fans cycling on overload
This came up last week and I am not completely satisfied with the way I "fixed" it. A refrigerated storage building operating at -5deg. C using 4 evaporators, each with 2, three horsepower fans. the defrost schedule has always been one evap at a time every six hours for 15min. using hot gas. the two coils nearest the access doors had evaporator motors ( 1 each) shutting off on overload when the frost built-up between defrosts. these things are checked several times each day and there are temp. recorders as well. when the evaps are clear the amp draw of each motor is 6.3 to 6.5 (rated 6.5). but when the coil begins to frost over the amps start to climb. Every one of the motors does this when I placed cardboard over the coil inlet. I thought the amps would go down when the incoming air was restricted. less air flow = less work = less amps? It has been quite humid lately, so I doubled the number of defrosts (every 3 hours) without changing the length of time. this seems like a temporary measure that works for now but still does not explain the rise in amperage as the air flow decreases. I expect to return the defrost back to the original schedule soon as nothing here goes unnoticed for long. three of the motors are original to the install over 10 years ago, while the rest are OEM replacements (century coil ). there doesn't appear to be any voltage supply issues, and the contactors, fuses, and connections all seam OK. The doors seal pretty good and have alarms if left open for more than 18min. I'm at this site maybe once a week for various things and will keep checking this until it gets back to normal. I also haven't seen any wobble or shifting of the motor shafts or fan blades, no unusual noise or shaking either. Any theories you might have would give me something to look for as I know this will not just "go away". THANKS
Axial flow fans load up more when airflow is restricted and will draw more current. Centrifugal blowers unload when airflow is restricted.
I would check that the coils are very clean. Those motors (assuming they are the design HP for the unit coolers) should not be running so close to RLA.
I see nothing wrong with more defrosts with hot gas. The defrost cycle is very short, no?
Here's a link to another thread discussing the difference between air load on centrifugal (squirrel cage) blowers and axial fan blades. GT Jets' example in post#13 has a good exercise using a shop vac and then a box fan that illustrates the difference:
Originally Posted by refrigeration johnny
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The first time I'd encountered an evap motor running with an internal overload (made by GE I think) I thought the motor was faulty. Same as your situation except, in MY situation, the coils in a W/I COOLER had frosted up. I'd initially thought that had happened because of the presumed faulty motor.
I discovered otherwise when I went to replace the motor with a Fasco off my truck and, while I had the old motor in my hand, I saw motor's diagram illustrating the overload function of the motor. I cleared the coils, put that old motor back in and it ran great. So...had to look elsewhere for the frosting issue.
We often learn as we go, DON'T we?