Running DC with blast gates closed??

[Thom2]

Larry and I started this discussion in another thread and decided to move it here to keep from further highjacking the OP's thread.

Here's a synopsis of the start of the conversation

John, I wouldn't let the DC run without at least one gate open -- I think that would put a strain on the motor that couldn't be good for its long-term health. With my system (essentially identical to yours), the debris in the bottom bag barely stirs at all when all the gates are closed; it picks up a little with one 2-1/2" gate open; with either two 2-1/2" gates or one 4" gate open, it swirls like it presumably should.

I do get some dust leakage around the top of the plastic bag. The solution is the usual one: duct tape.

I'm curious to see more comments on this.

My thinking says that if all the blast gates are closed, then you're starving the impeller of air, if there's no air for the impeller to move, then the motor's not doing any work. This thinking comes from the understanding that either running too large a pipe or "free-airin" the impeller will actually burn up a motor. The extra air puts more load on the impeller and take the motor to amperages it isn't designed for.

Is my thinking off base here??

I don't know whether your thinking is off-base or not. Mine says that even though the impeller isn't moving any air, it's still trying to move air.

You know the sound a shop vac motor makes when you put your hand over the end of the hose? To me that sounds like when the motor is working hardest. It would be the same for a DC. So maybe the two most dangerous circumstances for the motor are when it's moving no air at all, or moving too much?

I'd like to see more comments, too.

[Thom2]

You know the sound a shop vac motor makes when you put your hand over the end of the hose?

Larry,

I used to think this, but now I believe it is a bad comparison as the shop vac and DC use totally different impeller designs.

[thrytis]

I don't think shop vacs and DCs can be compared directly. Shop vacs use the air they are moving to cool the motor, so blocking the intake is a problem there. DC motors have their own fans to cool themselves.

It should be fairly easy to see who is right here. Just stick a meter on your DC and compare the current between having your blast gate open and closed. Now who is going to volunteer?

[Garasaki]

In terms of fan performance, it could go both ways.

In the past I've mentioned fan curves...these typically are airflow vs pressure, but often these also include a curve for horsepower vs pressure. The horsepower curve can go either direction, depending on the design of the fan...either lowest HP at 0 flow ("deadhead") or highest HP at that point.

But I don't know how that HP curve on a DC looks. Random glance at a centrifigal (sp?) fan crve, similar to the design of a DC, shows that the HP is lowest at 0 flow and highest pressure.

Really, DC manufacturer's SHOULD be providing us with true fan curves. This would allow us to compare apples to apples amoungst different models. This would also give us the info to answer this question!

[LarryG]

Thinking about this some more, I'm now inclined to think Thom is correct, for the reasons Eric states (motor cooling). When a DC is switched on with all the blast gates in the system closed, it would quickly empty the ducts of air and then the impeller is just sitting there spinning in a (theoretical) vacuum.

There's no question that running the motor with too little resistance (i.e., no ducts or gates to slow down the inrush of air) will soon burn up the motor -- that much is well-documented.

Since posting in the other thread I've had a quick look at the manual for one of the JET models, and also visited Bill Pentz's site. I didn't see any dire warnings about running a DC will everything closed up, so maybe Thom is right.

[LarryG]

Here's one vendor's answer.

From the FAQ section of the "Ductwork Design And Installation Guide," published by Oneida Air Systems:

What will happen if I turn the collector on with all of the blast gates closed? Will it burn out the motor? Why not?
It does not hurt the collector motor when all of the blast gates are closed. Closing the blast gates shuts off the air flow to the fan. The fan is doing “no work” so amperage draw drops. You can only overload the motor by not having enough resistance. For example, turning the collector on without the dust bin in place or running the fan without connecting it to a cyclone separator or ductwork.

-------
Hmm, well, okay then. I'll keep looking, but one would think a company like Oneida would know.

[Uncle Cracker]

Aside from whether or not harm would come to the motor, somebody wanna tell me why they would want to run their DC with all the gates closed? I'm not aware of any good reason.

And although motor cooling has been addressed here, what about cooling of the forward bearings on the motor shaft? Seems to me at least part of their cooling would be dependent on the passage of air through the system, as exterior exposure of the motor will cool rear bearings, armature and casing, but would offer little help to the forward bearings, on which the force and weight of the spinning fan impeller bears.

[Thom2]

Aside from whether or not harm would come to the motor, somebody wanna tell me why they would want to run their DC with all the gates closed? I'm not aware of any good reason.

I'll be honest here, it's not a matter of "wanting to" nor is it a "good reason". It's because I'm a moron and turn the DC on with all the gates closed and OCCASSIONALLY (whistles innocently) even makes a few cuts before ever realizing the gate isn't open.

