Help me understand dust collector motors and amp draw.

[joe milana]

Just one of those subjects that didn't interest me until now. When you close all your blast gates and your blower motor speeds up, is the motor drawing more or less amps, and inversely, when you open all the gates and the motor slows down due to the added load, is the motor drawing more or less amps? I'm sure there's more to it...

[roger wiegand]

Moving air takes work, so when the gates are open the motor is under load, works harder, slows down, and draws more amps. When it's just spinning in its housing and not moving masses of air it can speed up and will draw much less current.

[Ken Fitzgerald]

Roger's nailed it. When all the gates are closed, you aren't moving any air, and the motor draws less current. Years ago this was a topic of discussion and one of the members made a special cord up so he could use a clamp on amp meter. He verified the theory.

[joe milana]

OK, and correct me if I am wrong, but i believe i have heard people saying that they always have more than one blast gate open to prevent over amp-ing the motor.

[Jim Andrew]

I just open one gate at a time. Opening another gate cuts the suction in half on the machine I am using.

[Chris Parks]

OK, and correct me if I am wrong, but i believe i have heard people saying that they always have more than one blast gate open to prevent over amp-ing the motor.

That I don't follow and sounds weird to me as every gate that gets opened adds load. There are some funny urban myths to dust extrraction but that's a new one. Put simply no air = no load.

[David Kumm]

Opening gates will increase the amp draw up to the point where the impeller design can pull air through the main and push out the filters. After that any gates that are opened have no effect on the motor. Different impeller designs will change how much air can be moved through a certain system with given pipe size and filter and cyclone restrictions. Dave

[joe milana]

OK, thanks. I think I am beginning to understand. I am installing a torit cyclone along with an inverter so I will be able to monitor the amp draw under different scenarios. I want to have a basic understanding before I fire it up.

[Steve Peterson]

OK, and correct me if I am wrong, but i believe i have heard people saying that they always have more than one blast gate open to prevent over amp-ing the motor.

One concern about cyclones running with all blast gates closed is that the motor might overheat if it is in a closet that depends on air movement to cool the motor. Dust collectors are still drawing a few amps of current even in this state.

The opposite case occurs when running the cyclone wide open with no blast gates or filter. The motor can draw too much current and overheat or trip the circuit breaker. You have to resist the urge to test the motor/blower as soon as it is hung on the wall. It is OK to let it run for a second or two, but don't let it come up to full speed without some restriction.

Steve

[Ian Scofield]

Like he said above, if you let it run with all the blast gates open, aka with zero resistance, then the impeller will continue to speed up and speed up since there's no force working against it. When that happens, the motor burns up because it's spinning way faster then it's designed.

Conversely, if there's way too much load, IE all the blast gates are closed, you're creating a vacuum because the dust collector is attempting to pull air through your ducting but nothing is moving because there's no new air being pulled into this system. Therefore the dust collector draws more and more current until it trips the breaker.

[Chris Parks]

I have never had the wide open overload problem that Steve speaks of and I have installed quite a few Clearvue cyclones so I don't know if it is an urban myth or only some systems suffer from it. Possibly those that run a motor close to its limits for the system would have an issue.

[David Kumm]

Like he said above, if you let it run with all the blast gates open, aka with zero resistance, then the impeller will continue to speed up and speed up since there's no force working against it. When that happens, the motor burns up because it's spinning way faster then it's designed.

Conversely, if there's way too much load, IE all the blast gates are closed, you're creating a vacuum because the dust collector is attempting to pull air through your ducting but nothing is moving because there's no new air being pulled into this system. Therefore the dust collector draws more and more current until it trips the breaker.

If the gates are open the impeller will still only run at the rpm of the motor or pulley set. Same if gates are closed. The amps needed when the gates are open might overload the motor but not because the rpm changes. With the gates closed or open, as long as the amp draw is under FLA of the motor design, the motor can run pretty much forever. If the motor is TEFC it would take a pretty small closet to heat up enough to damage the motor. Dave

[John Donhowe]

if you let it run with all the blast gates open, aka with zero resistance, then the impeller will continue to speed up and speed up since there's no force working against it. When that happens, the motor burns up because it's spinning way faster then it's designed.

No offense intended, but I'm pretty sure this is not correct. As Roger said in post #2, when the gates are open, more air can move, so the motor does more work (draws more amps). With the greater load of air to move, impeller (and motor) speed slows down, not speeds up.

because the dust collector is attempting to pull air through your ducting but nothing is moving because there's no new air being pulled into this system. Therefore the dust collector draws more and more current

If you ignore friction losses from bearings, the only work that the motor is doing is from the impeller moving air. So, when the gates are closed, and no air is moving, the motor is actually doing comparatively little work, and therefore draws fewer amps. I know, it isn't intuitive, but it works less when gates are closed.

BTW, I agree that you shouldn't test a motor "naked", that is, with no ducts or filter attached, for fear of burning out. I don't think, though, that testing with just the cyclone and filter in line (no blast gates) would be a real risk. Remember that in designing a DC system, we're always urged to maximize air flow by using large diameter pipes, avoiding sharp turns, etc., which increase turbulence and inhibit air flow. Also, the filter resists air flow- think of it like a blast gate downstream of the impeller.

Since a cyclone by definition creates sharp turns in air flow, it will always reduce air flow significantly- the penalty paid for good dust separation. Since the air flow is reduced with a cyclone in place, the impeller will move less air, the motor will draw fewer amps, and there is less risk of motor burnout.

Any and all, please feel free to jump in and correct any errors you see in my logic

[Dennis Aspö]

The motor will only go as fast as it's designed to if there's no resistance. If you take an induction motor that's rated for say 3500RPM and just run it with the shaft so there's no resistance, it will not go faster than 3500rpm. Likewise if an impeller is mounted on this and there
is no resistance, it will spin at 3500rpm.

One should have a motor powerfull enough that it can spin the impeller at full speed though even when it takes in all air that it's design will allow (the inlet hole to the impeller logically decides this). That way it will never overload.

[Chris Parks]

Like he said above, if you let it run with all the blast gates open, aka with zero resistance, then the impeller will continue to speed up and speed up since there's no force working against it. When that happens, the motor burns up because it's spinning way faster then it's designed.

Conversely, if there's way too much load, IE all the blast gates are closed, you're creating a vacuum because the dust collector is attempting to pull air through your ducting but nothing is moving because there's no new air being pulled into this system. Therefore the dust collector draws more and more current until it trips the breaker.

Ian this is wrong but you are not the only person to get it opposite to what happens.