How to Save Money When Buying How Synchronous Motor Works
Apr. 28, 2025
Energy efficient motors, can someone explain to me how they save ...
I understand electrical theory, but the new "Energy efficient motors" have me baffled.
My company got a grant to install "Energy efficient motors" to save on power. I checked the amps before and after on all of the motors. They amped the same or worse on every one of the new motors! I'm old school, but I thought the rule was, "energy in energy out", and that appears to be the case.
Several of the new motors have a higher FLA than the ones we replaced! I understand the fact that they can endure the higher amps without self destructing.
How can they possibly save power?
Slightly more seriously, current alone isn't a measurement of power.
A typical cage induction motor takes 30% of full load current running uncoupled.
It clearly isn't running at 30% output power.
What changes is power factor.
Power is sqrt(3)VIcos(phi) for a three phase system. The issue that rcwilson points out is very real and not just theoretical, particularly on centrifugal fans and pumps where even a small increase in speed due to different slipage between the old and new motor can negate any power savings or even raise the power required. It is not unusual, for example, to have to machine an impeller in a pump to compensate for the different slipage to achieve the desired result.
It is imperative, as the others have noted, that you use an accurate true power meter (just amps are useless) on the existing motor setup (as a baseline) before installing the new motor.
In my view it is bad practice to just blindly change an existing motor for a high efficiency motor without analyzing the entire driven load as you may end up with undesirable results. There is more to efficiency than just input power also. You need to factor amount of power input vs amount of power output to get a true idea of efficiency. If the high efficiency motor is indeed running faster then the output power is not the same as the original motor so just measuring the input of both new and old will not tell you what input to output ratio is for each.
Also remember that heat is a result of inefficiencies. A more efficient motor may do similar work and give up less heat. That should be reflected in input to output ratios. To expand on the earlier points, the basic formula you thought of, "energy in = energy out" is a little too simplistic. It really is, "energy in = work performed plus losses in performing the work". The devil is in the details of the "losses in performing the work" part. In a motor, it takes a certain amount of energy just to MAKE IT a motor as opposed to a hunk of iron and copper. In that process, anything that reduces those losses will save energy, THROUGHPUT energy. We call that "efficiency".
You cannot use nameplate current data to asses what amount of energy a motor will consume, it must be considered when coupled to the load. HP is a shorthand expression of a mechanical construct that equates to "a certain amount of torque at a certain speed", but that's too difficult to say every time, so we shorten it to "HP". A load requires torque and speed, a motor delivers it. But a motor inherently delivers only as much as the load needs, it does not have a way to "force" a load to take more power. So regardless of what kind of motor you attach to a load and regardless of the FLC shown on the nameplate, that motor is only going to provide that needed load based power. What it CONSUMES from the energy source then is that load power requirement, plus losses.
So what you WILL see on an EE motor compared to a standard one is a difference in rated EFFICIENCY, often abbreviated "eff" on the nameplate. Old motors are 70-80% in most cases, EE motors are 90-95% eff. What they do to get the eff higher is to decrease the air gap between the stator and rotor, use higher grade steel so that there are fewer "eddy current" losses in the stator core, better bearings to reduce friction and either better cooling fans or no fans at all.
That would depend on rating.
This is for one we did a while back:
"2/4" "3/4" FL
96.7% 97.2% 97.3%
True, a motor with more torque will have less slip and be able to accelerate/maintain speed easier.
A motor with less torque will still try to attain synchronous speed, it will have more slip and will draw more current trying to do so. Any shortcoming of the load from receiving power is given up as heat. Power factor is not as big of a factor when motor is loaded but at same time has little to do with total efficiency. The reactive current is still doing no work, at the most it only increases voltage drop which will make some inefficiency in the supply and not so much in the motor itself.
I may contradict myself a little but yes more torque means a change in motor characteristics - meaning more horespower is available without overloading. If a centrifigal fan or pump has an increase in speed it will demand more power. If a constant torque load has an increase in speed it just does the work faster.
I still don't believe there will be a significant increase in torque in a high efficiency motor. There can be a change in torque from just replacing one motor with another one that is considered equivelant - they are mechanical devices and you can't expect them to all be exactly the same but rather fall within a certain zone of tolerance.
In the end is power is power. If it takes less input power to get same work done then you have higher efficiency.
Power not transfered to the load is given up as heat.
My company got a grant to install "Energy efficient motors" to save on power. I checked the amps before and after on all of the motors. They amped the same or worse on every one of the new motors! I'm old school, but I thought the rule was, "energy in energy out", and that appears to be the case.
Several of the new motors have a higher FLA than the ones we replaced! I understand the fact that they can endure the higher amps without self destructing.
How can they possibly save power?
I understand electrical theory, but the new "Energy efficient motors" have me baffled.By using less?
My company got a grant to install "Energy efficient motors" to save on power. I checked the amps before and after on all of the motors. They amped the same or worse on every one of the new motors! I'm old school, but I thought the rule was, "energy in energy out", and that appears to be the case.
Several of the new motors have a higher FLA than the ones we replaced! I understand the fact that they can endure the higher amps without self destructing.
How can they possibly save power?
Slightly more seriously, current alone isn't a measurement of power.
