[lit-ideas] Re: The Compressor Shorted To Ground
- From: Jlsperanza@xxxxxxx
- To: lit-ideas@xxxxxxxxxxxxx
- Date: Tue, 20 Nov 2007 22:18:58 EST
GEARY'S COLUMN
Q: Why does a compressor short to ground?
G: Because you _let_ it short to ground, that's because.
"The compressor shorted to ground -- and I know why"
J. M. Geary
_Subject: 50 HP Copeland Discus died - experiences?_
(http://www.elitesoft.com/sci.hvac/itmotfal.html) Be sure to check the
wiring between the
contactors to the compressor for a short to ground - don't laugh, I found one
like
this after "professionals" ...
www.elitesoft.com/sci.hvac/itmotfal.html
_Property Management Software for apartment managers, owners, and ..._
(http://www.softwareforlandlords.com/hvac6.php)
When a compressor fails short, what happens is that insulation on the wires
... resulting in insulation damage that shorts the rotor either to ground or
to ...
_www.softwareforlandlords.com/hvac6.php_
(http://www.softwareforlandlords.com/hvac6.php)
-----
"I lost a compressor last week :~("
"It was a pump-down unit."
"The other compressor works fine, BTW. Unit ohms and megs good. No history (
7 year old unit ) of failure. Oil good, freon good."
"Clean. All the way. Pulled off everything that pulls - 4 heads, end bell,
oil pump (and took pump apart)".
"You didn't say. Does the compressor attempt to spin? My only thought is
that you have a winding short."
"Are any of the windings discolored?"
"No -- why?"
"Well, you have to check the wiring between the contactors to the compressor
for a short to ground - don't laugh, I found one like this after
"professionals" condemned a 2 thousand dollar semi-hermetic compressor (which
had not
one ____ thing wrong with it)"
"You don't say!"
"Yep. Usually a short like this makes itself fairly obvious (smoke and so
forth), but if the lead(s) is(are) able to weld themselves and create a
connection that will trip the breaker every time with no further deterioriation
of
the leads, you will be hard pressed to see it."
"It's no laughing matter. I did check the wiring, to the extent of stripping
off the BX to examine every inch of every wire. I've seen the wrong
diagnoses things you speak of. I was called on one unit to second-guess a
contractor
who wanted to change a chattering contactor. The owner didn't want to pay for
a new 100 amp contactor. Turns out it was a 240V coil, in series with a
safety in the compressor. When I popped the compressor open ( 15 HP Carlyle ),
it
was major toast city - I couldn't have made it any blacker in there with
spray paint ! One side of the contactor control circuit was grounded inside the
compressor, the other side was hot w/ 120V, thus the chatter. I got to tell
the owner 'good news, bad news'... contactor's OK, compressor's gone ! :~):
"Sometimes they can wear such that they touch and ground out, then the force
of that spark pushes them away such that they ohm good, then the vibration
of start-up and running eventually brings the bare spot back into ground
contact ( another trip ), etc."
"I also ran across one case where a contactor was shorted to the backplate
of the relay panel it was installed on - the short was in the back bakelite of
the contactor (to the backplate) and was absolutely impossible to see."
"But you _did_ see"
"Well, yes. BTW, I caught both these problems with a Simpson 260 (no megger
required) and have learned to make sure that wiring downstream of the
contactor and upstream of the compressor is not shorted phase to phase or
phase to
ground. Hope this helps you."
"Sure. The new compressor is in now and spins like a top."
"Yes. I'm glad you learned. Once you know the why, there shouldn't be a
because! The symptoms would first indicate bearing failure, but you looked at
that and found nothing, right?
"Right"
"So, the only other thing it could be is a shorted stator winding. Not a
short to ground, which is easy to detect, but just a few shorted turns."
"Exactly -- but how would my _wife_ know?"
"Well, explain to her. The compressor, if shorted, will not absorb enough
energy from the stator flux, and act as a heavy drag on the motor, causing
increased current draw".
"Right, and the winding resistance may not be altered enough to read the
difference. The failed turns are nearly always at the inside of the winding, so
the burned insulation is rarely visible. This is because the innermost layer
of turns in a winding are farthest from the cooling effect (in this case of
refrigerant) of the motor's coolant, and also have been bent to the sharpest
radius, and also are subjected to the most intense alternating magnetic fields
(from all the windings above it), and so have the most eddy current heating.
The innermost windings often run 10-30 C hotter than the outermost. The heat
leads to insulation breakdown earlier in the innermost winding."
"Well, that would explain the price".
"Right. An inductance bridge will show the problem, but you probably don't
have one of those. The inductance of the bad winding won't differ much either,
but the shorted turns will cause the 'Q' (quality factor) of the bad winding
be approximately ZERO! That will be the indication of shorted turns. An
impedance bridge is similar to a wheatstone bridge, however it is used at AC
instead of DC, to measure the complex impedance of capacitors and inductors.
Not
only does it measure capacitance or inductance, but it measures the quality of
the component, as well. A stator winding (either in or out of the motor)
will show small resistance and large inductance. It should have high 'Q' when
out of the motor."
