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Discussion Starter · #1 ·
How does a 15 amp motor deliver 3.5 hp? (These are one of the router manufacturer's specs.) If one hp = 746 watts, at 110 volts that means you need about 22 amps to develop 3.5 hp. What am I missing? thanks. john
 

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Couple of things. First off, the power is likely calculated from 120 volts, not 110. Second, the motor is rated at 15 amps, but this may not be the peak amperage the motor is capable of handling. And the 3.5HP peak power is the peak rating, not the duty rating or average power of the motor.
 

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Discussion Starter · #4 ·
OK, thanks, Dan. Yes, I thought of the 120 V issue, but that does not explain much of the difference. I grew up talking about 110/220, so that is my habit, even though I never get 110 V when I meter anything. (Never get 115, either.) The peaks you mentioned may be the issues; I didn't think of that. I just don't want to put this "3.5 hp" motor on a 20 A breaker and be running to the breaker all the time to reset it. Bob: Do you mean to tell me that a manufacturer would fudge a tad? Nahh! Not in the US of A! cheers. john
 

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To really know what you're buying, you need to calculate the BS Factor. The formula is simple: BS= Rated HP/((Volts*Amps)/746). In your case, that's 3.5/((120*15)/746)=2.41 BS. That's a whole lot of BS.
 
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A router has a universal type motor and generally speaking, the HP increases as the RPM decreases, to point, then the opposite will occur.

In the OPs case, the HP listed is the maximum HP the router can produce. The amp rating is what it'll draw at some sort of 'normal usage'. The RPM at 'normal usage' is substantially higher than the RPM at peak HP resulting in lower amps.

Since the HP rating occurs at a lower RPM, the current will be higher than at 'normal usage'. The amp and HP ratings are not related to each other.

A typical air compressor has an induction motor, different than a universal one. An induction motor has 4 torque ratings, locked-rotor, pull-up, breakdown and full-load. Torque and HP are directly related (HP = Torque X RPM ÷ 5252).

The amp rating of a typical cheap air compressor is what it'll draw at normal operating pressure.

The HP rating is the maximum that the motor can produce. This will usually be very close to the breakdown torque RPM (usually about 70 - 80% of full load RPM).

As with the universal motor, the current at peak HP is much higher than normal operating current.

Also, as with the universal motor, the amps and HP are not related.

The way they get away with misleading ratings is the plain and simple fact that both ratings are true. The only problem is that the motor cannot produce the advertised HP for very long or it'll burn up. Further, it cannot produce its advertised HP at its rated amp draw.

Rob.
 

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Discussion Starter · #9 ·
Wow, Rob, now my brain is getting in "wound up", to be puny. I did a little digging now that I know what to look for. From here, http://www.pacontrol.com/download/Tutorial-Motor-Basics-Lecture.pdf, I see that torque and rpm are inversely related on a universal motor, which explains why they say to run larger bits at slower speed on a router.... I think.... As I dug, I realized that I need to really study electricity way more than I have to get a handle on this. There are a wide variety of motors in use; more than I ever knew. For now, I'll run the router on a 20 A circuit and call it good. cheers. j
 

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Discussion Starter · #10 ·
from Porter Cable

PC was kind enough to reply, and gave me permission to post this fyi. I just thought I'd add it to the mix. Enjoy. john.

"Hi John,
Thank you for contacting us.
We do apologize for the confusion. The HP rating is based on max output power which occurs at more than 15A. The 15A RATING is a thermally [and Compliance] driven number. For this test, we hold the motor under load until temperatures stabilize and then if said temperatures fall within the allowable range for Compliance [depending on motor construction, this can be upwards of 120° rise over ambient], then the tool can officially be rated for that current input."
 
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