Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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  #1   Report Post  
Chuck
 
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Default Compressor Motor: HP v.s. Amps?

I have an older Sears compressor that was given to me because it had
pinholes in the tank. I would like to replace the tank with a larger unit
(20 gal instead of the current 11 gal). I think I've located a suitiable
portable tank. But, in the process, I wanted to learn a bit more about the
actual compressor itself.

The compressor currently has a 3/4 hp motor. At first glance I was kind of
depressed thinking "Gee, just about every compressor I see these days has 3
+ hp. Will this thing even work well?" In asking around I was told that
amperage plays a big part in the actual torque of the motor. This is where
I get confused... I thougth torque was the direct product of the motor's
hp. How does the amperage come into play? Can you have a "strong" or
"weak" 3/4 hp motor? What factors actually determine the torque? Or, am I
looking at this equation in the wrong way?

Specs of the motor I currently have (from the mfr plate on the cover):

Doerr
Insul Class: A
Ser. Fact: 1.0
MC544
RPM: 3450
Duty: Cont
Type: K
Phase: 1
HP: 3/4
HZ: 60
FR: F56
Mtr Ref: 600702 H733
Thermally Prot: Manual
V: 115
A: 14.4

Specs from the capacitor:
STM
A-2131-10
375MFD
110VAC 60 Cycle
1160-7317

Any advice would be greatly appreciated. Please post replys to the group.

Thanks!
Chuck
  #2   Report Post  
Gunner
 
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Default Compressor Motor: HP v.s. Amps?

On Fri, 03 Oct 2003 12:28:57 -0000, Chuck wrote:

I have an older Sears compressor that was given to me because it had
pinholes in the tank. I would like to replace the tank with a larger unit
(20 gal instead of the current 11 gal). I think I've located a suitiable
portable tank. But, in the process, I wanted to learn a bit more about the
actual compressor itself.

The compressor currently has a 3/4 hp motor. At first glance I was kind of
depressed thinking "Gee, just about every compressor I see these days has 3
+ hp. Will this thing even work well?" In asking around I was told that
amperage plays a big part in the actual torque of the motor. This is where
I get confused... I thougth torque was the direct product of the motor's
hp. How does the amperage come into play? Can you have a "strong" or
"weak" 3/4 hp motor? What factors actually determine the torque? Or, am I
looking at this equation in the wrong way?

Specs of the motor I currently have (from the mfr plate on the cover):

Doerr


If its a Doerr, its a very good motor, and the data plate horsepower is
exactly what it produces, not Sears HP. As its a 3450 rpm motor, thats
where they are getting their rapid pumping from. A good unloader is a
requirement, with a "high speed" motor. The torque is related to the
size of the pully on the motor, and on the pump itself.

Gunner

Insul Class: A
Ser. Fact: 1.0
MC544
RPM: 3450
Duty: Cont
Type: K
Phase: 1
HP: 3/4
HZ: 60
FR: F56
Mtr Ref: 600702 H733
Thermally Prot: Manual
V: 115
A: 14.4

Specs from the capacitor:
STM
A-2131-10
375MFD
110VAC 60 Cycle
1160-7317

Any advice would be greatly appreciated. Please post replys to the group.

Thanks!
Chuck


Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.
  #3   Report Post  
Bob Swinney
 
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Default Compressor Motor: HP v.s. Amps?

Chuck sez:
"...I thougth torque was the direct product of the motor's
hp. How does the amperage come into play? Can you have a "strong" or
"weak" 3/4 hp motor? What factors actually determine the torque? Or, am

I
looking at this equation in the wrong way?"


Torque is the capacity of an engine to do work whereas HP is the rate at
which an engine does work. [Torque, in foot-pounds = (Horsepower x 5252)
divided by RPM.] For instance, an engine doing 250 HP of work and turning
at 1200 RPM has torque of 1094 ft. lbs. Torque is the force causing a shaft
to turn, sometimes called "turning moment". Torque, discounting friction,
is the same in each moving member of any transmission link - this is true
because of the equation above. In a machine working at any given rate (HP),
torque is the same at each link in the machine from the output shaft through
the transmission and on to the wheels. RPM varies from link to link because
of diameter differences but torque is the same everywhere. Consider the
equation, above: Torque is directly related to HP and inversely related to
RPM.

Electrical power is defined as the product of voltage x current. Electrical
power is measured in Watts and there are 746 Watts in 1 HP. For example, a
3/4 HP motor can develop [746 x 3/4 = 560 Watts] of power. A 3/4 HP motor
running on 120 volts would have an input current of [560 divided by 120 =
4.7] amps. Because small single phase electric motors have an overall
efficiency of around 50% it is customary, in rough calculations, to double
the amount of calculated input current. That would mean an input current of
9.4 amps if the 3/4 HP motor was delivering its full output capability of
560 Watts (3/4 HP of work). Key to this is that output power is always
related to input power. A motor has no inherent HP - HP always depends on
motion. Delivered HP (Watts of work), discounting friction, depends on the
amount of power (voltage x current) put into the motor. As seen above, the
motor running fully loaded would "draw" about 9 amps of current. Unloaded
and spinning freely, the current draw would be much less, probably less than
2 amps.

Bob Swinney


"Chuck" wrote in message
. ..
I have an older Sears compressor that was given to me because it had
pinholes in the tank. I would like to replace the tank with a larger unit
(20 gal instead of the current 11 gal). I think I've located a suitiable
portable tank. But, in the process, I wanted to learn a bit more about

the
actual compressor itself.

