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Forum topic by Davevand posted 04-19-2018 04:10 PM 527 views 0 times favorited 17 replies Add to Favorites Watch
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Davevand

63 posts in 980 days


04-19-2018 04:10 PM

Hi, I was thinking about building a small dust collector using my shop vac and a Dust Deputy for scroll saw work. I have a Kill-O-Watt meter and I checked the power on both my Jet DC and the Shop Vac, both using 120v.
The Jet DC uses 10.9 amps ans 900 watts when running
The shop vac uses 8.4 amps and 940 watts when running
I was thinking the Shop Vac would use a lot less power than my Jet DC, but it looks like that is not the case, correct?
My little Shop Vac actually cost more to run??


17 replies so far

View toddbeaulieu's profile

toddbeaulieu

815 posts in 3147 days


#1 posted 04-19-2018 04:41 PM

Your numbers don’t add up. P = I x E

With voltage consistent, you can’t have more current running through, with less wattage. Using your amperage readings, here’s your wattage:

10.9A = 1308W
8.4A = 1008W

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LesB

1816 posts in 3586 days


#2 posted 04-19-2018 04:59 PM

Volts X Amps = Watts

120 X 10.9=1308
120 X 8.4 =1008

I think your Kill-O-Watt meter is off.

-- Les B, Oregon

View dhazelton's profile

dhazelton

2789 posts in 2440 days


#3 posted 04-19-2018 05:02 PM

Yes – 11 amps (10.9, just rounding up to make it simple) times 120 volts is 1320 watts.

Only thing I can think of is maybe the universal motor on the shop vac has a high initial startup and the Killowatt is reporting that.

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GrantA

407 posts in 1551 days


#4 posted 04-19-2018 05:11 PM

If we’re going to nitpick, were they both in the same outlet for this test? Chances are neither is getting a full 120v, also I would expect the shop vac has more restriction (unless it’s got a brand new filter) therefore is under a load even at idle

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toddbeaulieu

815 posts in 3147 days


#5 posted 04-19-2018 05:16 PM

Maybe factors, but in the end, the original question is asking about relative power usage and power will always be higher with higher current and the same voltage.

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RobHannon

156 posts in 674 days


#6 posted 04-19-2018 05:19 PM

Does the meter calculate average watts over time? The DC probably has a lot higher startup draw than the shop vac does because the armature is significantly bigger. Also did you run both for the same period of time?

View Davevand's profile

Davevand

63 posts in 980 days


#7 posted 04-19-2018 05:48 PM

Same outlet, ran the number again and shows 119 volts for both and ~ the same numbers. I guess the meter is off.
Using my price per kWh I would be saving ~$2/month using the Shop Vac vs DC. Cost to build ~$36, ROI 18 months, I wonder if my Shop vac will last that long :-)

View Jim Dawson's profile

Jim Dawson

114 posts in 975 days


#8 posted 04-19-2018 06:27 PM

For $2.00/mo I’d use whichever is quieter.

View toddbeaulieu's profile

toddbeaulieu

815 posts in 3147 days


#9 posted 04-19-2018 06:31 PM

In the winter months I pay $30/day to heat my shop (crappy heater/insulation/old barn). I laughed when I saw the $2 figure.

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clin

929 posts in 1139 days


#10 posted 04-19-2018 06:58 PM



Your numbers don t add up. P = I x E

With voltage consistent, you can t have more current running through, with less wattage. Using your amperage readings, here s your wattage:

10.9A = 1308W
8.4A = 1008W

- toddbeaulieu

Your not including power factor. With AC, power is not simply volts x amps. This is only true if the current and voltage are in phase. It’s actually possible to have a million amps and a million volts and zero power. If the current and voltage were 90 degrees out of phase.

The power factor is the cosine of the angle difference between the voltage and current sine waves. So the OP’s values are likely correct, just that the power factors is less than 1.0, and therefore the real power, is less than the product of the voltage and current.

Power factor is a very big deal for the electric company. Power transmission losses are related to current. So in general the power company wants to see loads that have a power factor as close to 1.0 as possible. That way power isn’t wasted in the transmission lines with currents that are greater than needed to produce the actual real power being used.

-- Clin

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toddbeaulieu

815 posts in 3147 days


#11 posted 04-19-2018 07:06 PM

Interesting, but is this actually at play here? I suspect not – that this is a pedestrian use case.

View splintergroup's profile

splintergroup

2304 posts in 1366 days


#12 posted 04-19-2018 07:18 PM

My Kill-O-Watt meter (the cheap one) has a VA (volt-amps) setting and a PF (power factor) setting. These will let you know what is really going on.

View Fresch's profile

Fresch

321 posts in 2064 days


#13 posted 04-19-2018 07:32 PM

Watts or amps should be labeled on both pieces of equipment.

View clin's profile

clin

929 posts in 1139 days


#14 posted 04-19-2018 08:46 PM



Interesting, but is this actually at play here? I suspect not – that this is a pedestrian use case.

- toddbeaulieu

Almost certainly it is what is going on. Power factor is a known issue with AC motors, which is the load the OP is measuring. And as splintergroup said, the Kill-o-watt meter can measure it, and probably is measuring it.

Though I think a P.F. in the range is 0.9 is more likely. While he would seem to be getting closer to 0.7 on the DC. Bottom line VxA (VA) is not watts when it comes to AC.

-- Clin

View CWWoodworking's profile

CWWoodworking

141 posts in 322 days


#15 posted 04-20-2018 03:54 AM

How many more times will you dump you shop vac vs dc?

Your time is worth something.

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