Troubleshooting an Under-Loaded Dust Collector Fan Motor

      When a dust collector fan keeps drawing too many amps and trips the circuit breaker, troubleshooting involves comparing the system's capacity to its actual operating load. April 29, 2006

I just put in a used Torrit 15hp direct drive cyclone dc. It has a 30" diameter cyclone, a rotary air lock and outdoor baghouse with 380 sq. ft. of filter cloth and return air duct into shop. We have about 100' of ductwork from 12"to 6" in diameter. Everything seems fine except the motor draws way too many amps. It moves a tremendous amount of air for 15 hp. My electrician does not think it is the motor, but an air issue. If we close all the blast gates, the amps drop to normal and then raise as we open more gates. It should draw 37 amps, but draw up to 56 amps with 5 or more 6" gates open. Is it possible I have the wrong fan design in the housing for my use?

Forum Responses
(Dust Collection, Safety and Plant Operation Forum)
From contributor L:
As far as I can see, 56 amps would be normal for full load. I am assuming 208VAC. 1HP = 746 Watts of power at 100% efficiency. 15HP x 746W/HP = 11,190 Watts. 11,190 / 208VAC = 53.8 Amps. Remember, this is a 100% efficiency calculation. Does your motor have a phase multiplier? Not all motors have 1.0 rating, i.e. some have a 1.3 or higher, which would make current draw higher under load. I think your motor is doing fine, but I am not an electrician.

From contributor B:
Sounds to me like your system is way over capacity for your shop. You'd be fine with a 7.5 hp, 24" body Torit pulling 3200cfm through the 12" main. That would net you 4076 FPM velocity.

You also might consider just reducing your intake to 10" from the present 12". That would create a pressure drop, lowering airflow as well as amp draw. What is your shop's total air requirement? You could easily operate 3 6" drops, simultaneously with the 10" pipe.

You can also operate more drops simultaneously, but the reduction will increase SP resistance, reducing your present flow and amp draw. As is presently, your dc will pull between 4000-5600 cfm. That's 7-10 6" drops open simultaneously. Depending on how you do the reduction, you may not even require changing the 12" main. Just add a short reducer.

Let me know how many drops you want to run simultaneously, and I'll show you how to add that reducer without creating a potential clogging scenario behind the reduction. I need total number off drops, and total required cfm at any given moment.

From the original questioner:
Will I be wasting electricity by restricting? Can I put a smaller motor and fan in my unit instead of restricting? I know Torrit uses the same cyclone for several different hp ratings. I need 5-7 drops open. Mostly 6" diameter. My biggest dust requirement is a 2 head widebelt with 2 6" intakes. The rest are standard shapers, tablesaws, jointers and such.

From the original questioner:
I thought I would add this - I replaced an indoor 7.5 hp Aget cyclone unit with this Torrit. The Aget seemed too small if 4 or 5 gates were open. It had a 10" main. Was going to go with 10hp, but found a very good deal on this 15hp. I have a 20hp Torit in my moulder shop, which works great. It has a 14" main and only about 12' of duct before the 6 head moulder. Why would this one work so well and the 15hp doesn't? The ductwork seems proportionate for each to me. If anything, the 20hp is way less restricted.

From contributor B:
I'm not familiar with the 7.5HP Aget cyclone. As is, I have to work from assumptions, which are the following. 1. The motor appears to function properly, so the problem is not purely electrical. 2. You have all 7 drops, functioning simultaneously, no blast gates closed. 3. The 15 hp Torit is equipped with a B.I. (back-inclined) fan-wheel.

Based on those assumptions, you should be comfortable with 5600 cfm total draw, at all of the 6" drops, each pulling roughly 800 cfm, for a total 5600 cfm. If that's the case, you should be flowing 7140 FPM velocity through the 12" main.

External SP resistance, or the total resistance of your ducting, should not be less than 4.9" SP. If it is, you'll draw too much air, and amp draw will increase significantly. There are several ways to control this. You can do it on the intake side, or the outlet side.