The compressor currently has a 3/4 hp motor. At first glance I was kind

of
depressed thinking "Gee, just about every compressor I see these days has

3
+ hp. Will this thing even work well?" In asking around I was told that
amperage plays a big part in the actual torque of the motor. This is

where
I get confused...
Specs of the motor I currently have (from the mfr plate on the cover):

Doerr
Insul Class: A
Ser. Fact: 1.0
MC544
RPM: 3450
Duty: Cont
Type: K
Phase: 1
HP: 3/4
HZ: 60
FR: F56
Mtr Ref: 600702 H733
Thermally Prot: Manual
V: 115
A: 14.4

Specs from the capacitor:
STM
A-2131-10
375MFD
110VAC 60 Cycle
1160-7317

Any advice would be greatly appreciated. Please post replys to the group.

Thanks!
Chuck



  #4   Report Post  
Bruce L. Bergman
 
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Default Compressor Motor: HP v.s. Amps?

On Fri, 03 Oct 2003 12:28:57 -0000, someone who calls themselves Chuck
wrote:

snip
The compressor currently has a 3/4 hp motor. At first glance I was kind of
depressed thinking "Gee, just about every compressor I see these days has 3
+ hp. Will this thing even work well?" In asking around I was told that
amperage plays a big part in the actual torque of the motor. This is where
I get confused... I thougth torque was the direct product of the motor's
hp. How does the amperage come into play? Can you have a "strong" or
"weak" 3/4 hp motor? What factors actually determine the torque? Or, am I
looking at this equation in the wrong way?


Your old motor is rated in actual usable HP, while almost all small
compressors (and many other consumer appliances where they sell by the
Gee Whiz Factor) now are marked with "Peak HP" - basically, the
highest horsepower output recorded right before the motor stalls.
(And goes up in flames if you don't pull the plug fast.) Just like
diagonal size inflation on TV screens...

Note that on all the motors of these new compressors they don't have
a motor manufacturer's HP rating on the sticker - Some I've seen leave
a blank space there (so they don't contradict the sales literature),
other motor nameplates have the HP block marked "Special". You have
to go by current draw and voltage to get the real motor ratings, and
by CFM @ PSI ratings to do a reasonable comparison between units.

I'm not going to do the math, but your 3/4 HP motor would probably
be sold as a "2 HP (Peak)" compressor today. 3 if they like to lie.

-- Bruce --
--
Bruce L. Bergman, POB 394, Woodland Hills CA 91365, USA
Electrician, Westend Electric (#726700) Agoura, CA

WARNING: UCE Spam E-mail is not welcome here. I report violators.
SpamBlock In Use - Remove the "Python" with a "net" to E-Mail.
  #5   Report Post  
Dave Baker
 
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Default Compressor Motor: HP v.s. Amps?

Subject: Compressor Motor: HP v.s. Amps?
From: "Bob Swinney"
Date: 03/10/03 15:10 GMT Daylight Time
Message-id: t

Chuck sez:
"...I thougth torque was the direct product of the motor's
hp. How does the amperage come into play? Can you have a "strong" or
"weak" 3/4 hp motor? What factors actually determine the torque? Or, am

I
looking at this equation in the wrong way?"


Torque is the capacity of an engine to do work whereas HP is the rate at
which an engine does work. [Torque, in foot-pounds = (Horsepower x 5252)
divided by RPM.] For instance, an engine doing 250 HP of work and turning
at 1200 RPM has torque of 1094 ft. lbs. Torque is the force causing a shaft
to turn, sometimes called "turning moment". Torque, discounting friction,
is the same in each moving member of any transmission link - this is true
because of the equation above. In a machine working at any given rate (HP),
torque is the same at each link in the machine from the output shaft through
the transmission and on to the wheels.


Of course it isn't. If that were the case there would be no point in having
gearboxes. Horsepower is the same at any point in a machine, apart from
frictional losses. Torque varies inversely with rpm as altered by gearing. The
whole point of a gearbox is to multiply torque.

RPM varies from link to link because
of diameter differences but torque is the same everywhere.


If that were true then horsepower would be being created from nowhere or
dissipated to nowhere at different points in the machine, in violation of
everything that physicists hold sacred.



Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do though.


  #6   Report Post  
Bob Swinney
 
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Default Compressor Motor: HP v.s. Amps?

Dave sez: "...Horsepower is the same at any point in a machine, apart from
frictional losses. Torque varies inversely with rpm as altered by gearing.

The
whole point of a gearbox is to multiply torque...."


Another way of saying the same thing. Torque varies inversely with RPM.
Torque as well as HP is everywhere the same in a transmission link.

And Dave further sez:

"... RPM varies from link to link because of diameter differences but torque
is the same everywhere.
If that were true then horsepower would be being created from nowhere or
dissipated to nowhere at different points in the machine, in violation of
everything that physicists hold sacred."

Guess you missed the math, Dave! Horsepower is passed through each segment
of a transmission link - RPM varies, torque varies, but HP remains the same,
discounting frictional losses.

Bob Swinney



"Dave Baker" wrote in message
...
Subject: Compressor Motor: HP v.s. Amps?
From: "Bob Swinney"
Date: 03/10/03 15:10 GMT Daylight Time
Message-id: t

Chuck sez:
"...I thougth torque was the direct product of the motor's
hp. How does the amperage come into play? Can you have a "strong" or
"weak" 3/4 hp motor? What factors actually determine the torque? Or,

am
I
looking at this equation in the wrong way?"


