When translating amps to hp for a 120V appliance, is the formula essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?

"Aaron Fude" wrote in message ...
When translating amps to hp for a 120V appliance, is the formula essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is ridiculously high, no? Is my
formula not right, or is 2.41hp not ridiculous or is there something else I'm missing?

Well if it was a Craftsman shop-vac, it would make perfect sense... they have no problem claiming
they can get 5 or more horsepower out of a 120V outlet! That level of power is achieved by using an
old sales technique called "lying" ;^)

Your calculation is fundamentally correct but leaves out the effect of efficiency. 80% is a
reasonable, maybe a little optimistic level of efficiency for a small electric motor. So if you're
putting in 2.4 HP of electrical energy, you'd get 0.8 * 2.4 = ~1.9 horsepower out of the motor.
Then you're going to lose some more in the gears, bearings, etc. So you maybe get something like
1.5 HP at the blade?

It's also probably not rated to produce that kind of power continuously. It can do it for a few
seconds or maybe tens of seconds, but longer than that and you'd probably smoke the motor.

Eric Law

In article , Aaron Fude wrote:
When translating amps to hp for a 120V appliance, is the formula essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?

You're missing two things:
1) The amperage rating on any electric motor represents the locked-rotor
current, i.e. the current it draws when the rotor is unable to move. This is
*much* higher than the current the motor draws when it's in operation, and is
in no way representative of the motor's actual power output.
2) You're also assuming 100% efficiency, which will never be the case. There
will always be some losses due to friction in the bearings, etc.

GIYF

Aaron Fude wrote:
When translating amps to hp for a 120V appliance, is the formula
essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?

Missing several things; Doug pointed out a couple (LRA and efficiency,
specifically) -- there is at least the power factor in addition which
will be roughly another 20% for this motor, probably (Ryobi is not
particularly noted for high-end products).

So, that 15A wouldn't be running amps, but if it were the hp would be
something otoo 120*15*0.8*0.8/746 -- ~1.5 hp. It's probably about 1 hp
running at about 10A running under load.

There's a fairly nice discussion on the factors involved here--I posted
a response not terribly long ago either here or in the rec.woodworking
group that also had links to some motor spec sheets; don't have that
handy but a search of any of the motor manufacturers will show
representative numbers for various levels of quality/pricing motors.
Most "run of the mill" consumer-grade/homeowner tool-grade are not the
high efficiency types at all--they are more expensive than most tools
use that are carried by the BORGs. In part this is price but it's also
because they're relatively small as industrial tools/motors go...

http://www.bacharach-training.com/norm/electric.htm

--

Doug Miller wrote:
In article , Aaron Fude wrote:
When translating amps to hp for a 120V appliance, is the formula essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?

You're missing two things:
1) The amperage rating on any electric motor represents the locked-rotor
current, i.e. the current it draws when the rotor is unable to move. This is
*much* higher than the current the motor draws when it's in operation, and is
in no way representative of the motor's actual power output.

Nope. The nameplate will have the running amps at rated HP. Overload
protection of may motors is based on the nameplate current rating.
(Many motors also have a "service factor" greater than one which means
the motor can be operated at higher current (and HP) at a little shorter
life.)

Motors typically have a code letter for locked rotor amps. Locked rotor
amps is about 6x the running amps. Unlikely the running amps for a table
saw is 2.5A. (An even 15A is probably not a motor current rating.)

The "lie" in motor HP *may* be the max HP you can get from the motor
before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think that would make sense for a shop vac.

2) You're also assuming 100% efficiency, which will never be the case. There
will always be some losses due to friction in the bearings, etc.

Quite right.

--
bud--

"Aaron Fude" wrote in message
...
When translating amps to hp for a 120V appliance, is the formula
essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?

You are missing the advertisement junk. Many consumer products are way over
rated. The motor may use 15 amps for about .05 seconds when first started,
but will drop to way less when running. There is no real truth in
advertisement.

Just as some computer speakers are rated from 50 to 100 watts and are
powered by a small wall cube. If you open them up they will have marked on
the speakers 2 watts or less. That 50 watts comes from a music power
rating. It may hit 50 watts for .001 seconds when a drum is first hit. The
average power will probably be more like a half a watt or less.

