1800 rpm motor at 2500 rpm? 2

[FluidComp]

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?

 

[rbulsara]

Mostly likely answer is No, just because the word "continuously" in your question. All analyses aside for continuous operation get the right motor.

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.

 

[CJCPE]

Look at:

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.

 

[electricpete]

I wouldn't expect any problem with cooling fan, since it's running faster.

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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[jraef]

With lower HP motors you can sometimes get away with that, because some manufacturers don't bother stocking 2 sets of bearings for 4 pole or 2 pole motors. However, in larger motors like that there is a significant cost difference, so it's unlikely that the bearings will be rated for more than about 20% over speed. The rule of thumb is 75Hz max., but you can always ask the motor manufacturer if unsure.

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.

 

[DickDV]

Checking with the manufacturer is always a good idea as the hp gets up above 150hp or so but most likely the motor will runs just fine. Bearings will not likely be an issue and nor will cooling. Net efficiency will drop slightly.

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!

 

[waross]

I seem to remember a previous thread on a similar topic.
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

 

[LionelHutz]

waross has the best answer. Use a VFD with a higher voltage rating then the motor so you can keep the V/Hz equal at the higher speeds. In the USA, it is typically done with a 230VAC rated motor and a 480VAC rated VFD allowing the V/Hz ratio to remain equal up to 2x rated frequency or 120Hz.

 

[DickDV]

And so, by using the European technique of wiring the motor to 230V and using a 460V drive up to 120 Hz you magically double the hp of the motor! I don't think so!

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.

 

[itsmoked]

But DickDV think of a 400Hz motor. By virtue of raising the system frequency you are getting a magnetically appropriate size for that greater HP. (Within limits of course - like breaking shafts)

A 50kW 400Hz generator is about the size of a 5HP 60Hz 3Phase motor.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[aolalde]

But 400 HZ machines use core stacks made with very thin and “low loss” electric steel laminations

 

[Skogsgurra]

It is done routinely over here. Connecting a 400/230 V 50 Hz motor in delta and running it off a 400 V 87 Hz VFD to get 73 % overspeed is not a problem. At least not with four-pole machines. Bearing life is also usually not a problem, but better check with manufacturer. Bearing life is much more affected by the PWM stray currents than by the overspeed.

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

http://www.gke.org

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