Torque is the capacity of an engine to do work whereas HP is the rate at
which an engine does work. [Torque, in foot-pounds = (Horsepower x 5252)
divided by RPM.] For instance, an engine doing 250 HP of work and

turning
at 1200 RPM has torque of 1094 ft. lbs. Torque is the force causing a

shaft
to turn, sometimes called "turning moment". Torque, discounting

friction,
is the same in each moving member of any transmission link - this is true
because of the equation above. In a machine working at any given rate (H

P),
torque is the same at each link in the machine from the output shaft

through
the transmission and on to the wheels.


Of course it isn't. If that were the case there would be no point in

having
gearboxes.



Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do

though.


  #7   Report Post  
Jon Elson
 
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Default Compressor Motor: HP v.s. Amps?



Chuck wrote:

I have an older Sears compressor that was given to me because it had
pinholes in the tank. I would like to replace the tank with a larger unit
(20 gal instead of the current 11 gal). I think I've located a suitiable
portable tank. But, in the process, I wanted to learn a bit more about the
actual compressor itself.

The compressor currently has a 3/4 hp motor. At first glance I was kind of
depressed thinking "Gee, just about every compressor I see these days has 3
+ hp. Will this thing even work well?" In asking around I was told that
amperage plays a big part in the actual torque of the motor. This is where
I get confused...

That is intentional. Compressor ratings are largely a vast conspiracy
of lies.
You'll see lots of home shop type compressors at the local Home Depot with
standard 15A 115 V plugs rated at 5, and now even * 6 * Hp! Well,
there's no
way to get 6 Hp out of a 15 A wall socket. You can barely get 2 Hp, and
if you
run so much as a 100 W bulb on the same breaker, it will trip after a while.

Anyway, my rule of thumb is a decent single stage compressor will
deliver about
3.2 CFM (at 90 - 100 PSI) per (real world) Horsepower. I also will
throw out
another rule of thumb, which is that 2 (real) Hp is about the lower
limit you
should try to use in a home shop. 3/4 Hp is just too small for serious
air tool
work. It would be fine for an air brush or tire filling, but almost any
standard
air tool, like a die grinder, air paint sprayer, (very small) sand
blaster, air chisel,
etc. will use at least 6 CFM, and some will use a lot more. Unless you
want to wait
5 minutes out of every 6, you will soon be looking for a bigger compressor.
Forget HP, as some compressors sold for industrial use DON'T lie about it,
so it can't be used for comparisons. Just look at CFM at 90 PSI (don't
be foold by ratings
at 40 PSI, either.)

The other bad news is that the oilless compressors tend to be EXTREMELY
noisy,
and will drive you (or your neighbors) nuts! Oil filled compressors
generally
turn much slower, and some good ones can be rather quiet.

Jon

  #9   Report Post  
Bob Swinney
 
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Default Compressor Motor: HP v.s. Amps?

This may be the original statement that Dave took exception to; I still
stand behind it:

"...Torque, discounting friction,
is the same in each moving member of any transmission link - this is true
because of the equation above. In a machine working at any given rate (HP),
torque is the same at each link in the machine from the output shaft through
the transmission and on to the wheels. RPM varies from link to link because
of diameter differences but torque is the same everywhere. Consider the
equation, above: Torque is directly related to HP and inversely related to
RPM...."

Bob Swinney
"Dave Baker" wrote in message
...
Subject: Compressor Motor: HP v.s. Amps?
From: "Bob Swinney"
Date: 03/10/03 20:34 GMT Daylight Time
Message-id: t


Bob's first statement

"... RPM varies from link to link because of diameter differences but

torque
is the same everywhere.


Bob's second statement

Guess you missed the math, Dave! Horsepower is passed through each

segment
of a transmission link - RPM varies, torque varies, but HP remains the

same,
discounting frictional losses.


That isn't what you first posted. Whether you meant to say "hp is the same
everywhere" I have no idea but the first statement was categorically

wrong.


Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do

though.


  #10   Report Post  
Bob Swinney
 
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Default Compressor Motor: HP v.s. Amps?

OOOOPPPS sorry Dave! My statement should have been that HP is constant from
link to link in a transmission network. It follows that torque by the
equation: Torque = (HP x 5252)/RPM does in fact vary inversely with RPM.

Bob Swinney


"Dave Baker" wrote in message
...
Subject: Compressor Motor: HP v.s. Amps?
From: "Bob Swinney"
Date: 03/10/03 20:34 GMT Daylight Time
Message-id: t


Bob's first statement

"... RPM varies from link to link because of diameter differences but

torque
is the same everywhere.


Bob's second statement

Guess you missed the math, Dave! Horsepower is passed through each

segment
of a transmission link - RPM varies, torque varies, but HP remains the

same,
discounting frictional losses.


That isn't what you first posted. Whether you meant to say "hp is the same
everywhere" I have no idea but the first statement was categorically

wrong.


Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do

though.




  #11   Report Post  
Bob Swinney
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?


"Bob Swinney" wrote in message
. net...
OOOOPPPS sorry Dave! My statement should have been that HP is constant

from
link to link in a transmission network. It follows that torque by the
equation: Torque = (HP x 5252)/RPM does in fact vary inversely with RPM.

Bob Swinney


Erratum: Please see following correction notice sent to "Live Steam"
magazine:

Clover McKinley, Editor
Live Steam Magazine

One of the readers of the news group, rec.crafts.metalworking, has pointed
out a misleading statement made in my article. Page 19, next to last par:
"Wheel edge (pulling force) is everywhere the same in a transmission link."
While fundamentally true, this statement could be misleading. It should,
more accurately state: "Wheel edge power (horsepower) is everywhere the
same in a transmission link".

