Kind of a follow up to my pump RPM question.

I found a 1200 RPM 5 hp motor and an old Hitachi SJ200 5hp. vfd in the spare
stockroom (er junkpile)

Any reason I can't spin this combo at 3600 RPM? Still have 5 horse?

Karl

Karl Townsend wrote:
Kind of a follow up to my pump RPM question.

I found a 1200 RPM 5 hp motor and an old Hitachi SJ200 5hp. vfd in the spare
stockroom (er junkpile)

Any reason I can't spin this combo at 3600 RPM? Still have 5 horse?

You need to call the motor manufacturer and
ask them. I'm pretty sure the answer will
be no. I think 20% over nameplate would be
fine, but 100% is probably pushing it too far.

On Sat, 4 Apr 2009 08:33:52 -0500, "Karl Townsend"
wrote:

Kind of a follow up to my pump RPM question.

I found a 1200 RPM 5 hp motor and an old Hitachi SJ200 5hp. vfd in the spare
stockroom (er junkpile)

Any reason I can't spin this combo at 3600 RPM? Still have 5 horse?

Karl


More likely have more than 5hp at over double the speed. Not 10, but
mabee 7?

It will definitely be less than 5 HP due to losses at higher
frequencies. Call the motor mfr to find out max safe speed. No need to
speculate or outguess them. Yu will know the answer on Monday.

i

On 2009-04-04, Karl Townsend wrote:
Kind of a follow up to my pump RPM question.

I found a 1200 RPM 5 hp motor and an old Hitachi SJ200 5hp. vfd in the spare
stockroom (er junkpile)

Any reason I can't spin this combo at 3600 RPM? Still have 5 horse?

Karl

The usual rule of thumb for VFD's is that you have rated torque up to
the nameplate RPM (ie less than your 5 hp), and rated HP above nameplate
RPM (this is a function of the amount of power the motor can handle
without overheating. )

Your application would require running the motor at 180 hz and 3 times
the rated rpm. Pumps require pretty much full output for extended
periods of time. I doubt if the motor would take it for any reasonable
period. You could might get a test run in without too much grief.

Karl Townsend wrote:
Kind of a follow up to my pump RPM question.

I found a 1200 RPM 5 hp motor and an old Hitachi SJ200 5hp. vfd in the spare
stockroom (er junkpile)

Any reason I can't spin this combo at 3600 RPM? Still have 5 horse?

Karl

RoyJ wrote:
The usual rule of thumb for VFD's is that you have rated torque up to
the nameplate RPM (ie less than your 5 hp), and rated HP above nameplate
RPM (this is a function of the amount of power the motor can handle
without overheating. )

That's what I've heard (on RCM), but I don't recall seeing it in my VFD
documentation. And I've never understood it. Keeping the HP constant
above the rated RPM means that the torque/current is being
proportionally reduced. Why would you want to reduce the current above
the rated speed? It can't be to avoid overheating - that's just a
matter of current, not power. In fact, higher speeds would generally
mean more air flow & more cooling, allowing *more* current. Maybe it's
a matter of the VFD's limitations.

Your application would require running the motor at 180 hz and 3 times
the rated rpm. Pumps require pretty much full output for extended
periods of time. I doubt if the motor would take it for any reasonable
period. You could might get a test run in without too much grief.

Others have said "Check with the manufacturer" - I'll tell what they'll
say: "Don't do it, it's not rated for it". But you won't know if
they're just covering themselves or if they mean it. If you ask "Why",
you won't know whether you've gotten a straight answer to that, either.

I can see 2 effects of higher that rated RPM: centrifugal forces on the
rotor windings (do 3 ph motors have rotor windings?) and bearing
overheating. Now, many motors have a rated speed of 3600 rpm & I doubt
that there's much difference between their designs (rotor & bearings)
and 1200 rpm motors. I doubt that there would be enough cost savings to
justify different "technologies".

So, you gots to ax yourself "Do I feel lucky?". IOW, are you willing to
risk that motor to find out? I hope you do, 'cause I'd like to know.
So be sure to follow up if you do.

