Sizing 480 3ph electric motors

[Fallriverbryan]

I am hoping to gain some insight into the procedure for sizing an equipment motor. I have read that a motor should be operating at near FLA while it is in service on a piece of equipment. This situation would indicate that the motor is ideally sized for the load it is motivating. In all the years of dealing with 480 3ph motors, I have very rarely observed this situation. In general, I see motors under load at 70-85% of FLA. I understand that these motors will most likely last longer in service but may be more expensive on the front end. So the question...What calculations are performed to obtain the desired motor size?
Thank you for any insight

Sent from my SM-G950U using Tapatalk

 

[topgone]

I am hoping to gain some insight into the procedure for sizing an equipment motor. I have read that a motor should be operating at near FLA while it is in service on a piece of equipment. This situation would indicate that the motor is ideally sized for the load it is motivating. In all the years of dealing with 480 3ph motors, I have very rarely observed this situation. In general, I see motors under load at 70-85% of FLA. I understand that these motors will most likely last longer in service but may be more expensive on the front end. So the question...What calculations are performed to obtain the desired motor size?
Thank you for any insight

Sent from my SM-G950U using Tapatalk

If the motor is driving a pump, I guess it's better for you to look at the pump performance curve and start from there. Please remember that the Q-point of the pump will be located at the point in the curve called the "best efficient point". You could add about 10% fudge on power requirement of your motor calculations.

The Best Efficiency Point is defined as the flow at which the pump operates at the highest or optimum efficiency for a given impeller diameter.

 

[jumper]

I do not really understand the question.

Motors are sized to the load. Are you asking about choosing one with service factor or other ratings for consideration when choosing one for a particular application?

IOW, why one might to think about what is the most effective given two or more choices?

 

[Jraef]

In my opinion, you are up against two somewhat competing design philosophies; best practice for reliability, a concept that has prevailed for as long as the Industrial Age has been with us, vs best practice for efficiency, a push since the first 1970s “energy crisis”.

From an efficiency standpoint, an AC induction motor operates at its highest efficiency nearest when to being fully loaded. That’s because there is a fixed energy “burden” in making the collection of iron and copper into a motor in the first place. Most of that remains the same regardless of the load on the motor, so as a percentage of total energy consumed to accomplish a task, when the motor is loaded to its maximum capability, the fixed burden represents a smaller piece of the pie. So if your load requires 7HP, using a 7.5 HP motor that is 95% efficient at nearly full load is more efficient than using a 10HP motor at 70% load, because the fixed energy used to create flux in that 10HP motor is going to be higher than the fixed energy used to create flux in that 7.5HP motor.

But the long established practice of applying a 20-25% “fudge factor” to sizing motors is based upon empirical evidence for over a century that you get maximum reliability from equipment that is not pushed to its limits all the time. A 7.5 HP motor is not called that because it ceases to deliver power above that level, it’s called that because if it is asked to deliver more than that continuously, it fails sooner. Heat x time = failure.

So a machinery OEM that wants to impress a buyer with good efficiency numbers will use the 7.5HP motor, knowing it will likely at least outlast the warranty. But an end user, recognizing that 1 hour of unscheduled down time could wipe out a year’s worth of a few percent difference in energy use, will often opt for the 10HP motor and run it for longer without problems.

 

[winnie]

oint, an AC induction motor operates at its highest efficiency nearest when to being fully loaded. That’s because there is a fixed energy “burden” in making the collection of iron and copper into a motor in the first place. Most of that remains the same regardless of the load on the motor, so as a percentage of total energy consumed to accomplish a task, when the motor is loaded to its maximum capability, the fixed burden represents a smaller piece of the pie. So if your load requires 7HP, using a 7.5 HP motor that is 95% efficient at nearly full load is more efficient than using a 10HP motor at 70% load, because the fixed energy used to create flux in that 10HP motor is going to be higher than the fixed energy used to create flux in that 7.5HP motor.

I am going to disagree _slightly_ on the above.

The point at which a motor is most efficient can be adjusted by the design. By changing the ratio of copper to iron you can change the maximum efficiency point. As you put more load on the motor you decrease losses in the iron but increase losses in the copper.

I absolutely agree that most motors are designed with their max efficiency point at or near full load.

But if you are willing to design a motor for better life and efficiency, you could put the max efficiency point down at 75% of rated load. (Though you might call the lower hp value the rated load and make the service factor higher.) At some point the ratings numbers are marketing and not physics.

-Jon

 

[Fallriverbryan]

If the motor is driving a pump, I guess it's better for you to look at the pump performance curve and start from there.... the "best efficient point". You could add about 10% fudge on power requirement of your motor calculations.

Through I do have a pump system coming up, generally motivating conveyors or mixers/augers. Would be curious where you get the 10% fudge.

Sent from my SM-G950U using Tapatalk

 

[Fallriverbryan]

In my opinion, you are up against two somewhat competing design philosophies; best practice for reliability, a concept that has prevailed for as long as the Industrial Age has been with us, vs best practice for efficiency, a push since the first 1970s “energy crisis”.

From an efficiency standpoint, an AC induction motor operates at its highest efficiency nearest when to being fully loaded. That’s because there is a fixed energy “burden” in making the collection of iron and copper into a motor in the first place. Most of that remains the same regardless of the load on the motor, so as a percentage of total energy consumed to accomplish a task, when the motor is loaded to its maximum capability, the fixed burden represents a smaller piece of the pie. So if your load requires 7HP, using a 7.5 HP motor that is 95% efficient at nearly full load is more efficient than using a 10HP motor at 70% load, because the fixed energy used to create flux in that 10HP motor is going to be higher than the fixed energy used to create flux in that 7.5HP motor.

