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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Old October 6th 03, 05:23 PM
Bob Swinney
 
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Default OT?: Universal vs. Induction motor HP ratings


"Loren Coe" wrote in message
news:[email protected]
In article , Rick

Chamberlain wrote:
I've been looking at 14" abrasive chop saws and really don't like whats
out there. So, I turned my attention to 14" dry saws. Great idea, but
a little out of my price range.

I got to thinking, why couldn't I fab something? I have all the steel
tubing, nice 1" pillow blocks, and a 1725rpm motor. My only concern is
how big should the motor be?

Is there any difference in true HP for an induction motor vs. a
universal motor? If the universal motor on the PC Dry saw is 15A (about
1.5 hp), would I still need a 1.5hp induction motor for the same
performance? TIA for any advice.


not a technical response, they surely will follow, but the comparison is
sorta like two stroke vs four stroke engines. cc for cc, amp for amp
the 2-stroke, universal will be more powerful. there is a difference
in this anology, the univeral motor will have much more torque at
low rpms. --Loren


Actually a very good response, Loren. You are "right on" re. torque at low
RPM - that is probably the main reason that universal motors are used as
much as they are. Basically they are DC-type motors running on AC, and as
such they exhibit the "high torque at stall" characteristic of a DC motor.
Although I haven't tried it myself, I would be quite leery of powering a dry
saw with an induction motor. Certainly, a motor of sufficient HP to drive
the disc would be required (at least the same or larger than the universal
motor) Learning to saw all loads with a blade running at a single RPM might
be hard to do. I think it would work but it would work "best" when sawing
the same (optimum) size material every time. The old "chatter remedy" of
decreasing speed or increasing feed would not be at your disposal.

Bob (opinions are my own - feel free to disagree) Swinney




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Old October 6th 03, 07:45 PM
Loren Coe
 
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Default OT?: Universal vs. Induction motor HP ratings

In article , Rick Chamberlain wrote:
be hard to do. I think it would work but it would work "best" when sawing
the same (optimum) size material every time. The old "chatter remedy" of
decreasing speed or increasing feed would not be at your disposal.

Bob (opinions are my own - feel free to disagree) Swinney


Thanks Bob. Makes sense. I also have access to DC treadmill motors
with speed controls. Sound like a better alternative?


power-wise, yes, but many of these are sold in open frames, so be
prepared for that. in a portable tool, you may have to dink w/the
housing anyway, so that may not be an issue (for you). --Loren

  #4   Report Post  
Old October 7th 03, 04:48 AM
clare @ snyder.on .ca
 
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Default OT?: Universal vs. Induction motor HP ratings

On Mon, 6 Oct 2003 12:26:35 -0500, Rick Chamberlain
wrote:

In article ,
says...

"Loren Coe" wrote in message
news:[email protected]
In article , Rick

Chamberlain wrote:
I've been looking at 14" abrasive chop saws and really don't like whats
out there. So, I turned my attention to 14" dry saws. Great idea, but
a little out of my price range.

I got to thinking, why couldn't I fab something? I have all the steel
tubing, nice 1" pillow blocks, and a 1725rpm motor. My only concern is
how big should the motor be?

Is there any difference in true HP for an induction motor vs. a
universal motor? If the universal motor on the PC Dry saw is 15A (about
1.5 hp), would I still need a 1.5hp induction motor for the same
performance? TIA for any advice.

not a technical response, they surely will follow, but the comparison is
sorta like two stroke vs four stroke engines. cc for cc, amp for amp
the 2-stroke, universal will be more powerful. there is a difference
in this anology, the univeral motor will have much more torque at
low rpms. --Loren


Actually a very good response, Loren. You are "right on" re. torque at low
RPM - that is probably the main reason that universal motors are used as
much as they are. Basically they are DC-type motors running on AC, and as
such they exhibit the "high torque at stall" characteristic of a DC motor.
Although I haven't tried it myself, I would be quite leery of powering a dry
saw with an induction motor. Certainly, a motor of sufficient HP to drive
the disc would be required (at least the same or larger than the universal
motor) Learning to saw all loads with a blade running at a single RPM might
be hard to do. I think it would work but it would work "best" when sawing
the same (optimum) size material every time. The old "chatter remedy" of
decreasing speed or increasing feed would not be at your disposal.

Bob (opinions are my own - feel free to disagree) Swinney


Thanks Bob. Makes sense. I also have access to DC treadmill motors
with speed controls. Sound like a better alternative?

No.
Unless they are wound feild DC motors.
Then built with laminated silicon steel armature and feild poles they
are a universal motor - and you do NOT need speed control. Just more
to burn out when you croud it a bit.
Ideal motor is an undercompounded? motor - a series motor with a shunt
winding to limit top speed (no load) and a series winding to provide
more torque under load (lower speed and heavier load means more
current, and more torque)

  #5   Report Post  
Old October 8th 03, 02:54 AM
Robin S.
 