Awaiting your report,
Bob

Others have said "Check with the manufacturer" - I'll tell what they'll
say: "Don't do it, it's not rated for it". But you won't know if they're
just covering themselves or if they mean it. If you ask "Why", you won't
know whether you've gotten a straight answer to that, either.

Ya, I'm not going to bother.




I can see 2 effects of higher that rated RPM: centrifugal forces on the
rotor windings (do 3 ph motors have rotor windings?) and bearing
overheating. Now, many motors have a rated speed of 3600 rpm & I doubt
that there's much difference between their designs (rotor & bearings) and
1200 rpm motors. I doubt that there would be enough cost savings to
justify different "technologies".

So, you gots to ax yourself "Do I feel lucky?". IOW, are you willing to
risk that motor to find out? I hope you do, 'cause I'd like to know. So
be sure to follow up if you do.


I've run a 1800 RPM 7 1/2 HP at 3600 for 100s of hours. (CHNC spindle). As
no one seems to know, I'm just going to guess that it is better to find an
1800 RPM motor - pretty sure that will work. These are fairly easy to find.

Karl

On 2009-04-04, Karl Townsend wrote:


Others have said "Check with the manufacturer" - I'll tell what they'll
say: "Don't do it, it's not rated for it". But you won't know if they're
just covering themselves or if they mean it. If you ask "Why", you won't
know whether you've gotten a straight answer to that, either.

Ya, I'm not going to bother.

I thin that you are making wrong assumptions. Whenever I would ask VFD
companies or motor manufacturers I would get a straight answer such as
"do not do it because..." or "you can do it".

i




I can see 2 effects of higher that rated RPM: centrifugal forces on the
rotor windings (do 3 ph motors have rotor windings?) and bearing
overheating. Now, many motors have a rated speed of 3600 rpm & I doubt
that there's much difference between their designs (rotor & bearings) and
1200 rpm motors. I doubt that there would be enough cost savings to
justify different "technologies".

So, you gots to ax yourself "Do I feel lucky?". IOW, are you willing to
risk that motor to find out? I hope you do, 'cause I'd like to know. So
be sure to follow up if you do.


I've run a 1800 RPM 7 1/2 HP at 3600 for 100s of hours. (CHNC spindle). As
no one seems to know, I'm just going to guess that it is better to find an
1800 RPM motor - pretty sure that will work. These are fairly easy to find.

Karl

On 2009-04-04, Karl Townsend wrote:
Kind of a follow up to my pump RPM question.

I found a 1200 RPM 5 hp motor and an old Hitachi SJ200 5hp. vfd in the spare
stockroom (er junkpile)

Any reason I can't spin this combo at 3600 RPM? Still have 5 horse?

Well ... a lot of VFDs will only go to double the line frequency
(120 Hz from 60 Hz) so you could go no faster than 2400 RPM on the
motor. Some will go to 400 Hz -- if you tweak some limiting variables
in the setup parameters. You would need to go to 180 hz to get 3600
RPM.

Your torque will fall above the design frequency of the motor,
as the VFD can't increase the output voltage above the input voltage
level, and the required voltage is a function of RPM. As a result, you
probably won't get 5 HP out of the motor at that speed.

Since it is likely that the same motor series is made with the
same rotor for 3600 RPM and slower -- with only the field structure
(poles) changing, I consider it likely (though not *certain*) that the
rotor can survive 3600 RPM -- but no faster. Certainly don't stand in
line with the possible parts of an exploding rotor for at least the
first half hour of operation.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

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


Others have said "Check with the manufacturer" - I'll tell what they'll
say: "Don't do it, it's not rated for it". But you won't know if they're
just covering themselves or if they mean it. If you ask "Why", you won't
know whether you've gotten a straight answer to that, either.

Ya, I'm not going to bother.




I can see 2 effects of higher that rated RPM: centrifugal forces on the
rotor windings (do 3 ph motors have rotor windings?) and bearing
overheating. Now, many motors have a rated speed of 3600 rpm & I doubt
that there's much difference between their designs (rotor & bearings) and
1200 rpm motors. I doubt that there would be enough cost savings to
justify different "technologies".

So, you gots to ax yourself "Do I feel lucky?". IOW, are you willing to
risk that motor to find out? I hope you do, 'cause I'd like to know. So
be sure to follow up if you do.


