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Thread: SFA on Motor Nameplate

  1. #1
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    SFA on Motor Nameplate

    What is one to do with the SFA information on a motor nameplate? Can this SFA value be used as input to Square D tables to select thermal overloads?


    The SFA value found is 21.7, the FLA is 17.3.

    The motor strater overload relay occasionaly trips out....

  2. #2
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    It's the service factor amps. It looks as if your motor has a 1.25% servicew factor. 17.3 x 1.25=21.6a
    If you have selected your OLR heaters based upon the FLA, which I assume that you have done based upon your question, and are running into the SF amperes it would make sense that the OLR is tripping when it does.
    Use a clump-on ammeter to monitor the ampers.
    DAve

  3. #3
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    Thank you very much for your reply. The recorded measured values peaked at about 18.6A, and then tripped on overload. Possibly my "one time per minute" sampling was too slow...

    The B32 (Square D) element was selected on the "next size over" the 17.3A nameplate FLA. The B32 element current range listed in Square D information for my starter is 17.4 - 19.1A

    I think that the minimum current that the element should trip out would be 17.4A times 1.25, or 21.6A.

    If I select the next size up element, a B36, the element should trip out at 19.2A times 1.25, or 24A. Since this is just less than the NEC 430.32(C) maximum value of 17.3A times 1.4, or 24.22, I presume then that the installation would still be compliant.

  4. #4
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    Keep in mind, the service factor of the motor is considered the rating in which the motor can operate for limited periods of time. It is NOT supposed to be used for continuous operationat that rating. You should consult the manufacturer of the motor to determine how long the motor can operate at the SF without damage.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  5. #5
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    Follow the manufacturer's instructions to determine the motor Overload device setting. Do not simply take the motor FLA and multiply by 1.25.

    According to the Square D Digest:
    Step 1. Determine motor data
    a. Full load current (FLC)
    b. Service factor

    Step 2. If the motor and the controller are in the same ambient temperature
    a.1. For 1.15 to 1.25 service factor motors use 100% of motor FLC.

    Step 4. Locate proper selection table (based on enclosure size)

    Step 5. Optional calculation for protection level (see page 14-132)
    the trip current is 125% of the TU minimum current
    the protection level is the trip current/motor FLC
    Per NEC protection level can not exceed 125%

    In your case the B32 thermal unit minimum current is 17.4A and your motor draws 17.3A, so your protection level is (17.4*125%)/(17.3) = 126%. Technically you have selected overload protection in excess of that allowed by the NEC.

    Based on all of this your motor is undersized for the application.
    Just because you can, doesn't mean you should.

  6. #6
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    Jim Dungar is right, however my experience is that many OEMs routinely use the Service Factor when applying motors to their machinery. TECHNICALLY, there is no time limit on running in the Service Factor, however it is generally accepted that you will sacrifice service life by increasing winding temperature by doing so continuously. OEMS however sometimes don't care, they know that it will outlast their warranty so they go ahead and run that 5HP 208V motor into the SF and get 6.25HP rather than supply a 7-1/2HP motor at 83% load as they should have.

    I would replace the motor if it were me. You will find yourself doing it at some future time anyway, and Murphy's Law says it will be at the worst possible moment!

  7. #7
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    Motors are built in accordance with NEMA MG1, which states that when operated in their service factor rating the insulation will degrade 2x faster then when operated at the normal nameplate ratings.

    I'd say that is more then a generally accepted principle, and that is definitely a technical reason to not do so. A good specifcation will clearly state that under normal operation, operating in the service factor is not allowed.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  8. #8
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    Thanks for your input guys. I buckled down and did this write up, any questions or comments?

    National Electrical Code

    The basic premise behind the National Electrical Code overload protection requirements is that the operation of a motor in excess of its normal full-load rating for a prolonged period of time causes damage or dangerous overheating that may start a fire. Overload protection is intended to protect the motor and the system components from damaging overload currents. Motors are required to be protected from overloads. To protect a motor from an overload, the motor nameplate full-load current (FLA) is used to select the overload protection.

    A separate overload device that is responsive to motor current is required per National Electrical Code section 430.32(A)(1). This device shall be selected to trip or shall be rated at not more than 125% of the motor nameplate full-load current rating.

    When the thermal element selected in accordance with 430.32(A)(1) is not sufficient to start the motor or to carry the load, the National Electrical Code section 430.32(C) allows higher size thermal element to be used, provided the trip current of the overload relay does not exceed 140% of motor nameplate full-load current rating.

    Installation

    For example: a motor nameplate data shows FLA of 17.3A and a service factor of 1.25. Based on this information, Square D specifies a B28 thermal element using the "Thermal Unit Selection Table Number" for a Model 6 MCC. This table references Table 110 found on page 14-145 of the published Square D Digest. The B28 thermal unit is for motor FLA between 15.9 - 17.9, this element provides a Protection Level of 103.4% overload current. A B28 should have been initially installed.

    What was provided and installed is a B32 thermal unit. The B32 is the "next size over" the B28 thermal unit. The B32 thermal element violates section 430.32(A)(1) of the National Electrical Code, as shown below, however it is in compliance with the exception section of the Code 430.32(C), yet, it is still tripping out.

    The B32 thermal unit current range is 18.0 - 19.9A. The Trip Current Rating of this B32 thermal unit is 22.5A (18.0A times 1.25). Providing a Protection Level of 130% as calculated by the Trip Current Rating and the FLA time 100, (22.5 divided by 17.3 times 100). This 130% value exceeds the 125% limit imposed by NEC 430.32(A)(1). However, the Code section 430.32(C), does provide an exception to this 125% limit. The exception allows a Protection Level up to 140%. It is this exception which would allow the B32 to remain and not violate the Code. However, as previously mentioned, the B32 is still tripping out.

    Thermal Unit Up sizing

    The next size thermal unit listed by Square D is a B36, it has a current range of 20.0 - 22.8A. The Trip Current Rating of this B36 thermal unit is 25.0A (20.0A time 1.25). The Protection Level offered by the B36 thermal unit is 146% (25.0 divided by 17.3 time 100). This 146% value exceeds the 140% limit imposed by the exception allowed by NEC 430.32(C). A B36 thermal unit must not be installed under any circumstances.

    I am not aware of an incremental thermal unit for this motor starter, and would be reluctant to have one installed if one was found.

  9. #9
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    Quote Originally Posted by kingpb
    Motors are built in accordance with NEMA MG1, which states that when operated in their service factor rating the insulation will degrade 2x faster then when operated at the normal nameplate ratings.
    Respectfully, NEMA MG-1 (my copy is dated 2002) does not say anything about the rate of degradation being 2x, only that it will not be the same as running a motor under the SF.

    Here is a direct quote from section 9.15.1:

    "A motor operating continuously at any service factor greater than 1.0 will have a reduced life expectancy compared to operating at its rated nameplate horsepower."

    IMHO this is why many equipment OEMs feel they can get away with it, they are banking on the motor lasting out their warranty period. It's not right, it just is.

  10. #10
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    Quote Originally Posted by Jraef
    Respectfully, NEMA MG-1 (my copy is dated 2002) does not say anything about the rate of degradation being 2x, only that it will not be the same as running a motor under the SF.
    It seems that may be where your confusion lies, the current version of MG-1 is 2006, and 9.15 does not exist.

    The statement is found in 20.7.3.2 that degradation occurs at approx. 2x.
    "Just because you're paranoid, doesn't mean they're not out to get you"

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