Sincerely,

Bob Swinney


  #12   Report Post  
ATP
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Chuck wrote:
Can you have a "strong" or "weak" 3/4 hp motor? What
factors actually determine the torque? Or, am I looking at this
equation in the wrong way?


All 3/4 HP motors at full speed will do the same work. At the same RPM doing
the same amount of work they will output the same torque. Motors have
different power curves, and plotting torque vs. RPM is a useful way of
looking at the information. HP vs. RPM is usually graphed as well. To
compare torque values for full speed, steady state running conditions with
the same HP motors would be kind of pointless, unless I'm missing something.


  #13   Report Post  
Richard J Kinch
 
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Default Compressor Motor: HP v.s. Amps?

Chuck writes:

Any advice would be greatly appreciated.


See my page, "Evaluating True Horsepower and CFM Ratings of Air
Compressors" at:

http://www.truetex.com/aircompressors.htm
  #14   Report Post  
Richard J Kinch
 
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Default Compressor Motor: HP v.s. Amps?

Bob Swinney writes:

Torque is the capacity of an engine to do work ...


This, and many of your other comments here elided, are quite wrong.
  #15   Report Post  
Richard J Kinch
 
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Default Compressor Motor: HP v.s. Amps?

Bruce L. Bergman writes:

Just like diagonal size inflation on TV screens...


Worse. TV diagonal inflation is at least limited to sqrt(2), and bears
some meaningful proportion to the true size. "Peak HP" are many times true
HP and have little significance.

It is more like, "your TV screen would be X inches, if you put a giant
magnifying glass in front of it."


  #16   Report Post  
Richard J Kinch
 
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Default Compressor Motor: HP v.s. Amps?

Jon Elson writes:

It would be fine for an air brush or tire filling, but almost any
standard
air tool, like a die grinder, air paint sprayer, (very small) sand
blaster, air chisel,
etc. will use at least 6 CFM, and some will use a lot more. Unless
you want to wait
5 minutes out of every 6, you will soon be looking for a bigger
compressor.


Generally correct. But I found that an air chisel is so hard on the
skeleton, that you don't want to apply it at anything close to 100 percent
duty cycle. Now sand blasting, that is something else, you have an
infinite appetite for air with that.
  #17   Report Post  
Dave Baker
 
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Default Compressor Motor: HP v.s. Amps?

Subject: Compressor Motor: HP v.s. Amps?
From: "Bob Swinney"
Date: 03/10/03 20:34 GMT Daylight Time
Message-id: t


Guess you missed the math, Dave!


If you have a look at my website you will find three articles on power, torque
and their measurement which comprise one of the most comprehensive explanations
of the maths and physics that underly this topic anywhere on the web.


Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do though.
  #18   Report Post  
Bob Swinney
 
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Default Compressor Motor: HP v.s. Amps?

So, Richard - we would like to hear your definition of torque. And could
you elucidate a bit on the many "elided" comments? Please, give us your
"take" on the correct facts.

Bob Swinney

"Richard J Kinch" wrote in message
. ..
Bob Swinney writes:

Torque is the capacity of an engine to do work ...


This, and many of your other comments here elided, are quite wrong.



  #19   Report Post  
Dave Baker
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Subject: Compressor Motor: HP v.s. Amps?
From: "Bob Swinney"
Date: 04/10/03 02:58 GMT Daylight Time
Message-id: vPpfb.489338$Oz4.334706@rwcrnsc54


"Richard J Kinch" wrote in message
...
Bob Swinney writes:

Torque is the capacity of an engine to do work ...


This, and many of your other comments here elided, are quite wrong.



So, Richard - we would like to hear your definition of torque. And could
you elucidate a bit on the many "elided" comments? Please, give us your
"take" on the correct facts.


Torque bears no relation to any capacity to do work. Although the two
quantities are expressed in the same units of force and distance, torque is a
vector quantity and work is scalar one. Torque merely defines an instantaneous
twisting force about an axis. It could be expressed as a capacity to overcome a
given load applied to that axis but not to do a given amount of work. We need
to also know speed to calculate that and thus horsepower is what defines
capacity to do work.

Inherent in the use of the word "capacity" is a time element. Any engine could
theoretically do any amount of work if left running for long enough. The use of
the term capacity without recognising the inherent time element makes the term
meaningless.

For example, the question "can an engine producing X amount of torque lift
those bricks to the top of that building?" is meaningless. Appropriately geared
any engine could do that but only the horsepower defines how fast it could do
it.

All torque therefore tells one is the shaft speed at which an engine produces a
given amount of horsepower and the load the engine can overcome at that shaft
speed without further gearing.


Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do though.
  #21   Report Post  
Dave Baker
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Subject: Compressor Motor: HP v.s. Amps?
From: "Bob Swinney"
Date: 04/10/03 02:58 GMT Daylight Time
Message-id: vPpfb.489338$Oz4.334706@rwcrnsc54


"Richard J Kinch" wrote in message
1...
Bob Swinney writes:

Torque is the capacity of an engine to do work ...

This, and many of your other comments here elided, are quite wrong.



So, Richard - we would like to hear your definition of torque. And could
you elucidate a bit on the many "elided" comments? Please, give us your
"take" on the correct facts.


As to things elided..

Bob previously wrote

"Torque is the capacity of an engine to do work whereas HP is the rate at
which an engine does work."

The two statements "capacity to do work" and "rate at which work is done" are
actually the same. Capacity in this context is a rate by definition of the
inherent time element. Two different measures, torque and horsepower, have
therefore been used to define the same thing and clearly at least one of them
must therefore be wrong. The one that is wrong is torque as explained in
further detail previously.

By the same token...

