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Frank Davis
Joined: 05 Dec 2007
Posts: 3
Location: Minden, NV
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 Posted: Tue Feb 12, 2008 7:58 pm Post subject: Crowbar Application - off topic |
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I have a large air compressor plumbed into outlets around my shop, I
try to remember to turn the compressor off at the circuit breaker when I
leave the shop, but some times I forget. Recently, after forgetting, an
air line came loose dropping the air tank pressure below the compressor
turn on point (about 90 psi). The compressor could not keep up with the
air loss and ran continuously.
To prevent reoccurrences I came up with a crowbar circuit to shut the
compressor off in this situation, thanks to learning about crowbars from
the Z Figures. I wired a DPST 110V relay with the N/O contacts going to
the two hot legs of the 220V service and the fixed terminals to ground.
Energizing this relay put a dead short to ground on both hot leg and
opens the circuit breaker. Probably very high current through the
contacts for a very short time, but I haven't welded the contacts shut yet.
One side of t he relay coil goes to ground. The other side of the coil
goes to one leg of the 220v line (which is 110v above ground) through an
adjustable pressure switch (from McMaster-Carr) and a SPST switch in
series. The pressure switch is set to close below about 85psi. The
STSP switch is opened to allow compressor start up and is manually
closed to arm the system above 90psi. If the pressure drops below 85psi
with the system armed the circuit breaker opens, shuts off the
compressor, and removes voltage from the relay. It works well, but I'm
not sure how a building inspector would feel about it.
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Bob McC
Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON
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| Posted: Tue Feb 12, 2008 9:57 pm Post subject: Crowbar Application - off topic |
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Frank
Why not use a DPDT or DPST-N/C relay instead, and wire the contacts in
series with the compressor supply instead of shorting it to ground? That way
when the pressure drops low enough the compressor simply shuts off rather
than shorting out. When you are using large quantities of air of course you
have to defeat this system however just as you would with your scheme.
Sounds very inconvenient. Shorting the hydro line to ground isn't a
particularly good idea and is asking for trouble, probably sooner than
later. An alternative is to wire your DPST relay such that the compressor
supply is through this relay and then wire the relay coil to the lighting
circuit in your shop. You turn out the lights when you leave and the
compressor is automatically shut down. While you're in the shop working, the
compressor runs as intended, no dangerous shorts or other complications
involved and no extra circuits to arm or disarm as required.
Bob McC
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_________________
Bob McC
Falco #908
(just starting) |
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nuckolls.bob(at)cox.net
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| Posted: Wed Feb 13, 2008 4:33 am Post subject: Crowbar Application - off topic |
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At 12:37 AM 2/13/2008 -0500, you wrote:
| Quote: |
<robert.mccallum2(at)sympatico.ca>
Frank
Why not use a DPDT or DPST-N/C relay instead, and wire the contacts in
series with the compressor supply instead of shorting it to ground? That way
when the pressure drops low enough the compressor simply shuts off rather
than shorting out. When you are using large quantities of air of course you
have to defeat this system however just as you would with your scheme.
Sounds very inconvenient. Shorting the hydro line to ground isn't a
particularly good idea and is asking for trouble, probably sooner than
later. An alternative is to wire your DPST relay such that the compressor
supply is through this relay and then wire the relay coil to the lighting
circuit in your shop. You turn out the lights when you leave and the
compressor is automatically shut down. While you're in the shop working, the
compressor runs as intended, no dangerous shorts or other complications
involved and no extra circuits to arm or disarm as required.
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My shop in Medicine Lodge has an outlet driven through
a wind-up two-hour timer, not unlike the cooking timers
of yesteryear. I give 'er a good crank when I need air
but if I forget to turn the compressor off before leaving,
the timer does it for me.
I use a small contactor to relieve the timer's contacts
from having to carry the motor current. If I'm in the
shop long enough for the timer to run out, I'll generally
hear it drop out unless I'm making a lot of noise. Once
could also consider adding an indicator light that goes
out when the compressor power goes away.
The crowbar thing on AC mains breakers involves a much
higher energy event than for tripping a 5A breaker in
an airplane. It would probably function as advertised
but it's not the elegant solution. Bob's idea is probably
the best. Lowest parts count yet.
Bob . . .
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klehman(at)albedo.net
Guest
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| Posted: Wed Feb 13, 2008 5:47 am Post subject: Crowbar Application - off topic |
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Yes and a similar scheme for water may be even more useful. Water leaks
are frequent and can be very expensive as insurance companies well know.
There are commercial valve units with leak detectors available for those
on city water. However for well water, I wired in such a relay that
pulls in to kill power to the pump when the burglar alarm is armed. (BTW
the relay consumes far less power than any one of numerous household
electronic items do when they are turned OFF!)
If concerned about my compressor, I'd wire a relay to pull in and power
the compressor when shop lights were turned on or when Bob's wind up
timer was activated. Some guys do simply use a 230 volt switch box to
kill lights and shop power simultaneously when they leave the shop.
For my electronics bench, turning off the lights also kills power to the
soldering iron plug.
Ken
Robert McCallum wrote:
| Quote: |
Frank
Why not use a DPDT or DPST-N/C relay instead, and wire the contacts in
series with the compressor supply instead of shorting it to ground? That way
when the pressure drops low enough the compressor simply shuts off rather
than shorting out. When you are using large quantities of air of course you
have to defeat this system however just as you would with your scheme.
Sounds very inconvenient. Shorting the hydro line to ground isn't a
particularly good idea and is asking for trouble, probably sooner than
later. An alternative is to wire your DPST relay such that the compressor
supply is through this relay and then wire the relay coil to the lighting
circuit in your shop. You turn out the lights when you leave and the
compressor is automatically shut down. While you're in the shop working, the
compressor runs as intended, no dangerous shorts or other complications
involved and no extra circuits to arm or disarm as required.
Bob McC
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