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Author Topic: Sawdust blower motor failure due to inadequate restrictions....  (Read 575 times)

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Offline Percy

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I had the sawdust blower motor fry on my 3 month old Woodmizer MP360. One of the thermal protection shutoff wires was not properly installed at the factory. Gary from Salmon Arm Woodmizer fixt me up quickly with a warranty replacement(thank you soooo much). I took the failed motor to a local rewind shop(Terrace Rewind) and their diagnosis was the motor had been overloaded. I wanted to determine why it was/had been overloaded. After installing the new motor, making sure all connections were proper and tight, I fired up the generator, turned on the blower system and tested current draw. The motor was drawing 17 amps. The tag on the motor said 14.9 amps was its rating. Soooo, I disconnected the  discharge to cyclone pipe right at the blower itself and tested current draw....it went up to 20 amps. That confused the heck outta me till I did some reading....Apparently, the less restrictions there are on a centrifugal air blower, the harder the motor works.....When I built the blower system, I used 10 inch pipe thinking bigger was better....nyet!!! Im now replacing 30 feet of 10 inch with 8 inch. Im hoping to get it dialed in at around 14 amps by using a combination of 8 and 10 inch pipe...Ill let you know how that works out....
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Offline longtime lurker

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Re: Sawdust blower motor failure due to inadequate restrictions....
« Reply #1 on: April 13, 2019, 05:16:52 PM »
NOOOOOOOOOOOOOOOOOOOOOOOOO.

Do not replace the pipe.

You replace the pipe in 5 years you're upgrading blowers for more capacity and you need a pipe upgrade too.

Just put in a gate and create a restriction.... air cant flow in faster than the choke point so you get the same effect for a fraction of the cost. Think of it as putting a tap on a water pipe... no matter the size of the pipe you cant get more water out than the tap is open.

Theres a velocity effect - like big pipe with small tap will give you a pressure increase - you'll get more suck from it but you only need to drop the size of the last bit of pipe or put a second gate into the line to reduce that significantly.

I know a guy whos a backyard guru when it comes to dust systems... retired engineer type who understands the numbers and is happy to bore you to death with them... if you want I can give him your email address. 


The quickest way to make a million dollars with a sawmill is to start with two million.

Offline barbender

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Re: Sawdust blower motor failure due to inadequate restrictions....
« Reply #2 on: April 13, 2019, 07:04:15 PM »
Yep, I was surprised to learn that lessening the restriction made the blower motor work harder. I'm trying to think of an analogy, how about the restriction works like the rakers on a chainsaw chain? If you remove or lessen the restriction, it's like lowering your rakers too much. The tooth takes too big of a bite (or the blower vane gets too big of a bite). 
Too many irons in the fire

Offline YellowHammer

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Re: Sawdust blower motor failure due to inadequate restrictions....
« Reply #3 on: April 14, 2019, 12:46:46 AM »
Centrifugal fans can move more air if their airflow restrictions decrease or their RPM increases.  As they move more air, they require more power.  If you decrease the airflow restrictions by increasing pipe size, then as airflow goes up, the power required to move the greater amount of air increases also.  The electric motor has to work harder, and pulls more current. 

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Offline Don P

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Re: Sawdust blower motor failure due to inadequate restrictions....
« Reply #4 on: April 14, 2019, 07:51:30 AM »
Yup, air has weight and mass and this is a pump. The airflow has pressure, volume and velocity related to it. From memory ::) if you double the airflow you cube the hp requirement. General fan stuff I remember, width of the fan relates more to volume, diameter relates more to pressure, but if you take a large diameter high pressure fan with longer vanes and cut a large intake opening it doesn't create much pressure any more. I think the happy place is somewhere around 40-50% of fan diameter. What was the diameter of the stock intake?

 If you decrease the airflow by creating a restriction at either end of the pipe the velocity within the pipe decreases. That might be ok but watch for material dropping out of the airstream and filling the pipe.

Is the fan driven directly by the motor or belt driven? If belt driven the first cheapest place to play might be with pulley changes, slow it down a bit, that cubing works to your favor at that end, a small decrease in speed reaps the largest reward in hp requirement, if you don't drop material out.
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Offline Percy

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Re: Sawdust blower motor failure due to inadequate restrictions....
« Reply #5 on: April 14, 2019, 11:17:16 PM »
Yup, air has weight and mass and this is a pump. The airflow has pressure, volume and velocity related to it. From memory ::) if you double the airflow you cube the hp requirement. General fan stuff I remember, width of the fan relates more to volume, diameter relates more to pressure, but if you take a large diameter high pressure fan with longer vanes and cut a large intake opening it doesn't create much pressure any more. I think the happy place is somewhere around 40-50% of fan diameter. What was the diameter of the stock intake?

 If you decrease the airflow by creating a restriction at either end of the pipe the velocity within the pipe decreases. That might be ok but watch for material dropping out of the airstream and filling the pipe.

Is the fan driven directly by the motor or belt driven? If belt driven the first cheapest place to play might be with pulley changes, slow it down a bit, that cubing works to your favor at that end, a small decrease in speed reaps the largest reward in hp requirement, if you don't drop material out.
The fan is driven directly by the 7.5 hp 3phase motor. I changed most of the pipe to 8 inch and that only dropped the amp draw about 1 amp....The smaller pipe did increase the velocity before I put a restrictor in as Longtime suggested. After tons of reading, I placed the restrictor right at the blower output. 

