Can someone help me explain motor service factor to non-Electrician's?

[hockeyoligist2]

I am being asked to explain why we needed a more expensive motor for a piece of equipment.

Two identical machines, fully enclosed motors, no fan, 1/2 HP, FLA 1 Amp, SF 115, 480 V. A few weeks after they were installed by the contractor an operator burned his hand (blister) on one of the motors when he accidentally touched it. I checked the amps on both motors and they were pulling 1.30 amps.

I talked with engineering and asked them to have the manufacturer check them out. It was obviously under powered. Manufacturer said it is normal and the amps will drop after break in period. Engineering let it slide. The amps never dropped. A few weeks after the warranty ran out one motor finally let the smoke out. I wasn't there at the time and they replaced it with a Baldor, 1/2 HP, SF 1, it lasted 6 months.

When the Baldor let the smoke out, I bought a 3/4 HP, with a SF of 115, and replaced it. It was about $100 more than the first replacement motor. It is pulling 1 amp, barely warm to the touch.

I just got an E-mail from purchasing (non-electricians) wanting an explanation on my decision to spend the extra money. I am authorized to make that decision, however, I have to explain my decision to them. They are also asking why the original motor on the other machine is still running and the Baldor conked out so quick.

I've Googled for something to print out for them with no success. Trying to explain the difference in SF and an under powered motor to someone that doesn't know anything about motors (or anything electrical) is hard. All they know is $'s!

 

[iwire]

I am confused why is the SF is a big deal when you also increased the HP which is likely what fixed the problem.

 

[petersonra]

I am confused why is the SF is a big deal when you also increased the HP which is likely what fixed the problem.

I agree with Bob.

Think of SF as spare capacity. A 1 HP motor with a 1.15 SF is basically a 1.15 HP motor with a SF of 1.

 

[sgunsel]

Your motor is operating at 130% of rated power (Current = 1.3 amps/FLA=1.0 amps) and was overloaded even with a 115 service factor. Excess heat and premature failure should be expected.

 

[charlie b]

?Service Factor? is a measure of a motor?s ability to operate above its rated horsepower, without sustaining damage. If for example you have a 1/2 HP motor with a SF of 1.15, then you could run the motor at .575 HP (i.e., a 15% increase over rated HP), and that would not damage the motor. It would, however, shorten the life of the motor. It will cause an increase in heat generated, and in time that will wear out the winding insulation system or cause a failure in the bearings.

You did not say what current the Baldor motor was drawing, during its six month lifetime. My suspicion is that it ran at the same 1.3 amps as its predecessor. That means you were running it at a value of 30% over rated current. And since the Baldor motor had a SF of 1.0, it could not be expected to tolerate any overload for any significant amount of time. So naturally it was going to fail early. I would say that the extra money for the higher HP motor was a good investment, in that it is far more likely to run a long time without experiencing the same type of failure.

As to why the other (supposedly identical) motor continues to run without any sign of failure, I can only infer that the circumstances are not identical. Perhaps its windings were built with a larger wire size or better insulation. Perhaps the load it is driving has a lower amount of friction, or is run for a fewer number of hours per day. Perhaps the nearest ventilation fan is closer to the other motor, so it gets more cooling air. Without inspecting the installation, I cannot tell you anything. I would plan to deal with its eventual failure, and all things eventually fail, by replacing it with the larger HP motor.

 

[kwired]

Tell them it is plain and simple - the motor is undersized for the application. Unless something has changed to create more load than the application was designed for.

Also tell them the original on the second machine that is still working is likely to quit some time soon and they may as well get a larger one ready to replace it.


What is the cost difference between 1/2 and 2/3 hp? maybe 20-30 bucks. They are wasting more money than that in lost time from them and you wondering why you went with the larger motor.

Why was there not a period of time where overload tripping becoming a nuisance mentioned in the OP? Sounds like there was not proper overload protection either.

 

[sgunsel]

You didn't mention the temperature of the motor that did not fail yet. I see a lot of 0.5 HP, 1.0 FLA, 480VAC, 3 phase Baldor motors and they normally run pretty warm. But if it is pulling 1.3 amps it will likely fail sooner than later.

 

[petersonra]

You didn't mention the temperature of the motor that did not fail yet. I see a lot of 0.5 HP, 1.0 FLA, 480VAC, 3 phase Baldor motors and they normally run pretty warm.