I've run a 1800 RPM 7 1/2 HP at 3600 for 100s of hours. (CHNC spindle). As
no one seems to know, I'm just going to guess that it is better to find an
1800 RPM motor - pretty sure that will work. These are fairly easy to
find.

Karl




As the frequency applied to the motor increases, the internal reactance of
the motor increases so then you need a higher voltage to push the same max
current through the windings as limited by the wire size. Now if you
maintain the current by increasing the voltage you get a lot more power out
of the motor until you reach a frequency where the internal magnetic loses
of the motor generate too much heat. The other limiting factors are the size
of the bearings and the output shaft. They are limited as the HP they can
handle. Theroretically, you can double the frequency double the voltage and
get twice the HP out of the motor.



John

On 2009-04-05, John wrote:

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

[ ... ]

I've run a 1800 RPM 7 1/2 HP at 3600 for 100s of hours. (CHNC spindle). As
no one seems to know, I'm just going to guess that it is better to find an
1800 RPM motor - pretty sure that will work. These are fairly easy to
find.

[ ... ]

As the frequency applied to the motor increases, the internal reactance of
the motor increases so then you need a higher voltage to push the same max
current through the windings as limited by the wire size. Now if you
maintain the current by increasing the voltage you get a lot more power out
of the motor until you reach a frequency where the internal magnetic loses
of the motor generate too much heat. The other limiting factors are the size
of the bearings and the output shaft. They are limited as the HP they can
handle. Theroretically, you can double the frequency double the voltage and
get twice the HP out of the motor.

Except that you would need a 480V input power and a 480V VFD to
drive a 240 V motor at double RPM with full torque. A VFD being run
from 240 V just does not have the extra voltage to add as you increase
the frequency beyond the standard 60 Hz. That is why you get constant
torque from nameplate RPM on down -- because the VFD *can* reduce the
voltage to maintain the same current. But it *can't* increase the
voltage as you go above 60 Hz. It just isn't there to work with.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

Bob Engelhardt wrote:
RoyJ wrote:
The usual rule of thumb for VFD's is that you have rated torque up to
the nameplate RPM (ie less than your 5 hp), and rated HP above
nameplate RPM (this is a function of the amount of power the motor can
handle without overheating. )

That's what I've heard (on RCM), but I don't recall seeing it in my VFD
documentation. And I've never understood it. Keeping the HP constant
above the rated RPM means that the torque/current is being
proportionally reduced. Why would you want to reduce the current above
the rated speed? It can't be to avoid overheating - that's just a
matter of current, not power. In fact, higher speeds would generally
mean more air flow & more cooling, allowing *more* current. Maybe it's
a matter of the VFD's limitations.
No, it is the motor. The motor can deliver rated
torque as long as the voltage is proportional to
frequency. So, if the motor is rated for 220 V at
60 Hz, it would need 440 V at 120 Hz, and would
then deliver twice rated power output. But, the
higher combined voltage and frequency will
increase losses in the motor. Even though the
current would be the same, both increasing voltage
AND increasing frequency will deliver more heat to
the iron laminations, mostly, in the motor. Note
that this is NOT changing the motor wiring for the
high-voltage setting on a dual voltage motor, just
upping the voltage with frequency. So, the motor
is not rated for this service, and will overheat
pretty quickly. If you allow the voltage to level
off, while delivering rated current at higher
frequency, you give the iron a break, by reducing
at least the magnitude of the magnetic flux peaks.
This allows the heat loss in the motor to stay
within reason, and the speed-up of the integral
cooling fan mostly keeps it in check.

(Hmmm, this doesn't sound right, it seems as long
as the motor is drawing rated current, the Bmax
will still be the same, no matter the voltage.....
Anybody know?
Now, then, when the voltage no longer follows
frequency as you go up, the ROTOR magnetization
falls off, maybe that is what reduces the heating.)

Jon

Karl Townsend wrote:


I can see 2 effects of higher that rated RPM: centrifugal forces on the
rotor windings (do 3 ph motors have rotor windings?)
Kind of. Typical induction motors have "shorting
bars", which are shaped copper bars that are
shoved into the slots in the rotor laminations and
capped with copper plates at each end. The bars
are welded to the plates.