"Now, when you say, 'absorb energy from stator flux'?-- Is this something
like creating an opposing field?"
"Well, no. Exactly the opposite."
"How come?"
"Well, the iron in the stator (and the magnetic field in the rotor once it
is spinning) oppose the field of the stator, thereby preventing it from drawing
current from the line. The shorted turns remove the magnetic field, thereby
causing the rest of the winding to draw MORE current from the line."
"Very interesting! Are internal sensors usually placed there?"
"Yes, when the compressor is tested for insulation class, a thermocouple is
placed near the innermost turns of the winding, or some other location that
the motor designer determines will be the hottest winding location in the
entire motor. The temperature rise is then determined at full load. This
procedure is described in some detail in some versions of the NFPA's National
Electrical Code book."
"Will do".
"What is 'quality factor' ? It's a new term to me."
"Well, Quality Factor, or Q, is an electrical engineering term relating to
the loss in an inductor. Higher Q means less loss."
"How much then?"
"How much did I say?"
"Before you go, is there a way to use a capacitance meter to gain
information ? Or to home=brew an inductance bridge ( assuming one owns a good
digital
multi-meter with capacitance function ( Fluke 12 )?"
"Well, not really. The problem with the rotor inside the stator is that the
copper conductors in the rotor are SUPPOSED to be shorted. And, the stator
windings definitely see the short. The stator draws large current from the
line,
transfers large flux across the gap between stator and rotor, and causes
current to flow in the rotor winding, causing opposing magnetic flux. This is
what starts the motor spinning. But with the stator not conducting current,
and
the rotor not spinning, it looks like a short, and will make it much harder
to see the difference between a good and bad stator. It doesn't matter
whether the stator is in or out of the motor housing, but it will matter
whether
the rotor is inside the stator."
"Thanks again, for more really great information! It's really appreciated. I
always like to know the whys of things."
--------------------
SUMMARY
"Air conditioner compressors usually fail due to one of two conditions: time
and hours of operation (wearout), or abuse. There are some failures that can
occur elsewhere in the system that will cause a compressor failure, but
these are less common unless the system has been substantially abused."
"Usually abuse is a result of extended running with improper freon charge, or
as a consequence of improper service along the way. This improper service
can include overcharging, undercharging, installing the wrong starter
capacitor
as a replacement, removing (rather than repairing/replacing) the thermal
limiter, insufficient oil, mixing incompatible oil types, or wrong oil,
installing the compressor on a system that had a major burnout without taking
proper
steps to remove the acid from the system, installing the wrong compressor
(too small) for the system, or installing a new compressor on a system that
had
some other failure that was never diagnosed."
"The compressor can fail in only a handful of different ways. It can fail
open, fail shorted, experience a bearing failure, or a piston failure (throw a
rod), or experience a valve failure. That is pretty much the entire list."
"When a compressor fails open, a wire inside the compressor breaks. This is
unserviceable and the symptom is that the compressor does not run, though it
may hum. If the compressor fails open, and following the steps here does not
fix it, then the system may be a good candidate for a new compressor. This
failure causes no further failures and won't damage the rest of the system; if
the rest of the system is not decrepit then it would be cost effective to just
put a new compressor in."
"Testing for a failed open compressor is easy. Pop the electrical cover for
the compressor off, and remove the wires and the thermal limiter. Using an
ohmmeter, measure the impedance from one terminal to another across all three
terminals of the compressor. Also measure the impedance to the case of the
compressor for all three terminals."
"You should read low impedance values for all terminal to terminal
connections (a few hundred ohms or less) and you should have a high impedance
(several
kiloohms or greater) for all terminals to the case (which is ground). If any
of the terminal to terminal connections is a very high impedance, you have a
failed open compressor. In very rare cases, a failed open compressor may
show a low impedance to ground from one terminal (which will be one of the
terminals associated with the failed open). In this case, the broken wire has
moved and is contacting the case. This condition - which is quite rare but not
impossible - could cause a breaker to trip and could result in a misdiagnosis
of
failed short. Be careful here; do an acid test of the contents of the lines
before deciding how to proceed with repair."
"When a compressor fails short, what happens is that insulation on the wires
has worn off or burned off or broken inside the compressor. This allows a
wire on a motor winding to touch something it shouldn't touch - most commonly
itself a turn or two further along on the motor winding. This results in a
"shorted winding" which will stop the compressor immediately and cause it to
heat
up and burn internally."
"Bad bearings can cause a failed short. Either the rotor wobbles enough to
contact the stator, resulting in insulation damage that shorts the rotor either
to ground or to the stator, or end bearing wear can allow the stator to
shift down over time until it begins to rub against the stator ends or the
housing."
"Usually when one of these shorts occur, it isn't immediately a hard short -
meaning that initially the contact is intermittent and comes and goes. Every
time the short occurs, the compressor torque drops sharply, the compressor
may shudder a bit visibly as a result, and this shudder shakes the winding
enough to separate the short. While the short is in place, the current through
the shorted winding shoots up and a lot of heat is produced. Also, usually the
short will blow some sparks - which produces acid inside the air conditioner
system by decomposing the freon into a mixture of hydrochloric and
hydrofluoric acid."