In article , bud-- wrote:
Doug Miller wrote:
In article , Aaron Fude
wrote:
When translating amps to hp for a 120V appliance, is the formula essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?

You're missing two things:
1) The amperage rating on any electric motor represents the locked-rotor
current, i.e. the current it draws when the rotor is unable to move. This is
*much* higher than the current the motor draws when it's in operation, and is

in no way representative of the motor's actual power output.

Nope. The nameplate will have the running amps at rated HP. Overload
protection of may motors is based on the nameplate current rating.
(Many motors also have a "service factor" greater than one which means
the motor can be operated at higher current (and HP) at a little shorter
life.)

You really think that Aaron's Ryobi table saw pulls 15A while running??

Eric wrote:

Well if it was a Craftsman shop-vac, it would make perfect sense...
they have no problem claiming they can get 5 or more horsepower out
of a 120V outlet! That level of power is achieved by using an old
sales technique called "lying" ;^)

Well, if the exhaust port of the Craftsman shop-vac is exposed to a vacuum,
and the inlet of the Craftsman shop-vac is located in a hyperbaric chamber,
then it could easily generate five or more horsepower. g

Jon

Aaron Fude wrote:
When translating amps to hp for a 120V appliance, is the formula
essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?

Hi,
You are right except not adding power factor(efficiency) to the formula.
Do you have ammeter? Read the current while motor is running/working.
There you'll see it. Ryobi? My idea power tool is to buy the best and
keep it life time. Some times buying old used one is even better than
new crap.

Doug Miller wrote:
In article , bud-- wrote:
Doug Miller wrote:
In article , Aaron Fude
wrote:
When translating amps to hp for a 120V appliance, is the formula essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?
You're missing two things:
1) The amperage rating on any electric motor represents the locked-rotor
current, i.e. the current it draws when the rotor is unable to move. This is
*much* higher than the current the motor draws when it's in operation, and is
in no way representative of the motor's actual power output.
Nope. The nameplate will have the running amps at rated HP. Overload
protection of may motors is based on the nameplate current rating.
(Many motors also have a "service factor" greater than one which means
the motor can be operated at higher current (and HP) at a little shorter
life.)

You really think that Aaron's Ryobi table saw pulls 15A while running??

I would guess Aaron used an approximate number (exactly 15?) for the
sake of discussion.

What I can say is:
- the NEC gives a table value (approximate) of 16 full load running amps
for a garden variety 1 HP motor running on 115V.
- the current value on motor nameplates is running current
- locked rotor current is indicated on most motor nameplates as a code
letter
- as I wrote before, if 15A was a locked rotor current the full load
running current would be about 2.5A. Looking at the NEC table that is
the value for a motor well under 1/6 HP
- my table saw motor is 1 HP with a full load running current of 14.0A

I don't see any possibility that 15A is an LRA.

In answer - yes I do think it is entirely reasonable that Aaron's table
saw motor pulls *about* 15A when the motor is *loaded* to it's full
horsepower.

--
bud--

one HP is 748 watts, if that's any use. I suspect the
engineers and brainiacs out there have answered any possible
slant on the question.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..

120 volts x 115 amps = 1800 watts.

1800 watts / 748 = about 2.40. Works out OK if you figure
100% efficiency.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp
which is
ridiculously high, no? Is my formula not right, or is
2.41hp not
ridiculous or is there something else I'm missing?

Stormin Mormon wrote:
120 volts x 115 amps = 1800 watts.

1800 watts / 748 = about 2.40. Works out OK if you figure
100% efficiency.



Uh, 120 volts x 115 amps = 13,800 watts. You must have meant 15 amps.

Stormin Mormon wrote:
120 volts x 115 amps = 1800 watts.

1800 watts / 748 = about 2.40. Works out OK if you figure
100% efficiency.

115 amps?

YYes, I ddon't know how I ddid that. SSorry.

--
CChristopher A. YYoung
Learn more about Jesus
www.lds.org
..


"HeyBub" wrote in message
m...
Stormin Mormon wrote:
120 volts x 115 amps = 1800 watts.

