Table Saw Power, amps - Woodworking Talk - Woodworkers Forum
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post #1 of 25 Old 08-31-2020, 10:43 AM Thread Starter
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Table Saw Power, amps

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?
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post #2 of 25 Old 08-31-2020, 11:59 AM
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Depends on the type of motor. I'm not that knowledgeable, but I do know an induction motor on a watt basis has more power.

The Jet will have more power if the Rigid is a direct drive.

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post #3 of 25 Old 08-31-2020, 12:34 PM Thread Starter
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The Jet will have more power if the Rigid is a direct drive.
Both are belt driven
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post #4 of 25 Old 08-31-2020, 01:19 PM
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Older motors have more copper in the windings ......

Quote:
Originally Posted by rpschultz13 View Post
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/me...ctrical-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.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 08-31-2020 at 03:24 PM.
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post #5 of 25 Old 08-31-2020, 04:27 PM Thread Starter
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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?
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post #6 of 25 Old 08-31-2020, 04:52 PM
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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.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #7 of 25 Old 08-31-2020, 05:16 PM
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Quote:
Originally Posted by rpschultz13 View Post
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.


Quote:
Originally Posted by rpschultz13 View Post
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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post #8 of 25 Old 08-31-2020, 06:05 PM
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"...Second, you really can't compare motors by the current draw at rated power."


but Firstly because manufactures are exceedingly dishonest in their claims...
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post #9 of 25 Old 09-01-2020, 07:38 AM
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Quote:
Originally Posted by rpschultz13 View Post
Both are belt driven
OK. An induction motor also has a start capacitor.

The universal motors can have a small belt drive, but they will be much smaller and louder.
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post #10 of 25 Old 09-01-2020, 09:27 AM Thread Starter
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It's a Jet 708315 10" Benchtop. The motor has a small belt. 5000 rpm. I know I've seen the words INDUCTION and 15amps somewhere, just can't find it.
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post #11 of 25 Old 09-01-2020, 09:41 AM
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How to tell instantly ....

Quote:
Originally Posted by rpschultz13 View Post
It's a Jet 708315 10" Benchtop. The motor has a small belt. 5000 rpm. I know I've seen the words INDUCTION and 15amps somewhere, just can't find it.
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.


The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #12 of 25 Old 09-01-2020, 12:20 PM
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Quote:
Originally Posted by rpschultz13 View Post
It's a Jet 708315 10" Benchtop. The motor has a small belt. 5000 rpm. I know I've seen the words INDUCTION and 15amps somewhere, just can't find it.

That saw has a universal motor. Tool Parts Direct and eReplacement Parts both sell replacement brushes for it.

-----Dave
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post #13 of 25 Old 09-01-2020, 03:21 PM Thread Starter
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Thanks for the information:)
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post #14 of 25 Old 09-20-2020, 10:42 PM Thread Starter
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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 50í 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.


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post #15 of 25 Old 09-21-2020, 12:23 AM
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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.

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post #16 of 25 Old 09-21-2020, 06:40 AM
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Quote:
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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.

Quote:
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.

Quote:
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.

-----Dave

Last edited by HoytC; 09-21-2020 at 06:46 AM.
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post #17 of 25 Old 09-21-2020, 09:31 AM
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Quote:
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......................So yes, .............. making input Amps only a rough indication of output power.
Thank you.

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post #18 of 25 Old 09-21-2020, 12:26 PM
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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.


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post #19 of 25 Old 09-21-2020, 03:17 PM Thread Starter
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Quote:
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Is there any advantage in wiring up a 220v outlet? I could easily do it, just not sure if it would be any benefit.
So... is there any advantage for me?
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post #20 of 25 Old 09-21-2020, 04:20 PM
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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.
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The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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