HIGHER SERVICE FACTOR OF A MOTOR

[virata]

Hi guys,

I'd like to know what is the impact of having a 100-hp motor of 1.25 service factor. In our motor data sheet it is 1.15 SF. Is 1.25 SF safe to use and acceptable? Kindly let me know your thoughts.

Thanks,
V.

 

[waross]

Yes.
A service factor of 1.25% will allow an overload of 25%.
A service factor of 1.15% will allow an overload of 15%.
A service factor of 1.0% will not allow an overload.

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

 

[melspuds]

virata,
You might have to change the overloads if the motor is working in overload conditions.
Dave

 

[virata]

Thank you Warros.

Dave, no overloads here I believe.

 

[podobing]

V,
I think that a higher service factor indicates that the motor is mechanically able to run at 115% or 125% of nameplate horsepower. That would mean that the bearings and shaft are properly sized to operate at these horsepower levels without any adverse effects on their life. Aside from possibly setting the motors overloads, and possibly re-sizing the cables supplying power to the motor, the next biggest concern is the additional heat generated by the motor. Nameplate information such as rated HP is based on "x" degrees temperature rise in "y" degree ambient temperature. Operating in the service factor region will cause an increase in operating temperature.
Dave

 

[virata]

Thank you podobing!

 

[petronila]

Hi Virata

Just take account the Service Factor would works like accepted one overload percent if the motor is working in normal services conditions (rated voltage and frequency and max. altitude), other wise the motor will be receiving overload.

When the motor is overloaded will drain much current and much current minds much temperature, if the temperature increase the winding and bearing life will be shortenen, also note that more temperature is more looses so efficiency drops.

Best regards

Carlos

 

[virata]

All,

I understand the concern on overloads and all that. The vendor had actually sized the motor large enough to not get in to the service factor.

I just want to add that our project spec says that at 1.0hp and up the SF shall be 1.15. And what i just wanted to know if the SF of 1.25 is acceptable to Electrical's point of view. From what you guys are saying, having a higher SF is much safer and thus technically acceptable. Correct me if I'm wrong.

Thanks!

 

[waross]

When a motor is replaced with a motor of greater service factor, it should run at about the same current. It will be able to a greater degree of overload than the original motor.
However there may be Code issues.
Under many codes the conductors supplying a motor must be sized based on the rated current of the motor.
If you go from SF 1.15 to SF 1.25 the rated current may be greater. You should check that your supply conductors are large enough to meet code based on the rated current of the replacement motor.
If the efficiency and power factor of the replacement motor are similar to the original motor the existing overload settings are probably adequate.
However, under many codes, you will have the option of increasing the overload settings so that the motor may "ride through" a greater overload than the original motor.


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

 

[petronila]

Hi Virata

You are right, oversized motors keeps the system designer out of trouble but you the end user most waist an amount of energy and it is added costs, among others, the oversized motors motors will have problems like a high Inrush current and needs big motor protection (booth costs are very sensible when we speak about large motors).

M.O. is the vendor most provide one motor based in project calculation, so the vendor will be complete sure of the power that the application needs and don´t use an oversized motor.

Your Idea is good due to the use of one rightsized motor with any S.F will support eventual overloads ( if the overload is bigger the motor protection most act in order to avoid a damage).

Regards

Carlos

 

[panter]

And it always appears same question .
What is the difference between motor 100 kW with service factor 1.25 and 125 kW electric motor with service factor 1.0 ??

 

[waross]

panter: The difference is probably availability. It may be difficult to locate a 100 HP motor with a 1.25 service factor.

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

 

[lukin1977]

panter (28/Nov/15): I think that the difference is that the 100kW SF 1.25 can deliver 125kW only for a certain period of time and the 125kW SF 1.0 can continuously deliver 125kW

 

[zlatkodo]

I think the real question could be: Is it possible to convert a motor with SF = 1 to higher SF?
If an answer is: yes, then HOW?
BTW, each motor , even also the motor with SF=1 , can stand the overload for a while.
The price in any case is a higher working temperature and reduced life span.
That's why the term "SF" could be closely related to commercial rather then technical reasons.

 

[jraef]

Just because a Service Factor is there, does NOT mean you must use it. You do NOT need to increase the OL protection for it, there is NOTHING wrong with setting the OL for a 1.0 SF on a motor with 1.15 or 1.25. Your motor will just last longer and be more tolerant of line voltage swings.

Bottom line per the question asked; there is no problem using a motor with a HIGHER Service Factor than you require, just not a lower one (and only IF the OEM decided to utilize the SF rather than up-size the motor.) The one possible caveat might be that the 1.25SF motor efficiency may end up being slightly lower than that of a similar motor rated as 1.0 SF, but the nameplate will tell you that.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Many Baldor motors in that size range are standard with 1.15 SF, but if the motor is used in some hazardous locations the SF is de-rated to 1.0 WHEN THE MOTOR IS FED FROM A VFD. There is no de-rating if the motor is fed a sine wave.
When the argument/discussion concerning 100 HP motors with 1.25 SF is over, try to buy a 100 HP motor with a 1.25 SF. There are not a lot available.

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