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--