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Thread: Motor De-rating and FLA

  1. #1
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    Motor De-rating and FLA

    Not going to get into to too many details but here is what I have going on.

    Recently a failed 200H.P. motor (460VAC, 1.15 SF, Design B, 400A MCP, size 5 starter, 350MCM feeder) was replaced with a manufactured 250H.P. motor (460VAC,SF 1.15, Design B).

    The application is a PD boiler feed water pump. The replacement manufactured 250 H.P. motor has a name plate of 200H.P., per the vendor the motor “de-rated” from 250H.P. to 200H.P. Per the vendor I was assured that this is essentially a “200H.P.” motor.

    What I’m having trouble grasping is:
    Is the vendor correct on his statement of the de-rated motor?

    Is the FLA/inrush of the "250H.P. manufactured" motor per a 250H.P. or 200H.P.?

    Will the starter size still be 5? (cut off for 5 is 200H.P.)

    Will calculations for feeder cable, MCP settings, and OL be calculated from a size of 200H.P. or 250H.P.?

    Just having a little trouble getting my head around this.
    Thanks a bunch

  2. #2
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    Quote Originally Posted by MrJLH View Post
    Is the FLA/inrush of the "250H.P. manufactured" motor per a 250H.P. or 200H.P.?
    I think it unlikely.
    Can you check the respective nameplate data?
    Si hoc legere scis nimium eruditionis habes.

  3. #3
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    Quote Originally Posted by MrJLH View Post
    ...The replacement manufactured 250 H.P. motor has a name plate of 200H.P., per the vendor the motor “de-rated” from 250H.P. to 200H.P. Per the vendor I was assured that this is essentially a “200H.P.” motor.
    What the heck does that mean? De-rating of motors is done primarily to allow for increased duty cycles, or increased ambient temperature operation. But it's usually accomplished via just buying a larger motor than what the mechanical specs call for, not actually changing the nameplate of the motor. "HP" is a shorthand expression of torque at a given speed. So if the motor was built as a 250HP, but they put a name plate on it as 200HP, I fail to see how they could have LOWERED the torque capability of the motor.

    Here's what I think. I think it sounds as though you may have stumbled across a motor importer that has taken a standard IEC 315kW 380V 50Hz motor and is telling you to use it at 460V 60Hz. 315kW = 235HP, but IEC motor specs have no concept of "Service Factor", so if you take a requirement for 200HP with a 1.15SF, that's 230HP; close enough in their mind, that extra 30HP is covered in the converted HP of that kW rated motor.

    Why then are they calling is "250HP de-rated?" Because if you look at the TORQUE of a 315kW 50Hz motor and a 250HP 60Hz motor, they are closer to being the same. Torque = HP x 5250/RPM. Assuming (although not important) that this is a 4 pole motor, the 50Hz torque would be 235 x 5250/1455 = 847 ft-lbs, and a 250HP 60Hz motor would be 250 x 5250/1725 = 760 ft-lbs. The 315kW 50Hz motor is actually going to supply more torque than a 250HP 60Hz, but not enough to be called 300HP, so we jump from 250 to 300, they are calling it 250HP.

    What's important to you? In this case actual motor nameplate FLA. If you look at the specs for a NEMA size 5 starter it is rated for 270A, which is actually a little higher than the NEC chart for 200HP (240A). This is because the NEMA rating has to cover lower pole counts (slower base speed) motors. That may work for you here in not requiring you to upgrade to a Size 6 starter. As to conductor sizing, I would interpret the NEC as saying you need to go with the 250HP aspect.
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  4. #4
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    Quote Originally Posted by Jraef View Post

    Here's what I think. I think it sounds as though you may have stumbled across a motor importer that has taken a standard IEC 315kW 380V 50Hz motor and is telling you to use it at 460V 60Hz. 315kW = 235HP,
    You might want to review your numbers.
    Si hoc legere scis nimium eruditionis habes.

  5. #5
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    There are a number of possibilities here, but you might want to start by sending the serial number to the original motor manufacturer for verification of the rating.

  6. #6
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    Quote Originally Posted by Jraef View Post
    Torque = HP x 5250/RPM. Assuming (although not important) that this is a 4 pole motor, the 50Hz torque would be 235 x 5250/1455 = 847 ft-lbs, and a 250HP 60Hz motor would be 250 x 5250/1725 = 760 ft-lbs. The 315kW 50Hz motor is actually going to supply more torque than a 250HP 60Hz, but not enough to be called 300HP, so we jump from 250 to 300, they are calling it 250HP.
    I was tryting to calculate RPM which you have it as 1455 and 1725 RPM i used this formula..N=120*f/P...
    For 50Hz motor with 4 poles doesnt its RPM=120*50/4=1500 RPM?
    And for 60Hz motor with 4 poles RPM=120*60/4=1800 RPM?

    Am i missing something?

  7. #7
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    Quote Originally Posted by shortcircuit1 View Post
    I was tryting to calculate RPM which you have it as 1455 and 1725 RPM i used this formula..N=120*f/P...
    For 50Hz motor with 4 poles doesnt its RPM=120*50/4=1500 RPM?
    And for 60Hz motor with 4 poles RPM=120*60/4=1800 RPM?

    Am i missing something?
    You calculated synchronous speed. There is going to be some slip and a lesser rotor speed in a simple induction motor.

  8. #8
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    Quote Originally Posted by kwired View Post
    You calculated synchronous speed. There is going to be some slip and a lesser rotor speed in a simple induction motor.
    Is there any formula which includes that slip as well?

  9. #9
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    Quote Originally Posted by shortcircuit1 View Post
    Is there any formula which includes that slip as well?
    The nameplate will normally give you rated speed and rated frequency.From that you can, as you have done, calculate synchronous speed. The difference between that and nameplate speed is the slip. It is usually expressed as a percentage of synch speed. Values of 2-3% are typical. Sometimes less for larger motors.
    There is no simple formula that I know of.

    Can you calculate it?
    You can, or at least get close.
    But it's not simple.
    Si hoc legere scis nimium eruditionis habes.

  10. #10
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    The RPM on the nameplate should only be correct when motor has rated voltage, frequency, and load applied to it, as is other data that might be on the plate, like current, power factor, efficiency.

    Changing voltage, frequency or load will result in changes to any or all of actual running current, power factor, or efficiency.

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