Exhausting DC Outside: Is it safe for the motor?

[Augusto Orosco]

I have the option to exhaust my DC directly outside during the warmer months. My shop is in an attached two car garage. The garage has no heat or AC; so it doesn't share any piping with the house.

My question regards the DC motor (it's a Griz G10292Z). I read a few threads here that warn about the motor being taxed too much if it's used without the bags. Terms like negative pressure and others beyond my understanding were used, so I wanted to ask the question in plain terms:

Would it be safe for the DC motor if I get rid of the bags and exhaust directly outside? I do this as a hobby, so the DC (and all my tools for this matter) gets only very light use.

Thanks!

[Keith Outten]

Augusto,

I have been exhausting my dust collector outside for over 16 years and my DC motor seems to be just fine

No bags, no filters, no restriction, great performance

For the record my DC has a lot of hours on it mostly because my ShopBot CNC Router runs for long periods of time. I have a planer that has also put a lot of hours on my dust collection system through the years. If the motor in the DC died tomorrow it wouldn't owe me a thing.
.

[Bill Stoffels]

I beleive and willing to be corrected the issue is that with no discharge backpressure the rotor can "overspeed" and draw more than the required Amps in this condition.
most motors have an internal overload to prevent damage but I would monitor the motor temp as you first use it
Bill

[Rod Sheridan]

The issue is that as restrictions drop, airflow increases.

As you start pumping more air, you consume more power.

The motor can become overloaded, the only way to check is with an ammeter to measure motor current, when your fan is pumping the most air. (Blast gates open).

If your motor is overloaded a simple intake or exhaust restriction will lower the motor load.

Regards, Rod.

[Don Abele]

Augusto, I agree with what the others have said and have seen it in real life use. Most of the shops I've seen/worked in for the Navy all have LARGE central collection units (cyclones) that vent directly outside. Most use a dumpster for the collection bin. Never heard any problems with it.

However, the unrestricted running of a blower motor WILL overspeed it, causing it to draw too many amps. It will then either self-protect (breaker) or the magic smoke will be released.

With that said though, the ducting on the front end of your blower will restrict the air that the motor can draw. We (hobbyist woodworkers) tend to use 4" ducting, spiral, or long lengths which tend to add a lot of resistance. So I think the issue really is about turning your DC on with no ducting and/or bags attached, thereby allowing it to run unrestricted.

If I could vent outside, I would do as many Creekers do and nix the bags/filters.

Be well,

Doc

[Rick Christopherson]

The information regarding backpressure is backwards, and it relates to the inlet, not the outlet of the blower--at least as stated. Unrestricted exhaust is not going to increase motor load.

If you remove restriction from the inlet, then the blower can be over taxed because too much air is present than can be evacuated. If you block the inlet, then the blower operates in a vacuum condition, and motor load decreases.

The reverse is true at the outlet. When you remove restriction at the outlet, the pressure differential between the inlet to the outlet is reduced and the motor doesn't need to work as hard. When you obstruct the outlet, the pressure differential is greater, and the motor needs to work harder.

The volume of air to be moved is dictated by the inlet, not the outlet (within reason), and the pressure differential is dictated by how easily this volume of air can be expelled.

I am not sure where this misinformation originated, because I haven't heard it stated before, but I am willing to describe this in greater detail if necessary.

[Tom Veatch]

...Would it be safe for the DC motor if I get rid of the bags and exhaust directly outside?...

Augusto, since no one reading this (I assume) is completely familiar with the exact details of your system, there is no way anyone here can absolutely, definitively, and with certainty answer your question.

It is very, very unlikely that simply removing the filters will damage your motor, but the only way to know for sure is to measure the amperage through the motor when it is operating. If the amperage the motor draws under load is not greater than the full load amperage (FLA) shown on the data plate, the motor isn't being overworked and will not be damaged beyond normal wear and tear.

[Rick Christopherson]

Augusto, since no one reading this (I assume) is completely familiar with the exact details of your system, there is no way anyone here can absolutely, definitively, and with certainty answer your question.

No Tom. It has nothing to do with the details of his system. It is physics, and it can be definitely answered, and I just did.

Whoever is propagating this misinformation, I ask that they explain under what realistic condition a dust collection system will increase the motor load when the exhaust backpressure is reduced. Work is work, and the amount of work the motor is performing is directly proportional to the work of the blower, and there are proper physics equations for this. With the input unchanged, when you reduce the output pressure (pressure differential) you reduce the amount of work.

I am sorry if this is some sort of previously ongoing discussion that I was unaware of, but if I had known, I would have nipped it in the bud a long time ago.

[Tom Veatch]

Sorry, Rick, but I have to disagree. You're right that the work load on the motor is totally dependent on the product of flow rate and pressure rise across the blower.

