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Thread: Proper Implimentation of Motor Starter

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    Proper Implimentation of Motor Starter

    I'm trying to determine if I have to use any fusing before my motor starter w/Solid State motor protection. This starter is being integrated into a large HP VSD. It will be fed power from the output side of a circuit breaker that is located in a junction box on the VSD. From my reading the NEC I think that all I have to have is the power fed directly to the motor starter and out to the motor. When I look at off the shelf starters a lot of them have fusing before the motor starter. Does anyone have the answer on this?

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    Quote Originally Posted by freecrowder View Post
    I'm trying to determine if I have to use any fusing before my motor starter w/Solid State motor protection. This starter is being integrated into a large HP VSD. It will be fed power from the output side of a circuit breaker that is located in a junction box on the VSD. From my reading the NEC I think that all I have to have is the power fed directly to the motor starter and out to the motor. When I look at off the shelf starters a lot of them have fusing before the motor starter. Does anyone have the answer on this?
    Not sure exactly what you have going on there with a VSD as well as a motor starter.

    But if it were simply a circuit with just the motor starter - your branch circuit device provides short circuit and ground fault protection, and can be higher setting then conductor ampacity, the motor overload protection (which can go about anywhere in the circuit, but is often in close proximity to the contactor) not only protects the motor from overload but also protects the conductors from overload.

  3. #3
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    As kwired said, I too am not sure why you want a motor starter ahead of a VFD, it's not a good idea unless absolutely necessary per some safety requirement. Opening and closing a starter ahead of a VFD every time you start or stop a motor will stress out some components in the drive and lead to an early demise. Or is this to be used as a bypass in case the VFD fails? If so, you need a lot more than just one starter.

    But to your question, usually motor starters are listed to be used behind fuses OR thermal mag circuit breakers when field assembled. There may however be restrictions as to the size and possibly the brand of circuit breaker that the starter (or more specifically, the overload relay) can be used with. Only the mfr of the devices can tell you.
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    Here is a PIC of what I am asking.

    Motor Starter Schematic.pdf

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    Quote Originally Posted by freecrowder View Post
    Here is a PIC of what I am asking.

    Motor Starter Schematic.pdf
    Yes then, you will, because that 500A breaker cannot protect the 25A auxiliary motor circuit. Assuming this is 480V, I'll also assume this is 20HP, so per 430.52 you must size the SCPD (fuses or CB) for that starter at max. 250% of the NEC table value of 27A, so 67.5A, rounded up to 70A if it is a CB, or 175%, so 47.25A, rounded up to 50A if using time-delay fuses.

    Adding that 25A load may affect the 500A breaker as well, depending on the rating of the VFD...
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    Quote Originally Posted by Jraef View Post
    Yes then, you will, because that 500A breaker cannot protect the 25A auxiliary motor circuit. Assuming this is 480V, I'll also assume this is 20HP, so per 430.52 you must size the SCPD (fuses or CB) for that starter at max. 250% of the NEC table value of 27A, so 67.5A, rounded up to 70A if it is a CB, or 175%, so 47.25A, rounded up to 50A if using time-delay fuses.

    Adding that 25A load may affect the 500A breaker as well, depending on the rating of the VFD...

    What is the motor starter SS/Thermals protecting?

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    Quote Originally Posted by freecrowder View Post
    Here is a PIC of what I am asking.

    Motor Starter Schematic.pdf
    I'm from a different country and the rules are not the same. But I would neither design nor sanction such an arrangement without separate appropriately rated protection for the lower current circuit.
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  8. #8
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    Quote Originally Posted by freecrowder View Post
    What is the motor starter SS/Thermals protecting?
    They are to protect the motor and the branch circuit conductors from overloading. You still need short circuit and ground fault protection that complies with 430 part IV.

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