""Wheel edge (pulling force) is everywhere the same in a transmission link."
While fundamentally true, this statement could be misleading. "

This is not even fundamentally true, never mind just misleading, but strictly
speaking it isn't even clearly enough defined to be meaningful in any way. If
it is meant to mean the same sized wheel incorporated at different points in a
transmission system then it is false. The edge force will be defined by the
torque at that point and the wheel radius and the torque will differ at
different points according to gearing. If it meant to mean any sized wheel at
different points in a transmission system then it both false and misleading. It
can only be correct if the wheel is sized to account for the torque at that
point in the system and it then becomes just another part of the gearing system
anyway.


Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do though.
  #22   Report Post  
Daniel
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Jon Elson wrote ...

... 3/4 Hp is just too small for serious air tool work...



No argument in the context being discussed. However....

I've got three air conditioner compressors (one known good, two
unknown). Couldn't tell you the sizes, but I'm thinking it doesn't
matter much because I also have a 500 gallon propane tank that I'm
thinking of marrying one of them to. Now I may have to let the sucker
rung a day & a half to get up to pressure, but I'm thinking it would
take a LOT of serious air tool use to see much of a pressure drop from
a tank that size. Noise isn't an issue, the nearest neighbor is 1/2
mile & I can put the beast beyond the shop which is 100 yards from the
house. Any thoughts other than to pressure test it from a good
distance?






It would be fine for an air brush or tire filling, but almost any
standard
air tool, like a die grinder, air paint sprayer, (very small) sand
blaster, air chisel,
etc. will use at least 6 CFM, and some will use a lot more. Unless you
want to wait
5 minutes out of every 6, you will soon be looking for a bigger compressor.
Forget HP, as some compressors sold for industrial use DON'T lie about it,
so it can't be used for comparisons. Just look at CFM at 90 PSI (don't
be foold by ratings
at 40 PSI, either.)

The other bad news is that the oilless compressors tend to be EXTREMELY
noisy,
and will drive you (or your neighbors) nuts! Oil filled compressors
generally
turn much slower, and some good ones can be rather quiet.

Jon

  #23   Report Post  
Bob Swinney
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Dave sez, quoting facts originally set forth by myself:

""Wheel edge (pulling force) is everywhere the same in a transmission

link."
While fundamentally true, this statement could be misleading. "

This is not even fundamentally true, never mind just misleading, but

strictly
speaking it isn't even clearly enough defined to be meaningful in any way.

If
it is meant to mean the same sized wheel incorporated at different points

in a
transmission system then it is false. The edge force will be defined by

the
torque at that point and the wheel radius and the torque will differ at
different points according to gearing. If it meant to mean any sized wheel

at
different points in a transmission system then it both false and

misleading. It
can only be correct if the wheel is sized to account for the torque at

that
point in the system and it then becomes just another part of the gearing

system
anyway.


Well, Dave, aside from all your pomposity and references to "your" website,
I pretty well covered it in my earlier post which was an honest answer to
Chucks question. Strangely, though, you gave no mathematical examples to
lend clarity to your statements, and to gain position otherwise lost by all
the posturing. I incorrectly challenged you on one point and I apologized
to you for that. Kindly disregard that apology now, for the spirit in
which it was given has been sullied by your boorish, pompous and pedantic
attitude. The basic definitions I gave for HP, torque and RPM are all valid.
Won't you please spare the other readers further "enlightenment" - the point
has been beaten to death by now. If you wish to engage me in further
discourse, kindly take notice of the mistletoe hanging from my shirttail.

Bob Swinney



  #24   Report Post  
Tim Williams
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

"Chuck" wrote in message
. ..
The compressor currently has a 3/4 hp motor. At first glance I was kind
of depressed thinking "Gee, just about every compressor I see these days
has 3 + hp.


Take a closer look at *every single display* that says this. They will
*ALL* say "peak HP", which is completely and absolutely meaningless in
this instance. Pure marketing drivel.
Amps times volts equals watts, and for a motor, it'll take maybe 800 to
1000W for 1HP. Thus at 120V, it'll take at least 8A per HP. This you
can likewise apply to other posters' numbers.

Tim

--
"That's for the courts to decide." - Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms


  #25   Report Post  
Dave Baker
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Subject: Compressor Motor: HP v.s. Amps?
From: "Bob Swinney"
Date: 04/10/03 04:47 GMT Daylight Time
Message-id: zprfb.489686$Oz4.337254@rwcrnsc54


Well, Dave, aside from all your pomposity and references to "your" website,


Are you trying to imply that it isn't my website?

I pretty well covered it in my earlier post which was an honest answer to
Chucks question. Strangely, though, you gave no mathematical examples to
lend clarity to your statements, and to gain position otherwise lost by all
the posturing.


I'm not sure why you see it as posturing. You asked for comment on what torque
really was and on the other statements that were not correct. If I took the
liberty of replying on behalf of Richard Kinch then he can challenge my
position or add to it as he sees fit. I could go into the maths ad infinitum
but I see no point in repeating what is already explained in detail on my site.

I incorrectly challenged you on one point and I apologized
to you for that. Kindly disregard that apology now, for the spirit in
which it was given has been sullied by your boorish, pompous and pedantic
attitude. The basic definitions I gave for HP, torque and RPM are all valid.


Although you have quoted the formulae quite correctly you apparently don't
fully understand the underlying implications, or at least not to the point
where you can explain them clearly to others. Torque and work are fundamentally
unrelated.


Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do though.


  #26   Report Post  
Old Nick
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

On 3 Oct 2003 19:57:58 -0700, (Daniel) wrote
something
.......and in reply I say!:

Jon Elson wrote ...