But, it doesn't matter whether a flow restriction is placed on (or removed from) the inlet or the outlet. You can choke the flow through a blower by blocking the outlet (clogged filters) or by blocking the inlet (close the blast gates), or increase flow by removing blockage from the inlet (open the blast gates) or outlet (clean or remove the filters). The effect on the blower and motor is independent of whether it takes place upstream or downstream of the blower. The only thing affecting the blower is the flow rate through it and the pressure rise across it.

Will the blower do more or less work if the filters are upstream of the blower than it will if the filters are downstream of the blower? Will a battery put more amps through a load if a resistor is in the positive lead that it will if the resistor is in the negative lead?

I will agree that the static pressure inside the blower will be lower if the restriction is placed upstream and will be higher if the restriction is downstream. But the static pressure is immaterial. It's the differential in total pressure at the inlet and the outlet that's the significant parameter.

[Scott T Smith]

Tom and Rick - this is very informative, and I'm learning a lot. I have a question though...

I know that some pumps (air, water, or whatever) are designed to push, and work poorly at suction. Other pumps are designed to have a high suction, and a relatively unrestricted exhaust. This is especially relavent in pumps for moving water and fluids (some are designed for high head - ie high suction, at a loss of overall volume).

Wouldn't the influence of pressure reduction on the intake versus exhaust be dependant upon the impeller and pump chamber design (or whatever you call the chamber surrounding the impeller on a dust collection system)?

Also, since most electric motors are designed to operate at a specific RPM, unless there was pressure coming into the intake why would there be a chance of the motor overspeeding? You'd still have a degree of friction involved with the suction forces in the intake.

I'm not disagreeing with y'alls observations, merely trying to better understand all of the contributing factors and thus add to my knowledge. Thx. Scott

[Rick Christopherson]

The only thing affecting the blower is the flow rate through it and the pressure rise across it.

This contradicts what you implied in your previous posting, and it is what I had stated in mine. When you reduce the pressure differential, you reduce the work. It takes very little work to move a large volume of air across a low pressure differential. With all other things remaining equal, when you reduce this pressure differential on a DC, you will reduce the work. The flow rate will remain limited, and will not rise proportionally.

But, it doesn't matter whether a flow restriction is placed on (or removed from) the inlet or the outlet........Will the blower do more or less work if the filters are upstream of the blower than it will if the filters are downstream of the blower?

Yes, it makes a huge difference. When you obstruct the inlet, you reduce the volume of air available to the blower. When you obstruct the exhaust, you increase the pressure differential. The motor will work less and require less power when the pressure differential is reduced.

However, based on this last statement of yours, I suspect you are confusing the overall system throughput with the question that the Original Poster asked.

[Rick Christopherson]

Also, since most electric motors are designed to operate at a specific RPM, unless there was pressure coming into the intake why would there be a chance of the motor overspeeding?

You are absolutely correct. I can't remember where I just read this before you posted it, but there is absolutely no merit to it.

[Bill Stoffels]

Rick and Tom

If, there was a dust collector running with an 8" opening "inlet" and there was a direct discharge with no restriction on the outlet also say 8"
if the motor was running hot " meaning there is to much work.??
Would the best solution be to reduce the inlet to 4" or install filters on the back end??
that is to get the motor to run cooler take less amps??


Thanks in advance
Bill

[Tri Hoang]

I think Rick stated it well. I've vented outside for a period of time without any problem. The restriction is in the inlet.

The information regarding backpressure is backwards, and it relates to the inlet, not the outlet of the blower--at least as stated. Unrestricted exhaust is not going to increase motor load.

If you remove restriction from the inlet, then the blower can be over taxed because too much air is present than can be evacuated. If you block the inlet, then the blower operates in a vacuum condition, and motor load decreases.

The reverse is true at the outlet. When you remove restriction at the outlet, the pressure differential between the inlet to the outlet is reduced and the motor doesn't need to work as hard. When you obstruct the outlet, the pressure differential is greater, and the motor needs to work harder.

The volume of air to be moved is dictated by the inlet, not the outlet (within reason), and the pressure differential is dictated by how easily this volume of air can be expelled.

I am not sure where this misinformation originated, because I haven't heard it stated before, but I am willing to describe this in greater detail if necessary.

[Rick Christopherson]

Rick and Tom

If, there was a dust collector running with an 8" opening "inlet" and there was a direct discharge with no restriction on the outlet also say 8"
if the motor was running hot " meaning there is to much work.??
Would the best solution be to reduce the inlet to 4" or install filters on the back end??
that is to get the motor to run cooler take less amps??


Thanks in advance
Bill

Geez Bill, Ya hit me with an email, then PM, and now the same question here. For a second there, you had me doubting my sanity.

Yes, obstructing the inlet will reduce the load on the motor.

Here's an experiment for you:
Most shop vacs won't let you completely obstruct the discharge (for this very reason), but if you did, you would notice the motor slowing down. This is because the motor is working harder.

Conversely, we've all heard what it sounds like when you block the inlet--the motor speeds way up. That's because the motor is not working very hard at all.