When you increase the condenser airflow the system is no longer
perfectly tuned or matched. There is no other cause than that you put
the wrong fucking motor on it. The charge is good, by definition of
"perfectly tuned and matched". If you add refrigerant to fix it then
you've tried to make two wrongs into one right. It'll work, but it won't
ever be perfectly tuned again, period, unless you re-engineer the
remainder of the system. When you get done with all of that then your
wrong motor will now be the right motor, but now it's a completely
different system from what you started with. Just put the right fucking
motor on it and shut the hell up.
The OP probably has a dirty evaporator coil and/or low refrigerant
charge. Cleaning the condenser coil dropped the suction pressure even
lower than its already low value, causing the coil temp to drop and
causing the coil to freeze. If it had been 90deg outside the coil
wouldn't have frozen, but it was in the low 70's, so it did.
You totally misread what CB said, and I don't care whether you can
digest that or not, but I'll stop to say that this sort of behavior is
exactly why people got tired of your hack crap posts in alt.hvac. You
don't want to learn a fucking thing, you know very little about your own
occupation of 40 years, and while you think you have all the answers you
have virtually none at all. I'm sorry, but I didn't raise you, so it
isn't my fault. I'm done with trying to educate you, so from now on I'll
probably just tell people to run as far as they can from your advice on
these groups. If you happen to get one right, then I'll leave that one
First, you are not paying me to teach you. If you want to be taught, AFTER
FORTY YEARS in the trade, go back to school.
If you dont understand this simple, BASIC principle of AC operation, there
is NO WAY you have ever, in your life, charged a unit correctly, Mr Beer Can
Cold Ill pinch em off till its cooling guy.
You also have never worked on an old Trane/GE unit with two speed fans, or
knew how that worked either.
It is 73 degrees out right now (Miami winter...). I got some cleaner
for the outside coil because it looked really dirty. I also changed
the inside filter. I took the fan motor to the store and they replaced
with the "correct" fan motor. The existing one had 3 wires and the new
one had 4 wires. They sold me a small silver box to hook up the brown
wires to. If this does not work, I think I will call someone who knows
what they are doing...
Thanks to all for your help!!!
What kind of filter did you put in? Some of the washable filters and
most of the pleated filters are very restrctive to air flow. They can
cause the indoor coul to freeze on a system with marginal performance.
Let's just put it all into one list :)
Potential causes of evaporator coil freezing:
1) Insufficient indoor air flow, which can be caused by any obstruction
to airflow whatsoever. ( I Once found that the homeowners dog liked to
lie next to the intake grill, when I wasn't there of course, blocking
about 75% of the airflow. Finally caught the bastard on the third trip
out. Excessive dog hair on the air filter had me suspicious already:)
2) Low refrigerant charge
3) Wrong refrigerant
4) Metering device underfeeding, caused by either a restriction,
metering device failure, or wrong metering device size.
5) Refrigerant restriction in the refrigerant circuit.
6) Oversized compressor
7) Excessive condenser temperature drop, caused by excessively low
ambient temp, precipitation, water spray, oversized condenser fan motor,
condenser fan motor RPM too high, blade pitch to high, wrong blade,
wrong capacitor, voltage too high.
8) Undersized evap coil
9) Air bypassing evap coil
10) Restriction in evap coil tubing.
11) Indoor temp and or RH too low
12) Non-condensables in system
13) Indoor blower undersized
14) Indoor blower wheel dirty
15) Indoor blower running backward
16) 17) 18) ??? (add any causes that I missed here)
Oversized fan motor? That will not have any effect if it is turning the
same rpm. The speed of roation is going to change the air flow, bt if a 3/4
hp motor turns at 1140 rpm and a 1.5 hp motor turns at 1140 rpm, the cfm of
air moved by the fan will be the same.
I have two cars with the same transmission and differential. The new one
is 205 hp, the old one is 170 hp, but at 70 mph, they both turn at 2,000
rpm. Accelleration and top speed may differ, but the speed at 2000 rpm is
still the same.
Does not matter as long as the hp is sufficient to propel the car at 70 mph.
Both cars have the same gearing so the engine turns at the same rpm. I
could add a supercharger, put in a big V8, it would still run the same rpm
with the same gearing at the same speed. Just like a fan motor.
There are no locking torque converters on electric motors. They are free
to slip, meaning that the rotating field outpaces the rotor. The only
way to get the theoretical 1200 RPM, which is the RPM of the rotating
field, is to remove all friction and load from the rotor. 1075 is the
RPM at which the inherently 1200RPM motor is slowed down by the attached
load, which in turn ideally matches the HP rating of the motor. This is
why they call it a 1075 RPM motor rather than a 1200RPM motor, though it
actually is 1200 RPM under no load conditions.
