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Posted by Bob Engelhardt on February 8, 2008, 8:05 pm
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I have a 3450 rpm 3 phase motor on my drill press that I run up to 90 Hz
(5175 rpm). It hasn't self destructed yet and the opinion around RCM is
that it's probably OK (it's not a VFD rated motor). But any higher
would be pushing my luck.
In a thread here someone thought that a 1725 motor probably has the same
mechanical structure as a 3450 & hence could be run at 120 Hz. My
question is: if a 1725 motor is mechanically the same as a 3450 motor,
do you think that it could be run at 5175 rpm?
Part B: this 1725 motor is on my lathe. If I push it to 5175 rpm, the
max spindle speed will increase from 1000 rpm to 3000. Would that be a
Bad Idea?
Thanks,
Bob
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Posted by whit3rd on February 8, 2008, 9:38 pm
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> I have a 3450 rpm 3 phase motor on my drill press that I run up to 90 Hz
> (5175 rpm). =A0It hasn't self destructed yet
A lot depends on the lube, bearings and balance of the motor.
A Dremel tool, for instance, goes up to 35000 RPM, but some
care is taken to balance it dynamically (and the radius of
the rotor is kept small). If a thermometer says the motor is
operating under its limit, you should be safe until it starts to
sound bad (shaft flex or harmonic resonances will be in the
clearly audible range).
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Posted by Karl Townsend on February 9, 2008, 7:12 am
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> In a thread here someone thought that a 1725 motor probably has the same
> mechanical structure as a 3450 & hence could be run at 120 Hz. My
> question is: if a 1725 motor is mechanically the same as a 3450 motor, do
> you think that it could be run at 5175 rpm?
The only way to know is to test to destruction.
As a data point, I have several days of run time on a 7.5 hp 1725 at
4000rpm.
Karl
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Posted by Jerry on February 9, 2008, 1:52 pm
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>
>> In a thread here someone thought that a 1725 motor probably has the same
>> mechanical structure as a 3450 & hence could be run at 120 Hz. My
>> question is: if a 1725 motor is mechanically the same as a 3450 motor, do
>> you think that it could be run at 5175 rpm?
>
> The only way to know is to test to destruction.
>
> As a data point, I have several days of run time on a 7.5 hp 1725 at
> 4000rpm.
>
> Karl
Hi Karl
I like your suggestion that we dont have much real data on how fast we can
run motors before they fail and actually destroying one is probably the only
wat to learn its limit. I wanted to suggest that the relationship between
RPM and the force that throws the rotor windings out into the field is not
linear. The force is related to the square of the angular velocity. So,
some low RPM variations may not be a good guide for predicting for other
high RPM ranges.
Jerry
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Posted by Jon Elson on February 10, 2008, 6:53 pm
Please log in for more thread options Jerry wrote:
> I like your suggestion that we dont have much real data on how fast we can
> run motors before they fail and actually destroying one is probably the only
> way to learn its limit.
Hmmm, I don't recommend such an experiment without the
appropriate test site. While it is likely the first thing to
fail would be a copper bar in the rotor push outward enough
to make the rotor iron hit the stator and stop the motor,
there is the possibility the entire rotor would burst
completely! I wouldn't want to be annywhere near one of these
rotors coming apart at well above 5000 RPM.
When discussing these overspeeding topics some time ago, I
believe on this same list, somebody from the UK told about a
shop that had modified motors for operation on 800 Hz in a
woodworking shop. They had 2 diesel generators, one wound for
400 Hz, one for 800. They started the whole shop on 400 Hz,
then switched to 800 Hz, then shut off the 400 Hz generator.
I don't know what number of poles they used on the motors, I'm
pretty sure they couldn't be 2-pole, 4-pole, or even 6-pole,
that is just too fast for standard rotors. Guessing an 8-pole
winding at 800 Hz, that would be 12,000 RPM, the next
possibility is 12 pole, for 8000 RPM.
(I am not recommending doing anything like this, I tend to be
REALLY conservative when overspeeding anything.)
Jon
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> (5175 rpm). =A0It hasn't self destructed yet