As far as your current outlet is concerned, you're set up for very low outlet restriction with 380 sq.ft. of cloth. That's almost double the surface you usually find on the 15HP Torit, which is normally 200 sq.ft. This is not necessarily a bad thing. You'll have lower pressure drop as the filters tend to load; also, your small particle filtration will be enhanced, due to the low air/cloth ratio. Air velocity through each sq. ft. of filter is almost half of what it would normally be. The system is very free breathing.

On the intake side, with 7 gates open, there is very little resistance through each drop, and if each were operating at 800 cfm, you'd be at the peak air volume for that dc. I suspect the low resistance of those 7 open gates is allowing more than 800 cfm/drop, and that you're flowing more than 5600 cfm through the main. Velocity through the main may be more than 7140 fpm with all blast gates open, and that, would be the problem. The system would be too free-breathing.

If that is the case, you'd need to restrict airflow slightly by adding resistance. That's not a concern. You'd still have all the velocity required at all your drops, and more than enough through the 12" main. I suspect that as you start closing blast gates, you'll see the amp draw lower, and that is due to the increase in resistance. You can do two things. Either close some of your unused blast gates when they aren't required, or put the restriction in the main. Either will get you where you want to be.

Without actually doing a flow test, your amp draw will be the indicator. Try closing half of your gates, then take a reading of your amp draw. If it's normal, then proper restriction is the key. As I said earlier, that can be added either at the intake side or the outlet side. I'll help you with that. For now, take an amp reading by progressively closing some gates until your amp draw settles. Let me know how many gates are open, and we'll take it from there. This should be an easy solution.

From the original questioner:
Thanks. We had already done amp tests, which made me realize I have a problem. 43 amps with all gates closed and going up from there as gates were opened. We had also tried restricting the return air, which helped some but started popping #10 self tapping screws out that hold the one removable exterior panel onto the baghouse. I can't believe how much air this thing can move! I agree that it is moving too much air and overloading. I will try restricting the 12" main trunk. In reality, we will probably only have 4 or 5 gates open, but I would like some room to grow. As I asked earlier, is a smaller motor (10hp) and fan from Torit a more efficient (although costlier) answer?

From contributor O:
We installed a 15 HP Hartzell fan on an Aget bag house this summer - wonderful suction. The motor draws 32 amps - most branches do not have a gate on the last port on that branch. I'm almost sure that this amp rating is close to the motor plate rating. We have an 18" main duct, which I think is calculated at just over 3000 fpm. The salesman insisted that we put a damper on the outfeed side of the blower in case we did draw too much air and subsequently amps, but I have it close to wide open. I'd put a restriction there rather than in the main duct, as I think that would clog things up. Although that cfm through a 12" pipe means the air is moving right along.

From contributor B:
That's the way I see it so far, also, contributor O. Restriction at the outlet would be the least expensive and easiest to do. Won't hurt the filtration either; in fact, it will be enhanced. A new fan wheel may cost a bundle. Is there any way of checking the diameter of the current wheel? Perhaps it was modified from stock version. I wouldn't be surprised, after seeing the current filter arrangement with 380 sq. ft. of filter. Stock is usually 200 SQ. FT. Is the outlet a stock 14" outlet?

From the original questioner:
The outlet is a stock 14", which I have reduced to 12" diameter just before the baghouse. I bought the unit without a baghouse and married it to a baghouse I already had. The fanwheel is only about 1/2" smaller in diameter than the opening for it to fit through in the top of the fan housing. We had it apart to check it out, but did not measure it. It would be easy to measure. What is the stock diameter? I checked with a fairly local Torit dealer and they ran the numbers and also recommended restriction or a 10hp fan package to replace my 15hp at a cost of $1067. I am not opposed to the price if it fixes the problem. I also like the economy of running 10hp instead of 15hp if it is what is best for my setup. I would keep the 15hp in case I grow into it. Does this sound right to you or am I throwing money away?

From contributor B:
Actually, I'm shocked that price is so low. Is that with a trade in? You still may want to try reducing that outlet a little more. That certainly would be the cheapest fix, and easy to insert a short length of 10" pipe after your present 12" reducer. Your current shop requires roughly 5600 cfm, at fairly low external resistance. That Torit will deliver that flow even with 11" SP resistance. That thing is King Kong on steroids. Wouldn't surprise me a bit if you're pulling over 7000 CFM.

