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Default Maximum speed for 3 phase motor?


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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Default Maximum speed for 3 phase motor?


"Karl Townsend" wrote in message
anews.com...

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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Default Maximum speed for 3 phase motor?

On Sat, 9 Feb 2008 07:12:57 -0500, "Karl Townsend"
wrote:


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


I've run a lot of motors on VFD's at work. Inverter rated motors
quite often have the maximum rpm on the name plate. A lot of them
have been in the 4300 -- 4800 range. I doubt you're going to find the
data on a non-vfd rated motor, so personally I'd be hesitant to run
over 4800.

Below is a link to the manual for Marathon Black Max motors, which is
the type I use most frequently. Look at Table 3.4 for the recommended
maximum for those motors. Max speed is related to frame size, which
is no surprise due to the increase in rotor size. I doubt the numbers
are valid for your motor, but the trend should hold.

http://www.marathonelectric.com/moto...uals/5554E.pdf

Pete Keillor
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Default Maximum speed for 3 phase motor?

Pete Keillor wrote:
....
Below is a link to the manual for Marathon Black Max motors, ...
Table 3.4 for the recommended
maximum for those motors. ...


Thanks - good link! A couple of interesting things the
- for *belted" loads, their limit is 125% of rated speed. I'd guess
that's because of the radial bearing load. I'm going to go ahead and
ignore that.
otherwise (direct load), for my frame:
- a 3600 rpm motor has a 7200 (120 Hz) limit
- a 1800 rpm motor has a limit of 5400 (180 Hz)

I think that I'll compromise between the 125% limit and the 200/300% and
limit the 3600 to 150% (90 Hz, 5400 rpm) & the 1800 to 200% (120 Hz,
3600 rpm).

Thanks,
Bob
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Default Maximum speed for 3 phase motor?

On Sat, 9 Feb 2008 19:46:04 -0600, "Robert Swinney"
wrote:

This whole discussion reminds me of the old saw about giving a monkey a loaded gun and wondering how
long it will be before he shoots you in the ass with it.

Bob Swinney


Bob, you can do what you want of course, but I'll reiterate:

I doubt you're going to find the data on a non-vfd rated motor,
so personally I'd be hesitant to run over 4800.


I doubt the numbers are valid for your motor, but the trend
should hold.


I know inverter duty motors are insulated better due to the issues of
cooling at low speeds and high voltage transients from the VFD. It is
also entirely possible that the rotors in inverter duty motors are
built to withstand overspeed conditions, given the likelihood they'll
get used that way. Also, your application involves the side load of a
belt, although that would only relate to bearing failure.

The overspeed failure normally includes unwrapping of the rotor with
consequent sudden stoppage, and in big motors, likelihood that the
motor is going to wind up in a different location. I heard of one
instance in one of our plants in Texas of a 100 Hp. motor making it
clear across the ground level of the plant when the pump it was
driving seized. Yikes!

I personally would be cautious. Like one of the posters said, a
failure can be catastrophic.


"Bob Engelhardt" wrote in message
...
Pete Keillor wrote:
...
Below is a link to the manual for Marathon Black Max motors, ...
Table 3.4 for the recommended
maximum for those motors. ...


Thanks - good link! A couple of interesting things the
- for *belted" loads, their limit is 125% of rated speed. I'd guess
that's because of the radial bearing load. I'm going to go ahead and
ignore that.
otherwise (direct load), for my frame:
- a 3600 rpm motor has a 7200 (120 Hz) limit
- a 1800 rpm motor has a limit of 5400 (180 Hz)

I think that I'll compromise between the 125% limit and the 200/300% and
limit the 3600 to 150% (90 Hz, 5400 rpm) & the 1800 to 200% (120 Hz,
3600 rpm).

Thanks,
Bob



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Default Maximum speed for 3 phase motor?

On Sun, 10 Feb 2008 09:44:27 -0600, "Robert Swinney"
wrote:

Thanx Pete ! You did well to repeat the warning for those that might benefit from it. The "monkey
& loaded gun" analogy was aimed at anyone stupid enough have no regard for the simple physics
involved. I appreciate your reiteration although doubt those that can really use use it even
bothered to read it.

Are you going to N.A.M.E.S. this year?

Bob Swinney


I haven't decided yet, too much stuff going on. If I do, I'd probably
only spend one night or drive down and back same day (ugh).

Pete
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Default Maximum speed for 3 phase motor?

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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Default Maximum speed for 3 phase motor?

Pete Keillor wrote:

The overspeed failure normally includes unwrapping of the rotor with
consequent sudden stoppage, and in big motors, likelihood that the
motor is going to wind up in a different location. I heard of one
instance in one of our plants in Texas of a 100 Hp. motor making it
clear across the ground level of the plant when the pump it was
driving seized. Yikes!

I find this hard to believe. I would expect the shaft coupling
to shear easily and let the motor free. I would expect this
more catastrophic scenario would require a locking-up of the
rotor to the stator. Even if the bearings seized, there would
be huge shrieking and either the bearing burns up white hot, or
the motor protector cuts off. But, if a rotor bar came loose,
the rotor can seize to the stator very catastrophically. This
is pretty RARE.
I personally would be cautious. Like one of the posters said, a
failure can be catastrophic.

Yes, the faster you run it, the slimmer the safety margin gets.

Jon
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