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Thread: Motor Starts per hour

  1. #1
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    Motor Starts per hour

    We have a 150HP ,480V motor pump set started and stopped different times while running tests . It's a design B,480V ,3 phase, started across the line ,CONTINIOUS DUTY RATED MOTOR . How many starts a hour can this motor handle ????

  2. #2
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    Quote Originally Posted by Davebones View Post
    We have a 150HP ,480V motor pump set started and stopped different times while running tests . It's a design B,480V ,3 phase, started across the line ,CONTINIOUS DUTY RATED MOTOR . How many starts a hour can this motor handle ????
    I presume you're asking about number of starts PER HOUR.

    Here is what NEMA MG-1 says:

    c. Two starts in succession (coasting to rest between starts) with the motor initially at the ambient temperature or one start with the motor initially at a temperature not exceeding its rated load operating temperature.

    then:

    12.54.3 Considerations for Additional Starts
    When additional starts are required, it is recommended that none be made until all conditions affecting operation have been thoroughly investigated and the apparatus examined for evidence of excessive heating. It should be recognized that the number of starts should be kept to a minimum since the life of the motor is affected by the number of starts.

    +++++++++++++++++++++++++++

    Large motors in the range of greater than 500HP often do have the number of allowable starts specified as manufacturer supplied data.

    Thermal overload will help you to avoid immediate destruction; however it does not prevent long term thermal deterioration of the motor insulation.

    Winding embedded RTD go a step further. Combined with a microprocessor based thermal model they actually calculate the thermal dissipation capability of the motor and the protection module would actually calculate - and indicate - the cool down time and safe restart time. The caveat is that the RTD is mounted at a single location or maybe two in each phase winding, so you can still have hot-spots somewhere else. There is an allowance for an assumed hot spot in the calculations, but it is not a sure bet.



  3. #3
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    Multiple starts in succession can overheat the rotor to the point that the aluminum bars in the squirrel cage rotor melt, usually at the end ring connection points. While it is spectacular to see arcs and sparks flying out the end of the motor as the rotor disintegrates, it is quite dangerous. (We were trying to figure out why the 800 HP motor took so long to get the pump up to speed and tried too many starts close together. Multiple starts overheated the rotor.)

    A thermal overload or an RTD can protect the stator but it may not have the correct response time to protect the rotor on larger motors. (< 150 HP)

    Rotor overheating can be worse when the motor does not run very long between starts because no cooling air is flowing when the motor is stopped. The current and heating in the rotor is maximum during starting. If the motor runs a while after the start the flow of cooling air reduces the temperature much faster than the rotor at standstill.
    Bob Wilson

  4. #4
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    Quote Originally Posted by rcwilson View Post
    Multiple starts in succession can overheat the rotor to the point that the aluminum bars in the squirrel cage rotor melt, usually at the end ring connection points. While it is spectacular to see arcs and sparks flying out the end of the motor as the rotor disintegrates, it is quite dangerous. (We were trying to figure out why the 800 HP motor took so long to get the pump up to speed and tried too many starts close together. Multiple starts overheated the rotor.)

    A thermal overload or an RTD can protect the stator but it may not have the correct response time to protect the rotor on larger motors. (< 150 HP)

    Rotor overheating can be worse when the motor does not run very long between starts because no cooling air is flowing when the motor is stopped. The current and heating in the rotor is maximum during starting. If the motor runs a while after the start the flow of cooling air reduces the temperature much faster than the rotor at standstill.
    See IEEE paper No. PCIC-2009-14.

    New modeling and implementation into protective realying is narrowing this problem into non-existence, hopefuly soon. It does not seem that the OP needs to worry about this much. I tihnk that the breakpoint is 500HP and that is one of the reasons why MG-1 only goes up to that size.

    Back to the OP, somewhere in the back of my grey matter tells me that I have heard that 10,000 'normal' starts is the insulation life expectance. Of course that number is pretty meaningless without additional parameters. I could be off by couple of 0's. Hey, it ain't Memorex....

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