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Thread: What is the point of service factor?

  1. #11
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    Quote Originally Posted by Jraef View Post
    Bingo. I'm not going to bother to look it up, but the official NEMA definition of motor "service factor" states something to the effect that if you use the SF, you can expect that the performance specs will NOT be as stated on the nameplate,
    Are they even stated on the nameplate?
    Si hoc legere scis nimium eruditionis habes.

  2. #12
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  3. #13
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    OK, I found my copy of NEMA MG-1 from 1998, here's what it says:
    14.37.1 General
    A general-purpose alternating-current motor or any alternating-current motor having a service factor
    in accordance with 12.52 is suitable for continuous operation at rated load under the usual service
    conditions given in 14.2. When the voltage and frequency are maintained at the value specified on the
    nameplate, the motor may be overloaded up to the horsepower obtained by multiplying the rated
    horsepower by the service factor shown on the nameplate.
    When the motor is operated at any service factor greater than 1, it may have efficiency, power factor,
    and speed different from those at rated load, but the locked-rotor torque and current and breakdown
    torque will remain unchanged
    .
    A motor operating continuously at any service factor grater than 1 will have a reduced life expectancy
    compared to operating at its rated nameplate horsepower. Insulation life and bearing life are reduced by
    the service factor load.
    So I guess I was wrong about the torque, it specifically says the torque remains the same, but the other stuff does change. This is I think the last time they had those words about the life being reduced. I don't have a newer copy, but I've been shown excerpts of it (in a similar debate) pointing out that it is no longer including those last two sentences.
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  4. #14
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    Operating torque will increase
    but as stated temp will rise and life expectancy will decrease

    torque = hp x 5252/rpm
    load increases and rpm goes down and hp increases
    so operating torque increases
    lr and bd torque remain the same



  5. #15
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    motor service factor

    IT used to read [NEMA] that a service factor could be used continuously, if needed, then they backed it off to occasionally. The overall reason was to try and keep the frame size as small as possible.


    THey play a lot of games with service factors, many times they are the same motors, especially rotary phase converters, with a 1.0 SF they up the HP and get more $$$$$. But look closely and the one with the lower HP and SF is less $$$$$. I havent looked in years, but it was a common con thing.

    The average motor, runs at maybe 75% of rated HP. Rarely do you clamp a motor running right on the FLA.[there are situations where fans are restricted, and restricted by using a clamp meter, and pumps, but most things out there are close enough... Motors are rated with the FLA other data and even HP at the factory with a dynamometer. So they make 1000 motors, they sample 100, use the average and thats how it goes,

    A large above NEMA motor, is usually such an investment, it is speced and built to the PF, EFF, KVA CODE, DESIGN CODE, slip, on and on.

    One vanilla motor might run fine if it needed to be overloaded for a short while, another motor might struggle at FLA, it just doesn't happen that often, so they do the averaging thing. Look at the gross weight of identical motors from different manufacturers, It's not uncommon to have a 15 HP vanilla motor from MFG A to be wound with 10 pounds of magnet wire, and MFG B is wound with 21 pounds of magnet wire. Surely not, has to be a relationship between amount of wire and HP ? it really comes down to cheap imported steel for the laminations, lack of back iron, cheapened up conductor material in the rotor, cheaper bearings, rolled steel frame or cast frame, wherever they can cut and it still spins,

    Bottom line its a marketing thing, Think about motor HP ratings, 1, 1.5, 2, 3, 5, 7.5 10, 15, 20 so on, there are no 6 HP [at least no vanilla] there are no 2.5 HP motors, when in doubt find a motor guy/gal that knows their stuff and they are your best chance of getting what you need with enough wiggle room, and not more than you need, and NOT pay more upfront but everytime it runs. You will be told a rewound motor is less efficient than a new motor, more BS, sure it could happen, but who measures their motors eff? I promise a decent motor shop, will put more copper in the slots, [if it will fit and normally you can get some more in, you'll get insulation in between the end turn phases, the overall resistance will be as low or lower by both wire size and coil lengths, the windings will be hand bound every slot, the rotor balance is likely to be better, there are many improvements from a hand built motor over many new off the shelf. The warranty is the same in most cases, and the price might even be a motivator. Its difficult to compete with imported crap, especially in the lower HP ranges, but if it is a critical motor, or in some location it takes Houdini to get it out, consider having it rebuilt. Again find a motor gal/guy and let them SHOW you the benefits,

    We can also adjust for voltages say your 460 is constantly at 499,501,501 we can get the voltage rating up to the 500 by adding a few ampere turns and adjusting the conductor sizing.

    But if I had a critical motor[s] I'd have my old cast frame motor hand wound and refurbished. Plus the service factor thing can be dealt with if its an issue.

  6. #16
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    We used to have a customer in the HVAC industry who discovered that they could put motors with a 1.25 service Factor on some fans and pumps and save almost $50 a system. That is how cheap the HVAC industry tends to be. These systems were usually in the 20 to $30,000 range. And at least a couple of cases to accommodate that service Factor we had to go to the next size overload on some IEC starters.
    Bob

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