# Motor Service Factor



## Black Dog (Oct 16, 2011)

atjm88 said:


> I received 2 comparison spec from supplier.
> *Motor A*
> Motor: 5.5KW 415V 3P 50HZ 4 Poles
> Ratio: 75.63
> ...


*"Service Factor" — What's It All About?*

Most motor users are familiar with the concept of "service factor." It's been around for generations. The usual service factor for motors above 1 through 200 horsepower is 1.15 (per NEMA Standard MG1).
In NEMA Standard MG1-1.42 we find this definition: "The service factor of an 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..." Those "conditions," include a winding temperature rise at the service factor horsepower that exceeds the rated insulation system temperature by 10° C.
Thus, at 115% of rated horsepower, a motor can be expected to run 10° hotter than the standard limit applicable to rated load. A useful rule of thumb says that 10° higher temperature cuts insulation thermal life in half. That means a motor running continuously at the "permissible" service factor overload will have greatly reduced life.
What value, then, does a service factor have? It is not intended to simply allow an extra 15% load continuously. Rather, a service factor is meant to provide for occasional, sporadic, intermittent overload caused by variations in plant operation. These typically include process disturbances, such as random variations in the density of material being processed by industrial machinery. The service factor allows the motor to "ride through" such temporary conditions.
Many operators will also use a service factor to compensate for low or unbalanced line voltage; for harmonics in the voltage waveform; for seasonal ambient temperature swings; for temporary ventilation restriction caused by dirt buildup; or to allow for an increased power demand caused by process expansion or a higher production rate. But long-term overheating to the service factor temperature rating — 10° C above the design temperature of the insulation — will reduce winding life. If the motor is operating in a low ambient temperature (below the standard 40° C), that may not happen.
In any event, use a service factor with great care. Its existence will not increase available motor torque, nor permit more frequent or more severe starting.
If, say, a 45 hp load is to be supplied, a 40 hp motor having a 1.15 service factor will do the job. But the next larger rating (50 hp) without a service factor is normally the better choice. It offers longer life, higher torque, and probably a slightly higher efficiency at the actual load.


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## MDShunk (Jan 7, 2007)

Service factor is the safety net engineers use to compliantly overload their motors when they are a little unsure of the exact math for the requirements of the driven unit. For instance, in your case of this gear motor, this might be a tank agitator or stirrer of some sort. The motor horsepower might be fine for, say liquids with a density near water, but maybe the process sometimes changes and you're now going to agitate something thicker, like wallpaper paste, maybe. That's where the service factor will come in handy. I haven't found that you "pay" for service factor on the front end, so if the prices for both gearmotors you showed are near the same, go with the higher service factor. The higher the service factor, generally, the more durable the windings.


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## MDShunk (Jan 7, 2007)

atjm88 said:


> And why both Output Speed are the same 19RPM, but the ratio is different from both (75.63 and 76.39)? As I know, they should be the same as 4 poles system having around 1500RPM divide with 19RPM output, should get the same ratio right? How to get this different ratio as shown by supplier? And which info should I refer to if I want to get higher strength of Motor? Is that ratio or output speed?
> 
> Thanks.


They show different ratios because the two manufacturers undoubtedly use different combinations of gears inside the gear end of their gearmotors. The math on both still rounds to 19 rpm output from each. It appears they've used 1425 RPM as the basis for their math. 

Higher strength? The torque, without a doubt, although they're both near on the same. One is 30 nm (about 20 foot pounds) more than the other. The specs on both, save for the service factor, as essentially identical. Nothing that you should let rent space in your brain. 

If you gave me the specs on both of these, I'd declare them same in like kind and quality, ask you which one is cheaper, and tell you to buy that one.

A note on gear motors: The stator is generally the field replaceable part that you replace when the motor "burns out". If you already have gear motors deployed, stick with the same brand if you can. That causes your supply of replacement parts (the stator) to be smaller. For what it's worth, I like SEW when it comes to gearmotors.


