AC motor shaft load calculation 1

[nurujjaman]

Hi All,

I am new to this area.We have a rice mill plant. In the plant room plant directors are using a 3-phase induction motor of 250HP.But according to my basic knowledge that much of motor is not required.So now I want to optimize the size of that motor.

Can anyone suggest me a way to calculate the motor size.

please help.

Regards,
Nurujjaman

 

[ozmosis]

Nurujjaman
What you are thinking of doing is potentially quite a high investment and high risk.
There are lots of questions to ask but a basic one would be, why change? Is is because of energy savings or is the existing motor not working too well?
A first step would be measure the amps drawn by the motor under all conditions. This means your highest production now and try and determine if this is is the highest production it has ever been.
The difference between this and the actual rating of the motor is then known, approximately. Determine what level of overload/safety margin was designed for the system.
What is controlling the motor now? ( DOL, controlled start, variable speed?)
The cost of changing a motor is not just the motor; it will be switchgear, protection, maybe cables, mechanical couplings, gearboxes, etc
And then there is a risk that you have not taken into consideration future loading or increase production etc and have no scope to manufacture.
Think carefully about all implications.

 

[Compositepro]

Nurujjaman, what you probably do not realize is that a 250 hp motor does not consume 250 hp all the time it is on. The motor's power consumption is mostly determined by the mechanical load on it. 250 hp is just the limit of what it can supply on a continuous basis.

 

[waross]

And a 250 HP motor may drive a 125 HP load more efficiently and cheaper than a 125 HP motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nurujjaman]

Hi experts,
I have googling about tbe motor and its pheripherals and gathered a little knowledge on this.
what I unxerstand that our 250hp is running with less shaft load.so I need to measure the shaft loadon that motor.
so my main concern is about the shaft load.and I have to measure it.
please suggest me a good refdrence about the measuremsnt of shaft load.
Regards,
Nurujjaman

 

[waross]

Measure the Watts. Make an allowance for losses and calculate backwards to find the torque (shaft load?).
Re-read the previous answers a couple of times. It is possible to measure the torque, but for your application most of the experts here would not attempt to measure the torque.
You have answers that may be result of over one hundreds of years of experience with motors.
We would probably use an ammeter and a lot of experience but the current is not linear with loading.
You will achieve more accurate information with a Watt-meter.
Changing an existing motor for a smaller motor will be expensive and won't save any energy costs.
This is not the way to make a good first impression on management.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Just to pile on, you seem to be approaching this with an unyielding bias toward believing this is a big problem, yet it is NOT.

Motor electrical energy consumption is a factor of two things: The connected shaft load and the efficiency of converting the electrical energy into kinetic energy. You already know that your connected shaft load is lower, how much lower is actually irrelevant in that if it is 200kW, it is 200kW regardless of the size of the motor. then as a gross rule, motor efficiency is the only real issue here.

Motor efficiency peaks well below full loading and in small motors, drops again a tiny amount at above about 95% loading. The chart below is very typical for a small motor:

But as the motor size goes up, inherent design issues in the way they must be built lead to better efficiency, so the peak happens at much lower levels and stays higher throughout. This chart illustrates that as a comparison based on size:

That is why we are telling you that you have little, if anything, to gain by thinking that just because your motor is not fully loaded, it is consuming more energy. It might be, but it is a very small amount at worst, and the RISKS associated with UNDERSIZING the motor are worse, therefor not always worth the effort. For example if the original mill designer KNEW, from his specific experience AS a mill designer, that there was a "worst case scenario" which might require a very brief peak torque requirement from that motor, that would likely be the reason he chose a 250HP motor. But if you do NOT know about that, then a smaller motor may create a situation wherein the mill stalls and production is lost. That lost production has a very direct economic effect, usually MUCH greater than any perceived energy savings you might have realized.

"Will work for (the memory of) salami"

 

[jraef]

Damn, wish I could edit, I accidentally pasted the same link twice. Here is the 2nd chart.

"Will work for (the memory of) salami"

 

[elinBG]

And a 250 HP motor may drive a 125 HP load more efficiently and cheaper than a 125 HP motor.

Is this true? Because the test reports which I checked show smaller efficiency at lower load...

However, regarding the topic... I want to share an interesting tool which I found on the net

http://www.orientalmotor.com/support/motor-sizing.html

 

[elinBG]

sorry, forgot to attached the test report -

http://www.valiadis.gr/pool/ftp/drawings/K355M- 2-250KW-TEST_REPORT.pdf

Rated Power 250.00 kW / 340.00 hp
Speed 2985 RPM

 

[electricpete]

Quote (waross)
And a 250 HP motor may drive a 125 HP load more efficiently and cheaper than a 125 HP motor.

Is this true? Because the test reports which I checked show smaller efficiency at lower load...

Yes, generally true.
#1 - look at curves jraef posted, generally not a huge different between efficiency at 100% and efficiency at 50% load.
#2 - compare curves of different hp. The higher hp tend to be more efficient.

You already have a motor, purchased and working. What the heck are you trying to accomplish?

=====================================
(2B)+(2B)' ?

 

[petronila]

Hello nurujjaman,

Before replace any motor I will recommend you check first the WK2 or Load Inertia moment, then verify the Torque-Speed curves of new motor in order to know if will move that load without problems, if you found a small motor then you need to consider manny thinks more like Efficiency class, starting time, overload capacity, starter type, e.t.c.

Regards

Carlos

 

[jraef]

A couple more comments:

You apparently have a 250kW motor, not a 250HP!

When looking at data from people such as Oriental Motor, you must consider the source. Oriental Motors sell fractional HP motors, as in 500W, not 500kW. There is a very big difference in the designs of those motors. So their statement about having "smaller efficiency at lower load" may be true for THEIR class of motors, but you cannot equivocate that to what YOU are using here.

Per your motor data sheet, your efficiency only drops 1.84% between 100% load and 50% load. If your motor runs 24/7/365 and NEVER goes over 50% load, you might realize a small amount of energy savings. But here is what is missing; you cannot really compare the efficiency differences in THIS motor at 50% load, you must compare THIS motor at 50% load against another 125kW motor at 100% load, that is the only valid comparison. A quick look at the same mfr's 132kW motor shows an efficiency at 100% load of 94.98%. So that means at BEST, you will only realize a 1.22% net gain.

Then again, someone at some time SPECIFICALLY DECIDED to use a 250kW motor on your mill, when they COULD have selected a 200kW, 160kW or 132kW motor is that is all it needed. When a machine manufacturer decides what size motor to put on a machine, it's not usually a safe bet to assume they would go ahead and put in a LARGER MORE EXPENSIVE motor than was absolutely necessary, because that affects the size and price of their machine. If anything, I have seen it much more likely that the motor selection is marginal to begin with, they will often stretch the motor's capabilities even though they know it will affect the long term lifespan. But as long as it outlasts their warranty, it passes. So unless you can get hold of that person and discuss this issue in detail to understand exactly WHY they selected that motor, you are playing with fire here with very little to gain.



"Will work for (the memory of) salami"

 

[LionelHutz]

Measure kW input power when running with load and divide by 0.9 (just assume a rather low 90% motor efficiency to be relatively safe) and divide again by 0.746 to get the basic motor HP required.

Of course, without considering 100 other things besides the above the new motor may fail to work.

This is a tips site so going through each thing you need to consider when picking a motor is beyond the scope of a simple reply. It almost requires writing a book.

 

[ozmosis]

Jeff
I think it is still a 250 hp motor. The kW reference came from elinBG as an example I surmise.

 

[jraef]

Oh you're right, I got confused...

"Will work for (the memory of) salami"