From Curt Corum, forum technical advisor:
Super troubleshooting on behalf of all of you. I believe the model you have is the Torit 30-15 FM Cyclone. Multiple rating table is as follows :
5600 CFM @ 4.9" SPWG
5200 CFM @ 6.3" SPWG
4800 CFM @ 7.9" SPWG
4400 CFM @ 9.5" SPWG
4000 CFM @ 11" SPWG
Fan Size 17 1/2" x 3 1/2" Radial Design.

I have installed numerous Torit cyclones, but have not encountered a too low resistance situation. I have mostly had the problem with fume collection blowers, as those systems usually have a lot less external resistance. We would install a half blast gate just after the blower outlet to purposely induce resistance, as you have discussed. We would mount it on top of the duct, slide the blade 1/4 or 1/3 in, and pin it with the set screw. Sometimes they were so free wheeling that they would actually trip the breaker. I know your branches are larger than typical, being mostly 6", but with all of the duct work, bag house, and return duct, I would think you have at least 7" - 8" of external static. Even the 12" main should be creating resistance due to the fact that if you were even pulling 4800 cfm, it would be traveling in the 12" main at nearly 6,000 fpm and have a resistance of about 4.5" static pressure per 100 feet of run. Also, if you have a 12" 90 in the run, it is equal to 25 feet of straight pipe in equivalent resistance. Seasoned filters in the bag house would add some additional resistance if they are clean now.

I can only offer a few more possibilities. All duct connections must be sealed air tight. All of the machines should be fully tied in with flex, etc. The air lock must be attached to the bottom of the cone before you fire up the blower. Otherwise, I am baffled, especially with the 12" main, which is typical for a 10 hp duct system vs a 15 hp.

From the original questioner:
Curt, thanks for the response. I have tripped the breaker several times for no apparent reason, so I must be drawing too much air. All my connections are sealed and tight. My filter bags are fairly new.

Contributor B, thanks a bunch for your insight. Because of your recommendations I was finally able to restrict the exhaust side (before the baghouse) enough to make the system work for now. It is now pulling the max amps the motor plate allows and has average suction. I think I will spend the money for the 10hp fan package from Torit. I hope it works.

From contributor B:
Glad that quick fix worked out. The permanent solution might be a proper analysis of your present ducting. We really didn't examine that properly.

I gathered you have a 12" main, with all 6" drops. Seven of them, in fact. Does your main reduce in diameter, after the first one or two drops? It should. At the far end, for example, if you have two 6" drops close to each other, those should flow into an 8" pipe. Where the third drop meets the main, increase to 10". Fourth and fifth into 11", then the last two, into the 12" main. You may also want to take a closer look at those 6" drops. Do you really need 6" at all stations?

Seven 6" drops, each flowing 800 cfm, is fairly low resistance, since velocity through each is barely over 4000fpm. By decreasing the diameter of those which aren't necessarily a 6" requirement, you may find you flow the ideal air volume, albeit at a higher velocity. Suction will appear stronger. Try reducing three of them. Tweak it as desired, perhaps only two, or perhaps four.

The increased static pressure resistance should bring you to an ideal overall flow. A bit less volume than before, higher velocity through all the drops, and therefore, higher sp. Lower power consumption.

As for the other 6" drops, you'll move more air through those than you did before. That happens automatically because the system wants to balance itself, static-pressure-wise. So instead of 800 cfm though each of the 7 6" drops, you may get 700 cfm through each of the reduced ducts and an increase to 900 cfm at each of the unreduced ducts.

This would be very easy and cheap to test. Try the three closest to the dc, first. If you're happy with the results, I wouldn't bother upgrading the fan and motor yet. We can fine tune this even further. Just use cheap HVAC reducers for the test. Only about $2-$3 each. Try perhaps four or even 5 reducers. Don't worry about main velocity at this point. Just the amp draw and the suction/airflow at the intakes of the drops. We'll nail this sucker yet, and hopefully, for peanuts.

