1800 rpm motor at 2500 rpm?
1800 rpm motor at 2500 rpm?
(OP)
Can I continusly run a 4 pole 250hp 1800 rpm motor at 2500 rpm by increasing the hz up to 90 with a VFD and still maintain 250hp? Will this decrease the life of the motor? What risks do I have? Any suggestions?





RE: 1800 rpm motor at 2500 rpm?
As for factors affecting motor performance at varable speed using a VFD, refer to http://www.patchn.com/mtrwhtpaper.htm , for starters.
Some high points:
Bearings may not be rated for "continous" 140% overspeed.
What is it driving? Fan/ Pump? Is the driven equipment rated for 250HP at 2500rpm? Remember HP required by centrifugal fan/pumps increases in proportion to the cube of the speed (rpm).
Cooling fan may not keep up with cooling at such higher speed ( I may be wrong on this.)
The steel for motor is optimized for core losses at rated frequency. I beleive higher frequecy leads to higher core losses..more heating..
Most important of all: 250HP motor is not something to monkey around with. Get the right equipment. Insist on doing things only the right way. "Innovations" like this will NOT make you a hero.
RE: 1800 rpm motor at 2500 rpm?
NEMA Application Guide for AC Adjustable Speed Drive Systems
http://www.nema.org/stds/acadjustable.cfm (free download)
In section 5.1.2 Maximum safe operating speed, it is stated: “The maximum continuous operating speed of a direct coupled, Design A or B general-purpose motor operated in a 0°C to 40°C ambient temperature should not exceed the values given in Table 5-1.” Table 5-1 lists 2300 RPM as the minimum design speed for an 1800 RPM motor.
If the motor is operated up to 90 Hz with the voltage held constant at nameplate voltage between 60 and 90 Hz, you can probably get 250 Hp out of the motor between 60 an 90 Hz. Remember that Hp = Torque X RPM / 5252. Constant Hp with increasing RPM means reduced torque. If the motor produces 250 Hp and 750 lbs-ft of torque at 1750 RPM, it will produce no more than 250 Hp and 571 lbs-ft of torque at 2300 RPM. As rbulsara pointed out, the torque requirements of fans and centrifugal pumps increases as speed increases. Most other loads require constant torque as speed increases. Very few loads have decreasing torque and constant horsepower.
RE: 1800 rpm motor at 2500 rpm?
As you increase frequency above base speed without increasing voltage, the volts/hz decreases. The torque vs slip curve including breakdown torque all decrease by approximately volts/hz squared. At around 4/3 above base speed mentioned, you have around 3/4 of rated volts/hertz which reduces torque curve by 9/16. The machine may not be handle momentary overloads since breakdown torque will be so close to running torque.
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RE: 1800 rpm motor at 2500 rpm?
Besides, even though you will still have 250HP, your shaft torque will have dropped off significantly at 150% speed.
250HP 1800RPM = 729 lb-ft. of torque.( 250HP x 5250 / 1800RPM )
Same motor spun to 2500RPM = 525 lb-ft. of torque ( 250HP x 5250 / 2500RPM )
Now add to that the fact that any cooling fans on the motor will lose efficiency and you have a motor putting out less torque for the same load which will overwork the motor, plus its cooling system will not be functioning at its peak.
I can smell the smoke from here.
RE: 1800 rpm motor at 2500 rpm?
I do this often and the results are always an improvement in speed regulation, cooling, and a reduction in cost both for the drive and the motor. In many cases, a TEFC motor can be used in stead of an auxiliary cooled unit and the drive can often be resized as a normal duty unit (10% overload) rather than a heavy duty unit (50% overload).
This always gets an argument but, if in doubt, just look at Reliance's standard motor catalog. The torque-speed curves throughout the NEMA frame ratings all go to 90 Hz on 4 pole motors. I'll take their word for it!
RE: 1800 rpm motor at 2500 rpm?
I believe it had to do with reducing the cost of an installation by using an undersized motor wired for 230 volts and then driving it over speed and over voltage with a 480 Hz. VFD.
respectfully
RE: 1800 rpm motor at 2500 rpm?
RE: 1800 rpm motor at 2500 rpm?
If you decide to try it anyway and expect constant torque over 60Hz, keep a fire extinquisher handy, just in case!
It might work at constant torque up to 70hz or so, but not much further than that without some torque derating.
RE: 1800 rpm motor at 2500 rpm?
A 50kW 400Hz generator is about the size of a 5HP 60Hz 3Phase motor.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 1800 rpm motor at 2500 rpm?
RE: 1800 rpm motor at 2500 rpm?
Extra heating is also not a problem. It is the carrier frequency that heats the iron. Not so much the higher fundamental frequency. Internal circulation and external fan work much better at overspeed.
And it is not a "trick" - it is more like putting in an overdrive. And that has been done for decades in cars. And works very well.
Running above base speed without increasing V/Hz is another thing. As said by several posters.
Gunnar Englund
www.gke.org
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