I've notice many condenser fan motor draw current below motor rated current, and the motor run very hot. Is there a percentage formular as to how much current a motor should draw as related to the rate current shown on the motor plate?
Printable View
I've notice many condenser fan motor draw current below motor rated current, and the motor run very hot. Is there a percentage formular as to how much current a motor should draw as related to the rate current shown on the motor plate?
I believe you will have to contact the mfg to find that info as you do not know the specs for the winding resistance.
I have found that a fan running low amps is cavitating for one reason or another usually, does the thermostat ever satisfy? Could the fan motor be getting hot because of heat being conducted?
Cavitation.... check the condenser coil to see if it is plugged or restricted, another thing to check is the condition of the fan blades themselves, they may be bent and not drawing properly.
Is this original motor? Could be a wrong replacement motor,Is it an "air over" motor? Also is the voltage supplied within range? 3phase or single phase?Capacitor correct?
1) majority of these motor I refer to are replacement motors. some with replacement blade.Quote:
Originally posted by acman
Is this original motor? Could be a wrong replacement motor,Is it an "air over" motor? Also is the voltage supplied within range? 3phase or single phase?Capacitor correct?
2) There are no other information on condensing unit, it has long faded or tag has disappeared.
3) These replacement motors do have correct run capacitor.
4) most motor are psc type, single phase 208/230 vac. volatge read from my dmm are within acceptable variation. example: a 208vac may read 207v-213v. a 230v may read from 225v-242v.
5) all these motors are air over.
SUPPOSING SOMEONE DID REPLACE WITH A WRONG MOTOR, HOW CAN I TELL?. (example: supposing the original mortor was a 1/3hp motor, and some one slip a 3/4hp motor in it place with original fan blade, how can I tell this is the wrong motor and how can I determine what the correct motor should be?) BACK TO MY ORIGINAL QUESTION, IS THERE A FORMULAR OR GENERALLY ACCEPTED RULE OF THUMB as to HOW MUCH CURRENT CAN BE OVER/ UNDER DRAWN BY XXX% OF NAMED PLATE CURRENT. I AM LOOKING FOR MAXIUMN AND MINIUMN ACCEPTABLE LIMIT. ( I perfer not to call every motor manufacture each time I came across this situation)
I will have to dig on this one and let you know for sure. Without knowing the winding resistance... could be tough. You could measure the winding resistances, but do it after the motor has cooled.
I = V/R
Use your actual supply voltage and the measured resistance on the run winding. Just my opinion but I would tolerate a plus or minus of 5 percent on the amperage. Normally with a bad motor, low voltage or failing capacitor you see high amperage... not the low amperage you are seeing.
Perhaps if the capacitor is wrong (high mF) you would see low voltage?? You have mentioned that the motor is running low amperage... how do you know it is if you can't read the motor specs? Also... is the motor rated specifically for 50 or 60 hz?
One more suggestion... is the diameter of the fan blade correct for the shroud or is it too small?
Also, are the blades the correct pitch ?
I have noticed an effort over the years to reduce the size and weight of motors having the same horsepower. While this increases the efficiency (and reduces the cost), it leaves less surface area to reject heat, and they tend to run hotter.
Dean, not to nitpic, but the definition of cavitation is the formation and implosion of bubbles in a mechanical device, such as a pump or a boat prop. I believe what you are referring to here is airflow blockage. While airflow blockage through a squirrel cage reduces the amperage, airflow blockage through a propellor blade increases amperage.
Let me give you an ACTUAL CASE to my question. We pick up this account a month ago. Customer is call MEAT AND FISH FELLAS. Their office is located in a typical industrial complex. for several years, they used the general maintenance people who service this center (not necessary refrigeration people)
We were call in because one of their meat locker was @30F. Our tech found the condenser fan motor bad. customer indicate motor was replaced less than a year ago. here are the data: condenser is a copland.(tag has faded, no modle # avaible). It does have a copland semi-hermatic LAH -0310-TAC. FAN BLADE is six (6) blade, 24" diameter, approx 27 degree pitch, MOTOR is 1/2hp,A.O. smith model HE3l7171N', 1075rpm,208/230vac,TENV, plate amp rated @5.2
Tech purchase an exact replacement motor installed it with new run capacitor. current drawn is measured at 3.8amp (yes, that is not a mis type) 30day went by, customer complain motor we put in stop running. I went out to site, found motor shaft frozen, take motor apart, shaft was well scortched on both end of bell housing. wiring inside housing definiately indicate over heating.