1800 watts / 748 = about 2.40. Works out OK if you figure
100% efficiency.



Uh, 120 volts x 115 amps = 13,800 watts. You must have meant
15 amps.

Tony wrote:
Stormin Mormon wrote:
120 volts x 115 amps = 1800 watts.

1800 watts / 748 = about 2.40. Works out OK if you figure
100% efficiency.

115 amps?

Is it really that hard of a leap of logic for you to come to the conclusion
that the OP meant "15", and that the "115" value is a simple typographical
error?

Jon

In article , Stormin Mormon wrote:
one HP is 748 watts, if that's any use. I suspect the
engineers and brainiacs out there have answered any possible
slant on the question.

One horsepower appears to me to be 748 watts divided by efficiency of
the electric motor. At this moment, I like to say 900 watts for an
electric motor to deliver "1 horsepower".

Please keep in mind that AC electric motors, especially single-phase
(not requiring 3-phase power), and smaller than a horsepower or two,

appear to me to be likely to have efficiency at best in the 80's %,

and their power factor appears to me to be typically close to .8
or so, maybe .86 or so in some cases.

- Don Klipstein )

Stormin Mormon wrote:
one HP is 748 watts, if that's any use. I suspect the
engineers and brainiacs out there have answered any possible
slant on the question.


746 watts.

TDD

Watt did you say?

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"The Daring Dufas" wrote in
message ...
Stormin Mormon wrote:
one HP is 748 watts, if that's any use. I suspect the
engineers and brainiacs out there have answered any
possible
slant on the question.


746 watts.

TDD

On Aug 4, 6:33*am, "Stormin Mormon"
wrote:
Watt did you say?

--
Christopher A. Young
Learn more about Jesus
*www.lds.org
.

"The Daring Dufas" wrote in
...

Stormin Mormon wrote:
one HP is 748 watts, if that's any use. I suspect the
engineers and brainiacs out there have answered any
possible
slant on the question.

746 watts.

TDD

AC Volts times AC amps = volt-amps, not watts

Hp = (q*V*I*eff*pf)/746
N = number of horsepower
q= number of phases
V = voltage measured line to neutral
I = phase current
eff = motor efficiency
pf = power factor of motor

Approximate Hp for typical single phase, 120V motor.

******** Hp = 0.15 * motor amps ********

On Aug 3, 11:07*am, bud-- wrote:
Doug Miller wrote:
In article , Aaron Fude wrote:
When translating amps to hp for a 120V appliance, is the formula essentially

120*amps/745.699872

That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?

You're missing two things:
1) The amperage rating on any electric motor represents the locked-rotor
current, i.e. the current it draws when the rotor is unable to move. This is
*much* higher than the current the motor draws when it's in operation, and is
in no way representative of the motor's actual power output.

Nope. The nameplate will have the running amps at rated HP. Overload
protection of may motors is based on the nameplate current rating.
(Many motors also have a "service factor" greater than one which means
the motor can be operated at higher current (and HP) at a little shorter
life.)

Motors typically have a code letter for locked rotor amps. Locked rotor
amps is about 6x the running amps. Unlikely the running amps for a table
saw is 2.5A. (An even 15A is probably not a motor current rating.)

The "lie" in motor HP *may* be the max HP you can get from the motor
before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think *that would make sense for a shop vac.

2) You're also assuming 100% efficiency, which will never be the case. There
will always be some losses due to friction in the bearings, etc.

Quite right.

--
bud--

The "lie" in motor HP *may* be the max HP you can get from the
motor before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think that would make sense for a shop
vac. "

The way I heard it years ago is that the max-claimed HP of a shop-vac,
etc. was actually achievable. It was the based on the max amperage the
unit would draw if you jammed a screwdriver in the impeller. Just
before the motor fried, you will have achieved maximum HP.