Adding additional load beyond what it was designed for will reduce the
speed still further, but this will overload the motor, causing it to
output more HP than it was designed for. At some low RPM caused by
excessive drag, the HP curve will begin to drop again and this is called
the stall speed for the motor. On multi-tap blower motors the various
speed taps are actually just various HP configurations wrt the common
lead. IOW, the lowest speed is caused by overloading the motor at the
low HP obtained by using the low speed tap. This causes excessive
slippage between the field and the rotor and thus we get a lower speed
out of the motor. Multi-tap motors are engineered to accommodate the
overloaded condition on its lower speeds without burning.
IOW, over-sizing a motor will cause the actual RPM to run higher than
the nameplate rating, approaching 1200 RPM as the HP/LOAD ratio
approaches infinity. Under-sizing will cause the actual RPM to run lower
than the nameplate rating, approaching stall (or zero) speed as the
HP/LOAD ratio approaches stall value.
With proper gearing, you can prob get a car to 70 with a washing machine
motor....but thats not the point.
Your analogy is flawed in the fact that an electric motor, in the case we
are talking about, runs at, and Im using your example here, at WOT all the
time, limited in final RPM by blade pitch and load.
Your car example, has a variable speed control...your foot.
But a 1 hp motor capable of running a given fan blade at 1000 rpm is running
at 1000 rpm. A 2 hp motor with the same fan blade mounted running at 1000
rpm is till running at 1000 rpm. The fact that is has more HP does not make
it turn faster.
Are you asking or telling? Cause if your asking I'm ready to post a few
hundred web pages that will explain it to you. If you're telling then
you should probably stop telling and do a little more homework.
"[...]Actual RPM for an induction motor will be less than this
calculated synchronous speed by an amount known as slip that increases
with the torque produced. With no load the speed will be very close to
synchronous. When loaded, standard motors have between 2-3 percent slip,
special motors may have up to 7 percent slip, and a class of motors
known as torque motors are rated to operate at 100 percent slip (0
The slip of the AC motor is calculated by:
S = (Ns - Nr) / Ns
Nr = Rotational speed, in revolutions per minute.
S = Slip, in percent.
As an example, a typical four-pole motor running on 60 Hz might have a
nameplate rating of 1725 RPM at full load, while its calculated speed is
If you didn't believe me, then maybe you can believe the above. Happy
reading! It really is an excellent article, I recommend it to anyone who
deals with motors on a regular basis. Also good reading for those who
only want to learn about them. Now pardon me while I take time to read
the rest of it :)
BTW, according to the formula above, a PSC motor rated at 1050 RPM at
full load has a 12.5% slip. Assuming a nearly linear curve in the upper
region of the motor curve, doubling the HP would result in a slip of
approximately 6.25%, for a final RPM of approximately 1125. This is a
75 RPM increase. This may not seem like much, but the cfm increase due
to this much increase in RPM will be substantial. It more than likely
won't be an operational problem for the unit, but keeping the argument
in context, it could very well lead to a coil freezing up that wasn't
freezing before under the same ambient conditions, that is, when it was
already cool outside and the coil was already near the freezing point.
Is that understood, or would you like a bit more clarification. I can
talk about this shit all day long :)
OK so we put in a larger motor and now have 1125 rpm. There is a threshold
for everything and this may be enough to cause the freezup, but is it the
root cause? Given the design parameters and operating conditions of most
home ACs, do you think the oversized motor (additional cfm) is the problem
or it is just showing that there are other problems in the system? What is
the increase of cfm of the 75 rpm when compared to a very strong wind over
There is a tolerance, or should be. That doesn't mean that you should
purposely rig your system so that it's operating at one or the other
extreme. The root cause in the hypothetical set of conditions that I
referred to is that it's
1) cool outside.
2) too much cfm.
This is by definition of the conditions.
Now if I had said the unit is undercharged and running at a higher
ambient, and you then increase rpm, then the root cause would be the low
charge at a higher ambient, but not the motor, because in this case
correcting the charge will stop the freezing. OTOH, the system will now
freeze up at a higher ambient than before, so when the ambient falls
well be right back to no.1 above.
A wind will typically not affect head pressure significantly. Although
it assists draw-through on the side that it hits, it retards it on the
back side with a negative pressure. Very strong winds are a different
story. But if they cause freeze up, and they can at lower ambients, then
they would be the cause of the freeze up.
Just let me summarize the entire argument again:
Virtually anything that reduces head pressure below what it would run
with all components operating optimally, will also reduces SST, and thus
evaporator temperature. Thus any such change can lead to immediate
evaporator freezing if conditions are such that it was close to freezing
before that change was made to the system. Simply installing a more
restrictive filter can do it. Setting the t-stat down one more degree
can do it. Shutting a bedroom door can do it. The list is endless, but
all have in common that the coil skin temp must have been just above
freezing beforehand. No problems with the system are required for this
situation, since lowering either the indoor or outdoor temp tends to
shove the evaporator closer to the freezing mark. You wouldn't for
instance want to go off and leave the t-stat set to cool and 65F on a
night when the ambient is supposed to dip to 60. That is just asking for
a freeze up. HTH.
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