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Table Saw Power, amps

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#1 ·
Hi,

I have a 10" Jet benchtop tablesaw that boasts a 15 amp induction motor. I also have a 10" Ridgid 4520 hybrid that is 13 amps @ 120V. But when I compare the motors, the Ridgid motor is at least twice as big - seems like it would be more powerful. The sticky article on table saws said anything on 120V has a mathematical limit of 2HP, which makes sense. So is the Jet motor really more powerful than the Ridgid because of it's current rating?
 
#4 · (Edited)
Older motors have more copper in the windings ......

Hi,
I have a 10" Jet benchtop tablesaw that boasts a 15 amp induction motor. I also have a 10" Ridgid 4520 hybrid that is 13 amps @ 120V. But when I compare the motors, the Ridgid motor is at least twice as big - seems like it would be more powerful. The sticky article on table saws said anything on 120V has a mathematical limit of 2HP, which makes sense. So is the Jet motor really more powerful than the Ridgid because of it's current rating?

More copper or thicker wiring makes the older motors larger. They are "known" to have more power, and run cooler and longer. However, HP ratings are always suspect. Manufacturers and politicians often lie, so it's difficult to pin down an exact HP rating. Think about 6.5 HP shop vacs that have a 16 GA or 18 GA cords which is the first giveaway it's not a true rating.


One HP is 745 watts.
Divide 745 by 120 volt to get 6.20 AMPs
Now the question is whether that's starting or running AMPs, but
we know that starting AMPs is always greater.
So, there are definitely problems with HP ratings:
https://www.ridgidforum.com/forum/m.../725465-fake-hp-ratings-for-electrical-motors

Typical vintage Craftsman motor name plate info:



Notice the 14.0 AMP rating for 120 wiring which is the starting AMPs and may also be the full load requirement S.F.A. of 14.0 AMPs. But, is motor is rated at 1 HP, so there may be some confusion when doing the math, based on 745 watts = 1 HP.
 
#5 ·
So I understand that the 4512 could be converted from 120V to 240V, whereas the 4520 has a <10s blade brake that prevents the motor from being wired to 240V. I'm wondering if this the 4520 brake can be bypassed and allow the motor to be wired in 240V. I could make a 240V outlet in my shop, and I'm not afraid to dig into a motor if someone told me it was possible. Just not sure it's possible. Thoughts?
 
#6 ·
Here's all I know ....

In order to get 220 V you take each hot leg of 120 and measure between them. You would still have a 120 leg to operate the brake if that's what you wanted to do. Don't ask me how to do it, I just know the wiring has that capability inside/behind the motor cover.
 
#7 ·
Hi,

I have a 10" Jet benchtop tablesaw that boasts a 15 amp induction motor. I also have a 10" Ridgid 4520 hybrid that is 13 amps @ 120V. But when I compare the motors, the Ridgid motor is at least twice as big - seems like it would be more powerful. The sticky article on table saws said anything on 120V has a mathematical limit of 2HP, which makes sense. So is the Jet motor really more powerful than the Ridgid because of it's current rating?

First off, are you sure that the Jet has an induction motor? Most benchtop saws have universal motors. What's the part number of your saw?


Second, you really can't compare motors by the current draw at rated power. That current draw depends upon efficiency and power factor, both of which vary over quite a large range.


So I understand that the 4512 could be converted from 120V to 240V, whereas the 4520 has a <10s blade brake that prevents the motor from being wired to 240V. I'm wondering if this the 4520 brake can be bypassed and allow the motor to be wired in 240V. I could make a 240V outlet in my shop, and I'm not afraid to dig into a motor if someone told me it was possible. Just not sure it's possible. Thoughts?

Why? There's nothing to gain by doing that unless your power distribution has some serious deficiencies.
 
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#11 ·
How to tell instantly ....



Induction motors have sheet metal "humps" on the top to cover the capacitors, either one or two.

Universal AC/DC motors have small round buttons on opposite side which retain the brushes.

Circular saws, which are hand held and the cheaper table saws use universal brush type motors, sometimes direct drive, or with a multi-groove or gogged/toothed drive belt.

:vs_cool:
 
#14 ·
Back to this. My Ridgid 4512 has a 13 amp 3450 rpm motor that can be wired to 220v.

When I wired my shop, I used 12 gauge wire and 20 amp breakers. The outlet for the saw is probably 50’ from the sub panel.

Is there any advantage in wiring up a 220v outlet? I could easily do it, just not sure if it would be any benefit.


Sent from my iPhone using Tapatalk
 
#15 ·
Watts = Volts X Amps so Amps = Watts Divided by Volts.
This is purely an electrical factor and NOT a mechanical factor. Watts is electrical power , not mechanical power. In the case of electric motors, the power indicated by watts is the power drawn from the wall socket not the power output produced at the motor shaft. I dont remember the story but somehow some body a long time ago decided that 1 hp = 746 Watts.
So a very inefficient electrical motor can draw a lot of amps and not produce as much mechanical power as a very efficient electrical motor drawing the same wattage.
Bottom line is that the current method of determining mechanical power by applying 746 Watts = 1 HP is really quite useless.
 
