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Hot!what happen if external force make motor speed higher than expected speed in AN1078

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michael.li9
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2019/04/25 15:11:14 (permalink)
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what happen if external force make motor speed higher than expected speed in AN1078

Hi Experts,
In our application, water flow can make the motor rotation at same direction I like to control. Higher flow rate can make motor rotation faster. The question is if I like to control the speed at 500rpm for example, if flow rate make motor running faster than 500rpm. What happen, Can FOC make motor maintenance at 500rpm? I can't see it in code. I may miss something.
or what I can do to maintenance 500rpm?
 
 
 
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    PStechPaul
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    Re: what happen if external force make motor speed higher than expected speed in AN1078 2019/04/26 01:28:15 (permalink)
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    When an AC induction motor is mechanically driven faster than the drive frequency, it will act as a generator and will inject DC into the DC link bus. You can add a braking resistor which will provide drag on the motor shaft and help keep speed close to that at which it is being driven. For even better speed control you will need a shaft encoder to read RPM and then adjust drive frequency as needed.

     
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    Nikolay_Po
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    Re: what happen if external force make motor speed higher than expected speed in AN1078 2019/04/26 02:45:51 (permalink)
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    If motor rotor field will pass ahead the inverter field the motor become the generator automatically, without of any changes in control algorithm. When you will try to slowing down the flow, the voltage on the DC bus will rise. If there is none elements that will dissipate (or recuperate into the grid) the generated energy, the DC bus voltage will go higher and higher until a breakdown and catastrophic failure. Or bus overvoltage protection will shut off the PWM and the motor become just a rotating piece of metal without flow slowing down abilities.
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    du00000001
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    Re: what happen if external force make motor speed higher than expected speed in AN1078 2019/04/29 00:54:46 (permalink)
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    While the considerations about the motor becoming a generator given above are a general truth, there's another option to slow such a motor down - provided it's not an asynchronous machine (of which I'm just not sure whether this recipe might work as well):
    (Gradually) shorting the stator windings. This way, the excess power is dissipated in the stator windings (as opposed to being dissipated in some brake resistor). While this scheme is only applicable for short-term breaking  and not-too-high energies, it virtually comes for free.
    Not sure whether this concept comes readily with AN1078.

    PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)
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    michael.li9
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    Re: what happen if external force make motor speed higher than expected speed in AN1078 2019/05/01 14:45:29 (permalink)
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    thanks All. 
    Is there any way in AN1078 code know it is faster than expected? The issue is motor stop immediately after speed is higher then expected.
    There is a load resistor in our design. I can try that.
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