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Krunch
11-08-2012, 11:33 AM
Hello, all,

I was looking at some various incarnations of the age-old workhorse Skil worm-gear circular saw, and I noticed that there are now all different permutations of this unit, with motors that supposedly use different amounts of current:

13A "Old Standby" 7-1/4" (http://www.amazon.com/SKIL-HD77-4-Inch-Worm-Drive/dp/B0000223FC)
15A version of the same thing (http://www.amazon.com/SKIL-SHD77-4-Inch-Drive-SKILSAW/dp/B000P7MAEM)
13A lightweight version of the same thing (http://www.amazon.com/SKIL-HD77M-4-Inch-Worm-Drive/dp/B0000223FB)
15A version of the lightweight version (http://www.amazon.com/SHD77M-4-Inch-Drive-SKILSAW-Circular/dp/B000P7MAGA)
and
last but not least
13A version of the Old Workhorse with 8-1/4" blade (http://www.amazon.com/Skil-HD5860-4-Inch-60-Degree-Drive/dp/B0000223FF)

Now, my understanding of the way Universal motors work is, the harder you load them, the more current they draw, and the more power they put out – and this is the only way on earth that any tool manufacturer can, with a straight face, claim that a 110V/15A plunge router puts out 3-1/4 horsepower. Yeah, sure, if you bog it down, it'll draw 30A and MAYBE put out 3.25hp for a second or two, until the windings melt, but hey, it sells routers and "All manufacturers lie about this stuff, so we should, too."

Now, regarding the Skil saw motors, my hunch is that the exact same motor is used in every one of them, and if you put enough load on the motors, they ALL will draw 15A (or however much current your circuit will supply to them until either the circuit breaker throws or the windings melt)...and the different amperage ratings (with correspondingly higher prices) are used only as a way to charge more money for the supposedly higher-power units.

Do you folks agree with me on this? I really don't know much about this stuff...

Thanks in advance for any help.

MaxHeadRoom
11-08-2012, 11:53 AM
Maybe you are confusing HP with torque?
HP has a rate or dependent on RPM for the value.
Torque is a function of current.
Max.

Evan
11-08-2012, 12:11 PM
I don't know about the US but Canada now requires correct HP labelling on compressors. What used to be a 6 hp 115 vac compressor is now 1 hp. They obtained the 6 hp number as "instantaneous HP". There is no such thing but what they did was measure the torque produced if the rotating components were suddenly brought to an "instantaneous" halt and multiplied that by the rpm it was running at. Of course that was measuring angular momentum, not horsepower. Vacuum cleaners here are now mainly labelled by amp draw.

Universal motors have a certain rpm vs current where they generate the most power. They don't generate more power the faster they go or the more current they draw. The hp curve is a hump and above or below that current x rpm the output is lower. In pure theory a universal motor has no top rpm other than the speed at which magnetic fields propagate which is c. In practice they are limited by friction and windage.

cameron
11-08-2012, 12:33 PM
Canada may require it, but with inspection almost non-existent, it's probably meaningless.

Anyone like a nice Excel steak? No extra charge for the E-coli.

MaxHeadRoom
11-08-2012, 12:49 PM
Series motors are very high torque from zero, this is why your automotive starter is a series motor.
But because they operate in a runaway condition, there is should always a load of some kind fitted.
Note how a vacuum motor increases RPM when you block the hose, you are decreasing the load on the fan.
Max.

Krunch
11-08-2012, 03:37 PM
Thanks for the replies, fellas.

Evan, are you saying that you think the motors in the various "models" of Skil saw actually do put out different amounts of power?

In other words, at a blade speed of X rpms (where X is somewhere below the no-load speed), would the 15A motor actually be able to put more torque on the blade than the 13A motor?

What I'm trying to determine is, all other things being equal, would the saws rated at 15A actually be stronger than those rated at 13A? And my gut says, no.

Jon Heron
11-08-2012, 03:52 PM
Canada may require it, but with inspection almost non-existent, it's probably meaningless.

Anyone like a nice Excel steak? No extra charge for the E-coli.
Huh? are you confusing Ontario electrical inspection (ESA) with federal government food inspectors? Ontario has the strictest and most enforced electrical inspection program in the world, the same cannot be said about food inspectors...
Cheers,
Jon

Evan
11-08-2012, 04:00 PM
HP ratings are a matter of consumer advertising and that is entirely different again. The Office of Consumer Affairs is pretty active.


Evan, are you saying that you think the motors in the various "models" of Skil saw actually do put out different amounts of power?

Sure they do but without knowing other specifics such as gear ratios there is no way to compare.

