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Motor differences between Circular saws(120v/2.3hp) vs Table saw (120/1.5hp)?

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Forum topic by CodyJames posted 11-04-2011 01:55 PM 3739 views 0 times favorited 17 replies Add to Favorites Watch
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CodyJames

78 posts in 1157 days


11-04-2011 01:55 PM

What gives? How come they put more power in your hand but not on the table on 120v especially?

I see most of the higher end companies that make table saws in the 120v range, make them with either 1.50hp or with 1.75hp electric motors.

Then I look at circular saws and notice the 2hp and higher rating and wonder, gee, why not attach a circular saw motor to a table saw to get a bit more power at 120v?

It has just got me curious…
(BTW, I am a bit electrically ignorant so forgive me if this is a stupid question.)


17 replies so far

View richgreer's profile

richgreer

4525 posts in 1825 days


#1 posted 11-04-2011 02:27 PM

In my opinion, there is a problem with how horsepower is measured and reported for hand held tools.

In general, horsepower ratings are based on the amperage draw. With hand held tools they measure the draw when there is a surge in the amperage in the micro second before the circuit breaker pops. They are not measuring the amount of power that is being provided on a sustainable basis.

I don’t like it, but it does seem to be done on a consistent basis. I note that routers are often labelled as 1.75 hp, 2.25 hp or 3.25 hp and you can usually expect that all routers of a given hp rating have about the same power.

With stationary tools, they provide a more honest measurement of horsepower.

A comparison of the rating on a hand held tool and a stationary tool is not very meaningful.

Also note that most stationary tools have induction motors and most hand held motors have universal motors and these 2 types of motors have different characteristics. You can expect an induction motor to run quieter, last longer, be heavier and have a little less power at start up.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View PeteMoss's profile

PeteMoss

207 posts in 2221 days


#2 posted 11-04-2011 02:55 PM

Good comments Rich. To me, It is sort or analogous to big tractor trailer trucks. You look at a Kenworth or what have you and it may have a 425 horsepower motor in it. Then you think “hey, that isn’t so much, my Camaro has a 425 horsepower motor. Why not just use my engine in the semi”. The fact is that you could do that. Only it would have to run at 7000 rpm to produce that power and would soon give up the ghost, whereas the big diesel engine would happily run along for year upon year. Sort of the same thing in tool motors. Induction motors on stationary tools are big heavy things that hum away quietly for a log time. Universal motors are noisy obnoxious things that tear up more quickly, but have a much higher power to weight ratio thus making them suitable for handheld use.

Although, a lot of the smaller portable table saws also use universal motors for the same reason. So they pretty much did strap a circular saw into a table frame.

-- "Never measure......cut as many times as necessary." - PeteMoss

View CodyJames's profile

CodyJames

78 posts in 1157 days


#3 posted 11-04-2011 03:00 PM

Thank you for the explanation and confirming what I somewhat suspected, I knew that the quality between the two motors were much different. Obviously, the stationary tools motors are meant to run for longer intervals and need to run cooler than the more instantaneous handtools that run for seconds at a time on average rather than minutes.

Thank you, Rich

View Craftsman on the lake's profile

Craftsman on the lake

2420 posts in 2189 days


#4 posted 11-04-2011 03:07 PM

I don’t know about the horsepower thing but I do find that using a fence on a table saw can slow things down if the wood isn’t straight. I make sure my jointer has given me a straight edge to put into the table saw. If there is a slight bow the blade might not bind but it groans a little. You usually don’t have a fence with a circular saw and even if you did the saw is short enough for it not to be noticable.

Also, The mechanical advantage. I have an older delta/rockwell 10” contractors table saw. Sometimes if I have a lot of hard wood that isn’t too thick I’ll put in a 7 1/4” blade made for a circular saw. The smaller blade is like changing the pulley size on the saw. More torque. but it still appears to spin as fast because of the reduced size… I think.

-- The smell of wood, coffee in the cup, the wife let's me do my thing, the lake is peaceful.

View CodyJames's profile

CodyJames

78 posts in 1157 days


#5 posted 11-04-2011 03:27 PM

I never thought about using a circular saw blade on the tablesaw. It would seem to make sense that it would reduce the effort by the 1.5hp motor to cut, almost like “faking” more power. I am going to give that a whirl when my saw gets delivered.

I just bought a powermatic 64 10inch artisan saw, and of course the first thing I worry about is “is that enough power”. I am not cutting foot thick curly maple or anything of the sort, and if I did, that’s what a bandsaw would be for. I am a newbie woodturner and the table saw is not the center of my shop, but, my lathe is.
I had a Ridgid table saw before my basement shop got flooded in my old house, it would get severely bogged down with 4/4 maple and would burn even just inch thick maple, just before it flooded I measured up the blade vs the mitre and noticed that the trunion was off a 1/8th of an inch, causing binding against the fence at the end of the cut, I was JUST working on filing down the trunion holes when it flooded that weekend.