Many times motors running at or near their design HP are quite warm to the touch. But, i have never run across one hot enough to burn someone, uncomfortable, but not damaging.

It is curious that a motor that is running at 30% over its rated current did not trip out, as another poster pointed out.

Not to denigrate a whole industry, but it seems like a lot of packaging machines are made as cheaply as possible. It would not surprise me one bit if the motor was tripping out and rather than replace the motor someone just changed the OL settings so it wouldn't trip anymore, rather than get the right sized motor.

 

[hockeyoligist2]

I am confused why is the SF is a big deal when you also increased the HP which is likely what fixed the problem.

Sorry, I left something out.

I could have bought another 1/2 HP with a SF of 115 or a 3/4 HP with a SF of 1 cheaper.

The reason I need to explain the extra money for a motor with a higher HP/SF boils down to:

Purchasing asks "Why do we need this, the manufacturer installed 1/2 HP 115 SF and one is still running, why did one fail and the other is still running?"


My answer so far is:

They are under powered and in a small building that will get up to 120 degrees or more when it is 90 degrees outside, we sometimes have temps above 100 here. With that kind of temps during the day I think the SF could come in to play. It is a lime mixer and the lime feed is not perfect so there will be times it is thicker than normal and need the extra protection.



In the long run I think it will pay off, a more efficient motor saving power, longer life. Purchasing only see's replacement costs, they don't see longer life of the motor.

 

[kwired]

...someone just changed the OL settings so it wouldn't trip anymore..

A piece of #12 solid shunted across heater elements works pretty good for this on those type of applications. On motors that draw more than 50 amps you may need to use larger conductor

 

[One-eyed Jack]

I am confused why is the SF is a big deal when you also increased the HP which is likely what fixed the problem.

I am thinking it is a big deal because OEM convinced the OP's Eng.Dept. that the 115 SF was the solution to a "temporary" overload and it would go away after"break in" ??????? They only put enough motor to get thru warranty. Buyer Beware after that. If the OP's Engineering is anything like most in house eng. I have worked with they believed the first story ;make that excuse that they heard. All else they ignore. OEM said it would work so you must be lying thru your teeth. Disregard the excess current and the 30 degree above max design temp that job site test shows. The extra money to make it a long term solution is ignored by the bean counters. They can only count beans one day at a time. Two underpowered motors that last 6 months each plus downtime cost more than the one 3/4 hp that runs for six years. One bean count per day is all they understand. You,me,and the OP know. The beaners ain't got a clue.

 

[kwired]

.. Purchasing only see's replacement costs, they don't see longer life of the motor.

Then ask them if they wish to purchase a new motor every six months to a year or every ten years or maybe even longer.

 

[kwired]

I am thinking it is a big deal because OEM convinced the OP's Eng.Dept. that the 115 SF was the solution to a "temporary" overload and it would go away after"break in" ??????? They only put enough motor to get thru warranty. Buyer Beware after that. If the OP's Engineering is anything like most in house eng. I have worked with they believed the first story ;make that excuse that they heard. All else they ignore. OEM said it would work so you must be lying thru your teeth. Disregard the excess current and the 30 degree above max design temp that job site test shows. The extra money to make it a long term solution is ignored by the bean counters. They can only count beans one day at a time. Two underpowered motors that last 6 months each plus downtime cost more than the one 3/4 hp that runs for six years. One bean count per day is all they understand. You,me,and the OP know. The beaners ain't got a clue.

It is possible this machine was designed for limited duty cycle and they are trying to run it longer than that or even continuously.

 

[hockeyoligist2]

Many times motors running at or near their design HP are quite warm to the touch. But, i have never run across one hot enough to burn someone, uncomfortable, but not damaging.

It is curious that a motor that is running at 30% over its rated current did not trip out, as another poster pointed out.

Not to denigrate a whole industry, but it seems like a lot of packaging machines are made as cheaply as possible. It would not surprise me one bit if the motor was tripping out and rather than replace the motor someone just changed the OL settings so it wouldn't trip anymore, rather than get the right sized motor.

Yep, I run in to the "cheap" often. Most machines are maxed out on HP. The setting on the overloads was 1.4 amps, Which I suspect the contractor set it that high on the installation because it was tripping.

