Which Motor ... - Redux

[jraef]

I have a similar application to the one posted in thread237-94599

I am working on a machine currently using 2x 400HP 460V motors connected to a common shaft. Motors have a nameplate FLC of 458A. The original machine designer apparently used a "better to overkill than stall" method of selecting motors. Under normal operation we now find that they draw only 220A each, and full load "worst case scenario" conditions, each motor pulls 300A max., roughly only 66% of full load. They are rebuilding the machine and I have proposed they change to 300HP motors rated at 346FLA each because they will be running at about 87% of capacity at full load and I believe at better efficiency. The additional starting torque is not an issue here, the machine (a rock crusher) cannot start loaded under any circumstance anyway. Unloaded it is basically just a big air pump pulling no more than 600A at startup.

Am I off base in proposing this as a way to save energy in the long run? BTW, they have plenty of uses for the 400HP motors elsewhere so nothing will go to waste.

"Venditori de oleum-vipera non vigere excordis populi"

 

[electricpete]

I'm not that familiar with crushers but my assumption is when a rock gets stuck a certain way it can create a momentary high torque requirements.

Is there flywheel that helps it ride through tjat tu[e pf momentary high load torque requirements? If not may need to oversize motor to provide high enough breakdown torque?


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[electricpete]

Wow. Must have been some gremlin that got in there an garbled my typing (it couldn't have been my fault ;-)

Should have been
"Is there flywheel that helps it ride through that type of momentary high load torque requirements?"

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[jraef]

Actually, it is a Vertical Shaft Impact crusher which works by flinging the rocks with centrifugal force to the outside and letting them smash against each other. No moment of inertia to worry about really. The rocks drop onto a rotating disc which, in effect, is the flywheel. It kind of looks like a centrifugal pump upside down without the volute. The HP is needed just to overcome the acceleration of the rock mass wanting to slow down the rotor.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

Jraef:
As I understand the two motors are in Tandem. If both have identical electric construction, they will share equally the load and are able to provide 800 HP nominal.

When each motor draws 220 Amperes, the Efficiency (EFF) and Power Factor (PF) are very low. I guess EFF= 0.75 and PF=0.6. The power demanded by the load could be:
BHP= 1.732*480*220*0.75*0.6/746 = 110.33 (each motor). 220.7 HP total in tandem.

When the motors draw 300 Amperes (worst case scenario), the EFF and PF improve but still are not optimum. I assume EFF=0.85 and PF=0.8. The load demand is:
BHPmax = 1.732*480*300*0.85*0.8/746 = 227.3 HP (each motor) and 455 HP total.

Based on those figures I propose that you leave those motors (400HP). The Change is ENERGIZE ONLY ONE OF THEM.

That way you will push the motor load close to the nominal capacity of the motor, improving the operating EFF and PF. Your regular load is around 220 HP.

Even at critical load condition your motor will be only with 14% overload and that is not the regular condition of operation.

The other motor could be a spare, ready to be energized whenever it is required (use space heaters to keep it dry).

If for whatever reason extra power is required you have it in stand by.

You will save the cost of replacing motors.

 

[tarzan56007]

Do you think it is possible that the conveyor or whatever system that is feeding this unit with media is undersized, possible if rocks were fed into this machine a little quicker it would work harder and therefore more efficiently,
another thought is it may be designed for zero down time so as to be ran with one motor but if it were to fail the other could be brought online until the first one could be repaired.

 

[jbartos]

Suggestion: If a custom-made motor happens to be affordable, then the maximum efficiency of the induction motor is possible to engineer and design at 66% or so of the shaft load for the 400HP motor. Also, Service Factor might be applied, and it may be custom-made, e.g. more than 1.15. However, such motor will not be possible to compare to industry standard motors, e.g. to NEMA motors.

 

[sreid]

From your original post I get the sense that there is resistance to your proposal. There can be a number of reasons for this.

1) Change has risk. If it ain't broke don't fix it.

2) The Return On Investment (ROI)time in energy savings may be too long.

3) Corporate survival. If one sticks his neck out, the head could get chopped off.

 

[charlierod]

Thermal aspects should also be considered, besides not always more loading implies more efficiency. Check the document "Motor Life: The effects of Motor Loading, Service Factor and Temperature Rise on Insulation Life" at

http://www.sea.siemens.com/motors/product/mtallpapers.html

 

[rbulsara]

I am with sreid and charlierod. Let the 'businessman' in you dictate , rather the 'engineer', in this case.

Plus I dont think you are going to save any significant energy. What price explaining if some problem crops up in the furture to Production or Management?

 

[aolalde]

Jraef:
I think this is an engineering decision not an administrator one.
My opinion is that your first proposal is not the best solution. Not from the engineering point of view, nor for the financial return on investment.

 

[jraef]

All points well taken. Thanks for the input. As a result of my initial inquiry I am being connected to the original design engineer next week. You have all given me thoughtfull issues to discuss (and consider for my own sake!).

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Suggestion: The electrical energy savings (or efficiency) should not be underestimated if the machine runs 24/7.

 

[jraef]

For those still interested...

I met with the original machine mechanical engineer today. As it turns out, he originally determined that he needed 545HP at peak load, but rather than run 2x 250HP motors into the service factor, he selected 300HP motors for it, "just to be safe". At time of delivery, the motor supplier only had 350HP motors available so they offered them at the same price and that was how it shipped. At some later date, a user somewhere else must have needed to replace the motors and probably ran into the same deal, i.e. 400HP motors for the price of 350, so now we have 400HP motors on a machine that could have used 250HP to begin with! We are going back to 300HP, but mainly because this engineer is concerned for the wieght on the frame, it was never designed to carry much more than the 300HP motors.

For what it is worth pertaining to other comments:

Here is a link to a similar machine (it shows a good drawing of the basic concept)

http://www.oresizer.com/products.html

The machine design is such that at this HP, the motors must counter each other on each side of the vertical shaft to avoid torsional stress, so it doesn't work to use only one motor.

The feed conveyor sizing / rate was a valid point, but that is essentially what had happened here. The new owner does not need (and cannot supply) the maximum throughput capacity of the original design. We tested it at his maximum requirement when we determined the motor loading earlier.

Thanks again for everyone's input. It helped me to look more objectively at this before I went at it.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Suggestion/Question: How come that there are so many problems with a motor size, if there are so many motors, either new ones or used ones available?