Thanks for all the info folks, I am pretty certain I made the right decision. I am thinking that if I really need a more powerful 3hp saw in the future, I can buy a motor to replace the 1.5 on it now.

View crank49's profile

crank49

3524 posts in 1722 days


#6 posted 11-04-2011 03:29 PM

About the most true HP you can generate with a 120 volt, 15 amp circuit is 1.75 HP. I suppose with super attention to efficiency, the best bearings, the best wire, etc. you could get 2 HP; but that’s it.

How do you explain a shop vacuum with a big bold 3” tall label claiming 6.5 HP (peak, in tiny letters)?
Simple, they LIE.

Table saws usually have induction motors. They are designed to run all day at the rated power. If you overload an induction motor it will simply start to build up heat and eventually get so hot it burns the insulation from its wiring, shorts out and shuts down. If you designed the induction motor to run on 60 volts and then plugged it into a 120 volt circuit, 200% overvoltage, it would not create 200% more horse power, it would over heat more quickly and burn out faster.

Vacuums, routers, hand held circular saws, and even job site type table saws have universal, series wound motors. If you design one of these motors to run on 60 volts and then plug it into 120 volts it will overheat also, but while it’s running it is producing 200% more power than its rating. To overcome the overheating problem, the manufacturers install additional cooling fan capacity to bleed off most of the excess heat. In a shop vacuum the air coming through from the filter is routed through the motor to keep it from melting down. Hence, the exaggerated HP rating.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View CodyJames's profile

CodyJames

78 posts in 1157 days


#7 posted 11-04-2011 03:44 PM

God I love electricians…. and engineers…. and electrical engineers.. =)

View TheBossQ's profile

TheBossQ

82 posts in 1444 days


#8 posted 11-04-2011 06:39 PM

And your PM 64 is plenty powerful for a hobbyist woodworker. Unless you’re ripping 100 board feet of 8/4 Hard Maple, I think you’ll find the saw performs quite well. A good quality blade and well tuned saw are essential parts of the formula. If your Ridgid was 1/8” out, that is a lot. Easily 5x the ideal.

If you have the option, I would put a 220v receptacle in and run the saw on 220v.

I love my PM 63. It does everything I ask of it, and does it well.

View 8iowa's profile

8iowa

1495 posts in 2512 days


#9 posted 11-04-2011 07:02 PM

I can’t add a whole lot to Rich’s comments, his pointing out that hand held tools are powered with universal motors and table saws use induction motors is the crux of the situation.

Induction motors are standardized by NEMA. The HP rating is determined by the long term temperature rise running on full load. With a relitively flat speed torque curve, an induction motor can deliver considerably more power than the nameplate rating for short periods of time. Some manufacturers like to call this “peak power”, however it has no meaning as far as NEMA is concerned.

Universal motors are not standardized. Manufacturers can pretty much tell you anything. Many of these motors are rated in amps, which gives you very little indication of HP.

-- "Heaven is North of the Bridge"

View CodyJames's profile

CodyJames

78 posts in 1157 days


#10 posted 11-04-2011 07:26 PM

Thanks TheBossQ, that statement helped put my mind at ease.

8iowa, YEAP! I notice that most of the companies offer their circular saws in Amps, while some like Porter Cable are in HP, which is what got me curious.

By the way, I absolutely LOVE my Porter Cable circular saw, the only thing that I don’t like, No safety… the trigger is freely clickable. I found myself the first time picking it up oddly and I hit the trigger, I hadn’t used it yet even. Surprised me when it started up and I was like “woah, no safety”! It is something to be very aware of if anyone descides to purchase this saw.

View TopamaxSurvivor's profile

TopamaxSurvivor

15088 posts in 2427 days


#11 posted 11-04-2011 07:34 PM

I didn’t read everything above, but basically they are calculatiing HP by locked rotor current these days instead of work preformed; ie, they are lying!! ;-((

For small motors, look a the wattage or amps x volts. Use 1500 watts per hp and you will get close to reality.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View crank49's profile

crank49

3524 posts in 1722 days


#12 posted 11-04-2011 10:18 PM

Topo, there for a minute I thought we might be agreeing on something, :-) but my references say 1HP is 746 watts.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View TopamaxSurvivor's profile

TopamaxSurvivor

15088 posts in 2427 days


#13 posted 11-05-2011 12:21 AM

If you have a dc motor operating under optimum conditions you might gt clkse to that number.

For practical purposes in the real world with small ac motors, double it.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View crank49's profile

crank49

3524 posts in 1722 days


#14 posted 11-05-2011 01:04 AM

See, 120 volts X 15 amps = 1800 watts / (746 /0.9 effeciency) = 2.17 HP

In a perfect world.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View TopamaxSurvivor's profile

TopamaxSurvivor

15088 posts in 2427 days


#15 posted 11-05-2011 01:24 AM

In the real world, a 2.17 hp motor on 120 vac will draw about 25 or 26 amps ;-(( You probaly will have a tough time starting it on the 15 amp GFCI protected outlet in your garage.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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