... 3/4 Hp is just too small for serious air tool work...



No argument in the context being discussed. However....

I've got three air conditioner compressors (one known good, two
unknown). Couldn't tell you the sizes, but I'm thinking it doesn't
matter much because I also have a 500 gallon propane tank that I'm
thinking of marrying one of them to. Now I may have to let the sucker
rung a day & a half to get up to pressure, but I'm thinking it would
take a LOT of serious air tool use to see much of a pressure drop from
a tank that size.


Not very much actually. There is no substitute for sheer input. The
only exception is where you have intermittent use and the compressor
_nearly_ meets the max requirement of the tool.

My figuring.

For a Random Orbital Sander

At 120 PSI in the tank (90 PSI at the regulator), you will have:

120 / 15 (pressure / atmos pressure) * 500 (gallons) / 6.4 (imperial
gallons per ft3) = 625 ft3

The tool will stop running properly below 90 PSI in the tank (and at
the regulator), at which stage you will have:

90 / 120 * 625 = 470 ft3

You get to use 156 ft3.

At, say 8 CFM (a kind estimate of the requirement for a ROS), you will
get twenty minutes continuous workable use out of the tank, absolute
max.

If it took a day and a half to get up from atmos pressure, then you
will take something over 156 / 625 * 1.5 days = 9 hours to get back up
to 120 PSI and start again.

A spraygun that could run down to say, 40PSI would be better, if it
used maybe 10CFM(???). You get to use 625 - ( 40 / 120 * 625) = 400
ft3, so you get 40 minutes continuous spraying, which would do a heck
of a lot.

You will then have the pump up time again, although you would only
have to go to 50PSI in the tank to get working again......but then you
would run out faster etc.

Give yourself 3/4 of these times if you mean 500 US Gallons.

Noise isn't an issue, the nearest neighbor is 1/2
mile & I can put the beast beyond the shop which is 100 yards from the
house. Any thoughts other than to pressure test it from a good
distance?


Pressure test with water, not air. Fill with water, until there is the
tiniest pocket left, then use air. Then the only air that can escape
explosively is the tiny bit you put in. You should also test to way
above what you intend to use it for.

************************************************** ****************************************
Those who can, do. Those who can't, teach.
The rest sit around and make snide comments.

Nick White --- HEAD:Hertz Music
Please remove ns from my header address to reply via email
!!
")
_/ )
( )
_//- \__/
  #27   Report Post  
Jon Elson
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?



Richard J Kinch wrote:
Jon Elson writes:


It would be fine for an air brush or tire filling, but almost any
standard
air tool, like a die grinder, air paint sprayer, (very small) sand
blaster, air chisel,
etc. will use at least 6 CFM, and some will use a lot more. Unless
you want to wait
5 minutes out of every 6, you will soon be looking for a bigger
compressor.



Generally correct. But I found that an air chisel is so hard on the
skeleton, that you don't want to apply it at anything close to 100 percent
duty cycle. Now sand blasting, that is something else, you have an
infinite appetite for air with that.


It depends on the job. I haven't done much metalwork with an air
chisel. But, I found a really odd way to use one. I have some of the
hardest concrete ever found in the world. It is great for a tough
foundation, until you want to drill a hole in it. I had to make a
~ 1.5" hole for a drain pipe from a "laundry tub" in my shop to the
utility room where there is a floor drain. A Starrett carbide hole
saw was instantly destroyed on this stuff, and hardly even made a mark
on the concrete. So, I drilled a bit into it with a carbide masonry
drill, and banged on things with a hammer a bit, and then the idea
hit me to try the air chisel! I had a short pointed tool that came
with it. It went through the concrete almost like butter. You just
keep the point moving, and it shatters a little cement at a time.
Eventually, the incredibly hard Jasper aggregate falls out, and there's
new cement to attack. When I ran out of length on the provided tool,
I made a 1 foot long version of it out of a McPherson strut rod that
somebody gave me. I had to turn the air tool end down to match
the tools that came with it. It was amazing! The added weight
of the tool seemed to make it even more powerful. I made the entire
hole through in one night, and then finished it to where a pipe would
fit through it the next night.

So, I ran the thing practically full blast for about 5 hours one night
and 3 or so the next. I didn't find it particularly horrible to the
bones and joints. Maybe it is worse in metal, with the rebound or
something. It WAS a DIRTY job, I came up from the basement looking
like a comics character, just 2 blinking eyes in a shapeless grey
blob!

Jon

  #28   Report Post  
Richard J Kinch
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Bob Swinney writes:

The basic definitions I gave for HP, torque and RPM are all valid.


No, you said, "Torque is the capacity of an engine to do work", which is
just wrong.
  #29   Report Post  
Richard J Kinch
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Dave Baker writes:

If I took the
liberty of replying on behalf of Richard Kinch then he can challenge my
position or add to it as he sees fit.


You answered it fine, thank you.
  #30   Report Post  
Richard J Kinch
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Daniel writes:

I also have a 500 gallon propane tank that I'm
thinking of marrying one of them to.... Any thoughts other than to
pressure test it from a good distance?


Hold your nose.


  #31   Report Post  
Richard J Kinch
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Jon Elson writes:

... the idea
hit me to try the air chisel! I had a short pointed tool that came
with it. It went through the concrete almost like butter. You just
keep the point moving, and it shatters a little cement at a time.


You have reinvented the star drill.

So, I ran the thing practically full blast for about 5 hours one night
and 3 or so the next. I didn't find it particularly horrible to the
bones and joints.