I have also ran in to GC's ordering under powered machines so that they would trip instead of damaging things that would cost more to fix if something else went wrong before the warranty expired.

An example, A machine kept tripping out and they just kept resetting it, under warranty. After the warranty ran out, GC gone, I called the manufacturer and they said that the GC ordered it with a lower HP motor than recommended........

 

[petersonra]

I am thinking it is a big deal because OEM convinced the OP's Eng.Dept. that the 115 SF was the solution to a "temporary" overload and it would go away after"break in" ???????

A lot of times the decisions on what machine to buy are made by the buyers who only care about the price. Often times the buyers get some kind of bonus based on the amount of money they allegedly save the company.

It's not like this is something new though. The place I worked 25 years ago used to compete with another company on many of our larger projects.

We would supply a complete system that was factory assembled, tested, and then disassembled for shipping.

This other company supplied what we used to jokingly call a "kit". A bunch of parts would show up on site and have to be assembled on site by the installing contractor. Pipe would have to be cut to length, internals installed, even a lot of wiring and tubing had to be done on site. We would lose 3 or 4 jobs a year to this other company just on price, because we were normally about 10% more than their kit price.

Sometimes there was what was called an "evaluated" cost, which included the initial cost, the install, and operating cost over some period of time. We were always much lower than the other company, but even then the buyers would often buy the cheaper one. i was told more than once that the reason was that the buyers were evaluated based solely on the capital cost, and installation and operating cost was not considered.

 

[ControlFreak]

Are the bean counters really that bad down in Greenville?

All kidding aside, I find it odd that the 3/4 HP motor is only pulling 1 amp as well. What overload protection are you running?

 

[iwire]

All kidding aside, I find it odd that the 3/4 HP motor is only pulling 1 amp as well. What overload protection are you running?

It is 480 volt

 

[winnie]

If the mechanical load is the same, and the motors have similar efficiency, then the current drawn should be the same. The fact that the _overloaded_ 1/2 hp machine was drawing more current than the safely loaded 3/4 hp machine suggests that it was operating much less efficiently, or that the load has somehow reduced.

One possibility: '1 hp' doesn't just describe the continuous power output of a machine. Instead you have a whole slew of standards and expectations that are encompassed in the nominal 1hp value. Very crudely: A 1hp machine with a 1.15 SF has the starting characteristics of a 1 hp machine, but the ability to produce 1.15 hp on a continuous basis.

I wonder if the 1/2 hp motor was never actually accelerating the machine to proper running speed, and thus operating inefficiently. It may be that a special design 1/2 hp motor with better acceleration characteristics might have been sufficient for the job.

I would also wonder if the machine is being mechanically overloaded, and the motors burning out simply presage a mechanical failure...with the replacement of a larger motor the mechanical equivalent of overfusing.

-Jon

 

[Robert Martin]

Service factor

Service factor

Some good info.
SF is for intermitant loads.
Electric Motors - Service factors:
A change in NEMA standards for electric motor service factors and temperature rise has been brought about because of better insulation used on electric motors. For instance, a 1.15 service factor -- once standard for all open electric motors -- is no longer standard for electric motors above 200 hp.
Increases in electric motor temperature are measured by the resistance method in the temperature rise table. Electric motors feature a nameplate temperature rise, which is always expressed for the maximum allowable load. That is, if the electric motor has a service factor greater than unity, the nameplate temperature rise is expressed for the overload. Two Class-B insulated electric motors having 1.15 and 1.25 service factors will, therefore, each be rated for a 90?C rise. But the second electric motor will have to be larger than the first in order to dissipate the additional heat it generates at 125% load.

Electric motors feature a service factor, which indicates how much over the nameplate rating any given electric motor can be driven without overheating. NEMA Standard MGI-143 defines service factor of an ac motor as "...a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor..." In other words, multiplying the electric motor's nameplate horsepower by the service factor tells how much electric motors can be overloaded without overheating. Generally, electric motor service factors:

Handle a known overload, which is occasional.
Provide a factor of safety where the environment or service condition is not well defined, especially for general-purpose electric motors.
Obtain cooler-than-normal electric motor operation at rated load, thus lengthening insulation life.

 

[ceb58]

Just tell them its what you get for going with the lowest bidder. They were specked to the lowest quality standards. Given the load and environment that the motors are working under it is a given that the other will fail.