|Posted: Mon May 28, 2018 7:02 pm Post subject: Is the OVP 5 A CB supposed to trip when I switch off the
At 03:03 PM 5/28/2018, you wrote:
With the screen shot of the O scope...Â Â
What signal did you use to trigger the scope (capture the screen shot)?
The 'M'...Â Is that for Manual Mode?
Where do you have the leads attached?
Where are you measuring the ~3 ms from & to?
What is your definition of: "Fire across contacts" and Fire goes out here"?
Referring to the article at:
The first figure is the performance baseline for
the contactor . . . a Stancor/White-Rogers 70
series, 12v, 'whisky-barrel'.
One terminal of the coil is grounded. The other
terminal gets 12vdc applied through a switch.
The scope Ch1 reads votlage across the coil.
Trigger is ac coupled for a negative going
excursion of the coil voltage (initiated by
first opening of the controlling switch). Yes,
"M" is for manual mode (single sweep with manual
reset for next trigger). Using the 4th trace
from the article we see that the first major
event (the field collapse transient) begins
at the trigger point (little black arrow at top)
and is pretty much over 6 major horizontal
divisions at 0.5mS per division.
The first figure of the article is the baseline coil
collapse signature for that contactor. For this
measurement, I needed a switch with a VERY fast contact
spreading velocity. As the article states, this was
achieved with a piece of 22awg wire strung between to
c-clamps and tensioned with a bunch of rubber bands.
The switch was 'opened' by simply cutting the wire.
You will note that dv/dt (slope) for Emf during field-collapse
is on the order of 1v/microSecond. The transient's amplitude
exceeds 300 Volts.
The last figure, repeated above, explores the collapse
signature when coil current is controlled by an ordinary
toggle switch. Here again we see a sharp, negative-going
transient that begins with switch opening is 0.05v/microSecond
or 1/20th that of the coil when controlled with a fast switch.
The ONLY explanation for this difference is that as the switch
contacts first opened, coil collapse begins and an arc forms
in the gap. This prevents coil current from going to zero
and produces a t=L/R retardation of collapse. ~1250 uS
after switch opening, we see a transient that marks an increase
in the slope of the curve . . . this has to be where the fire
goes out and the DC current flow in the coil goes zero.
The narrative accompanying the other two figures is, I
believe, self explanatory.
This particular study focuses on arcing at the controlling
switch and does not address arcing at the contacts of the
contactor as driven by contact spreading velocity. That's
Bob . . .
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