20 minutes with an air chisel, and my hands are suffering vibration
sickness. Half-paralyzed and tingly, like running a string trimmer for too
long. Anti-vibe gloves help, but it still is too fatiguing for long
sessions. This sort of thing can injure you seriously.
  #32   Report Post  
Roger Head
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Hi Tim,

Nothing personal, but as some aspects of this thread have become fairly
pedantic, maybe we should continue in the same vein...

Strictly speaking, volts times amps gives volt-amps (VA). If they are
exactly in phase, then it's the same as watts. When the phase starts to
shift, the line current increases *for the same true power delivered*
(to an electric motor, for example). That is why the electric companies
are always on about power-factor correction, because they need to keep
their cable currents as low as possible - remember, resistive power
losses in their cables is I-squared * R, which doesn't care about phase
angles, power-factor, etc. Also, IIRC, your power meter on the wall
measures true power, not VA, so you only get charged for the power you
consume, not the VA that your equipment draws.

So there's just a little more (useless?) trivia - like so many threads,
we have drifted a long way from the original question...

Roger



Tim Williams wrote:

"Chuck" wrote in message
. ..

The compressor currently has a 3/4 hp motor. At first glance I was kind
of depressed thinking "Gee, just about every compressor I see these days
has 3 + hp.


Take a closer look at *every single display* that says this. They will
*ALL* say "peak HP", which is completely and absolutely meaningless in
this instance. Pure marketing drivel.
Amps times volts equals watts, and for a motor, it'll take maybe 800 to
1000W for 1HP. Thus at 120V, it'll take at least 8A per HP. This you
can likewise apply to other posters' numbers.

Tim

--
"That's for the courts to decide." - Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms




  #33   Report Post  
Bob Swinney
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Richard J Kink sez:

"...No, you said, "Torque is the capacity of an engine to do work", which is
just wrong..."

Uh huh. And would you care to comment on exactly how that statement is
wrong? Torque is a turning moment that defines the force applied to a shaft
in order to do work. Without torque the shaft cannot turn, work cannot be
done.

Bob Swinney


"Richard J Kinch" wrote in message
. ..
Bob Swinney writes:

The basic definitions I gave for HP, torque and RPM are all valid.




  #34   Report Post  
ATP
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Bob Swinney wrote:
Richard J Kink sez:

"...No, you said, "Torque is the capacity of an engine to do work",
which is just wrong..."

Uh huh. And would you care to comment on exactly how that statement
is wrong? Torque is a turning moment that defines the force applied
to a shaft in order to do work. Without torque the shaft cannot
turn, work cannot be done.

Bob Swinney

These terms are all defined quite specifically. You can find them in any
physics textbook. Look them up and compare them to your statements. You're
just digging yourself in deeper.


  #35   Report Post  
Tim Williams
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

"Roger Head" wrote in message
...
Strictly speaking, volts times amps gives volt-amps (VA).


Ok, so step that up to 10 or 12A per HP due to reactive currents.

Tim

--
"That's for the courts to decide." - Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms




  #36   Report Post  
Dave Baker
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Subject: Compressor Motor: HP v.s. Amps?
From: "ATP"
Date: 04/10/03 16:10 GMT Daylight Time
Message-id: t

Bob Swinney wrote:
Richard J Kink sez:

"...No, you said, "Torque is the capacity of an engine to do work",
which is just wrong..."

Uh huh. And would you care to comment on exactly how that statement
is wrong? Torque is a turning moment that defines the force applied
to a shaft in order to do work. Without torque the shaft cannot
turn, work cannot be done.

Bob Swinney

These terms are all defined quite specifically. You can find them in any
physics textbook. Look them up and compare them to your statements. You're
just digging yourself in deeper.


Just the top of a flat cap and a few stray locks of hair visible over the lip
of the grave now. At regular intervals a shovel blade appears and another clod
of earth flies over the edge and lands on the growing pile. As we get closer
the sound of muttering and grumbling becomes audible from down below; it's a
very warm day. Crickets chirp onomatopoeically in the long grass of the meadow
behind the churchyard. An itinerant engine salesman wanders by and takes his
ease on a tree stump. A clod of earth flies out and lands on his foot and he
jumps up startled and lets out a small exclamation.

The sound of digging ceases abruptly. A flat cap pops up above ground to reveal
the red, sweaty, dust besmirched face supporting it. The face peers round and
apprises the situation.

"Sorry 'bout that guvnor. I didn't hear you come by."

The salesman merely waves a languid hand by way of pardon and settles himself
down again. He shakes the last grains of earth off his foot and takes in the
labourer with a frank stare.

"That's hard work for such a hot day my friend." he volunteers.
"Too true guvnor. It's feckin hot work and no mistake." says Bob, for it is he.
"Perhaps I can be of help then," says the salesman, "if an engine would ease
your toil? I trade in such for my living."
"Hmmmpf, engines." grumbles our worthy. "Tried one once and it b'aint no use."
"How so?" queries the salesman.
"Not enough torque. Weren't up to the job."

The salesman raises an eyebrow and enquires further. "Torque?"
"Aye guvnor, torque. For that's the capacity of an engine to do work and this
one we tried didn't have enough. Oh it got there in the end but in hot weather
like this it were a bit late if you take my drift."

The salesman was apparently no expert at taking drifts and raised his eyebrow a
fraction higher. Bob leaned forward confidentially.
"The parson was doin' his nut, guvnor, and rightly so. By the time we was
finished down 'ere they was starting to smell. It was alright in the winter
like when you's got a day or two in hand but in the summer...well you can
imagine what's it's like if we's late in this line of work." He grasps the end
of his nose with two fingers by way of further explanation.

