You also have the choice of hand starting (rope?), or capacitor start
with associated switching for every tool motor you're wanting to run or
having one idler to start and then having the self start plus immediate
reversibility features available at each tool. Big three phase motors
being as cheap as they are, seems simple enough to use the idler.

bob g.

wrote:

On Fri, 27 Aug 2004 18:09:20 GMT, "Jerry Martes"
wrote:

snip

I have a little dyno with which I can measure the efects
of changing idler
size on a 3 phase motor's ability to deliver power.
The improvement in "tool motor power delivering ability"
is almost
undetectable for NO idler or for an idler 1 1/2 times tool
motor HP when
the tool motor is loaded up to 2/3 of its name plate Max HP.

It impressed me that the 3 HP 3 phase motor that I use
produces 3 HP at
1725 RPM to the load even when fed from single phase without
idler.

It is probably obvious to those who have thought about it,
that a heavily
loaded (near full name plate HP) 3 phase motor does run alot
smoother when
it is fed thru a big-big idler.

It might be concluded that 3 phase motors that arent
loaded heavily for
long periods can be run from single phase without an idler.
But, a 3 phase
motor will produce its full name plate HP even when fed
single phase with no
idler.

Jerry

snip


All the 3 pjase motors I've tested will produce more than their name
plate rated HP when fed good 3 phase. A 3 HP motor will deliver 5 HP to the
load when fed good 3 phase. It does get exactly as much hotter as you'd
guess so you'd want to put an external fan on it.
Heating of the motor is the principal factor that limits the amount of
power that an induction motor can safely deliver. That could be improved by
external fans and aditional heat sinks. Since the motor conducts heat away

from the windings thru its frame, the entire frame becomes a significant

factor in determining the max power rating. The 2/3 name plate HP 'rule of
thumb' for guessing at the max power available from a 3 phase motor, fed
single phase, must be altered to allow the fact that the heat sink and fan
is working as though it is a 3/3 HP motor.

It might be interesting to note that many 3 phase motors that are used in
Rec Craft applications are often commercial tool motors that will not be
loaded to their original ratings so the need for producing 'good 3 phase' is
often misunderstood.

A tool motor can easily spun up with an idler much smaller than the tool
motor's name plate HP, contrary to the implication that Big idlers are
needed. Most tools that use 3 phase motors can be unloaded while being
started, so even a switch and capacitor can spin up most 3 phase tools.

As I noted above, the 3 phase motor will provide the load with very nearly
the same HP when fed either single phase or 3 phase, when the motor is
loaded to only 1/2 its name plate HP. At those "lightly loaded" conditions,
I've been unable to descern the difference between the performance of the 3
phase motor fed either 3 phase or single phase.
The instanteous loss of torque every 1/120th of a second exist in either a
single phase motor or a 3 phase motor fed single phase. That "pulsing"
doesnt become measurable when the motor is loaded to less than about 3/4 its
rated HP. For loads under about 3/4 its name plate rated HP (2 1/4 HP for
a 3 HP motor) a 3 phase motor fed single benefits little from the use of in
idler. The pulsing is probably the first noticable result of having no
idler (or small idler) when using single phase to drive a heavily loaded 3
phase motor.

I plotted (with Excel) the HP vs RPM for the 3 HP 3 phase motor, fed
single phase, with a 5 HP idler and with no idler. I'll send you a copy if
you'd want to see it..

Jerry



A pair of good posts. It really is refreshing to see
some solid input on 3 phase phase conversions which is both
soundly based and backed up by direct measurement. I hope it
will dispel some of the myths on minimum sizes of idlers and
the practical usefulness of precision "tuning".

Jim