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COIL INRUSH CURRENT

 
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user9253



Joined: 28 Mar 2008
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Location: Riley TWP Michigan

PostPosted: Thu Jun 14, 2018 4:07 am    Post subject: COIL INRUSH CURRENT Reply with quote

Below is a quote from a VansAirforce post. Why is the inrush current higher than steady state current? Inductors oppose any change in current. Why doesn't that opposition limit the inrush current? I am not questioning the accuracy of the measurements, just trying to understand the laws of physics.
Quote:
All of this is for the four-post starter solenoid (ACS 11-03162):
Without flyback diode: -510V peak pulse
Current through flyback diode: 6.3A decaying over about 40 ms
Coil inrush current: 7.3A
Coil steady state current: 3.6A


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charlesdavis(at)iuncapped
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PostPosted: Thu Jun 14, 2018 4:51 am    Post subject: COIL INRUSH CURRENT Reply with quote

From what I understand, once there is an iron core inside the coil, the
current through the coil will decrease, if the coil is energised for a
longish period without the iron, it will overheat - have seen this many
times with solenoids controlling gas in refrigeration systems where the
coil housing melts to destruction if not fitted onto the solenoid properly

Charles
On 14-Jun-18 2:08 PM, user9253 wrote:
Quote:


Below is a quote from a VansAirforce post. Why is the inrush current higher than steady state current? Inductors oppose any change in current. Why doesn't that opposition limit the inrush current? I am not questioning the accuracy of the measurements, just trying to understand the laws of physics.

> All of this is for the four-post starter solenoid (ACS 11-03162):
> Without flyback diode: -510V peak pulse
> Current through flyback diode: 6.3A decaying over about 40 ms
> Coil inrush current: 7.3A
> Coil steady state current: 3.6A

--------
Joe Gores


Read this topic online here:

http://forums.matronics.com/viewtopic.php?p=480876#480876




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nuckolls.bob(at)aeroelect
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PostPosted: Thu Jun 14, 2018 5:55 am    Post subject: COIL INRUSH CURRENT Reply with quote

At 07:08 AM 6/14/2018, you wrote:
Quote:
--> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>

Below is a quote from a VansAirforce post. Why is the inrush current higher than steady state current? Inductors oppose any change in current. Why doesn't that opposition limit the inrush current? I am not questioning the accuracy of the measurements, just trying to understand the laws of physics.

> All of this is for the four-post starter solenoid (ACS 11-03162):
> Without flyback diode: -510V peak pulse
> Current through flyback diode: 6.3A decaying over about 40 ms
> Coil inrush current: 7.3A
> Coil steady state current: 3.6A

It's probably vagaries of semantics and
test setup. This is why it's important to
publish schematics of test setups, screen
shots of traces and showing all the
steps in the math.

We KNOW that unlike the charging profile
for a capacitor (infinite onset current
flow) the inductor has a zero onset
current flow. The begs explanation
for the author's definition of 'inrush'
current.

I'm guessing that the 7.3A figure was
measured at the greatest stabilized
current observed after application of power.
This would be the t=L/R plot coming
asymptotic to I=E/R several time constants
after switch closure. Not what we normally
refer to as inrush current. I'm guessing
that the 'steady state' value is due to
coil heating up after an extended period
of power on.

Here's what the numbers look like for
a continuous duty battery contactor:

https://goo.gl/gvX4mu

https://goo.gl/cpzthL

The starter contactor may well present
a higher ratio of cold to hot because it's
an intermittent duty device suffering a
greater copper temperature rise than the
battery contactor.

The 6.3A figure for diode current is
probably taken after a short but stabilized
contactor coil current of that same value.
The initial discharge of a capacitor is
the same as the voltage to which the capacitor
is charged. Discharge of the inductor yields
and initial current flow equal to that which
was flowing in the inductor . . . i.e. the
contactor was de=energized at 6.3A. Hence
catch diode current would peak at 6.3A
and decay along the t=L/R plot.

Without seeing the test setup and plots, it's just
guessing . . .






Bob . . .


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PostPosted: Thu Jun 14, 2018 6:04 am    Post subject: COIL INRUSH CURRENT Reply with quote

At 07:50 AM 6/14/2018, you wrote:
Quote:
--> AeroElectric-List message posted by: Charles Davis <charlesdavis(at)iuncapped.co.za>

From what I understand, once there is an iron core inside the coil, the current through the coil will decrease, if the coil is energised for a longish period without the iron, it will overheat - have seen this many times with solenoids controlling gas in refrigeration systems where the coil housing melts to destruction if not fitted onto the solenoid properly

Adding a core material does INCREASE the
inductance of a coil. The time constant
for charging an inductance is t(seconds)=
L(henries)/R(ohms).

Adding the core increases L but the resistance
is unchanged. Hence, t= goes up. I.e. time to
charge the coil is longer. Keep in mind that
the values of t are on the order of tens of
milliseconds.

Heating is purely a function of the DC resistance
of the coil and the applied voltage which
has nothing to do with the magnitude
of inductance. The time constant for heating
is about 1000 times longer than for current
rise. I.e. measured in minutes.

The classic definition of 'inrush' is that
current which flows the instant power is applied
to the device. For capacitors, inrush is very
large, for inductors it is zero.



Bob . . .


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PostPosted: Thu Jun 14, 2018 6:25 am    Post subject: COIL INRUSH CURRENT Reply with quote

If we are thinking of similar "longish periods" perhaps the iron core is
just conducting heat away perhaps assisted by the refrigerant flow.
Ken

On 14/06/2018 8:50 AM, Charles Davis wrote:
Quote:

<charlesdavis(at)iuncapped.co.za>

From what I understand, once there is an iron core inside the coil,
the current through the coil will decrease, if the coil is energised
for a longish period without the iron, it will overheat - have seen
this many times with solenoids controlling gas in refrigeration
systems where the coil housing melts to destruction if not fitted onto
the solenoid properly

Charles
On 14-Jun-18 2:08 PM, user9253 wrote:
>
>
> Below is a quote from a VansAirforce post. Why is the inrush current
> higher than steady state current? Inductors oppose any change in
> current. Why doesn't that opposition limit the inrush current? I am
> not questioning the accuracy of the measurements, just trying to
> understand the laws of physics.
>
>> All of this is for the four-post starter solenoid (ACS 11-03162):
>> Without flyback diode: -510V peak pulse
>> Current through flyback diode: 6.3A decaying over about 40 ms
>> Coil inrush current: 7.3A
>> Coil steady state current: 3.6A
>
> --------
> Joe Gores
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=480876#480876
>
>
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