By the expression of mild distaste on the salesman's face the drift has clearly
now been well and truly taken. "I think I can supply you something that will
meet your needs." he says. He reaches inside a battered valise and withdraws a
well thumbed catalogue. He leaves through it briefly.

"There are two engines I would recommend to you sir. One has a maximum torque
of 50 pounds feet and the other has 75. Which of these would have the higher
capacity to do work would you suggest?"

Bob ponders this question but as he starts to reply our camera zooms out. The
scene recedes into the distance until just the vague outline of a hump of earth
and two people are visible. We can hear nothing further.

So how did Bob reply?


Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do though.
  #37   Report Post  
jim rozen
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

In article rMAfb.683556$uu5.112144@sccrnsc04, Bob Swinney says...

Uh huh. And would you care to comment on exactly how that statement is
wrong? Torque is a turning moment that defines the force applied to a shaft
in order to do work. Without torque the shaft cannot turn, work cannot be
done.


Although to be a spoilsport, no work is done by a shaft
that is providing a torque, but is not turning. A wrench
on a stuck bolt, for example.

Torque is still RXF but because the angular velocity is
zero, work being done is zero.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

  #38   Report Post  
jim rozen
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

In article , Richard J Kinch
says...

No, you said, "Torque is the capacity of an engine to do work", which is
just wrong.


Possibly incomplete at worst.

If there is no torque on a shaft, it cannot perform any work.

That's close enough for me.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

  #39   Report Post  
Bob Swinney
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Aw Jeeze, Dave. I guess I was wrong about you. I had no further comment
until I read your little story, even if I was the butt of it. I do love a
cockney story and who would have thought you were such a writer? Surely you
have missed your true calling. Why, I'd even say your pomposity is exceeded
only by your verbosity.

Bob Swinney

"Dave Baker" wrote in message
...
Subject: Compressor Motor: HP v.s. Amps?
From: "ATP"
Date: 04/10/03 16:10 GMT Daylight Time
Message-id: t

Bob Swinney wrote:
Richard J Kink sez:

"...No, you said, "Torque is the capacity of an engine to do work",
which is just wrong..."

Uh huh. And would you care to comment on exactly how that statement
is wrong? Torque is a turning moment that defines the force applied
to a shaft in order to do work. Without torque the shaft cannot
turn, work cannot be done.

Bob Swinney

These terms are all defined quite specifically. You can find them in any
physics textbook. Look them up and compare them to your statements.

You're
just digging yourself in deeper.


Just the top of a flat cap and a few stray locks of hair visible over the

lip
of the grave now. At regular intervals a shovel blade appears and another

clod
of earth flies over the edge and lands on the growing pile. As we get

closer
the sound of muttering and grumbling becomes audible from down below; it's

a
very warm day. Crickets chirp onomatopoeically in the long grass of the

meadow
behind the churchyard. An itinerant engine salesman wanders by and takes

his
ease on a tree stump. A clod of earth flies out and lands on his foot and

he
jumps up startled and lets out a small exclamation.

The sound of digging ceases abruptly. A flat cap pops up above ground to

reveal
the red, sweaty, dust besmirched face supporting it. The face peers round

and
apprises the situation.

"Sorry 'bout that guvnor. I didn't hear you come by."

The salesman merely waves a languid hand by way of pardon and settles

himself
down again. He shakes the last grains of earth off his foot and takes in

the
labourer with a frank stare.

"That's hard work for such a hot day my friend." he volunteers.
"Too true guvnor. It's feckin hot work and no mistake." says Bob, for it

is he.
"Perhaps I can be of help then," says the salesman, "if an engine would

ease
your toil? I trade in such for my living."
"Hmmmpf, engines." grumbles our worthy. "Tried one once and it b'aint no

use."
"How so?" queries the salesman.
"Not enough torque. Weren't up to the job."

The salesman raises an eyebrow and enquires further. "Torque?"
"Aye guvnor, torque. For that's the capacity of an engine to do work and

this
one we tried didn't have enough. Oh it got there in the end but in hot

weather
like this it were a bit late if you take my drift."

The salesman was apparently no expert at taking drifts and raised his

eyebrow a
fraction higher. Bob leaned forward confidentially.
"The parson was doin' his nut, guvnor, and rightly so. By the time we was
finished down 'ere they was starting to smell. It was alright in the

winter
like when you's got a day or two in hand but in the summer...well you can
imagine what's it's like if we's late in this line of work." He grasps the

end
of his nose with two fingers by way of further explanation.

By the expression of mild distaste on the salesman's face the drift has

clearly
now been well and truly taken. "I think I can supply you something that

will
meet your needs." he says. He reaches inside a battered valise and

withdraws a
well thumbed catalogue. He leaves through it briefly.

"There are two engines I would recommend to you sir. One has a maximum

torque
of 50 pounds feet and the other has 75. Which of these would have the

higher
capacity to do work would you suggest?"

Bob ponders this question but as he starts to reply our camera zooms out.

The
scene recedes into the distance until just the vague outline of a hump of

earth
and two people are visible. We can hear nothing further.

So how did Bob reply?


Dave Baker - Puma Race Engines (
www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do

though.


  #40   Report Post  
Richard J Kinch
 
Posts: n/a
Default Compressor Motor: HP v.s. Amps?

Bob Swinney writes:

Torque is the capacity of an engine to do work


And would you care to comment on exactly how that statement is
wrong?


No. Consult any post-medieval physics textbook on the topic of "rotational
dynamics". Force, torque, work, power, etc., are all tidy concepts.
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