Calculating energy savings for replacing an electric motor

[gar]

121202-1531 EST

iceworm:

I have been thinking more on my black box analysis. Can you pick any holes in it.

If line voltage is constant, and I think that is the assumption we must make, then is there any normal type of load that as a result of increased efficiency of the induction motor would have a lower RPM than the less efficient motor?

Input power is watts to the motor. Output motor shaft power is input minus losses. Since the motor is more efficient that means more output power for the same input power.

Is it likely that the motor design criteria would be shifted such that more slip would occur at our operating point, but still have higher efficiency. I do not expect this would be a design direction. It would imply increasing the rotor resistance, and more than offsetting that increased rotor loss with a much more reduced loss in the magnetic circuit area.

My guess is that both magnetic and rotor losses would be reduced to gain higher efficiency. But I have not seen any comparative motor curves.

.

 

[iceworm]

121202-1531 EST
...I have been thinking more on my black box analysis. Can you pick any holes in it. ....

No. But I can't plug any holes either.

ice

 

[weressl]

LZ -
I not getting this. Why would it run faster? How would one calculate how much faster?

ice

Two examples of current production motor data, one is simply EPact (lower efficiency, lower FLrpm) the other is high efficiency (higher efficiency, higher FLrpm). Older, standard efficiency motors would even have lower full load RPM's than either of these.

http://www.reliance.com/pdf/pdf/aced/W00214-A-A005.pdf

http://www.reliance.com/pdf/pdf/aced/W00805-B-E003.pdf

Page 10:
http://www.cee1.org/ind/motrs/CEEMotorGuidebook.pdf

Page 36:
http://webbooks.net/freestuff/motor_selection.pdf

(Unfortunately it does NOT elaborate WHY should you select the next highest HP, but it is for the higher FLrpm and the resulting higher load on the new motor.)

"For centrifugal loads, the replacement motor selected should be the next nameplate size above the motor output
when operating under fully loaded conditions. It is recommended that voltage, amperage, kW draw, power
factor, and slip be metered for a variety of motor operating conditions such that the maximum load point is
known with confidence. The slip technique should not be used for rewound motors or motors operating
at other than their design voltage."

BTW the motor load can be determined from an accurate stroboscope reading the shaft RPM and scaled 0-100% between the No-Load rpm and Full-Load rpm, of course you need to have a bench tested motor, not just the factory typical production data.

 

[weressl]

121202-1531 EST

iceworm:

If line voltage is constant, and I think that is the assumption we must make, then is there any normal type of load that as a result of increased efficiency of the induction motor would have a lower RPM than the less efficient motor?

I refer you to the affinity laws of centrifugal loads.

http://en.wikipedia.org/wiki/Affinity_laws

 

[iceworm]

... it does NOT elaborate WHY should you select the next highest HP, but it is for the higher FLrpm and the resulting higher load on the new motor.)

"For centrifugal loads, the replacement motor selected should be the next nameplate size above the motor output
when operating under fully loaded conditions. ...

I refer you to the affinity laws of centrifugal loads. ...

All this is true. But the OP is not dealing with a centrifugal loads

Hello, I am looking into replacing an old air compressor motor with a new energy efficient motor ...

Doesn't apply

ice

 

[iceworm]

Two examples of current production motor data, one is simply EPact (lower efficiency, lower FLrpm) the other is high efficiency (higher efficiency, higher FLrpm). Older, standard efficiency motors would even have lower full load RPM's than either of these. ....

Okay I see that. It's more than I thought it would be.

... Unfortunately it does NOT elaborate WHY should you select the next highest HP, but it is for the higher FLrpm and the resulting higher load on the new motor.) ....

This is one part that still baffles me. Taking your example, the premium motor is 1765 rpm FL. The other is 1759 rpm FL. Giving a speed increase of 1.0034. For a PD type compressor (screw, scroll, piston) the power increase is linear. Even if it was a pump/affinity Law load, the increase in power is (1.0034)^3 = 1.0103 A 1% increase. Not exactly a big deal.

... BTW the motor load can be determined from an accurate stroboscope reading the shaft RPM and scaled 0-100% between the No-Load rpm and Full-Load rpm, of course you need to have a bench tested motor, not just the factory typical production data.

I guess I just don't see this happening. Its a 10 hp motor. Nobody is going to spend that kind of time or money for the data.

I don't even see oversizing the motor. Going from a 10hp to a 15hp doesn't look like a great idea.

I am certainly not saying you are wrong. I'm saying I don't see that it matters.

ice

 

[weressl]

Okay I see that. It's more than I thought it would be.


This is one part that still baffles me. Taking your example, the premium motor is 1765 rpm FL. The other is 1759 rpm FL. Giving a speed increase of 1.0034. For a PD type compressor (screw, scroll, piston) the power increase is linear. Even if it was a pump/affinity Law load, the increase in power is (1.0034)^3 = 1.0103 A 1% increase. Not exactly a big deal.

Except that the data was for an EPact motor that meets the minimum efficiency of todays requirement. Older motors may have as low as1745RPM FLprm.

I guess I just don't see this happening. Its a 10 hp motor. Nobody is going to spend that kind of time or money for the data.

I don't even see oversizing the motor. Going from a 10hp to a 15hp doesn't look like a great idea.

I am certainly not saying you are wrong. I'm saying I don't see that it matters.

ice

I know, it would need to be a perfect strom to all those ifs to go in the wrong direction. However, Murphy is alive and well. Even if in this case the issue is not even marginal at least the readers/followers of this Post are aware of the issue as something to think about.

My worst case scenario with a replacement was when the mechanical guys decided to change the motor and former chain drive on a ribbon blender to a new motor and cog-belt drive. Same HP and RPM. Not the breakaway torque though.....I think the mechanical guys face eventually reached the shade of mauve.:lol: