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Old January 3rd, 2018, 08:52 AM   #1
rupej
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Motor Service Factor Question

I have a typical motor that can be wired for 230/460 but is also rated for 208VAC operation. It has a typical 1.15 service factor. If I wire this motor to a 208V feed, can I still set the overload to FLA (@208v) x 1.15? It would seem to me that powering with 208V, which is 10% less than 230V, should effectively reduce the service factor.

Thanks in advance!
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Old January 3rd, 2018, 02:14 PM   #2
James Mcquade
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rupej,

the motor service factor is how much the motor can run above its rated current for a short time without damaging the motor.

for example, you have a motor rated at 230 volts and 2.1 amps.
the service factor means that the motor can run at 2.1 x 1.15 = 2.415 amps
for a short period of time without damaging the motor. (I made the numbers up)

as the voltage goes down, the motor current will go up, so a motor at 208 volts will require more current than a motor at 230 volts.

you must look at the motor legend for full load amps or look at the motor specs. the SF has nothing to do with the overload, you must use the contactor selection chart for motor overloads.

james
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Old January 3rd, 2018, 03:20 PM   #3
Bit_Bucket_07
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Quote:
Originally Posted by James Mcquade View Post
rupej,

the motor service factor is how much the motor can run above its rated current for a short time without damaging the motor.

for example, you have a motor rated at 230 volts and 2.1 amps.
the service factor means that the motor can run at 2.1 x 1.15 = 2.415 amps
for a short period of time without damaging the motor. (I made the numbers up)

as the voltage goes down, the motor current will go up, so a motor at 208 volts will require more current than a motor at 230 volts.

you must look at the motor legend for full load amps or look at the motor specs. the SF has nothing to do with the overload, you must use the contactor selection chart for motor overloads.

james
I agree, with one exception. The SF represents the percentage of rated HP that the motor is designed to endure temporarily, rather than the percentage of FLA that the motor can temporarily endure.

Many motors will list the SF Amperage as "SFA" on the motor nameplate.
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Old January 3rd, 2018, 05:17 PM   #4
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Quote:
Originally Posted by Bit_Bucket_07 View Post
I agree, with one exception. The SF represents the percentage of rated HP that the motor is designed to endure temporarily, rather than the percentage of FLA that the motor can temporarily endure.

Many motors will list the SF Amperage as "SFA" on the motor nameplate.
I am under the understanding Service Factor refers to Amps, not HP. Because a motor could be run indefinitely at double its rated amps and half its rated voltage without exceeding the SF rating, if it applies to HP. This is potentially a very real scenario if we are talking about VFD applications. And if it's a VFD application running 200% FLA @ 50% nameplate voltage, that means it's probably running half-speed and half cooling. Doubling the amps while halving the cooling doesn't sound kosher. So I'm going to go with amps unless someone can explain why I'm wrong.
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Old January 3rd, 2018, 05:42 PM   #5
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Quote:
Originally Posted by strantor View Post
I am under the understanding Service Factor refers to Amps, not HP.
That's a common misunderstanding. That's why I responded to James Mcquade's post. SF refers to HP.
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Old January 3rd, 2018, 07:26 PM   #6
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Quote:
Originally Posted by strantor View Post
... Because a motor could be run indefinitely at double its rated amps and half its rated voltage without exceeding the SF rating, if it applies to HP.
I don't know where you came up with this, but you are dead wrong...

Understand that if you RECONNECT a 460V motor as 230V, yes, the current in each phase doubles, but so do the NUMBER OF COILS used in parallel within the motor, so the current on EACH WINDING remains the same.
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Old January 3rd, 2018, 08:47 PM   #7
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Quote:
Originally Posted by jraef View Post
I don't know where you came up with this, but you are dead wrong...

Understand that if you RECONNECT a 460V motor as 230V, yes, the current in each phase doubles, but so do the NUMBER OF COILS used in parallel within the motor, so the current on EACH WINDING remains the same.
The section you quoted leads me to believe you interpret me as condoning running a motor at half it's voltage and double its current. That is NOT what I'm saying, and I'm not talking about reconnecting it for 230V.

