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Old January 3rd, 2018, 07:52 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, 01:14 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, 02:20 PM   #3
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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, 03:06 PM   #4
NetNathan
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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, 03:13 PM   #5
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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, 03:25 PM   #6
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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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Old January 3rd, 2018, 04:07 PM   #7
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There is a very good explanation of this issue in a White paper issued by (the old pre-ABB) Baldor written by Edward Cowern, a staff PE there. The collection of his works are called "the Cowern papers" and are considered the "gold standard" for describing what motor mfrs do to address issues like this. Here is an on-line copy:
http://www.baldor.com/Shared/manuals/pr2525.pdf

On page 60 of that pdf, he discusses the issue of applying 208V to 230V motors and what most motor mfrs do to accommodate this.

He addresses Service factor in several places and confirms that SF is an increase in HP, not necessarily FLA (although that would likely follow), but the one most relevant to this discussion is on page 14. As I interpret this (and I think most motor suppliers would agree), if the motor is actually showing 208-230/460V on the nameplate, and says a HP value with a 1.15SF, you can operate it into the SF at 208V just as the nameplate implies. But... expect a less than maximum lifespan out of that motor.

I always council end users to not use SF, it is the "fudge factor" you want in order to ensure uninterrupted production. Any money you save on an under rated / over stressed motor goes right out the window the very first time you get unexpected down time.

The only people who regularly use SF are OEMs who want to avoid the cost of a larger motor, knowing that the smaller one run into its SF will out last the warranty, which is all they care about. That decision by the way is at the EXPENSE of that future unexpected down time suffered by the end user.
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Old January 3rd, 2018, 04:17 PM   #8
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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, 04:19 PM   #9
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Quote:
Originally Posted by jraef View Post
The only people who regularly use SF are OEMs who want to avoid the cost of a larger motor, knowing that the smaller one run into its SF will out last the warranty, which is all they care about. That decision by the way is at the EXPENSE of that future unexpected down time suffered by the end user.
Please understand this is NOT why we buy our motors this way and our motor are made to run for extended periods of time at 1.25 SF.

The customer wants to fastest cooling they can get on some parts, but a lot of other parts do not require that fast of cooling time.
Because it is a cooling fan motor, the hotter the cooling gas is the less load on the fan blade, but as furnace cools, the load increases, this is where we go into SF.
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Last edited by NetNathan; January 3rd, 2018 at 04:35 PM.
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Old January 3rd, 2018, 04:42 PM   #10
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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, 06:26 PM   #11
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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, 07:47 PM   #12
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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, 08:09 PM   #13
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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, 09:21 PM   #14
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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, 09:53 PM   #15
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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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