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Thread: Motor stalling

  1. #1
    T.M.Haja Sahib Guest

    Motor stalling

    Quote Originally Posted by kwired View Post
    As long as the power delivered starts the motor and there is no other problems the undersizing of the transformer is likely POCO's problem.
    Does the capacity of a transformer has any thing to do with motor stalling ?

    Suppose the under capacity transformer is able to start the loaded motor. Would not the motor stall due to excessive voltage drop as a result of under capacity of the transformer ?

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    Quote Originally Posted by T.M.Haja Sahib View Post
    Does the capacity of a transformer has any thing to do with motor stalling ?

    Suppose the under capacity transformer is able to start the loaded motor. Would not the motor stall due to excessive voltage drop as a result of under capacity of the transformer ?
    How is it a motor will be able to start yet load is so great it will stall? Opening of a protective device is not stalling, I'm talking about stalling because not enough torque is available to move the load, not because some protective device opened the circuit. In order for motor to start moving enough torque must be produced to allow load to move. Accelerating to proper speed is another issue but not being able to reach that speed is not stalling.

    If voltage drop is excessive enough that the motor is not going to accelerate to full speed, current will be high, voltage drop will be even worse, something is going to give and it will shut down or burn up.

    If connected to a variable speed drive then we have a different situation - starting current will be low, overloading will start to happen somewhere before full rated speed is reached. The transformer will attempt to deliver the load that is demanded but will have increased heating as a result. If transformer is not able to deliver power needed voltage will drop, current will go up - making voltage drop even worse - something will either respond and shut it down or something will eventually burn up.

  4. #4
    T.M.Haja Sahib Guest
    Quote Originally Posted by kwired View Post
    How is it a motor will be able to start yet load is so great it will stall?
    The loaded motor, fed by an under capacity transformer, is able to start because the rated voltage is available to the motor at the instant of starting.But voltage drops soon after due to under capacity of the transformer.This could cause stalling of the motor.

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    Quote Originally Posted by T.M.Haja Sahib View Post
    The loaded motor, fed by an under capacity transformer, is able to start because the rated voltage is available to the motor at the instant of starting.But voltage drops soon after due to under capacity of the transformer.This could cause stalling of the motor.
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    In regards to thoughts about a loaded motor- If a loaded motor is able to start then there is enough capacity for it to run.
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  6. #6
    T.M.Haja Sahib Guest
    Quote Originally Posted by ActionDave View Post
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    Quote Originally Posted by ActionDave View Post
    If a loaded motor is able to start then there is enough capacity for it to run.
    This may not be true in all cases.

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    Quote Originally Posted by T.M.Haja Sahib View Post
    Sorry, I can't help it. You better bear with me or quit reading me.

    This may not be true in all cases.
    Can you cite a case in which this might occur?? I have never seen a case in which there was enough of a voltage drop (under capacity transformer) to cause a motor to stall after the motor had already started. The highest "drop" would be when the motor is first starting up, not after the initial inrush.

  8. #8
    T.M.Haja Sahib Guest
    Quote Originally Posted by eric7379 View Post
    Can you cite a case in which this might occur?? I have never seen a case in which there was enough of a voltage drop (under capacity transformer) to cause a motor to stall after the motor had already started. The highest "drop" would be when the motor is first starting up, not after the initial inrush.
    See below.
    Quote Originally Posted by kwired View Post
    If connected to a variable speed drive then we have a different situation - starting current will be low, overloading will start to happen somewhere before full rated speed is reached. The transformer will attempt to deliver the load that is demanded but will have increased heating as a result. If transformer is not able to deliver power needed voltage will drop, current will go up - making voltage drop even worse - something will either respond and shut it down or something will eventually burn up.

  9. #9
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    Quote Originally Posted by T.M.Haja Sahib View Post
    Does the capacity of a transformer has any thing to do with motor stalling ?

    Suppose the under capacity transformer is able to start the loaded motor. Would not the motor stall due to excessive voltage drop as a result of under capacity of the transformer ?
    Seems like a déjà vu moment...

    If the motor is connected direct on line you have this sort of characteristic:



    Clearly, the highest current is at starting. Thus is a double whammy for the supply voltage. Not only is it the highest current but it is at poor power factor which results in greater voltage regulation than the same current at good power factor. Then you're also hit about the lowest torque per amp. If the motor can get past that, things get better all round, so stalling isn't like to be caused by transformer capacity.
    This is just confirming what others have already pointed out.

    If the motor is fed by a variable frequency drive (not mentioned in your original post) and the supply voltage drops then so will the DC link voltage. Most VFDs default to fixed V/f ratio so there at at least a couple of possible outcomes. Depending on how low the voltage is I can see a couple of possible outcomes. The low DC link might prevent the motor getting maximum frequency thus limiting its speed. And if it's lower than the drive settings the drive will trip. Again, the motor stalling isn't a likely outcome.

  10. #10
    T.M.Haja Sahib Guest
    Quote Originally Posted by Besoeker View Post
    ..... the motor stalling isn't a likely outcome.
    With VFD drive and under capacity supply transformer,when will the motor stalling be likely?

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