From contributor B:
By the way, when you do that test, remove the outlet restriction you added, but only after you see a drop in amperage from reducing a few drops. Don't even think about the suction, or volume, at this point. After removing the outlet restriction, you should be amazed at how much suction you'll have at the unrestricted 6" drops. Just keep reducing more of the ones that aren't totally necessary. What I'm trying to achieve is to put your system in proper balance to the very high static pressure capabilities of your dc. Just trying to do it in a way that you'll gain the most benefit - increased suction strength.

From contributor B:
One more observation. As Curt pointed out, velocity through the 12" main should be high enough to get you the proper resistance, but on closer observation, it is not. Only a very short portion of that 100 foot long 12" main is operating at that velocity, and that is where your closest two drops to the dc meet the main. All of the other length is way under, and that's why you're moving so much air and using so much power. I'd bet you're moving well OVER 5600 cfm. Perhaps as much as 6400-7000.

From the original questioner:
My main does reduce 1" in diameter after each 6" drop. Each 6" drop serves 1 or 2 tools like 2 shapers or a small bandsaw and spindle sander. The 2 tools may or may not be used together but are independently gated with 4" gates after a 6x4x4 pant or lateral "Y". My total duct length is about 100". From the cyclone we have ~14' of 12", two 6'drops, then 10" main for 6' a 6" drop, 9" main to next drop, etc.

From the original questioner:
When I did the amp test, I was at max amps with all gates closed. We had to put in a restrictor with only a 5.5" diameter hole after the cyclone exhaust to get the amps even that low. A smaller hole in the damper made the amps go down, but there was not much suction. A larger hole made us draw too many amps. Also, many of the 6" drops have two 4" gates that were opened independently during the amp tests. Would that restrict the 6" drops in the manner you suggest? When we open only 1 or 2 gates, we are already maxed out on amps.

From contributor B:
Okay. Now I have a much clearer idea of what your main duct looks like, and that should help. I want to be clear on something else. You said you closed all gates at every drop? And still had excess amp draw? Let's eliminate something right off. Is the rotary air-lock functioning properly? Any chance that its drawing air? Are the joints on the main sealed properly? What happens (amp-wise), if you close half of your 4" gates, say one of every two on the 6" drops?

I'm trying to get you up to at least 4.9" WC resistance in your intake ducting. I suspect we'll have to go a bit higher in fact, and that's because you have so little outlet restriction. You're running twice the filter surface usually found in the 15hp model. Typically, that's 200 sq.ft. You have 380.

The Torit chart I'm using shows that at 4.9" WC External SP, you'll flow 5600 cfm, but that's with the 200 sq.ft. of filter. You're undoubtedly drawing more air than 5600 cfm. The excess amp-draw is actually caused by a combination of both SP and CFM. In the absence of an actual air test, or pressure test, we have to do a little detective work to balance the two to your advantage. If we get this right, you'll still be able to use your current dc, and have that future add-on capability without having to change dc's.

PS. If you have any documentation with your dc, especially a flow range chart, that would be a big help. It may say something like 4,000-5,600cfm at 11"WC- 4.9"WC External SP.

From the original questioner:
I have no paperwork for the dc. All the joints are taped. How do I check the airlock for leaks? It seems to be okay. If I close all the gates, I pull 37 amps. That is what the motor plate also says (Baldor motor). I can have about 2-6" and 1-4" open and after that it starts drawing about 40 amps. I do have a restrictor between the baghouse and collector. It has a 5.5" diameter hole in it. The exhaust of the cyclone is about 9"x14" without the restrictor. The intake of the baghouse is 12" diameter.

From contributor B:
I'm wondering if your blower looks like the one in the picture I'm looking at. The outlet you describe is not at all what the chart shows. Also, the airflow chart shows the air delivery at 8" sp. That's about 6400 cfm.

Something really odd here. When I calculate a static efficiency of 85% for your fan, and use 6400 cfm as a volume rate, well, I get a figure of 17.77 BHP, but only if you're pulling that 6400cfm at 12" SP... I'm wondering if the fan hasn't been changed at some point.

