2 cycle of current as voltage is applied to the coil and the magnetic field is built up in one direction. As the current reverses the magnetic field also reverses which causes an opposing voltage or counter EMF which opposes current flow. Thus, after the first 1/2 cycle current is reduced to a given level dependent upon the "impedance" (AC resistance measured in ohms) of the coil. The same thing happens with motors and transformers.
This is how I ubderstand it:
Please note that there is no inductive reactance as with AC so it's the resistance of the coil and as jim dungar stated, hole or cold coil which determines the resistance. That's as simple of a "rule of thumb" that you can get. Inrush? the current will rise to a point as determined be the simple resistance of the wire that makes up the coil which can vary depending upon the temperature of the coil. You see an AC coil has 2 things that are basically different from the DC coil. The first is the iron core that must be laminated to reduce the eddy currents that are caused by the alternating magnetic field which causes the core to heat. You see the iron core has resistance to the eddy currents that are induced which results in heat.
The second thing is the counter EFM generated by the alternating magnetic field opposes the AC current flowing through the coil. That's where petersonra's question is addressed. Inrush is that first 1/2 cycle of current as voltage is applied to the coil and the magnetic field is built up in one direction. As the current reverses the magnetic field also reverses which causes an opposing voltage or counter EMF which opposes current flow. Thus, after the first 1/2 cycle current is reduced to a given level dependent upon the "impedance" (AC resistance measured in ohms) of the coil. The same thing happens with motors and transformers.
Re: coils
OK now,
The phenomenom which creates inductive reactance is present in any circuit. That is, the back emf of an inductor is given by,
e = - L*di/dt
From this equation, we derive inductive reactance for AC circuits.
But, if we apply a step function voltage, E, to a pure inductor, the resulting current is,
i(t) = Et/L, a ramp function with no limit
If the inductor has resistance, the current is,
i(t) = (E/R)(1 - exp(-tR/L), current is limited to E/R
Ignoring inrush current of course!
[ October 21, 2005, 10:43 PM: Message edited by: rattus ]
OK now,
The phenomenom which creates inductive reactance is present in any circuit. That is, the back emf of an inductor is given by,
e = - L*di/dt
From this equation, we derive inductive reactance for AC circuits.
But, if we apply a step function voltage, E, to a pure inductor, the resulting current is,
i(t) = Et/L, a ramp function with no limit
If the inductor has resistance, the current is,
i(t) = (E/R)(1 - exp(-tR/L), current is limited to E/R
Ignoring inrush current of course!
[ October 21, 2005, 10:43 PM: Message edited by: rattus ]
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