A typical cage induction motor takes 30% of full load current running uncoupled.
It clearly isn't running at 30% output power.
What changes is power factor.
Power is sqrt(3)VIcos(phi) for a three phase system. The issue that rcwilson points out is very real and not just theoretical, particularly on centrifugal fans and pumps where even a small increase in speed due to different slipage between the old and new motor can negate any power savings or even raise the power required. It is not unusual, for example, to have to machine an impeller in a pump to compensate for the different slipage to achieve the desired result.
It is imperative, as the others have noted, that you use an accurate true power meter (just amps are useless) on the existing motor setup (as a baseline) before installing the new motor.
In my view it is bad practice to just blindly change an existing motor for a high efficiency motor without analyzing the entire driven load as you may end up with undesirable results. There is more to efficiency than just input power also. You need to factor amount of power input vs amount of power output to get a true idea of efficiency. If the high efficiency motor is indeed running faster then the output power is not the same as the original motor so just measuring the input of both new and old will not tell you what input to output ratio is for each.
Also remember that heat is a result of inefficiencies. A more efficient motor may do similar work and give up less heat. That should be reflected in input to output ratios. To expand on the earlier points, the basic formula you thought of, "energy in = energy out" is a little too simplistic. It really is, "energy in = work performed plus losses in performing the work". The devil is in the details of the "losses in performing the work" part. In a motor, it takes a certain amount of energy just to MAKE IT a motor as opposed to a hunk of iron and copper. In that process, anything that reduces those losses will save energy, THROUGHPUT energy. We call that "efficiency".
You cannot use nameplate current data to asses what amount of energy a motor will consume, it must be considered when coupled to the load. HP is a shorthand expression of a mechanical construct that equates to "a certain amount of torque at a certain speed", but that's too difficult to say every time, so we shorten it to "HP". A load requires torque and speed, a motor delivers it. But a motor inherently delivers only as much as the load needs, it does not have a way to "force" a load to take more power. So regardless of what kind of motor you attach to a load and regardless of the FLC shown on the nameplate, that motor is only going to provide that needed load based power. What it CONSUMES from the energy source then is that load power requirement, plus losses.
So what you WILL see on an EE motor compared to a standard one is a difference in rated EFFICIENCY, often abbreviated "eff" on the nameplate. Old motors are 70-80% in most cases, EE motors are 90-95% eff. What they do to get the eff higher is to decrease the air gap between the stator and rotor, use higher grade steel so that there are fewer "eddy current" losses in the stator core, better bearings to reduce friction and either better cooling fans or no fans at all.
HP is a shorthand expression of a mechanical construct that equates to "a certain amount of torque at a certain speed", but that's too difficult to say every time, so we shorten it to "HP".Elsewhere, it is given as kW. A 55kW motor for those of us not still using Imperial units is one that delivers 55kW at the output shaft.
So what you WILL see on an EE motor compared to a standard one is a difference in rated EFFICIENCY, often abbreviated "eff" on the nameplate. Old motors are 70-80% in most cases, EE motors are 90-95% eff.
That would depend on rating.
This is for one we did a while back:
"2/4" "3/4" FL
96.7% 97.2% 97.3%
I understand electrical theory, but the new "Energy efficient motors" have me baffled.Your apprehension towards energy efficient motor taking higher running current is justified.The energy efficient motor takes higher starting current than a standard motor.But Its power factor is not lower than that of a standard motor.So its running current should be lower.But if the running current is higher,it means a mismatch between the motor and its load. The more I think about it the less I am buying into the idea that the more efficient motor may have more torque and because of that less slip resulting in more speed at the load.
My company got a grant to install "Energy efficient motors" to save on power. I checked the amps before and after on all of the motors. They amped the same or worse on every one of the new motors! I'm old school, but I thought the rule was, "energy in energy out", and that appears to be the case.
Several of the new motors have a higher FLA than the ones we replaced! I understand the fact that they can endure the higher amps without self destructing.
How can they possibly save power?
True, a motor with more torque will have less slip and be able to accelerate/maintain speed easier.
A motor with less torque will still try to attain synchronous speed, it will have more slip and will draw more current trying to do so. Any shortcoming of the load from receiving power is given up as heat. Power factor is not as big of a factor when motor is loaded but at same time has little to do with total efficiency. The reactive current is still doing no work, at the most it only increases voltage drop which will make some inefficiency in the supply and not so much in the motor itself.
More torque means a change in motor characteristics.The motor may then be unsuitable to drive the load the standard motor was initially driving.A mistake in the choice of the energy efficient motor,I would say.
I may contradict myself a little but yes more torque means a change in motor characteristics - meaning more horespower is available without overloading. If a centrifigal fan or pump has an increase in speed it will demand more power. If a constant torque load has an increase in speed it just does the work faster.
I still don't believe there will be a significant increase in torque in a high efficiency motor. There can be a change in torque from just replacing one motor with another one that is considered equivelant - they are mechanical devices and you can't expect them to all be exactly the same but rather fall within a certain zone of tolerance.
In the end is power is power. If it takes less input power to get same work done then you have higher efficiency.
Power not transfered to the load is given up as heat.
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