From the original questioner:
I mistyped - the exhaust is 9 x 12, not 9 x 14. The motor/fan looks similar, but not just like mine. I do not have the railing around the motor. The fan itself looks different. My fan blades radiate straight out from the center like spokes on a wheel, but of course do not go all the way to the hub. Hope this info helps.

From contributor B:
OK. I think I know the fan wheel you're referring to. It has 11 or 12 blades, right? That's a caged radial fan wheel, if I'm not mistaken.

From the original questioner:
That is exactly what my fan looks like.

From contributor B:
I have zero info on that fan wheel, since it no longer appears on the Torit site. I think they mostly use the B.I. (back-inclined) fan wheel nowadays. I'll crunch a few numbers and see what turns up. I'd need the static and mechanical efficiency numbers, so I'll try a few scenarios and get back to you. It may be possible that the problem is not with volume, but SP resistance in relation to volume. Seems very odd at this point, especially with that 5 1/2" ring you mentioned. Are your drops all flex-pipe? Are they long? Can you estimate total length of all 7 for me? Are they plain-end ducts at the machines, or are you using tapered hoods? As for your rotary air-lock, are you using that feature? Does it appear to function properly? Any chance that it's sucking air?

From contributor B:
I ran a few numbers, and here's what I come up with so far. In each case, I had to use fairly low SE (static efficiency) ratings, which I find a bit surprising. 60%-65%.

6000cfm@12"WC,65%SE= 17.43BHP(brake Horsepower) ,,,,6000cfm@ 11"WC,60%SE,= 18.88 BHP.
With 5000cfm @ 12" WC, and 60%SE, I get 15.73 BHP, and for 4000cfm @ 16" WC, I get 16.78 BHP.

All of these high resistance scenarios would lead to excess amp draw to varying degrees. At, the Bernuolli Manometer is a simple project to test airflow without instruments. Easy to make, using a simple piece of plastic tubing.

From the original questioner:
Each drop is 14-20' long. They have short (2') flex or none at all. Each drop consists of (at the ceiling) a lateral Y off the main followed by a 45 into ~6' of 6" diameter to a 90 going down toward floor connecting to tool with short flex or a 90. Some Y into 2 45's and connect the same way. I use the air lock to dump into an auger to deposit dust on a wagon. It seems to work fine.

From contributor B:
I have no info on that model of the Torit. If you can do a test on one or two drops perhaps that can help. As it is, I'm puzzled at this point. There is no doubt that the B.I. fan, who's data chart I'm using, is more efficient, statically and mechanically. I'll keep my eyes peeled for a chart similar to your current model. One that provides the BHP curve, as well as the SP and CFM. I'll get back to you.

One thing that's really throwing me is the mention of the 5 1/2" reduction on the outlet side. Maybe I'm misunderstanding that. Is your current ducting the same configuration as it was when you were using the other dc? Can you be specific as to why you upgraded? Your ducting, sounds pretty efficient in design, and you should have no trouble flowing 5600 cfm total at fairly low resistance, since each drop would be operating at barely over the required 4000 fpm. The new model BI fan would require less HP to move that same volume of air, so it's understandable that your current setup requires more hp to do the same job. That part is easy. It's that outlet reduction that I don't get. If you're flowing 5600 cfm (800cfm/drop x 7 drops), then I'm sure that's beyond your blower's capacity, but 5600 cfm through a 5 1/2" outlet port?

From contributor B:
One more thing. Do you know if your motor is wired "Y" or "Delta"? Is it taking more than a few seconds for the blower to get up to speed? More than about 4 seconds? Then that would be "Y" most likely. Is the motor drawing excess amps at full speed? Or just at start-up?

From the original questioner:
My motor draws excess amps at full speed any time more than about 4 gates are open. I went with a bigger dc because I relocated it outside from inside and thought I would add some more needed suction. I have 4 guys in the shop and the 7.5 hp wasn't cutting it any more. I increased the main duct diameter 2" for most of its length, but maybe I should not have. I also reconfigured some of the drops because we moved and replaced some equipment. Redoing the drops really didn't change much as far as the dc is concerned, because they are gated anyway. It was suggested to me (by you and others) that my system is too free flowing and I should try to restrict it. I discovered through trial and error restricting on the exhaust between the blower and baghouse to 5.5" worked best, but far from great. If I restricted to a larger diameter, the motor would pull even more amps; to a smaller diameter, less amps, but not much suction. My motor gets up to full speed in about 4-5 seconds. Thank you for all the time you are putting into this - I really appreciate it.