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Default OT?: Universal vs. Induction motor HP ratings


"Rick Chamberlain" wrote in message
...

I'm looking for an alternative to spending $450-500 for a Porter Cable
1410P or Dewalt 872 dry cut chop saw. Both run at 1300 rpm, and use 15A
AC/DC. Both have a 1" arbor.


Carefull with those saws. I sell them at work and have heard reports of
blades costing up to about $1/cut.

We've started to sell the 7-1/4" steel-cutting carbide tipped saw blades.
Apparently they cut very well (I've seen samples from customers) and are
cheaper (on a blade/cut/$$$ basis) than abrasive blades.

Once you add the steel cutting capability to a 7-1/4" saw, you can saw
virtually any material you can get your hands on (perhaps no aerospace
super-alloys) by using different blades. May want to go with a worm-drive
for sawing solid stock.


The machine is nothing more than a nice chop saw for metal, and it will
take up much less room in my shop than a traditional 4x6 or 5x7 bandsaw.


How big of a work envelope do you need? Are you willing to use a hacksaw in
some instances?


Neither machine has variable speed, although it sure would be nice to
have.


Why? At the price you're going to pay for a blade, you probably want to run
the saw at its *correct* speed. Too slow and you could have issues with
carbide... I don't think we sell a more expensive 14" saw blade....

Just some thoughts...

Regards,

Robin




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Old October 8th 03, 03:00 AM
clare @ snyder.on .ca
 
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Default OT?: Universal vs. Induction motor HP ratings

On Tue, 07 Oct 2003 12:12:57 GMT, Rick Chamberlain
wrote:



My preliminary design is really quite simple - a pivoting box frame with
the blade arbor mounted using high grade pillow block bearings. I would
drive the arbor with a belt/idler combination to take up slack when
bringing the blade down. Or I could change the pivot mechanism so that
the idler is mounted on the pivot point and eliminate the slack problem.

In this application, what motor would you suggest? And, where could I
find one?

Of course, if there is an inexpensive source for 15A universal motors
with a 1" arbor running at 1300 rpm, I'd be all for it. :-)

Thanks again for all of the responses.

How about a 5HP Briggs and stratton running at half speed?

Really, you need TORQUE, and in electric motors that means a wound
rotor,.
To get the power you want you need a geared motor, or a honking HUGE
rotor to develop the torque.
My advise is to keep your eye open for a good used cutoff saw, or a
good deal on a quality import.
  #7   Report Post  
Old October 8th 03, 03:42 AM
Don Foreman
 
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Default OT?: Universal vs. Induction motor HP ratings

The difference is that if an induction motor is loaded to much below
nominal speed at rated power, the torque it can produce starts to
decrease so the motor stalls. A series-wound universal motor will
slow down more under load, but torque keeps increasing clear down to
stop. For that reason, HP ratings on universal motors (and DC PM
motors as well) are pretty meaningless unless they're specified at a
stated speed. "Sears" horsepower is stall torque * no-load speed.
The motor can't actually deliver anything like that much power under
any load condx, but it can deliver a lot of torque under heavy load,
though it will overheat if heavy overload is sustained for any
length of time.


Induction motors vary a lot in how much max torque they can deliver
(at considerably higher than rated current) before they stall. Some
can deliver as much as 5X rated torque -- while drawing nearly 5X
rated current as well. Not for long, though! I've measured the
current on a 2 HP 3450 RPM 117 volt capacitor-run (no start winding or
switch) induction motor at over 90 amps for a fraction of a second
during startup. Rated current on that motor was 19 amps.

In general, a universal motor of given current rating will deliver a
lot more torque (at reduced speed) than an induction motor of same
current rating because all of the load current goes thru the field in
a series-wound universal motor while the magnetizing current (and
hence field flux density) in an induction motor is essentially
constant. More field current --- higher field flux density ---
more torque but lower speed.

If a dry saw operates more like a cutting tool than an abrasive
chopsaw, then an induction motor may be OK if you can just feed stock
at a rate the saw can handle. An abrasive chop saw, like a drill,
sometimes needs to be leaned on to keep cutting rather than just
heating the work, and that takes torque.

On Mon, 06 Oct 2003 14:58:17 GMT, Rick Chamberlain
wrote:

I've been looking at 14" abrasive chop saws and really don't like whats
out there. So, I turned my attention to 14" dry saws. Great idea, but
a little out of my price range.

I got to thinking, why couldn't I fab something? I have all the steel
tubing, nice 1" pillow blocks, and a 1725rpm motor. My only concern is
how big should the motor be?

Is there any difference in true HP for an induction motor vs. a
universal motor? If the universal motor on the PC Dry saw is 15A (about
1.5 hp), would I still need a 1.5hp induction motor for the same
performance?

TIA for any advice.


