Air Conditioner Freezing Up

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TURTLE wrote:

You aren't going to ever understand.
hvacrmedic
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TURTLE wrote:

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 alone.
hvacrmedic

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Bullshit? 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.

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One other thing, just what is the outside air temperature now ? It may be too cold to run the air conditioner.
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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!!!
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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.
Stretch
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Stretch wrote:

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)
hvacrmedic
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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.
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Edwin Pawlowski wrote:

At what rpm is the Hp rating? And torque? And the weight and shape of the car?...... Tony
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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.
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Edwin Pawlowski wrote:

This is Turtle.
they are missing what your saying because of cause of the problem and not what your statement was. i think we are tring to bullshit somebody here but not sure.
TURTLE
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Edwin Pawlowski wrote:

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.
hvacrmedic
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wrote in message

Ummmm..no.
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.
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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.
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Edwin Pawlowski wrote:

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.
hvacrmedic
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I'm always looking to learn.
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Edwin Pawlowski wrote:

Start here:
http://en.wikipedia.org/wiki/Electric_motor
An excerpt:
"[...]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 RPM/full stall).
The slip of the AC motor is calculated by:
S = (Ns - Nr) / Ns
where
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 1800."
_______________________
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 :)
hvacrmedic
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RP wrote:

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 :)
hvacrmedic
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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 the condenser?
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Edwin Pawlowski wrote:

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.
hvacrmedic
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