But the long established practice of applying a 20-25% “fudge factor” to sizing motors is based upon empirical evidence for over a century that you get maximum reliability from equipment that is not pushed to its limits all the time. A 7.5 HP motor is not called that because it ceases to deliver power above that level, it’s called that because if it is asked to deliver more than that continuously, it fails sooner. Heat x time = failure.

So a machinery OEM that wants to impress a buyer with good efficiency numbers will use the 7.5HP motor, knowing it will likely at least outlast the warranty. But an end user, recognizing that 1 hour of unscheduled down time could wipe out a year’s worth of a few percent difference in energy use, will often opt for the 10HP motor and run it for longer without problems.

This makes sense to me and is what I have experienced over the years. Most times we have discussions with our motor supplier and get their recommendations for motor/equipment requirements. I really am just curious where perhaps a common line is drawn for determining fudge factors. My next line of questioning would probably involve implementing VFDs into the equation. We use them quite often for speed control. Thanks for the replies.


Sent from my SM-G950U using Tapatalk

 

[retirede]

When I sized motors for the equipment we designed, we relied on Engineering calculations and verified the results with testing. “Fudge factors” are rarely employed any longer in today’s competitive environment.

In fact, we designed the machinery to fully utilize the motor capacity.

 

[topgone]

Through I do have a pump system coming up, generally motivating conveyors or mixers/augers. Would be curious where you get the 10% fudge.

Sent from my SM-G950U using Tapatalk

My experience tells me I have to add some factor for system aberrations that could happen. If you think 10% is not enough, then it's your call. Statistics tells me things do not follow as designed, so better be on the safe side than be sorry.

 

[jumper]

When I sized motors for the equipment we designed, we relied on Engineering calculations and verified the results with testing. “Fudge factors” are rarely employed any longer in today’s competitive environment.

In fact, we designed the machinery to fully utilize the motor capacity.

Agree.

 

[topgone]

When I sized motors for the equipment we designed, we relied on Engineering calculations and verified the results with testing. “Fudge factors” are rarely employed any longer in today’s competitive environment.

In fact, we designed the machinery to fully utilize the motor capacity.

I have inherited circa 60's type installation in my other life. Honestly, the motor sizing were so underrated that the initial corrective measures done were to limit the opening of discharge valves/ dampers if only to continue operating!
Lately, been trying to correct motors that were troublesome-->always running above FLA due to wrongly installed piping!:weeping: This thing goes on and on if designers don't see the possible problems during actual operation.

 

[electrofelon]

My experience tells me I have to add some factor for system aberrations that could happen. If you think 10% is not enough, then it's your call. Statistics tells me things do not follow as designed, so better be on the safe side than be sorry.

Seems like certain things would be harder to accurately calculate required horsepower. Take a pump with a very stable head and pressure, that could be calculated very accurately. Other things with variable and/or unknown loads, perhaps not so much.

 

[drcampbell]

This question is best delegated to mechanical engineers, not electricians or electrical engineers.
The other "new" variable is efficient & economical VFDs, which have the potential to improve both longevity and energy consumption.

 

[Fallriverbryan]

Read an article from the US department of energy. It details the calculations to determine motor load which was very in depth. A summary of part is as follows.

Most electric*motors*are designed to run at 50% to 100% of rated load. Maximum*efficiency*is usually near 75% of rated load. Thus, a 10-horsepower (hp)*motor*has an acceptable load range of 5 to 10 hp; peak*efficiency*is at 7.5 hp. A*motor's efficiency tends to decrease dramatically below about 50% load.

Is this a reasonable baseline that I can refer to in your guys opinion?

Sent from my SM-G950U using Tapatalk

 

[topgone]

This question is best delegated to mechanical engineers, not electricians or electrical engineers.
The other "new" variable is efficient & economical VFDs, which have the potential to improve both longevity and energy consumption.

You could be right. But the problems after the commissioning stage will likely be handled by the electrical engineers at site. When you missed the opportunity, it can bite your bottom line.:happysad:

 

[winnie]

Most electric*motors*are designed to run at 50% to 100% of rated load. Maximum*efficiency*is usually near 75% of rated load. Thus, a 10-horsepower (hp)*motor*has an acceptable load range of 5 to 10 hp; peak*efficiency*is at 7.5 hp. A*motor's efficiency tends to decrease dramatically below about 50% load.

Is this a reasonable baseline that I can refer to in your guys opinion?

I think it might be a reasonable rough approximation, but only very rough.

Thinking about this topic, I looked through the toshiba motor catalog and picked out a few motors. The smaller motors all had their highest efficiency point at peak horsepower. But the 100 hp motor had its peak efficiency at 75%.

Most motor datasheets have a table listing speed, torque, current, and efficiency at 25,50,75,100, and 125% of full load. Like I said, I glanced at 3 or 4 different motor datasheets; I'd suggest you do the same with a larger sample to get a feel for efficiency versus % load. (Note: going through a big stack of datasheets and generating a table of this information is the sort of thing that I would be willing to do for hire, but perhaps one of the motor companies already has such a table or database that they would share.)

The efficiency curves are pretty flat above 50%, such that other factors (the mechanical design) will dominate the efficiency picture. In other words, it doesn't matter if you have a 94% efficiency motor or a 95% efficient motor if your pump system is 30% efficient when it could be 50%.

Finally, larger motors tend to be more efficient than smaller motors, so it _may_ be the case that a 10 hp motor operating at 7.5hp is more efficient than the 7.5hp motor at 100%.

-Jon