I automatically knock off 20-30% from ANY manufacturers claim. On ANY product…HP, BTU, fuel mileage, battery life, etc. Even with that, I think a 3hp 110 spindle is asking for a fire.
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There’s a lot of BS in motor horsepower ratings in the consumer space. For example, my shop vac is advertised as 5 HP “peak horsepower”. It’s also specified at 9 amps at 120V. Well, 9 x 120 = 1080 watts, or 1080/746 = 1.45 horsepower. Huh? “Peak” horsepower is based on the maximum current the motor can draw; i.e., the locked rotor rating, measured with the motor in a laboratory fixture. It’s the biggest number the writers of the advertising copy could in some way justify using, and the average consumer doesn’t understand much beyond “bigger is better”. Yes it’s misleading, but routinely done in consumer products. Can the motor deliver 5 mechanical horsepower on a 15A/120V circuit? No that’s impossible.
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I’m not sure about your comments. Although the motors are different, my table saw has a 3hp motor running off a 15 amp breaker on 110v. In all the years of use i’ve never popped the breaker during a saw cut.
Hey Art, hey all,
That’s what was discussed above. Power tools don’t have to comply to IEC 60034-1 so the power with which they are sold can be fantasy.
3 hp is possibly what it will draw if it is blocked in a lab. Of course it will pop a 15 A breaker then. Even if you calculate with the ancient “horsepower” unit, power tools are often sold with current measured in a load situation and multiplied with the voltage according to P = U * I. Sometimes they seem to add a hp rating of what it draws before the tool is slowed down by the mechanical load and begins to get hot. This means you pay for power consumption, not for mechanical power delivered. The electrical power draw says nothing about a motor’s mechanical power delivered (motor efficiency), means whether it is a strong or a weak power tool.
Regarding circuit breakers, if it is rated with 15 A, this means that on 120 Volts * 15 Ampères which is an electric power draw of 1.8 kW (2.4 hp), the fuse will pop.
However a spindle has to comply to IEC 60034-1 so the power rating on the nameplate should mean the mechanical power delivered at the shaft, not the electric power draw. However, the power rating on cheap chinese spindles is often wrong, as is shown in this example. This means a spindle with a correct nameplate rating of let’s say 3 hp / 2.2 kW (so this is the mechanical power delivered) will draw between 3.4 and 4.5 kVA (see here and here). That’s on the spindle input which is at the output of the VFD. Then add the VFDs loss and the power draw at the VFD input is even higher. This explains the current draw values on the two VFD nameplates for 2.2 kW (3 hp) spindles above.
Also power tools like routers are usually so-called universal motors (carbon-brush commutated series-wound motors) that have a much worse efficiency than spindles (induction motors), which means with the same mechanical power delivered, universal motors need much more power drawn than spindles, or the other way round: With a specific electrical power consumption, a spindle delivers much more mechanical power than a router. See spindle vs. router motor characteristics comparison.
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Hey stuperduck, hey all,
But this would be for single-phase AC. Spindles run with three-phase AC whose formula is
P = U * I * √3
and the current (I) here would be the current per phase, and you have three of them, each shifted by 120°. Producing such a three-phase electricity is the purpose of the VFD. Also the VFD that is in front of the spindle has its own efficiency (or better said, lack of), which means the power draw would be even higher.
Anyway, at the input side of the VFD, there is usually single-phase electricity, and the formula for single-phase AC is
P = U * I
This gives:
120 V * 15 A = 1800 W
Conversion:
1800 W / 0.745 kW = 2.4 hp = power at which a 15 A circuit breaker will pop.
This also explains that for the 100 V class, Hitachi offers only VFDs for max. 0.75 kW spindles. This is because they don’t dare to endanger you when connecting it to a usual domestic U.S. supply circuit, like the cheap chinese spindle sellers at am*zon, Alib*ba, Ali*xpress or eb*y do, who simply omit the VFDs input current draw rating to let you believe you can run such a spindle like a 3 hp @110 V on a U.S. domestic supply circuit safely.
To know what supply circuit and circuit breaker rating you need, it has to be on the VFD nameplate and in the VFD manual. So I would not buy a VFD where this rating is willingly omitted. If you buy a serious VFD, like a Hitachi, Omron, Invertek, then you will always find the input current draw of the VFD, as shown on the two nameplates above. The cheap no-name chinese VFDs omit that, may they have a Hu*ny*ng name on it or not. I would never buy such a VFD.
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I’ve never heard of a 3 HP table saw that runs on 120V/15A. I have 3 HP cabinet saw, jointer, and planer, and they all run on a 240V, 15A or 20A circuit.
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My 13A tablesaw “claims” 3.25 HP…But I can also run it, a 1500w space heater, and a shopvac on a 20A circuit, so…somebody is lying 
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It is highly unlikely that the table saw gets run near its peak power, not sure on the heater and shop vac, seems like the breaker would trip if starting all at once.
Pat
I don’t do it intentionally, but on several occasions I have forgotten to turn off the heater before using the table saw but never tripped a breaker. If either the saw or the shopvac were ANYWHERE close to the “spec”, this could not be possible. But, manufacturers and their marketing departments know full well that americans are stupid, and will happily pay more for a bigger number on the box. After all, we still think oranges are the best way to get vitamin c.
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It’s a bit of a marketing gimmick, unfortunately, and likely something most people would miss. They’re listing Peak Watts, rather than RMS Watts.
Input voltage is 120Vrms.
Vpeak = Vrms × sqrt(2) ~ 170Vpeak
Wrms = 120Vrms × 13A = 1560Wrms
Wpeak = 170Vpeak × 13A = 2210Wpeak
So, the peak wattage, using only input power consumption and ignoring everyrhing else about the system, is 2210Wpeak (close to 3HP). A bit misleading, but nit “technically” false advertising.
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There seems to be some confusion in some quarters about the horsepower ratings of motors. Even if motor actually can produce 3 HP of mechanical power, most of the time it’s not going to be drawing that much power. Just like if you have a car with a 200 HP engine; at idle and most other times, it is producing far, far less than 200 HP. Good thing; otherwise you’re gas tank would run down very, very quickly. The 1500W space heater is using 1500 watts all the time since it’s basically a big resistor, but the motors at idle won’t draw anywhere near their ratings while they are idling.
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