How to use a DC

[Dale Thompson]

Dale,

Closing the airflow off to a vacuum also ends up drawing minimum amps but can burn up the motor. A closed airflow again means the motor is pushing the least air, so doing the least work and drawing the least amps. Because vacuums are normally configured to be on the clean side of the filters, most use the main air stream for cooling instead of a separate motor cooling fan. So closing off the airflow to a vacuum will soon leave your motor burning up.

bill

Bill,
EGADS!!! I think that I am starting to drown in my own perfection! What a way to go! Now you can see why I am unemployed and virtually unemployable. Anyway, in my grief, I think that I'll just walk out on my dock and feed myself to one of the big Muskies lurking under that fallen tree about 10 yards up the shore! Maybe I can save myself if I tell them the TRUTH about DCs and shop vacs.

Thanks for the info, Bill. I can easily bow to and salute intellect even though I have no concept of the term.

Dale T.

[Tom Jones III]

You shouldn't assume that simply because the startup doesn't blow a 20 Amp breaker that the motor isn't pulling more than 20 Amps.

Quite a few motors use a capacitor to power the startup avoiding the large power spike on the wires upstream but still using a massive power spike inside the motor itself. I believe that the baldor 3HP motor in my PM 66 TS uses a capacitor at startup, but I don't remember where I saw that.

Sometimes you'll run across a motor that you are unable to start, stop then restart quickly because the capacitor has not yet recharged.

[Bill Pentz]

Bill,
EGADS!!! I think that I am starting to drown in my own perfection! What a way to go! Now you can see why I am unemployed and virtually unemployable. Anyway, in my grief, I think that I'll just walk out on my dock and feed myself to one of the big Muskies lurking under that fallen tree about 10 yards up the shore! Maybe I can save myself if I tell them the TRUTH about DCs and shop vacs. ...

Dale,

I'm pretty jealous of your perfect record. I seem to make more mistakes than just about anyone I know. Plus I get to make them a few times before finally figuring things out. Good thing I am so hard headed that I just keep trying.

Anyhow, break out the fishing rod and a little bait! I remember as a kid catching those big Muskies as a lot of fun. Too bad I gave up fishing ages ago. *smile*

bill

[Charlie Plesums]

Quite a few motors use a capacitor to power the startup avoiding the large power spike on the wires upstream but still using a massive power spike inside the motor itself. I believe that the baldor 3HP motor in my PM 66 TS uses a capacitor at startup, but I don't remember where I saw that.

Sometimes you'll run across a motor that you are unable to start, stop then restart quickly because the capacitor has not yet recharged.

Sorry Tom, but the starting capacitor is to shift the phase slightly, to allow a single phase motor to start, and is charged in 1/120 of a second. With single phase power, the magnetic field pulls equally on opposite sides of the rotor, so the motor will just sit and hum on starting. The capacitor allows one side to pull a little more, and once things get moving, the motor runs fine, and the capacitor is switched out of the circuit.

My 5 hp motors normally draw about 20 amps. When I start my jointer or table saw, they peg the 60 amp ammeter for a couple seconds - so are drawing far more than 60 amps, but just briefly. When I start my bandsaw (with wheels that weigh over 50 pounds each), that 5 hp motor pegs the 60 amp scale for almost 5 seconds. Some people with that bandsaw have to use a 40 amp circuit and larger wire to handle the starting load, but mine hasn't popped the 30 amp breaker ... yet.

Residential power is sold on actual power used, with no special charge for the distribution lines and power capacity that the electric company has to have available to start the motors. Commercial power is sold differently... a lower cost per unit for the actual power used, and a separate "demand" charge for the distribution lines required to handle the peak loads (such as starting large motors)

[Tom Jones III]

Sorry Tom, but the starting capacitor is to shift the phase slightly, to allow a single phase motor to start, and is charged in 1/120 of a second.

Wow, you learn something every day. Wonder what I was thinking? Thanks for the correction.