#16 · (Edited)
Watts = Volts X Amps so Amps = Watts Divided by Volts.
Almost. Watts = Volts x Amps x Power Factor. Power Factor takes into account phase shift and harmonics.

This is purely an electrical factor and NOT a mechanical factor. Watts is electrical power , not mechanical power. In the case of electric motors, the power indicated by watts is the power drawn from the wall socket not the power output produced at the motor shaft. I dont remember the story but somehow some body a long time ago decided that 1 hp = 746 Watts.
No. The Watt is the metric unit for power, whether electrical or mechanical. Horsepower is the Imperial or US Customary unit of power.

Metric: One Watt = one Newton-meter per second. (Volts and Amps are defined in terms of mechanical units)

Imperial: One horsepower = 550 foot-pound per second.

Plug in the conversion factors for meters to feet and Newtons to pounds and you get 746 W/hp.

So a very inefficient electrical motor can draw a lot of amps and not produce as much mechanical power as a very efficient electrical motor drawing the same wattage.
Bottom line is that the current method of determining mechanical power by applying 746 Watts = 1 HP is really quite useless.
A NEMA rated motor (US) has its shaft power rated in horsepower. An IEC rated motor (Europe) has it's shaft power rated in Watts. By convention, the conversion factor used for this rating is actually the rounded value of 750 Watts/horsepower.

A 1.5 kW IEC motor is the same as a 2 hp NEMA motor. Same motor, different label.

So yes, output power = input Volts x Amps x efficiency x power factor, making input Amps only a rough indication of output power.
 
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#18 ·
You need to measure them to see which manufacturer (or both) are lying.


I've got a Husky air compressor in my garage that boasts 7HP at 220V which isn't possible with a 220V 15 amp motor (4.4HP max). Marketing departments are filled with liars who think they are clever.



You can figure, ponder and guess all day long but if you really want to know you'll need to put a clamp meter on one line and start it up. You'll want to measure startup current, running current no-load, running current while cutting a board and then the other factor is stall current, if you hit a knot and the blade jams what is the motor's current? The stall current is important because you want to determine if a stall will trip the breaker due to over current or if it will just hum until it overheats and trips the motors thermo-switch. Since overheating is bad for a motor it would be better if a jam blows the circuit breaker.


JayArr
 
#20 ·
I must have 25 -240 volt breakers ....

Between the woodshop, the metal shop and the garage I have lots of 240 machines, table saws, planers, RAS, drum sanders, air compressors, welder, heaters etc. I believe that it's always "better" to run less current in a wire than more for the same gauge wire.

There is a safety factor for in doing that such that the wires are less likely to overheat from excess current draw. Whether the dual voltage motors produce "more power" I don't know. Any 3 HP motors I have need 240 volts anyway. The 5 HP air compressor and table saw also need 240 volts.

If you can run the wires since there's not much expense there, so may as well do it. :vs_cool:
 
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#22 ·
Universal motors use RPM to generate horse power numbers. (Key word NUMBERS.)

Just look at your routers. I look at one of my table mounted routers that is rated as 3½ horse power. But it does it at about a zillion RPM. Would that motor drive my UniSaw? With belts / pullies or gears, the router motor might run the saw but I wouldn't want to try or use it.
 
#24 ·
THANK YOU, @NoThankyou!! Your post really clicked for me, and put it all together.

I know a little about motorcycles, and the same debate about horsepower goes on there. The discussion usually revolves around engine torque vs. engine horsepower. People like to compare the large displacement big bikes with two large cylinders and the smaller displacement bikes with four smaller cylinders. The maximum engine RPMs on those large twin-cylinder bikes is perhaps 6000 RPM, while the maximum on those smaller four cylinder bikes is 11000-12000+ RPM. That drives the horsepower ratings on those four cylinder bikes much higher than their large displacement twin-cylinder counterparts.

I have a Porter-Cable 890 2-1/4 HP mid-size router and a much smaller Makita RT0701C 1-1/4 HP compact router. I have been using the Makita compact router a lot the last few days, and marveled at how much horsepower could be put into a router motor that is so small. The Makita router was burning the wood on test pieces. I slowed it down, and that solved the issue. There is a chart in Makita manual relating the 1-6 numbers on the dial to actual RPMs.

Your post brought it all together for me.

The top speed of the Porter-Cable router is 23,000 RPM. The top speed of the Makita compact router is 30,000 RPM. That's how Makita boosts their horsepower ratings on that small motor.
 
#23 ·
There is one more possible advantage. It involves the electrical code so you'll have to check your local state or province for this but in some places the current rating in a shop or garage can be calculated by using the machine with the largest load. The theory is that a homeowner won't run more than one machine at a time so it's OK to have a welder, an air compressor a Radial arm saw and a table saw all on one breaker since only one will get used at a time. This saves space in your breaker panel.
 
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