There are wound-rotor induction motors that are
used with commutators and brushes for slow
starting of large inertial loads like blowers and
carousels. Some have spring-loaded "bracelets"
that expand by centrifugal force to short the
commutator when the motor is up to speed.

Jon

John,
Excellent description. You are correct, but I know of no VFD unit that
increases voltage with frequency. The effect is that current drops from
rated with frequency above rated frequency, but speed increases limiting
somewhat the drop in net power. Overheating of the motor occurs at frequency
below the motor rating without reducing the applied voltage. Again, I know
of no VFD that reduces voltage either.
Steve

"John" wrote in message
...

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


Others have said "Check with the manufacturer" - I'll tell what they'll
say: "Don't do it, it's not rated for it". But you won't know if
they're just covering themselves or if they mean it. If you ask "Why",
you won't know whether you've gotten a straight answer to that, either.

Ya, I'm not going to bother.




I can see 2 effects of higher that rated RPM: centrifugal forces on the
rotor windings (do 3 ph motors have rotor windings?) and bearing
overheating. Now, many motors have a rated speed of 3600 rpm & I doubt
that there's much difference between their designs (rotor & bearings)
and 1200 rpm motors. I doubt that there would be enough cost savings to
justify different "technologies".

So, you gots to ax yourself "Do I feel lucky?". IOW, are you willing to
risk that motor to find out? I hope you do, 'cause I'd like to know. So
be sure to follow up if you do.


I've run a 1800 RPM 7 1/2 HP at 3600 for 100s of hours. (CHNC spindle).
As no one seems to know, I'm just going to guess that it is better to
find an 1800 RPM motor - pretty sure that will work. These are fairly
easy to find.

Karl




As the frequency applied to the motor increases, the internal reactance of
the motor increases so then you need a higher voltage to push the same max
current through the windings as limited by the wire size. Now if you
maintain the current by increasing the voltage you get a lot more power
out of the motor until you reach a frequency where the internal magnetic
loses of the motor generate too much heat. The other limiting factors are
the size of the bearings and the output shaft. They are limited as the HP
they can handle. Theroretically, you can double the frequency double the
voltage and get twice the HP out of the motor.



John

"DoN. Nichols" wrote in message
...
On 2009-04-05, John wrote:

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

[ ... ]

I've run a 1800 RPM 7 1/2 HP at 3600 for 100s of hours. (CHNC spindle).
As
no one seems to know, I'm just going to guess that it is better to find
an
1800 RPM motor - pretty sure that will work. These are fairly easy to
find.

[ ... ]

As the frequency applied to the motor increases, the internal reactance
of
the motor increases so then you need a higher voltage to push the same
max
current through the windings as limited by the wire size. Now if you
maintain the current by increasing the voltage you get a lot more power
out
of the motor until you reach a frequency where the internal magnetic
loses
of the motor generate too much heat. The other limiting factors are the
size
of the bearings and the output shaft. They are limited as the HP they can
handle. Theroretically, you can double the frequency double the voltage
and
get twice the HP out of the motor.

Except that you would need a 480V input power and a 480V VFD to
drive a 240 V motor at double RPM with full torque. A VFD being run
from 240 V just does not have the extra voltage to add as you increase
the frequency beyond the standard 60 Hz. That is why you get constant
torque from nameplate RPM on down -- because the VFD *can* reduce the
voltage to maintain the same current. But it *can't* increase the
voltage as you go above 60 Hz. It just isn't there to work with.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---



Exactly, you get a 240 to 460 transformer and a 480 volt vfd and set up the
parameters to limit the starting current and get twice the hp out of the
motor. I was amazed at the little 1hp motors they used in the DC-8
emiciator pumps that ran on 400 cps. They were less than the size of a 1/4
hp 60 cps motor.


