Running DC with blast gates closed??

[radhak]

Thanks Larry, I think I am sorta getting it now : running it without the bag or the filter would be bad, right?

[LarryG]

Thanks Larry, I think I am sorta getting it now : running it without the bag or the filter would be bad, right?

Exactamundo.

(Word chosen specifically to meet the 10-char minimum, otherwise I'd have just put "Right.")

[DaveStL]

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.

Remember it's about 28" of Water Column per psi. A typical hobbyist DC has effectively no flow at around 10" WC, so you're only going to get to 0.4 psi vacuum (14.3 psia). Since it's not really airtight there will be a small flow through the fan.

[ironhat]

Sorry - I was reading both threads and posted to the wrong one so I removed it.

[radhak]

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.
.

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

Jeff, an after-thought question : did you run into that problem yourself? What happened? (Just trying to really understand how less resistance could kill a motor).

[LCHIEN]

Jeff, an after-thought question : did you run into that problem yourself? What happened? (Just trying to really understand how less resistance could kill a motor).

Resistance but no movement is low amount of work.
Moving air is work.
Moving the most amount of air = most work.
least resistance = most amount of air = most work.

Whether fully open ducting burns up your motor or not depends on how it was designed... to draw max amps with some ducting (resistance) or to draw max amps with fully open input (no ducts, no bags) (no resistance).

[Jeffrey Schronce]

When I relocated my cyclone I had to take a couple inchs off of the legs to make it fit overhead and give is a couple inchs breathing room for motor fan. This made my barrel a little too tall to fit. I *had* to do a few operations and turned on cyclone while doing the operations. I then went inside the house (leaving the cyclone on as I had not gotten used to it being outside in its own room and hearing no noise) for about 30 minutes. Thus machine ran wide open for about an hour. When I went back to the shop there was a whiff of ozone in the air and a tripped breaker. Tried firing it back up and sounded like a train. Wilke put a new motor on it for me for a very reasonible price.

Moral of the story : No resistance, before or after the impeller = burned up motor.

[radhak]

Thanks both of you, that helps.

So...if my DC (Jet 650 with a clear plastic bag below and Wynn filter above) is attached just to the TS, and I keep it running for cuts and in between while I am adjusting the stop-block, marking the measures, adjusting the fence, etc. Say all this (cut, stop, cut again) goes on for an hour. Should I be concerned about the motor and keep shutting the DC off in between? Or is that independent of the 'lack of resistance' issue?

[Rslaugh]

My brutha Thom is correct. (don't look surprised. Actually he often is) Centrifugal fans in large buildings have traditionally been controlled with inlet vanes or outlet dampers both of which can close off the airflow almost completely and potentially run for long periods of time in this condition.

Today most of these applications use AC Drives or VFD's which vary both the frequency and voltage to a standard 3 phase induction motor to vary the speed of the motor. HP varies as the cube of speed to about 20% of speed. (80% speed is about 50% HP) It's not recommended to run a standard motor below 20% for long as most are fan cooled themselves. IIRC If you let the motor run at full speed and throttle the airflow HP varies as the square of airflow so there is a enough energy savings with the drive to pay for itself usually in less than 2 years.

Because the designs vary as Loring pointed out above you should check the current being drawn in that condition to be sure but i would think the filter and the line to the table saw would be enough resistance.