motor service factor

[david90]

Why does a 3 phase AC motor need an HP rating and a SF HP rating? Why not replace the HP rating with the SF HP rating since the SF HP rating is the maximum permissible load that the motor can operate at?

 

[DamianInRochester]

http://www.engineeringtoolbox.com/service-factor-d_735.html

 

[Lancie1]

Why does a 3 phase AC motor need an HP rating and a SF HP rating? Why not replace the HP rating with the SF HP rating since the SF HP rating is the maximum permissible load that the motor can operate at?

David90,
Your question implies a view of the world where everything is black or white, with no shades of gray. What I mean that in my many years of engineering experience, I have come to believe that many devices in the real world cannot be easily divided into nice neat categories. That is true of motors. How much horsepower any one motor can produce is dependent on how long the motor will run without burning out the winding insulation. In other words, a 2 horsepower motor will not suddenly stop running if the load jumps up to 2.1 horsepower. That 2 HP motor might easily produce 3 HP - for a few minutes! It is not a cut-and-dried number, so therefore the Service Factor ratings were developed to measure the amount of reduction in life if the basic horsepower rating is exceeded.

Therefore, if someone wants a motor that can be overloaded by 15% and still not have its life-time reduced, then they need a 1.15 SF motor in the same HP rating.

 

[david90]

David90,
Your question implies a view of the world where everything is black or white, with no shades of gray. What I mean that in my many years of engineering experience, I have come to believe that many devices in the real world cannot be easily divided into nice neat categories. That is true of motors. How much horsepower any one motor can produce is dependent on how long the motor will run without burning out the winding insulation. In other words, a 2 horsepower motor will not suddenly stop running if the load jumps up to 2.1 horsepower. That 2 HP motor might easily produce 3 HP - for a few minutes! It is not a cut-and-dried number, so therefore the Service Factor ratings were developed to measure the amount of reduction in life if the basic horsepower rating is exceeded.

Therefore, if someone wants a motor that can be overloaded by 15% and still not have its life-time reduced, then they need a 1.15 SF motor in the same HP rating.

Why even bother with SF HP rating at all since it's safer to design using a motor's HP rating. I am right?

 

[leitmotif]

Why even bother with SF HP rating at all since it's safer to design using a motor's HP rating. I am right?

NOPE.
The HP rating is for the HP motor can deliver at 100% load. The SF rating is the allowable overload (sometimes listed on the nameplate with the time factor). If I recall correct unless it is stated on nameplate the Service Factor can be assumed for a 15 minute period.

This is the same type of number you see for duty factor on welding equipment. At a DF of 50% you can run full output 50% of time 25% maybe full output 75% of time etc etc.

Moral of story
KNOW THY LOAD
and choose the motor that can meet the demands of the load. It is often a good idea to oversize your motor selection by 10%. It does not hurt them at all to run them at 90% load
but you may damage insulation at 110% load if run for extended periods as a standard practice.

Dan Bentler

 

[DickDV]

Oh Boy! Service Factor! There is probably nothing else that has stirred so many false claims over the years. Some of them appear in the posts above.

To get the record and the understanding straight, consider this. Service factor is simply the amount of extra cooling capacity build into the motor at full load. So, a motor with a 1.0 service factor is right at it's insulation temperature limit at full load. A motor with a 1.15 service factor has 15% extra cooling capacity at full load. That's it! Nothing else.

Now, that extra cooling can be used up in a number of ways. Note that the extra cooling is a continuous rating. There is no time limit since the extra is always there.

Service factor is used most commonly to overload the motor. Fifteen percent overload is about 15% extra heat so the service factor is used up. Note that the overload can go on forever without exceeding the temperature limit of the insulation class.

Other uses of service factor are placement of the motor in an environment which is hotter than 40 degrees C. Or, the motor can be utilized above its rated level of 3300ft elevation. In all of the above cases, the extra cooling provided by the service factor is used up.

The use of the service factor is not without its disadvantages as the motor life is significantly shortened. Good design does not deliberately use the service factor but sizes the motor for full load or less operation. Service factor should be reserved for those situations that arise unexpectedly of for special situations like emergencies.

 

[jstolaruk]

Thanks DickDV for clearing that up; that's the way I understood it and was beginning to wonder if I had been missing something.

 

[Lancie1]

A motor with a 1.15 service factor has 15% extra cooling capacity at full load. That's it! Nothing else.

Yes, but the effect for the user is what? Does the typical motor user care about the cooling capacity, or about the extra power that the motor can produce due to the extra cooling?

Think of it like going to Burger King. Do you want regular, or do you want it super-sized? It is still basically the same product.

 

[Tom Jenkins]

Yes, but the effect for the user is what? Does the typical motor user care about the cooling capacity, or about the extra power that the motor can produce due to the extra cooling?

Think of it like going to Burger King. Do you want regular, or do you want it super-sized? It is still basically the same product.

The key to service factor is in the last line of Dick's post. You never design to use the service factor - it's there for things you don't know about, like any other safety factor. For example, a fan will draw more hp if the air is cold, all other things being equal, but an HVAC contractor is likely to pick the fan and motor off a standard curve for 68 F (20 C). Assuming the fan is indoors and drawing outdoor air, the service factor lets the owner avoid cooking his motor on a cold January day. It will stay cold for more than 15 minutes, so it's good that the SF is continuously available. On the other hand, had the contractor gone cheap and used the service factor in picking the motor, the owner would be doing a rewind!

 

[HarleyHetz]

In my plant, it is OFTEN over 110 deg F with 95% humidity. So, the end effect for this user is this, if I have a service factor of 1.15 on my motor, typical motor insulation is rated at 40C or 104F. This means that my motor insulation is rated for 104F * 1.15 = 119F. Doesn't leave much additional room for cooling, hence I had better not overload it much at all!! In fact, as suggested here already, if I want it to last, I would be better off if I upsize it a bit. So, if the load indicates that I should use a .75HP motor, I'll up it to 1 horse, and get the 1.15SF to boot!!

 

[DickDV]

Lancie, if you have an application for a motor that never encounters longterm overload, longterm overtemp, or any other unexpected similar disturbance, then, whether you buy a 1.0, 1.15, or 1.40 service factor makes no difference. It will be of no benefit to overspecify the motor.

Interestingly, metric IEC motors never have a service factor other than 1.0. Service factor is a uniguely NEMA thing.

 

[Lancie1]

Lancie, if you have an application for a motor that never encounters longterm overload, longterm overtemp, or any other unexpected similar disturbance, then, whether you buy a 1.0, 1.15, or 1.40 service factor makes no difference. It will be of no benefit to overspecify the motor.

Thank, Dick. I know that, and have used 1.0 in most cases, and occasionly 1.15 if there is a chance of an overloaded conveyor or some other type of unexpected event. In fertilizer plants, they like 1.15 because some humid days can cause the fertilizer to set up like concrete on conveyors and chutes and it takes more power to move it. I don't think I have ever used a 1.4 SF.