I found a "horsepower-amps-volts" clarification posted to this
newsgroup by John T. Horner. There's probably other good ones, too. I
thought it would be constructive to repeat it here, especially for
those ignorant of the horsepower-amp-volts thing, like me. John said
(with some minor edits by me)
First, Sears isn't the only company to use the misleading Developed
Power labeling .... but may be the most egregious about it.
"Developed" horsepower is much like "Peak RMS Power" was in the world
of amplifiers. In the US the advertising of "Peak Power" was outlawed
in the 1970s as misleading advertising.
"Developed" horsepower is calculated by jamming the motor in a stall
(not rotating) and turning on the power. The power drawn by the motor
under those conditions (amps x volts = watts) is measured. The
measurement is then converted to horsepower though the conversion 1
horsepower = 746 watts. A 3 "Developed Horsepower" motor is thus one
which draws 2,238 watts at a dead stall. Using standard 115 volt house
wiring we have 19.5 amps being drawn at a stall. A plain 6.12 ohm
resistor of sufficient power dissipation capacity (that is one very
physically large resistor!) thus also "Develops" 3 horsepower. Of
course this is all pure Horsepucky.
Electric motors are not 100% efficient at converting electrical power
into rotational mechanical power. Real "rated" horsepower is measured
as the mechanical power output of the motor at speed and under load.
The www.baldor.com site has good information on the specifications of
their ac motors for background reading if anyone is interested. An
excellent (big bucks!) single phase motor might have a full load
efficiency of 80%. The more common ones are in the range of 55% to
Now let us do some simple math:
A true 3 horsepower motor of 65% efficiency running at full load needs
(3HP X 746 watts/HP)/0.65 eff = 3,443 watts input power. At 115 volts
this implies 29.94 amps, obviously a serious overload for a standard 15
amp wall socket. At 230 volts the current draw drops to 14.97 amps,
within the capabilities of a 230 volt, 20 amp feed.
Doing the same calculations for a 1.5 HP motor you need about 15 amps
at 115 volts. Thus a power tool with a 1.5 HP motor running on 115
volts should be connected to a 20 amp circuit, not a 15 amp circuit.
This is why the rated horsepower of woodworking machines generally
doesn't go above 1.5 HP for 115 volt equipment.
There are issues of starting current, line losses, temperature margin
and such which have been left out of this quick primer.
I wish the government would put an end to this nonsense of developed
horsepower in the same manner they squashed the analogous situations
which once prevailed in the worlds of audio equipment and automobile
Basically a good explanation but a few corrections are in order. "Peak" and
"RMS" power are two different things. The advertisement of peak power was
the problem. RMS is a true measure of power. The audio industry is notorious
for padding their numbers even today. Car audio is a good example. There are
many systems sold that, if the actual output was as high as claimed, would
kill the engine when the volume was turned up.
Standard line voltage is 120 volts RMS (169.68 volts peak), not the 115
volts in the article. Small point and does not reduce the validity of the
article. Also, it is common practice, even among industrial suppliers, to
rate small motors in input horsepower as it makes it easier for the end user
to match a motor to his supply.
Did you check that both legs of the 240v circuit are hot? The light may
well run off one leg (at least in the US). I had a similar problem with
a 240v pool pump once. The breaker had failed and only supplied power to
Mike, would it be possible to jumper around the switch? Sudden failure,
smooth spinning, no smoke, brushes not showing damage (I presume you
also took a peek at the armature). Hmmm ... doesn't seem likely that the
router is getting juice.
There are two kinds of light--the glow that illuminates, and the glare
It probably would, but - well I am not very informed on things
electrical, and would not want to take a chance on doing anything silly.
Your guess seems right - I figure the speed control - which I looked at -
its an epoxy filled block, or the cut out on the switch guard. Def no
problem with brishes/armature/bearings etc - its a switching on/isolation
I might feel more confident next look (lol)
I have one, but touch wood, it has given me flawless service.
From the sound of your symptoms, I'd suspect a problem with the little
electronics board that does the soft start/speed control.
Hard to know what the best course is, maybe buy another identical one,
and get the other fixed when you are next in town?
On Fri, 29 Dec 2006 21:27:56 GMT, "Mike Richardson"
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