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## atjm88 (Feb 27, 2015)

MDShunk said:


> Service factor is the safety net engineers use to compliantly overload their motors when they are a little unsure of the exact math for the requirements of the driven unit. For instance, in your case of this gear motor, this might be a tank agitator or stirrer of some sort. The motor horsepower might be fine for, say liquids with a density near water, but maybe the process sometimes changes and you're now going to agitate something thicker, like wallpaper paste, maybe. That's where the service factor will come in handy. I haven't found that you "pay" for service factor on the front end, so if the prices for both gearmotors you showed are near the same, go with the higher service factor. The higher the service factor, generally, the more durable the windings.


You are right, it's some kind of tank agitator to stir the glue to ensure that the glue will not be harden.

I'm interested to know from which spec you can know it's for tank agitator? Is that because of the higher torque? Normally how much torque consider as a minimum requirement for a good tank agitator?

Thanks.


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## atjm88 (Feb 27, 2015)

MDShunk said:


> They show different ratios because the two manufacturers undoubtedly use different combinations of gears inside the gear end of their gearmotors. The math on both still rounds to 19 rpm output from each. It appears they've used 1425 RPM as the basis for their math.
> 
> Higher strength? The torque, without a doubt, although they're both near on the same. One is 30 nm (about 20 foot pounds) more than the other. The specs on both, save for the service factor, as essentially identical. Nothing that you should let rent space in your brain.
> 
> ...



Both are from the same manufacturer which is SEW.

It's SEW Parallel Helical Geared Units.

The price for both is around 1000USD different.

Shaft size for Motor A (70mm Diameter) is bigger than Motor B (60mm Diameter).

I wonder if the Shaft Size is the major cause for the price to differentiate this much instead of the Service Factor?

Extra inquiry: How the maker design the Service Factor? Is that the winding inside the motor? Or the Gear Box? 

Thanks


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## MDShunk (Jan 7, 2007)

atjm88 said:


> You are right, it's some kind of tank agitator to stir the glue to ensure that the glue will not be harden.
> 
> I'm interested to know from which spec you can know it's for tank agitator? Is that because of the higher torque? Normally how much torque consider as a minimum requirement for a good tank agitator?
> 
> Thanks.


How did I guess that? The low RPM. How much torque totally depends on what you're stirring, so I didn't guess off that. Could be water or it could be tar. :laughing: There's a pretty decent horsepower/torque chart SEW has out for how fast you want to agitate fluids of various centipoise viscosity with various mixer, paddle, and scraper designs. That goes out the window, a little bit, when you're dealing with non-newtonian fluids like glues, but they're close enough in my experience.


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## MDShunk (Jan 7, 2007)

atjm88 said:


> Both are from the same manufacturer which is SEW.
> 
> It's SEW Parallel Helical Geared Units.
> 
> ...


1000 bucks different? wow. I might guess that's because the one shaft size is standard fare, and the other shaft size is special order? 

How is the design different? Mostly I think it's just higher temp varnish on the windings, but I'm no expert. Has nothing to do with the gearbox.


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## JRaef (Mar 23, 2009)

Interesting that they show a Service Factor at all. SEW is German. There in no such thing as Service Factor with IEC motor specs, SF is strictly a NEMA concept, so it's likely something they essentially made up just for marketing here in NA. No telling what that really means. 

The RATIO difference with the exact same shaft speed of 19RPM would be a difference in the gearbox ratio. Notice that the motor shaft speeds are different. Motor A has higher slip; 8.7% vs 7.7%. So in order for the final shaft speed to be identical, the gear ratio MUST be slightly different.

For $1000 difference though, there is probably more to it than that. Most likely the more expensive one is rated for higher efficiency. There are many ways to increase efficiency, but generally they will use better methods of winding the coils so they are more tightly packed, there will be better grades of steel with different magnetic qualities, and the air gap will be smaller, which means the bearings must be more precise. All of those things cost money.


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## atjm88 (Feb 27, 2015)

MDShunk said:


> How did I guess that? The low RPM. How much torque totally depends on what you're stirring, so I didn't guess off that. Could be water or it could be tar. :laughing: There's a pretty decent horsepower/torque chart SEW has out for how fast you want to agitate fluids of various centipoise viscosity with various mixer, paddle, and scraper designs. That goes out the window, a little bit, when you're dealing with non-newtonian fluids like glues, but they're close enough in my experience.