J Tiers
11-08-2012, 10:06 PM
Universal (series) motors have another interesting characteristic, which the typical universal motor has to less extent than a DC series motor..........

When you slow them down, their magnetic field goes up, which means that they do not have the grossly increasing current draw that one might expect.... Until saturation, the more current, the higher the field, and the slower the motor has to go to generate the same back EMF..... It's as if they had an electrical "downshift". The torque per amp also goes up.

I suspect that cheap universal motors don't do this as well, due to minimum iron, but a decent series motor such as a traction motor does it very well. It's one reason a locomotive could start so slow with such large drawbar pull.

Appliance series motors are a thing of the past , now, new appliances are going brushless DC because it is more efficient. A portable drill can go half again longer before recharging with the brushless DC as with the old-fashioned brush-type motor.

darryl
11-08-2012, 10:43 PM
I would expect that if the amount of iron in the various motors is the same, then what they've done is change the number of turns in the field windings, and changed the gauge of the wire. They may have done the same for the armature. What it boils down to then is that the higher amp motor is going to get hotter quicker. Maybe they are betting that the typical duty cycle in average use is going to take care of things. Maybe they are allowing the motor to run hotter and are betting they will last past the warranty period before they actually burn out.

In one sense it kind of balances out. If you don't lose as much rpm under load, the cut will go quicker, it will maintain a little more self-cooling with the fan at a higher rpm, and the duty cycle could be less.

But perhaps the iron is used near to it's saturation point at full power. It's possible then that the higher amp motors may actually be longer, both the field and the armature. This would allow a higher power output, but with the same rate of heating, and of course a higher power draw.

There's only about a 25% difference between an 11 amp motor and a 15 amp. Adding only that much iron doesn't increase the overall length of the motor by that much. The brushes won't need more room, and neither will the bearings.

camperkn
11-09-2012, 02:42 AM
You are correct in assuming that all the Skil 7 1/4 worm drive saws use the same motor. Same amount of laminations and same wire size and turn count. The diference in amp ratings is based on other components and insulation spacings. These are determined by requirements of UL. As I recall they required a different switch for the 15 amp tool.

Skil didn't list horsepower on their commercial tools, only their consumer grade products. You could buy a 2 hp, a 2 1/2 hp, and a 3 hp circular saw all with the same motor. All hp ratings were true, driven by marketing.

Evan
11-09-2012, 04:18 AM
You can't run a three horsepower anything on a 15 amp circuit. Maximum allowed amp draw for more than a few seconds is 12.5 amps, here and the US. That limits a 15 amp circuit to 2 hp at most.

Krunch
11-09-2012, 09:22 AM
You are correct in assuming that all the Skil 7 1/4 worm drive saws use the same motor. Same amount of laminations and same wire size and turn count. The diference in amp ratings is based on other components and insulation spacings. These are determined by requirements of UL. As I recall they required a different switch for the 15 amp tool.

Hmmm...of the saws I listed above:
• the 15A version of the "Old Standby" claims to have a 20A switch;
• the 15A lightweight version claims to have a 20A switch; and
• the 13A version with 8-1/4" blade claims to have a 22A switch.

And then there's this little overlooked gem, which I just noticed in Amazon.com's review:

"We couldn't feel any difference in performance between this Skil's 13 amp motor and others drawing 15 amps."

So I'm tempted to stick with my original assessment, that as far as power goes, unless the gear ratios are different, they're all the same...

Thank you all again for your replies.

P.S. I think I solved the mystery. If you go here (http://www.amazon.com/SHD77M-4-Inch-Drive-SKILSAW-Circular/dp/B000P7MAGA), under the "Product Features" list of bullet points, you'll see the first two features listed are that it has "increased Performance - 15 amps" and "an Upgraded Motor - High Temp Wire." So I guess the only difference in the 15A models is that they use wire that has higher-temperature insulation on it...in other words, under equal loads, the 15A models won't burn up as fast...?

J Tiers
11-09-2012, 09:32 AM
You can't run a three horsepower anything on a 15 amp circuit. Maximum allowed amp draw for more than a few seconds is 12.5 amps, here and the US. That limits a 15 amp circuit to 2 hp at most.

It's more than a "few seconds"....... but likely less than say 15 to 30 min at a time. UL has been playing about with the definition of "continuous" as far as current draw.

The 15A breaker should allow 20A for considerably longer than 3 seconds, but you would have to consult the rating curves to see what the typical let-through is. It depends a bit on the type, since thermal (the most common) have variable ratings depending on ambient temp, how many other breakers nearby are heavily loaded, box bus bar temperature, etc.