"Over time (possibly a couple of weeks, usually less) the shuddering and the
sparking and the heat and the acid cause insulation to fail rapidly on the
winding. Ultimately, the winding loses enough insulation that the inside of the
compressor is literally burning. This will only go on for a few minutes but
in that time the compressor destroys itself and fills the system with acid.
Then the compressor stops. It may at that time melt a wire loose and short to
the housing (which can trip your house main breaker) or it may not. If the
initial cause of the failure was bad bearings causing the rotor to rub, then
usually when the thing finally dies it will be shorted to the housing."
"If it shorts to the housing, it'll blow fuses and/or breakers and your
ohmmeter will show a very low impedance from one or more windings to ground. If
it doesn't short to the housing, then it will just stop. You still establish
the type of failure using an ohmmeter."
"You cannot directly diagnose a failed short with an ohmmeter unless it
shorts to the housing - a shorted winding won't show up with an ohmmeter though
it would with an inductance meter (but who has one of those?) Instead, you have
to infer the failed short. You do this by establishing the the ohmmeter
gives normal readings, the starter capacitor is good, power is arriving at the
compressor, AND an acid test of the freon shows acid present."
"With a failed short, just give up. Change everything, including the lines if
possible. It is not worth fixing; it is full of acid and therefore is all
junk. Further, a failed short could have been initially induced by some other
failure in the system that caused a compressor overload; by replacing the
whole system you also will get rid of that potential other problem."
"Less commonly, a compressor will have a bearing failure, piston failure or a
valve failure. These mechanical failures usually just signal wearout but
could signal abuse (low lubricant levels, thermal limiter removed so
compressor
overheats, chronic low freon condition due to unrepaired leaks). More rarely,
they can signal another failure in the system such as a reversing valve
problem or an expansion valve problem that winds up letting liquid freon get
into
the suction side of the compressor."
"If a bearing fails, usually you'll know because the compressor will sound
like a motor with a bad bearing, or it will lock up and refuse to run. In the
worst case, the rotor will wobble, the windings will rub on the stator, and
you'll wind up with a failed short."
"If the compressor locks up mechanically and fails to run, you'll know
because it'll buzz very loudly for a few seconds and may shudder (just like any
stalled motor) until the thermal limiter cuts it off. When you do your
electrical checks, you will find no evidence of failed open or failed short.
The acid
test will show no acid. In this case, you might try a hard-start kit but if
the compressor has failed mechanically the hard-start kit won't get the
compressor to start. In this case, replacing the compressor is a good plan so
long
as the rest of the system is not decrepit. After replacing the compressor,
you must carefully analyze the performance of the entire system to determine
whether the compressor problem was induced by something else."
"Rarely, the compressor will experience a valve failure. In this case, it
will either sit there and appear to run happily but will pump no fluid (valve
won't close), or it will lock up due to an inability to move the fluid out of
the compression chamber (valve won't open). If it is running happily, then
once you have established that there is indeed plenty of freon in the system,
but nothing is moving, then you have no choice but to change the compressor.
Again, a system with a compressor that has had a valve failure is a good
candidate for a new compressor."
"Now, if the compressor is mechanically locked up it could be because of a
couple of things. If the compressor is on a heat pump, make sure the reversing
valve isn't stuck half way. Also make sure the expansion valve is working; if
it is blocked it can lock the compressor. Also make sure the filter isn't
clogged. I once saw a system that had a locked compressor due to liquid lock.
Some idiot had "serviced" the system by adding freon, and adding freon, and
adding freon until the thing was completely full of liquid. Trust me; that
doesn't work."
"Should diagnosis show a clogged filter, then this should be taken as
positive evidence of some failure in the system OTHER than a compressor
failure.
Typically, it will be metal fragments out of the compressor that clogs the
filter. This can only happen if something is causing the compressor to wear
very
rapidly, particularly in the pistons, the rings, the bores, and the bearings.
Either the compressor has vastly insufficient lubrication OR (and more
commonly) liquid freon is getting into the compressor on the suction line.
This
behavior must be stopped. Look at the expansion valve and at the reversing
valve
(for a heat pump)."
"Often an old system experiences enough mechanical wear internally that it is
"worn in" and needs more torque to start against the system load than can be
delivered. This system will sound just like one with a locked bearing; the
compressor will buzz loudly for a few seconds then the thermal limiter will
kill it. Occasionally, this system will start right up if you whack the
compressor with a rubber mallet while it is buzzing. Such a system is a good
candidate for a hard-start kit. This kit stores energy and, when the
compressor is
told to start, dumps extra current into the compressor for a second or so.
This overloads the compressor, but gives some extra torque for a short time
and
is often enough to make that compressor run again. I have had hard-start kits
give me an extra 8 or 9 years in some old units that otherwise I would have
been replacing. Conversely, I have had them give only a few months. It is
your call, but considering how cheap a hard-start kit is, it is worth trying
when the symptoms are as described."
"And this, in a nutshell, is what can happen to an air conditioner compressor
when it shorts to ground, and what you can do about it."
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