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Compressor Motor: HP v.s. Amps?



 
 
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  #1  
Old October 3rd 03, 01:28 PM
Chuck
external usenet poster
 
Posts: n/a
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
Ads
  #2  
Old October 3rd 03, 02:16 PM
Gunner
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Posts: n/a
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  
Old October 3rd 03, 03:10 PM
Bob Swinney
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Posts: n/a
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  
Old October 3rd 03, 06:34 PM
Bruce L. Bergman
external usenet poster
 
Posts: n/a
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  
Old October 3rd 03, 07:20 PM
Dave Baker
external usenet poster
 
Posts: n/a
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  
Old October 3rd 03, 08:34 PM
Bob Swinney
external usenet poster
 
Posts: n/a
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  
Old October 3rd 03, 08:41 PM
Jon Elson
external usenet poster
 
Posts: n/a
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  
Old October 3rd 03, 10:51 PM
Bob Swinney
external usenet poster
 
Posts: n/a
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  
Old October 3rd 03, 10:58 PM
Bob Swinney
external usenet poster
 
Posts: n/a
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.


 




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