[Thom2]

... one would think a company like Oneida would know.

Good enough for ME!

[LCHIEN]

The DC operation is:
When the input is open, e.g. minimal ductwork or hoses on the inlet, open to the air, and the bags are removed, then the impellor is working onto the least resistance and is moving the most air. The impellor is working the hardest at this point and the current will be at a maximum.
Now, many DC motor impellors are sized together so that this current is below the maximum current for the motor and impellor with no resistance and will be safe at all input conditions.
Some manufacturers think this condition is not normally ocurring so they size the impellor a bit bigger so that with some minimal ducting (still wide open at the inlet of the duct) and a set of bags on the output side, the motor draws maximum current with that amount of resistance. This according to them is the minimal resistance load and should not be exceeded. Take off the bags and duct, and it will be a bit overcurrent. Someone really trying to wring out the max performance out of the motor might size an impellor to use the maximum power with a full set of ducting and bags and this would be OK given they understand they should not ever run the system with no ducts for more than a minute or so!

Now, closing off the inlet (e.g. all blast gates closed) to the DC impellor has the impellor working on a partial vacuum and not moving much if any air. It sounds very loud because what air does move makes a lot of noise as it screams by. So contrary to what your senses tell you, the DC motor is working at a minimum load, as low as it can get with an that impellor attached (Well you could go lower if you surrounded it with a complete vacuum I suppose). So the motor could run forever this way and not damage it - you won't like it because its making so much noise and appears to be in distress.

For shop vacs, its a little bit different story. I think the current is still lowest at the point where you have the hose blocked off, but, most shopvacs (but not all, e.g. Fein and the old Ryobi vacs uses a separate cooling fan) use the air flowing thru for cooling the motor, so doing this for an extended period of time would be bad for the shop vac.

This all is relatively easily confirmed by attaching and watching an AC Ammeter to the units (DC or Vac) while alternately blocking off the intakes and allowing full air inlet.
I haven't tried all cases for a complete experiment but I've done enough playing to know that as you unblock the hose inlet to a DC, the amperage will increase to a maximum when the duct is completely open and a bit higher if the duct is removed completely.

[Jeffrey Schronce]

You can only overload the motor by not having enough resistance. For example, turning the collector on without the dust bin in place

I wished I had read that preciously 1 3hp motor ago! LOL!

[Uncle Cracker]

I'll be honest here, it's not a matter of "wanting to" nor is it a "good reason". It's because I'm a moron and turn the DC on with all the gates closed and OCCASSIONALLY (whistles innocently) even makes a few cuts before ever realizing the gate isn't open.

Thom, that's about as honest an answer as I have ever heard. Thanks for the chuckle...

[radhak]

From the FAQ section of the "Ductwork Design And Installation Guide," published by Oneida Air Systems:

You can only overload the motor by not having enough resistance. For example, turning the collector on without the dust bin in place or running the fan without connecting it to a cyclone separator or ductwork.

Hold on - i am a bit slow today, but does the 'dust bin' mean the bag for collecting? And the 'fan' refers to something other than the DC itself?

In my case I have a Jet 650 with the Wynn filter on top with a clear bag beneath, and have a very short duct (6 ft) to connect the TS or the Router (only one at a time). So if I start the DC up and not the TS, how does it give it more resistance than if it the connecting duct were removed?

[ragswl4]

I'll be honest here, it's not a matter of "wanting to" nor is it a "good reason". It's because I'm a moron and turn the DC on with all the gates closed and OCCASSIONALLY (whistles innocently) even makes a few cuts before ever realizing the gate isn't open.

I caught myself doing the same thing but now I have changed my shop practice. I first open the gate to the machine I am going to use, close all other gates (3 in my case), turn on the DC and then the machine I will be using. Turn off the machine I have been using then turn off the DC.

Now its automatic for me and I never have all the gates closed when the DC is turned on.

[LarryG]

So if I start the DC up and not the TS, how does it give it more resistance than if it the connecting duct were removed?

It doesn't -- that part of the FAQ answer is poorly written; I interpreted it the same way as you on my first reading.

See if the way I've edited it, below, is better:

"You can only overload the motor by not having enough resistance -- for example, by turning the collector on without the dust bin in place, or by running the fan without connecting it to a cyclone separator or ductwork."

(These instructions are referring to a cyclone system, so "dust bin" means the collection barrel. On a bag-type DC, the bags would serve the same function of providing resistance to the air flow, so that the impeller doesn't move so much air the motor burns up, as Loring describes in theory and as Jeff unfortunately found out in actual practice.)