DerbyDad03 wrote:
On Aug 3, 11:07 am, bud-- wrote:
Doug Miller wrote:
In article , Aaron Fude wrote:
When translating amps to hp for a 120V appliance, is the formula essentially
120*amps/745.699872
That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?
You're missing two things:
1) The amperage rating on any electric motor represents the locked-rotor
current, i.e. the current it draws when the rotor is unable to move. This is
*much* higher than the current the motor draws when it's in operation, and is
in no way representative of the motor's actual power output.
Nope. The nameplate will have the running amps at rated HP. Overload
protection of may motors is based on the nameplate current rating.
(Many motors also have a "service factor" greater than one which means
the motor can be operated at higher current (and HP) at a little shorter
life.)

Motors typically have a code letter for locked rotor amps. Locked rotor
amps is about 6x the running amps. Unlikely the running amps for a table
saw is 2.5A. (An even 15A is probably not a motor current rating.)

The "lie" in motor HP *may* be the max HP you can get from the motor
before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think that would make sense for a shop vac.

2) You're also assuming 100% efficiency, which will never be the case. There
will always be some losses due to friction in the bearings, etc.
Quite right.

--
bud--

The "lie" in motor HP *may* be the max HP you can get from the
motor before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think that would make sense for a shop
vac. "

The way I heard it years ago is that the max-claimed HP of a shop-vac,
etc. was actually achievable. It was the based on the max amperage the
unit would draw if you jammed a screwdriver in the impeller. Just
before the motor fried, you will have achieved maximum HP.

AC compressors are rated in RLA Run Load Amps and LRA Locked Rotor Amps.
Of course when you're putting the freon in a system, you can watch the
current draw increase as the compressor does more work. My favorite
question to use to tax the mind of a neophyte is "Will the blower draw
more or less current if the inlet is blocked off?"

TDD

On Aug 4, 9:11*pm, The Daring Dufas
wrote:
DerbyDad03 wrote:
On Aug 3, 11:07 am, bud-- wrote:
Doug Miller wrote:
In article , Aaron Fude wrote:
When translating amps to hp for a 120V appliance, is the formula essentially
120*amps/745.699872
That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?
You're missing two things:
1) The amperage rating on any electric motor represents the locked-rotor
current, i.e. the current it draws when the rotor is unable to move. This is
*much* higher than the current the motor draws when it's in operation, and is
in no way representative of the motor's actual power output.
Nope. The nameplate will have the running amps at rated HP. Overload
protection of may motors is based on the nameplate current rating.
(Many motors also have a "service factor" greater than one which means
the motor can be operated at higher current (and HP) at a little shorter
life.)

Motors typically have a code letter for locked rotor amps. Locked rotor
amps is about 6x the running amps. Unlikely the running amps for a table
saw is 2.5A. (An even 15A is probably not a motor current rating.)

The "lie" in motor HP *may* be the max HP you can get from the motor
before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think *that would make sense for a shop vac.

2) You're also assuming 100% efficiency, which will never be the case.. There
will always be some losses due to friction in the bearings, etc.
Quite right.

--
bud--

The "lie" in motor HP *may* be the max HP you can get from the
motor before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think *that would make sense for a shop
vac. "

The way I heard it years ago is that the max-claimed HP of a shop-vac,
etc. was actually achievable. It was the based on the max amperage the
unit would draw if you jammed a screwdriver in the impeller. Just
before the motor fried, you will have achieved maximum HP.

AC compressors are rated in RLA Run Load Amps and LRA Locked Rotor Amps.
Of course when you're putting the freon in a system, you can watch the
current draw increase as the compressor does more work. My favorite
question to use to tax the mind of a neophyte is "Will the blower draw
more or less current if the inlet is blocked off?"

