"jim rozen" wrote in message
...
In article P6yXc.1002$yP4.369@trnddc08, Jerry Martes says...
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.
This is an unexpected, yet fascinating result. It may be that
the hp ratings on some motors is fairly conservatve ("anti-sears
power rating...") which could account for some of this.
I wonder if the same 3 hp motor would actually do 4, if it were
run on utility 3~?
Jim
Jim
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
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