I reasoned with that size of blade, it must have more horse power, so I put in a 3/4hp G.E. 1075rpm, 208/230v, plate amp 5.4amp. My meter show it draw 3.8amp. motor run hot. Thinking I got the wrong motor in there. I went to another system with the same condensing unit. It has original fan motor. guess what, it is a Emerson, capacitor start, 1/2hp, 1100 rpm motor. plate amp is 4.0. actual measured current draw 3.8amp.
Another example: went to a customer yesterday, call Az COMMERCIAL COIN AND LAUNDRY SUPPLY HOUSE. they have a G.E. air condition unit 3.5Ton. one of their own tech (laundry machine repair person put in a G.E. MOTOR, 1/2hp, 1075 rpm, plate amp is 3.6amp) 18"dia three blade, 30degree pitch. fan blade mounted backward. fan blade tangle up with some loose wires stopped the motor. From past experienance, I kown this unit required a 1/4hp, 1075rpm, 1.4amp motor with either a 3or 4 blade. So that is what is did. working perfect now. I am lucky on this one, what about the many others I don't recognize.
This is just two case out of many, We come across this type of problem often, so I like to know how can I tell right away if a particular situation has a wrong motor?? or fan blade??
These are all 6 pole motors. Without a load (blade), they would rotate at 1200RPM (assuming 60Hz). This would soon overheat and burn out the motor. They are rated at 1075RPM (with the proper load), because they are designed to run best at that speed. If run at a higher speed (less than proper load) they will overheat. Replacing with a higher HP motor is not better. You should be able to obtain charts from a fan blade manufacturer showing proper motor/blade combinations.
Subzero*psia: your formula I=V/R will only work on a resistive load. It is not accurate when dealing with an inductive load ( not trying to nitpic!)
I agree with what you say. I think you must be sitting on a desk or something. sitting on a desk and working out in the field is quit different.Quote:
Originally posted by Gary
.Replacing with a higher HP motor is not better. You should be able to obtain charts from a fan blade manufacturer showing proper motor/blade combinations.
Imagine, you are out in the field, you walk up to this old unit, many other tech has gone thru it before you, you have no idea who manufactured the fan blade (normally no data avaible on blade or who manufacture the blade) or if the motor is the correct replacement. But you need to come up with a solution quickly (normally a customer will complain if you charge them more than an hour or two to replace a motor, normally not possible to spend a day trying to find the blade manufacturer, or investigate what is the correct motor or fan blade etc...)
can you imagine telling the customer "I will come back tomorrow to replace that bad motor???" when his meat locker is at 30-40F.
how can I tell if someone had replaced the blade with correct pitches or # of blade??. keep in mind you are out in the field - not sitting in an office- no reference book avaible. normally not possible to tell the customer, "I will go back to our office check on all this and come back to fix your problem in a few days"Quote:
Originally posted by WebMaster
Also, are the blades the correct pitch ?
I have been working in the field for 33 years.
If you know the diameter, number of blades, and pitch, you can look it up on ANY blade manufacturer's chart. It doesn't matter what brand it is. The chart will tell you what motor goes with that blade.
Thank for your reply, Today I went to one of our local distributor, ask for the chart as you mentioned. They pulled out a 3 ring binder contain many manufacturer fan blade and its specification. each manufacturerer has a small pamplet of varieou blade design, size, pitches, demensions. it tells me CFM deleived at varieous static pressure, etc. however, nowhere in the book does it say what size of motor goes with a particular blade. Unforturnately, they don't have any spare copy to give me. and too many pages for me to photocopy. Is there is web site avaible to look up such information?.Quote:
Originally posted by Gary
I have been working in the field for 33 years.
If you know the diameter, number of blades, and pitch, you can look it up on ANY blade manufacturer's chart. It doesn't matter what brand it is. The chart will tell you what motor goes with that blade.
The paragraph below is just academic. I am still puzzled by the following; how can a 24"dia. six blade, 27degree pitch, can be powered by a 1/2hp, 1100rpm motor rate at 4.1amp. actual current drawn is 3.8amp. the motor used here is: EMERSON k55hxjz -3123 part# 050-0248-00 condenser is Copland (data plate faded not readable) compressor is Copland semihematic; LAH1-0310-tac, low temp.