NOOOOOOOOOOOOOOOOOOOOOOOOO.

Do not replace the pipe.

You replace the pipe in 5 years you're upgrading blowers for more capacity and you need a pipe upgrade too.

Just put in a gate and create a restriction.... air cant flow in faster than the choke point so you get the same effect for a fraction of the cost. Think of it as putting a tap on a water pipe... no matter the size of the pipe you cant get more water out than the tap is open.

Theres a velocity effect - like big pipe with small tap will give you a pressure increase - you'll get more suck from it but you only need to drop the size of the last bit of pipe or put a second gate into the line to reduce that significantly.

I know a guy whos a backyard guru when it comes to dust systems... retired engineer type who understands the numbers and is happy to bore you to death with them... if you want I can give him your email address.



Was a little late reading your post. Thanks for the offer about your guy. I think Im good. Im using that snaplock pipe in 5 foot lengths and 30 feet of it was less than 100 canadian so its not breaking me .....PLus I got some 10 inch laying around with all my other mistakes...We call it "The Field Of Broken Dreams" :D


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Offline pineywoods

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Re: Sawdust blower motor failure due to inadequate restrictions....
« Reply #6 on: April 15, 2019, 10:33:18 AM »
Hmmmm. Learn something new every day.. Maybe that explains why I have gotten away with using a 2 hp harbor freight sawdust  blower. My inlet is about 4 inches square ,that's 4X4 = 16 sq inches. Output is 5 inch pipe.  Lucky beats good most of the time.. ;D
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Offline moodnacreek

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Re: Sawdust blower motor failure due to inadequate restrictions....
« Reply #7 on: April 15, 2019, 01:23:07 PM »
This is interesting. I have used blowers for a long time, always belted to something with plenty of power. I wonder what happens to the power draw when pulling sawdust.

Offline YellowHammer

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Re: Sawdust blower motor failure due to inadequate restrictions....
« Reply #8 on: April 15, 2019, 03:07:38 PM »
With a blower unit (blower and motor), there are two things happening, first, the blower fan is designed to be most efficient at a certain RPM and flow rate, this is called the "Design Point" or if designed to operate over a range of RPM its called the "Design Curve".  The power that is requires to operate is called "absorbed power" but is really a measure of absorbed shaft power required for it to maintain RPM.  Most fans are designed and tested using a device that measure absorbed shaft per per RPM, such as a dynamometer.  The size of the ducts are taken into the design and are consider basic inlet and exhaust losses.  Easy to match and add to the calculation.

Second, only after a blower is designed, built and tested and its absorbed power characteristics "mapped", is a power unit matched to that fan.  Generally a fixed RPM electric motor is used in an industrial application, especially ac motors because they will operate at fixed RPM within their power range.  DC motors are different, but can be stepped using an encoder to be controlled to maintain RPM or even change RPM as desired.  

Anything that disturbs the flow rate and moves the fan off its design point will cause the fan to become less efficient and move less air.  As it moves less air, it requires less power.  Basically, it starts "slipping". Some fans are designed as "Clean Air" fans or blowers, some are designed as "Trash Fans".  A good example is that many of the blowers mounted on top of cyclone dust collector systems are only designed for clean air, and the cyclone must spin the debris out before it gets to the fan for it to operate efficiently.  In this case, the fans can be designed to ingest clean air, with very close tolerances, and minimal efficiency losses.  A good example of a clean air fan is any fan mounted to the top of a vacuum bag cyclone duct collection system where the debris is spun out before the clean air reaches the fan.

On the other hand, trash fans are intentionally designed for debris and dirty air, with larger tolerances, more open blade clearance, and therefore are much less efficient per HP.  Basically a trash fan will let debris laden air slip through its sloppy tolerances, but will not jam or fail like clean air fan would.  A good example of a trash fan is any dust collector where the wood chunk and chips are blown directly through the fan then exhausted into bags.

This can have real life consequences, as some of you might remember some years ago when I bought an Oneida cyclone for a pressure side dust collector but could not use an Oneida blower for that configuration.  Oneida only produces clean air fans used on the low pressure side of the cyclone, where I needed a trash fan that operated on the high pressure or exhaust side of the cyclone.  Since I was operating the Oneida cyclone backwards, I couldn't use their fans, and they sold it to me with no warranty and with no blower.  However, Timberking/Woodmaster makes a very excellent trash fan that worked perfectly.  Its all in the design and intended use.  

Anyway, probably more than anybody wanted to know.

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Offline samandothers

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Re: Sawdust blower motor failure due to inadequate restrictions....
« Reply #9 on: April 15, 2019, 07:50:05 PM »
Yellow Hammer the explanation is good.  I have an old Grizzly bag DC. This was designed to take the dust/chunk laiden air and push it into the bags.  The input was four 4" ducts reduced into a 9 or 10" opening on the center of the fan housing.  The fan is about 14" or so with extra space around it in the housing for the fan/blower. Then there are two out puts from the fan housing 180* apart into the bag stacks.

My plans are to use this as the basis for a cyclone based system.  Use 6" duct from shop into the cyclone and then duct into the center of the fan housing from the cyclone.  The fan will then be seeing clean air.  This will then feed out to two filter canisters versus the old bags which were larger micron.


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