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Old October 8th 03, 04:19 AM
Don Foreman
 
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Default OT?: Universal vs. Induction motor HP ratings

On Tue, 07 Oct 2003 12:12:57 GMT, Rick Chamberlain

My preliminary design is really quite simple - a pivoting box frame with
the blade arbor mounted using high grade pillow block bearings. I would
drive the arbor with a belt/idler combination to take up slack when
bringing the blade down. Or I could change the pivot mechanism so that
the idler is mounted on the pivot point and eliminate the slack problem.

In this application, what motor would you suggest? And, where could I
find one?

Of course, if there is an inexpensive source for 15A universal motors
with a 1" arbor running at 1300 rpm, I'd be all for it. :-)


Some of the treadmill motors are universal motors. The one I have
is. Just look for brushes: if the motor has brushes it's almost
certainly a universal motor.

Noload speed of nearly any universal motor will be considerably higher
than 1300 RPM but you can manage that with belt and pulleys. In
general, the larger the diameter of the motor the lower the running
speed. Small diameter motors rated at 10 to 15 amps, as used in
portable power tools, run at very high speeds with two-stage planetary
gear reduction. I seem to recall that the treadmill motors run at
about 5000 RPM no-load. You could "dial down" the speed with a
speed controller, but you'll want the torque multiplication that you'd
get with a gearbox or belt 'n pulley speed reducer.

A DC PM treadmill motor might also work OK here. They don' t have the
stall torque that a series-wound universal motor does, but are
considerably better than an induction motor. Again, you'll need
speed reduction. See, e.g.
http://www.sciplus.com/category.cfm?category=174

4800 RPM, 11.2 amps, 1.5 to 2.0 HP, $49.50 for a new one. A
suitable bridge rectifier will set you back another $2.


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Old October 8th 03, 06:39 PM
clare @ snyder.on .ca
 
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Default OT?: Universal vs. Induction motor HP ratings

On Tue, 07 Oct 2003 22:19:38 -0500, Don Foreman
wrote:

On Tue, 07 Oct 2003 12:12:57 GMT, Rick Chamberlain

My preliminary design is really quite simple - a pivoting box frame with
the blade arbor mounted using high grade pillow block bearings. I would
drive the arbor with a belt/idler combination to take up slack when
bringing the blade down. Or I could change the pivot mechanism so that
the idler is mounted on the pivot point and eliminate the slack problem.

In this application, what motor would you suggest? And, where could I
find one?

Of course, if there is an inexpensive source for 15A universal motors
with a 1" arbor running at 1300 rpm, I'd be all for it. :-)


Some of the treadmill motors are universal motors. The one I have
is. Just look for brushes: if the motor has brushes it's almost
certainly a universal motor.


Actually, that is not accurate. If it has brushes it will run on DC.
A universal motor is specially designed to reduce eddie current loss
in the iron. IF the motor has a laminated feild "core" and a laminated
armature it MAY be a universal motor.
Solid pole peices or armature most definitely is not, and permanent
magnet motors have brushes, but absolutely can NOT be run on AC.

Noload speed of nearly any universal motor will be considerably higher
than 1300 RPM but you can manage that with belt and pulleys. In
general, the larger the diameter of the motor the lower the running
speed. Small diameter motors rated at 10 to 15 amps, as used in
portable power tools, run at very high speeds with two-stage planetary
gear reduction. I seem to recall that the treadmill motors run at
about 5000 RPM no-load. You could "dial down" the speed with a
speed controller, but you'll want the torque multiplication that you'd
get with a gearbox or belt 'n pulley speed reducer.

A DC PM treadmill motor might also work OK here. They don' t have the
stall torque that a series-wound universal motor does, but are
considerably better than an induction motor. Again, you'll need
speed reduction. See, e.g.
http://www.sciplus.com/category.cfm?category=174

4800 RPM, 11.2 amps, 1.5 to 2.0 HP, $49.50 for a new one. A
suitable bridge rectifier will set you back another $2.


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Old October 8th 03, 07:56 PM
Don Foreman
 
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Default OT?: Universal vs. Induction motor HP ratings

On Wed, 08 Oct 2003 17:39:24 GMT, clare @ snyder.on .ca wrote:



Some of the treadmill motors are universal motors. The one I have
is. Just look for brushes: if the motor has brushes it's almost
certainly a universal motor.


Actually, that is not accurate. If it has brushes it will run on DC.
A universal motor is specially designed to reduce eddie current loss
in the iron. IF the motor has a laminated feild "core" and a laminated
armature it MAY be a universal motor.
Solid pole peices or armature most definitely is not, and permanent
magnet motors have brushes, but absolutely can NOT be run on AC.

Absolutely right. Essentially all universal motors have brushes, but
not all brush motors are universal. Thanks for catching that, Clare.

A better discriminator might be whether or not the motor is spec'd
to operate on either AC or DC. I think that might even be where the
term "universal" originated.


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