Shopsmith Forums

Many think that when you "open up" the flow on a blower and let it push the air thru against less resistance that the load on the motor will decrease. Actually, just the opposite is the case.

An AC induction motor has a very flat speed torque curve. Thus the motor can provide considerably more torque ( and HP) with very little loss in speed. However, if operated continuously in this manner it will overheat and perhaps burn up. The nameplate full load HP and amps is based on the allowable temperature rise of the motor, in most cases 55 degrees C, under continuous operation.

Thus, if all the back pressure is removed, the motor will move further out on it's torque curve and try to move as much air as possible. The amount of flow, and the torque/HP required, will increase dramatically.

8iowa wrote:Many think that when you "open up" the flow on a blower and let it push the air thru against less resistance that the load on the motor will decrease. Actually, just the opposite is the case.

An AC induction motor has a very flat speed torque curve. Thus the motor can provide considerably more torque ( and HP) with very little loss in speed. However, if operated continuously in this manner it will overheat and perhaps burn up. The nameplate full load HP and amps is based on the allowable temperature rise of the motor, in most cases 55 degrees C, under continuous operation.

Thus, if all the back pressure is removed, the motor will move further out on it's torque curve and try to move as much air as possible. The amount of flow, and the torque/HP required, will increase dramatically.

I thought the original concern was operating it with lower load(No Fan Blades). That would move it down on the torque curve. ???<<<Please IGNORE this!

Heat caused by increased currents squirrel cage(internal to armature) or increased input current(or both)????

His measuring an extremely low current(line cord) has me befuddled! Kinda wonder if he clamped it around the entire line cord and the motor has some frame leakage current which is what was indicated on the ammeter(.04A)!

JPG40504 wrote:I thought the original concern was operating it with lower load(No Fan Blades). That would move it down on the torque curve. ???<<<Please IGNORE this!

Heat caused by increased currents squirrel cage(internal to armature) or increased input current(or both)????

His measuring an extremely low current(line cord) has me befuddled! Kinda wonder if he clamped it around the entire line cord and the motor has some frame leakage current which is what was indicated on the ammeter(.04A)!

The .04 A puzzled me, too.
The amp probe is the type that opens like a large pliers except it only closes at the end of the jaw. leaving 3-4 inches of open area.
Measured 1/2 way from the outlet to the motor. Did a couple quick reads after running it for 20 minutes. Measured those within 6" of the motor and again within 6" of the outlet. Never got a reading over the .04 A. (probe set @ 20 A)
After running for 20 minutes, the motor was barely warm to the touch.
Motor cord is #12, as is the line feeding the outlet which is less than 20' from the breaker box.
The probe is good. We use it to check the start/run capacitors on our water well pump about every 3 months.

Gene,
What does your neighbor think of you blowing sawdust at them?

Gene Howe wrote:The .04 A puzzled me, too.
The amp probe is the type that opens like a large pliers except it only closes at the end of the jaw. leaving 3-4 inches of open area.
Measured 1/2 way from the outlet to the motor. Did a couple quick reads after running it for 20 minutes. Measured those within 6" of the motor and again within 6" of the outlet. Never got a reading over the .04 A. (probe set @ 20 A)
After running for 20 minutes, the motor was barely warm to the touch.
Motor cord is #12, as is the line feeding the outlet which is less than 20' from the breaker box.
The probe is good. We use it to check the start/run capacitors on our water well pump about every 3 months.

You need to clamp over ONE wire not the line cord with 3 conductors. It will measure the NET current flowing through the 'clamp'. If you put it over the entire linecord, the current flowing 'in' on one wire will be canceled by the current flowing 'out' in the other. Also make sure the 'clamp' is closed tightly.

curiousgeorge wrote:Gene,
What does your neighbor think of you blowing sawdust at them?

If the cyclone 'lid' is working, there is NO(or at least very little) sawdust being blown. All the neighbor will see is air and will think it is an exhaust fan(whose function it IS performing).

JPG40504 wrote:If the cyclone 'lid' is working, there is NO(or at least very little) sawdust being blown. All the neighbor will see is air and will think it is an exhaust fan(whose function it IS performing).

Yeah. I knew that. I'm not a total dumb a.. .Didn't you notice the smiley at the end of the question? I was just poking a little fun at Gene. You engineer types need to loosen up a little and quit trying to over analyze everything.

JPG40504 wrote: All the neighbor will see is air and will think it is an exhaust fan(whose function it IS performing).

IF ya can see the air there, ya have bigger PROBLEMS.

We are in an area that rural people even call the boonies. No neighbors.
JPG..What a dummie I am. Of course! When I check the capacitors, it's always on one wire at a time. (slapping forehead...HARD)
Redo the test tomorrow.

beeg wrote:IF ya can see the air there, ya have bigger PROBLEMS.

OK! Make that see the effects of the air flow!:D

GH You ain't the first!