AC induction motor - V/f control

  • #1

Main Question or Discussion Point

What will happen if an ac inductions motor is running, an suddenly an error occurs in the voltage pr. hertz motor control so the stator frequency goes down below the rotor frequency for 1-2 sec?
 

Answers and Replies

  • #2
jim hardy
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the motor will attempt to return electrical power to its source, decelerating as it does so.

Keep your thinking simple.
What would be the slip under the condition you postulate?
What do yur formulas tell you about power vs slip?
 
  • #3
Okay thanks.
Let say the rotor frequency is 60Hz and the stator frequency suddenly become 2 Hz or 0 Hz(DC) in 1-2 sec the motor will generate a lot current?

Lets say your AC motor accelerate from stand still and your voltage and frequency is controlled properly, but suddenly the frequency (an error occur) is not proper compared to the voltage (V/f ratio). The stator frequency are still in front compared to rotor frequency. Do the AC motor draw a big current?
 
  • #4
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Condition of rotor is not important in this case. When the frequency of power supply drops close to zero without any voltage reduction, this case is same that you connect an ac motor to the dc power supply. Generally reduction of motor internal inductance due to frequency drop (Xl=2*pi*f) can be caused a big current.
 
  • #5
Okay. if the this happens in 1 sec or less etc. Does the motor still draw a big current?

How about if this happens while the motor is running does it still draw a big current because now the motor produce CEMF? the inductance value does not only change by stator frequency.
 

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