Thanks. If you don't mine, may I know where to get the SEW Chart as mentioned? I can't find it online.


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## atjm88 (Feb 27, 2015)

JRaef said:


> Interesting that they show a Service Factor at all. SEW is German. There in no such thing as Service Factor with IEC motor specs, SF is strictly a NEMA concept, so it's likely something they essentially made up just for marketing here in NA. No telling what that really means.
> 
> The RATIO difference with the exact same shaft speed of 19RPM would be a difference in the gearbox ratio. Notice that the motor shaft speeds are different. Motor A has higher slip; 8.7% vs 7.7%. So in order for the final shaft speed to be identical, the gear ratio MUST be slightly different.
> 
> For $1000 difference though, there is probably more to it than that. Most likely the more expensive one is rated for higher efficiency. There are many ways to increase efficiency, but generally they will use better methods of winding the coils so they are more tightly packed, there will be better grades of steel with different magnetic qualities, and the air gap will be smaller, which means the bearings must be more precise. All of those things cost money.


At first I only choose the higher torque which is around 1k Nm and rpm is 30+ and SF less than 1. Then the supplier told me to see on the Service Factor. If it's more than 1, then that should be a better choice to stir the Glue solution in a tank which he suggested as the 1st posts. I'm not sure if Service Factor is quite important to consider too.


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## Electric_Light (Apr 6, 2010)

10 hp 1.1 SF = you can load it 11 hp all day long. 
10 hp 1.6 SF = you can load it to 16 hp with no damage all day. 

I read somewhere that they 10 hp 1.6SF and something like 15 hp 1.1SF are essentially identical. 

In the US, 4 poles are commonly 1,725 RPM, so multiplying by 5/6, I get 1,437.
/75.63 gets me 19.000. /76.39 gets 18.811. But, you only showed two digits, so if they were rounded to nearest 1RPM, they're both 19RPM. 

So, the difference could be the slip, or rounding. If they would've showed 19.00, then we'd know for sure, but with just "19", I don't know. 

Starting torque is important, and it's shown as the % of full load torque. 
Also check if number of starts allowed per hour is consistent with your production need.

SF = can the bus keep up with the mountain roads for hours without overheating? 
starting torque = can it get going if it stops on a steep hill? 

starts / hour: tour bus type service or city bus type service?


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## telsa (May 22, 2015)

*$1000 means SOMETHING*

If you poke around you might find that the pricey unit is wired to be reversed -- and has an exotic plug that brings the magic leads out.

Many gear-motors don't need the reversal -- and many do.

You can only imagine the ADDITIONAL torque stress on a gear-motor when it has to reverse direction -- and it's stuck in goop.

19RPM must be more popular than I thought. I have no less than three of 'em in my garage -- right now... and they are reversible, too.

Not only are they used for blending, they were commonly used for dinky conveyor belt systems. [ Mine ]

The OTHER price adder would be if one is explosion proof/ food grade (wash down)/ TEFC vs drip proof, etc.

You pay up if the gear set is rated to be flipped on its axis -- as a blender -- vs sitting horizontal propelling a conveyor. Some gear-motors can't tolerate a vertical shaft position.

It'll all be in the fine print. 

BTW, in normal practice, these are bought in pairs/ triples etc. They are NOT the kind of item that you can purchase -- at ANY price -- on short notice. So every sane buyer purchases at least one spare.

In many batch mixers, two gear-motors -- with paddles are used. The prospect of an unstirred batch stirs the purchase of redundancy.

If you are cost sensitive, then you can't play the game. 

Forget about the price -- make sure that the gadget TOTALLY meets your process requirement. The wrong buy is like short feeder conductors: scrap to you. 

The latest rage in gear-motors is feed-back -- typically slapped on, encoder style, so that a SCADA set-up can monitor the actual shaft speed. For many processes, this is an important metric.

You may find that you really have to kick this decision around the table.


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## bjjohns (Jun 10, 2015)

*SEW Info*



atjm88 said:


> Thanks. If you don't mine, may I know where to get the SEW Chart as mentioned? I can't find it online.


I think he might be referring to this, it's not a chart so much as it is a design & spec manual.
http://download.sew-eurodrive.com/download/pdf/11509031_Rev1.pdf


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