greenhouse fan motors

Yes, there is a large contactor box, and yes it would seem as though

220V would have been feasible.

The installation was done by professional electricians; perhaps if I had been doing it I might have made different choices.

I suppose that if the wire size were the same then a 220V motor would have less loss, but I suspect that the maker uses a smaller size wire and ends up with about the same waste heat.

The pulley change is an interesting idea; I should look into that. I don't know what the curves look like for these propeller fans, but I assume that slowing down the fans would reduce the load on the motors and might cause them to last longer. It would have to make them last a LOT longer to be really useful.

The motor is mounted above the fan hub, and is directly in the air stream, so it is getting as much cooling as it could ever hope for.

Bob

The simple calculations shown in the Grainger catalog shows the hp required varies withe Cube of the speed. Their example shows increasing the rpm fr om 5000 to 6000 requires 1.73 times as much hp. So reducing the speed a li ttle will have a large effect on the hp and life.

Dan

Bob, I dont think you have ever really told us how these motors fail. You seem to be saying that it might be bearing failure. Then all the discussion about current, voltage etc is wasted. Motor bearings last for decades if rated and cooled correctly. Are yours cool? Is there an undue side load? eg belts too tight. Is the atmosphers corrosive?

3/4hp? I thought they were 1/3 hp?

Running a belt and in the airstream, check. Thats good.

No cap? Not very many single phase motors of that size, dont have start/run caps. Sure there arent any inclosed in the fan housing somewhere?

As for no lube points...I think Id pull down a motor and check. Some indeed are "permanantly lubricated" on the name plate. But I think Id look for either plugs in the end bearing covers or little holes that you can spritz oil into the bearings.

Id also take a volt meter and put it on the fan circuit (s) somewhere and check your voltage on both the line and on the input to each fan occasionally. You said that there were several that were running slow...thats either a sign of bearings about to freeze up, shorted windings, low voltage at the motor...or bad run caps or even bad start caps on some..some motors.

Get some measurements and you might determine whats going on.

Gunner

Reviewing the data..they are 1/2 hp

There are two different failure modes.

One mode is that the motor is extremely rough-starting on low speed. It is rough enough to shake the greenhouse structure. It seems as though the torque is shifting rapidly from high to low and back again, which I take to mean that the starter circuit is cutting in and out, which in turn I take to mean that either the motor starter is defective or the motor does not have enough power to accelerate the fan.

The other failure mode is that the motor runs unreasonably slowly. The motor may run at full speed when new, but before very long it gets slower and never runs at full speed again.

The bearings do not seem to be seized up, they do not seem to have excessive friction, they do not make noises.

Coolness is relative... The motors are not running hot enough to raise immediate blisters if I touch the casing, but I would not want to leave my hand pressed firmly against the casing for very long.

Belt tightness: If I push on them they move inward a short distance. They are not loose enough to flap around and not tight enough to go "twang". They seem of ordinary tightness to me. I have no specification and no way of measuring it anyway.

Corrosive atmosphere? Well, it is a greenhouse, there is a lot of moisture in the air, fertilizers and occasional insecticides are used. The fans are sold specifically for greenhouse use and the company is well-known and has been around for a long time, so I can only hope that they have designed with the environment in mind.

Bob

Right, 1/2 HP rating.

I am wondering whether that is really enough to drive a 36 inch fan at the speed it is trying to go.

The voltage seems to be fine, I have looked a couple times. The current is running right up to the nameplate rating, so there is certainly no spare capacity in the motor.

Bob

Im going out on a limb here and saying the motor is too small or its geared too high. Probably geared too high would be my guess..

Ive put a lot of motors into machinery over the years..and a 1/2hp motor seems a bit small for a 36" fan run at "high speeds"...but its not part of my expertise. That being said..Ive got pedestal fans out in the shop that are spinning 36" fans out in the shop on 1/2 hp motors..but they are turning fairly slowly and are direct drive with no venturi effect.

Mine are all about 1500 rpm if I recall correctely.

Gunner

What does the Company say? He should have better knowledge than we can.

Have you tried running with the belt off? That should give you an idea if they are overloaded. Do the pulleys increase the speed or decrease it at the fan? IE what's the ratio? You should be able to get the expected temp rise from the motor specs.

Progressive failure as you describe sounds very strange. There really should be some failure to observe and fix.

Muffin fan motors are simple shaded-pole driving a small blade, not an induction driving a large blade.

It's easy to rig a belt-drive so the motor is overloaded - You have to look at the Full Load Amps and the Service Factor, and make sure the actual draw is under the nameplate ratings. If it's not, you have to make the motor pulley smaller to drop the fan speed and the load.

If it doesn't have the variable-diameter pullies, you need to add them. Or you have them and they've rusted solid at the old setting, which means dismantle and derust.

And check the line voltage available at the fan motors - If they are

120V motors, anything below 110V is trouble. You could have a bad wirenut that has rusted out halfway to the greenhouse too. This is why they use the 240V motors - half the current means half the voltage drop. You might have 120V at the house, but the Greenhouse is 250' from the house which means 500' of wire...

If the pipe is big enough you can run #6 Copper or #4 Aluminum wire and solve the voltage drop issues - if it's 1/2" conduit and already at fill, you'll need to bump the voltage up instead.

Are these 1725 (1800) rpm motors when 1125 (1200) should be used?

Well, there is some good news. The company is going to send replacement motors at 3/4 HP rather than 1/2 HP. If it is a simple overloading issue then maybe that will solve it.

Bob

They sent three new motors, rated at 3/4 HP rather than the original

1/2 HP.

They are physically more impressive. They have the same diameter as the original motors, but they are 9 1/2 inches long rather than 7 1/2 inches long. They also have an external capacitor cover; the original motors presumably have the capacitor within the motor housing, so the actual motor part is not as long as it looks.

The new motors are rated at 12 A rather than 7A.

I opened the first box and pulled out the motor, and the shaft was frozen so it wouldn't even turn. I know that some of you have good results with these guys, but I have serious doubts about their QC.

The other two motors are installed and running, at least for the first few days...

Thanks for all the ideas.

Bob

Pull off that cover on the ass end and see if the fan is hanging up on the tail end of the cover.

Ive had a few recently where the handling left something to be desired where they evidently dropped the motors on the fan housing and bent the sheet metal. I hammered the sheet metal back in place and they ran fine

And I've had one where the fan was pressed on too far and rubbing on the front housing. Eyes open, guys! QC ain't what is used to be.

Any more, "ISO 9001" means 'Someone filled out the right paperwork.' They can still build 'em wrong, pack 'em wrong, send 'em to the wrong address, etc.

ISO 9000 just means you are getting the best documented junk you can buy.

I would look into incorrect application.

If the atmosphere has excessive humidity, salt dust, and such, perhaps the OP needs to buy totally enclosed hostile duty motors. Also, possibly, motors are undersized for the fans that they are turning, and are overloaded.

i