John

"Steve Lusardi" wrote in message
...
John,
Excellent description. You are correct, but I know of no VFD unit that
increases voltage with frequency. The effect is that current drops from
rated with frequency above rated frequency, but speed increases limiting
somewhat the drop in net power. Overheating of the motor occurs at
frequency below the motor rating without reducing the applied voltage.
Again, I know of no VFD that reduces voltage either.
Steve


You can set a number of parameters on the newer VFD's including current
limiting which in effect does this by voltage limiting. Much of the motor
heat comes from resistance losses. some from friction and the rest from eddy
current losses in the magnetic material. you can ramp the current and
therefore voltage in relation to frequency similiar to a soft start unit
used on larger hp motors.


John

wrote:

Kind of a follow up to my pump RPM question.

I found a 1200 RPM 5 hp motor and an old Hitachi SJ200 5hp. vfd in the spare
stockroom (er junkpile)

Any reason I can't spin this combo at 3600 RPM? Still have 5 horse?



Speed.

Whenever a problem involves safety there is always one or
more ultra conservative health and safety fan who will paint a
picture of remotely possible extreme dangers arising from
deviation from manufacturers rated conditions.

Certainly motor operation outside normal manufacturers
published rating carries with it a degree of risk. This has to be
a personal decision and this is the way I look at it.

In the 50/60Hz induction motors I have handled I have not
seen mechanical design changes related to the different operating
speeds of 2, 4 and 6 pole machines. Manufacturers appear to use
the same rotor and bearing construction throughout - only the
stator changes. The rotor construction is extremely strong and
the same arrangement is used inaircraft machines operating at
24,000 RPM.

A possible but unlikely problem is the degree of mechanical
unbalance in the rotor. Rotors are routinely trimmed for
mechanical balance. The forces arising from the remaining
residual unbalance rise as the square of the speed. If the
balancing is poor it will become evident as excessive vibration.

On this basis I would have no hesitation in operating a 4
or 6 pole motor at 2 pole speed.

With a suitable VFD I would also be prepared to go well
beyond this.However, in this case, I would first test the motor
in a position wheremechnical failure was not a safety hazard and
never subsequently use it at more than 2/3 test speed.

Motor electrical behaviour.

The copper behaviour is roughly independent of frequency so
the heating resulting from copper losses remains directly
proportional to current squared. This is easily checked and,
provided it remains below manufacturers rated full load current,
there is no problem.

Iron losses are significant. Dependent on design, total iron
loss may be comparable with total copper loss. When operating at
a different frequency the iron losses change. Iron losses are non
linear but, for a first approximation, at flux densities and
frequencies appropriate for our motors, losses rise as the square
of the flux density and directly with frequency.

Although common VFDs can increase the motor drive frequency
up to and beyond several times supply frequency they cannot
correspondinly increase the motor voltage above supply voltage.
Since this means that the motor flux density drops (and
correspondingly reduces the torque per amp of current), the
reduced flux density loss more than compensates for the loss from
increased frequency so the total iron losses reduce.

This meas that there is no overheating problem resulting
from increased frequency at constant motor voltage. With a near
ideal motor the available output power would stay at rated power
as the speed is increased

However with real motors not all of the flux generated by the
ampere turns of the stator windings couples directly to the
rotor. This failure to couple is leakage inductance and because
it effectively appears as an inductance in series with the
windings it reduces the useful voltage reaching the motor. This
effect is directly proportional to frequency and is very
dependent on the layout of the stator windings. Most motors will
behave pretty well as as expected up to at least 1 1/2 times
supply frequency. Good operation is possible well beyond this but
much depends on the detail design of the windings.

Your proposal to treble the supply frequency is certainly
well into the uncertain area. I think there's a good chance of
getting a large fraction of 5HP at the higher speed but you need
to watch for possible large RPM drops. Normal operation and the
above comments assume that the motor is operating within a
hundred or so RPM of synchronous speed. If the rotor speed drops
well below synchronous speed the motor efficiency drops and a
large fraction of the input power is dissipated in the rotor
bars. This excess power loss is directly proportional to slip
speed (RPM below synchronous) and can result in excessive motor
dissipation even if the supply current remains below rated
current.

One other possible problem is fan loading - fan power
absorption rise as the cube of the speed. The power absorption of
any fan attached to your motor rises 27 times!!

Test results on your setup wuld be very interesting -
keep us posted.

Jim