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.