TDD- Hide quoted text -

- Show quoted text -

Less

DerbyDad03 wrote:
On Aug 4, 9:11 pm, The Daring Dufas
wrote:
DerbyDad03 wrote:
On Aug 3, 11:07 am, bud-- wrote:
Doug Miller wrote:
In article , Aaron Fude wrote:
When translating amps to hp for a 120V appliance, is the formula essentially
120*amps/745.699872
That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?
You're missing two things:
1) The amperage rating on any electric motor represents the locked-rotor
current, i.e. the current it draws when the rotor is unable to move. This is
*much* higher than the current the motor draws when it's in operation, and is
in no way representative of the motor's actual power output.
Nope. The nameplate will have the running amps at rated HP. Overload
protection of may motors is based on the nameplate current rating.
(Many motors also have a "service factor" greater than one which means
the motor can be operated at higher current (and HP) at a little shorter
life.)
Motors typically have a code letter for locked rotor amps. Locked rotor
amps is about 6x the running amps. Unlikely the running amps for a table
saw is 2.5A. (An even 15A is probably not a motor current rating.)
The "lie" in motor HP *may* be the max HP you can get from the motor
before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think that would make sense for a shop vac.
2) You're also assuming 100% efficiency, which will never be the case. There
will always be some losses due to friction in the bearings, etc.
Quite right.
--
bud--
The "lie" in motor HP *may* be the max HP you can get from the
motor before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think that would make sense for a shop
vac. "
The way I heard it years ago is that the max-claimed HP of a shop-vac,
etc. was actually achievable. It was the based on the max amperage the
unit would draw if you jammed a screwdriver in the impeller. Just
before the motor fried, you will have achieved maximum HP.
AC compressors are rated in RLA Run Load Amps and LRA Locked Rotor Amps.
Of course when you're putting the freon in a system, you can watch the
current draw increase as the compressor does more work. My favorite
question to use to tax the mind of a neophyte is "Will the blower draw
more or less current if the inlet is blocked off?"

TDD- Hide quoted text -

- Show quoted text -

Less

You win a Kewpie Doll!

TDD

The Daring Dufas wrote:
DerbyDad03 wrote:
On Aug 4, 9:11 pm, The Daring Dufas
wrote:
DerbyDad03 wrote:
On Aug 3, 11:07 am, bud-- wrote:
Doug Miller wrote:
In article , Aaron Fude
wrote:
When translating amps to hp for a 120V appliance, is the formula
essentially
120*amps/745.699872
That would give a 15amp ryobi table saw a rating of 2.41hp which is
ridiculously high, no? Is my formula not right, or is 2.41hp not
ridiculous or is there something else I'm missing?
You're missing two things:
1) The amperage rating on any electric motor represents the
locked-rotor
current, i.e. the current it draws when the rotor is unable to
move. This is
*much* higher than the current the motor draws when it's in
operation, and is
in no way representative of the motor's actual power output.
Nope. The nameplate will have the running amps at rated HP. Overload
protection of may motors is based on the nameplate current rating.
(Many motors also have a "service factor" greater than one which means
the motor can be operated at higher current (and HP) at a little
shorter
life.)
Motors typically have a code letter for locked rotor amps. Locked
rotor
amps is about 6x the running amps. Unlikely the running amps for a
table
saw is 2.5A. (An even 15A is probably not a motor current rating.)
The "lie" in motor HP *may* be the max HP you can get from the motor
before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think that would make sense for a shop
vac.
2) You're also assuming 100% efficiency, which will never be the
case. There
will always be some losses due to friction in the bearings, etc.
Quite right.
--
bud--
The "lie" in motor HP *may* be the max HP you can get from the
motor before stall (which which would be a lie), which you might very
occasionally use. Wouldn't think that would make sense for a shop
vac. "
The way I heard it years ago is that the max-claimed HP of a shop-vac,
etc. was actually achievable. It was the based on the max amperage the
unit would draw if you jammed a screwdriver in the impeller. Just
before the motor fried, you will have achieved maximum HP.
AC compressors are rated in RLA Run Load Amps and LRA Locked Rotor Amps.
Of course when you're putting the freon in a system, you can watch the
current draw increase as the compressor does more work. My favorite
question to use to tax the mind of a neophyte is "Will the blower draw
more or less current if the inlet is blocked off?"

TDD

Less

You win a Kewpie Doll!


I was at an industrial plant that had a large pump on the water well -
10 HP or larger. The motor was on the surface with a shaft down the well
to a centrifugal pump at the bottom. The plant engineer (actually a
mechanical engineer) said when they first started the pump the motor
current was too high, so he closed the discharge valve a little until
the current was at the rated motor current. I was not happy with the
idea and measured the motor current while I closed the discharge valve
on a 4" submersible pump. He was right. Mechanical HP is basically flow
rate times head pressure at the pump. Closing the valve raised the head
pressure but apparently lowered the flow rate faster.

--
bud--