What I am saying, is that (efficiency and 3ph equations disregarded) If you ran a 4.8kw/480V/10A motor with a 1.25 sf at 4.8kw/240V/20A (still wired for 480V), it would burn up despite being ran below the the specified service factor. Therefore service factor can only apply to amps, not HP/Kw.
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Old January 3rd, 2018, 09:09 PM   #8
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^ I agree, and what drives or overloads for that matter calculate the HP and trip based on that? None that I regularly use, so why not just go by FLA x SF?
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Old January 3rd, 2018, 10:21 PM   #9
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Quote:
Originally Posted by rupej View Post
^ I agree, and what drives or overloads for that matter calculate the HP and trip based on that? None that I regularly use, so why not just go by FLA x SF?
I certainly didn't establish the standard, but I am aware of what the standard is.
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Old January 3rd, 2018, 10:53 PM   #10
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Originally Posted by Bit_Bucket_07 View Post
I certainly didn't establish the standard, but I am aware of what the standard is.
Per whatever standard you're referencing, does the service factor applying to HP (not current), only hold true when voltage is equal to nameplate value?
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Old January 3rd, 2018, 11:41 PM   #11
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Originally Posted by strantor View Post
Per whatever standard you're referencing, does the service factor applying to HP (not current), only hold true when voltage is equal to nameplate value?
Never mind, I answered my own question. You must be referencing this standard?

Quote:
The National Electrical Manufacturers Association (NEMA) defines service factor in section MG1 – 1.43 of their manual as: “The service factor of an alternating current (AC) motor is 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.” The conditions under which service factor may be applied are described in NEMA MG1 – 14.36 as: “When the voltage and frequency are maintained at the value specified on the motor’s nameplate, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate.”
So in order for service factor to apply, V and F have to match nameplate. They're constants.
P=IE.
If E is a constant, then there is only one variable which can affect P, and that is I. Current.
So your argument is nothing but semantics. Service Factor is based on Current (at nameplate voltage), and therefore HP (at nameplate voltage). And the way you broadly stated it to be based on HP alone (no mention of nameplate voltage) makes the argument less than true.

Last edited by strantor; January 3rd, 2018 at 11:47 PM.
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Old January 4th, 2018, 11:40 AM   #12
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Quote:
Originally Posted by strantor View Post
Never mind, I answered my own question. You must be referencing this standard?



So in order for service factor to apply, V and F have to match nameplate. They're constants.
P=IE.
If E is a constant, then there is only one variable which can affect P, and that is I. Current.
So your argument is nothing but semantics. Service Factor is based on Current (at nameplate voltage), and therefore HP (at nameplate voltage). And the way you broadly stated it to be based on HP alone (no mention of nameplate voltage) makes the argument less than true.
I haven't made an argument. The Service Factor standard is what it is. You should not apply the SF for a motor that is designed to run on a variable frequency drive, nor should you apply the SF for a motor that is designed to operate in a very high altitude location, due to lower density ambient air providing less efficient cooling.

That said, the SF relates to HP, rather than to motor amperage. Of course, horsepower and amperage are inherently intertwined, but you will calculate an invalid maximum amperage value for a motor by applying the SF to amperage, rather than to HP.
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Old January 3rd, 2018, 04:06 PM   #13
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Motors can also be ordered to run in SF for extended periods.
We order our cooling fan motors that way...... with even a 1.25 SF.
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Old January 3rd, 2018, 04:13 PM   #14
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Motors can also be ordered to run in SF for extended periods.
We order our cooling fan motors that way...... with even a 1.25 SF.
Honestly, if a motor is not operating in harsh conditions or at a very high ambient temperature, it can operate indefinitely at SF. You'll decrease the lifespan of the motor winding insulation by doing so, but it's not as if motors don't eventually wear out anyway.
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Old January 3rd, 2018, 04:25 PM   #15
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Quote:
Originally Posted by Bit_Bucket_07 View Post
Honestly, if a motor is not operating in harsh conditions or at a very high ambient temperature, it can operate indefinitely at SF. You'll decrease the lifespan of the motor winding insulation by doing so, but it's not as if motors don't eventually wear out anyway.
We get our motors wound for higher temp and VFD (if we use VFD or not).
Our motors approach 300 plus degrees for about 30 minutes while they are running, due to the fact they are enclosed in a "cooling can" and only have Argon gas as atmosphere.
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