Energy efficient motors, can someone explain to me how they save money?

jaggedben

Senior Member
The other factor here is time. We don't know what purpose these motors are serving, but it could be something intermittent. If the new motors push air or water faster, such that they can push the same amount as the old motors in less time and then turn off for a while, then they are saving energy even if the power requirements are the same.
 

iwire

Moderator
Staff member
Actually, we don't know that.
All we know from the OP is that more current is being drawn under the load conditions for which it was measured.
Of course, let's make it complicated, maybe this one can go 2500 posts.

In most cases more current at aprox the same voltage is more energy, not always but most times.

But I will go back to my code and let you guys have fun picking the fly poop out of the pepper. :happyyes:
 

MSU

Member
The real question on torque is what does the load (torque requirement) require? The motor will only make the torque required by the load.

Typical FL RPM of an NEMA B motor may range from 1770 to 1790. As previously stated, most energy efficient motors have a slightly higher FL RPM than a standard efficiency machine, since rotor resistances are usually lower. If you replace a 1770RPM motor on a pump (or other variable torque load) with an 'energy efficient' motor rated 1790RPM, the motor will attempt to run faster (dictated by the pump's speed torque curve), and make more HP as a result of that effort. the additional work may be unwanted, not needed, and therefore wasted energy. The tool of choice in that application would be a VFD. Speed can then be matched with load flow requirements.

Losses in a motor are generally described as Load (or copper) Losses, No Load Losses (or Core Losses), and stray losses. Mechanically, windage and friction account for most of the losses.

KW flow to the motor primarily produces torque (and also heat losses and mechanical losses). KVAR flow primarily excites the core. If the PF of the motor is known (and it is frequently on the nameplate), and the efficiency is known (and it is frequently on the nameplate), then the VAR flow at FL can be calculated using voltage and current ratings along with EFF and PF.

Excitation Vars can also be ascertained at no load by measuring the NLA. Essentially, the only KW flow at NL is NL Losses and a small percentage of L Losses, along with windage and friction. Once knows the VAR flow at NL, and FL, one can estimate the VAR flow to be pretty much linear in between. Knowing all of that, reasonably close estimates can be made concerning Watt requirements at loads along the speed torque curve.

From a general estimation point of view, E Eff motors are more likely to save energy on constant torque or loads whose torque requirements are varying (rollers, crushers, presses). Variable torque loads are generally considered to be pumps, fans, other fluid movers whose speed torque curve clearly defines a torque requirement which changes predictably with output at varying RPM. Variable torque loads are usually better served with application of a VFD to control flow in the place of some other downstream throttling device. in cases of long run times, E Eff motors may also be applicable.

Run time is the largest single parameter affecting savings. KW saved times hours equals KWH saved, If the hours aren't there, there is no satisfactory payback.

Since HP = (Torque x RPM)/5252 (using Torque in Ft-Lbs) HP and Torque of the E Eff and Std motors will be in the same ball park, but they are not determined by the motor's speed torque curve, but rather the LOAD's speed torque REQUIREMENTS. As long as the Load's torque requirements are beneath the motors speed torque curve (for the applicable applied voltage and frequency), the motor will satisfactorily drive the load. When the motor's RMS Thermal loading exceeds the thermal capacity of the motor, it will either trip or fail.

Motors (on sine power) are dumb devices which RESPOND to the loads demands as long as the demand does not exceed the capability of the motor.
 

kwired

Electron manager
Of course, let's make it complicated, maybe this one can go 2500 posts.

In most cases more current at aprox the same voltage is more energy, not always but most times.

But I will go back to my code and let you guys have fun picking the fly poop out of the pepper. :happyyes:
Even if you assume unity power factor, a motor with less friction or windage will be more efficient, that would show up reading current only.

Now a motor that is only 5% more efficient seems like nothing, just like a single higher efficiency lighting ballast seems like nothing. Take that times thousands of units and it adds up. 5% on large motors starts being noticeable at user end of things.
 
Top