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Service Factor Of Motor


AB2005

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Dear All,

 

I understand about the service factor of motors. But here is some confusion i.e. 1.15 SF means that a typical motor can run continuously at 115% power. For example, a 10KW motor can run at 11.5KW without any trouble. Why we don't consider this motor as 11.5KW? A 10KW motor with 1.00 service factor can’t run continuously at 11.5KW.

 

"Don't assume any thing, always check/ask and clear yourself".

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First off, you must understand that Service Factor is a term that only applies to NEMA designed motors. So if you are looking at a kW rated motor, it was actually designed around NEMA specifications for the North American marketplace. NEMA specifications are (now) very clear as to what Service Factor means. Using the Service Factor is supposed to be a temporary thing, NOT continuous. The salient point for your question is that when a motor is run continuously at it's Service Factor rating, the motor life will be 1/2 of what you can expect if run at it's normal rating. That is why they don't just re-list it as a 11.5kW motor. Unfortunately a lot of OEM equipment manufacturers take advantage of SF ratings because they only care that the equipment lasts out its warranty period, but end users should NOT use it continuously IMHO.
"He's not dead, he's just pinin' for the fjords!"
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Using the Service Factor is supposed to be a temporary thing, NOT continuous. The salient point for your question is that when a motor is run continuously at it's Service Factor rating, the motor life will be 1/2 of what you can expect if run at it's normal rating. That is why they don't just re-list it as a 11.5kW motor.

Dear Mr. Jraef,

 

Thanks for your reply. Its mean that the RPM of a motor, which SF is 1.15, would not dropped out if we increase its load close to 115% for a short time. But if we exceed the load on a motor up to 115% which SF is 1.00, its RPM would be dropped.

 

 

 

"Don't assume any thing, always check/ask and clear yourself".

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No, it doesn't affect the RPM, just the loading. When you increase the load on an AC induction motor, you may briefly lose RPM, but that increases slip, which causes the motor to pull more current to regain the designed slip. More current always means more heat. So if you use a motor rated at 1.0SF, and you put a load on it that is 115% of rated, the motor will overheat and become damaged very quickly. If, on the same load, that motor were rated 1.15SF, the thermal damage would take years to manifest. Either way the RPM will remain the same.
"He's not dead, he's just pinin' for the fjords!"
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JRaef

 

Any time you increase the load on an asynchronous motor, the slip also increases.

(Remember that old Frank Sinatra song...... "Love and Marriage" ?... line from song says ...

(Ya can't have one without the oth-er.)

Likewise, when you operate a motor at it's 1.15 S.F. rating,

the actual shaft rpm will be slightly less than it would be if operated at it's 1.0 S.F.

 

Consider a 4-pole motor that only has a 1.0 S.F. and is rated 10 HP, 460/3/60, 14 amps, and 1765 rpm.

As you increase the loading on this motor beyond 100%, the current goes up, the torque output goes up, and the RPM drops accordingly, the efficiency drops a tiny amount ... the Power Factor goes up, and the Power delivered to the shaft increases ..... True, the motor is "overloaded" according to its design limitiations, and you cannot operate overloaded forever without damaging the motor (as you can with a S.F. rated motor).

 

The same laws of physics apply to S.F. rated and non-S.F. rated motors alike; NEMA and IEC alike.

 

And to expand upon your point, motors rated with a 1.0/1.15 S.F. are typically rated Class B temperature rise at 1.0 S.F. and Class F temperature rise at 1.15 S.F.

 

Even at that, Jeff, it is not unusual to find that such motors are wound with Class H insulation .... The point being, that even if the user runs the motor continuously at 1.15 S.F. .... the life of the motor will not be seriously impacted (when insulation is Class H).

 

Cheers,

 

 

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Yes, I see your point. IF it were to pull back into its original slip speed, it would no longer need to draw more current to try to get back to it, which could only happen if the load were no longer excessive, which means the current would also drop.

 

My thinking was more along the lines of a drastic RPM change, but you're right, any increase in load above rated would result in a sligh drop in RPM, it would have to. Thanks for the correction.

"He's not dead, he's just pinin' for the fjords!"
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