From contributor D:
Have you checked the voltage at the motor when it is drawing high amps? You could have a low voltage problem. Possibly caused by wire size being smaller than required.

From the original questioner:
We did check voltage. I am on 240 volt and the meter showed actual of 239 v.

From contributor B:
I was going to ask the same question contributor D did, so glad we got that out of the way. I'd like you to try something else, just to eliminate the possibility of dirty filters. Close half of your drops, but totally disconnect the outlet. Let it breathe freely, while you do a quick amp test. Let me know what happens.

From contributor R:
The amount of knowledge being shown here is awesome. One thing I haven't seen addressed is the size of the connection between the outdoor baghouse and the shop. Inside the shop, it doesn't matter; outside is a different deal. The unit can't suck any more than it can blow - that is, if it pulls more air from the shop than it can blow back into the shop, it adds work to the motor. More work increases the amp load. Again, I know little, but whatever the airflow into the cyclone is, it must equal the airflow back into the shop, or you're dealing with some negative pressure in the shop, and more amps to pull it.

From the original questioner:
Haven't had a chance to try your latest idea yet. However, I am quite certain the motor will be very overloaded.

Contributor R, good point, though I do not think that an undersized return air duct is my problem. It is 16" in diameter. The duct going into the dc is 12".

From contributor B:
I too think it might get overloaded, based on what you've described to date, however BHP is a combination of both volume (cfm) and SP. Very little makes much sense so far, and in the absence of an air-test, there is precious little else to go on at this point.

The shop-made Bernouilli Manometer takes minutes to make, and costs pennies, and at least would provide some indication of airflow, however limited it is. While disconnecting the outlet may seem a little extreme at this point, it would simply rule out dirty filters. Don't forget to open only a few of your drops first. The second reason I suggested that was because it's quick.

From contributor D:
When you checked the voltage, did you check at the motor and under full load?

From the original questioner:
I checked the motor amps under full load.

From contributor R:
16" sounds okay, but with the system running and at the full amperage, open an outside door to the bagroom. See if it makes any amp difference. Then open a shop door after doing the above, and check for any difference.

From contributor E:
Is it possible you have a bad winding? You can look it up on the web on how do to this with a tester. Have you checked the amperage draw individually for each line? Is the wiring correct? My guess is because you've checked amperage when the fan is under no load, "all blast gates are closed" and it reads high, you've probably got a motor problem. Start there and rule it out first by having it tested.

From contributor E:
I have a 15 hp motor wired 230v 3 phase and the rated amperage is 36 amps. The amperage of your motor should be significantly less than that under no load. Blast gates closed… by chance is your motor running backwards? It's possible.

From the original questioner:
I am starting to agree with contributor E. The motor is not running backwards. I have not checked amp draw on each line. I will do that. I did have the electrician back to double check the wiring very early on and it was okay. We did also early on take a baghouse side off. No change. The bags are only three years old. Is there a way to check the motor without removing it?

From contributor D:
Contributor E is on the right track. Do all of your checking at the motor. Close everything necessary to get the amps down to the rated load and check the voltage and amperage on each leg of the 3 phase at the motor. With the motor running at full load amps, let it run for some time. It should run all day without any problems unless your motor is bad. Early on you said the circuit breaker kicked off - that should not have happened if the motor starter has the proper heaters. The breaker is there only to protect wiring in your building and should be sized according to the wire's current carrying capacity.

From the original questioner:
Thought I'd let everyone know what I ended up doing. I purchased the new 10hp fan package from Torit. Seems to work fine. Amp draw is right where it should be for 10hp, suction is good and I am happy. I should have the 15hp motor checked out to see if it was flawed, but I don't want to spend any money on it. Thank all of you very much for your input.

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