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Capacitor motor starting


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Were doing a motor start study at work and one method that has been mentioned is using capacitors. I've never come across this method of starting motors before. No other start method will be used just cpacitors, any ideas or suggestions on how this will work much appreciated.

It will be used mainly at MV.


Best regards and a happy christmas


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  • 2 weeks later...

Hello thewarb


Capacitive starting is commonly used for single phase motors. The single phase motor needs to have a second phase to start, see our page on single phase motors, and the capacitor is used to create a phasehift to drive the start winding.


It is not commonly used with three phase motors, however, at zero speed, the induction motor has a very low power factor. If there is capacitance added, the power factor can be improved and the start current reduced. The difficulty is that as the motor accelerates, the power factor improves, reducing the amount of correction required. To do this effectively, requires many steps, and these need to be synchronsed to the motor speed. - not an ideal situation.


Best regards,

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Capacitor assited starting for large AC 3 phase motors is a very old attempt to solve a common problem, that of not having a stiff enough supply to get the motor moving from a dead stop when the motor ciruit power factor is at it's worst. The capacitors supply the short boost of VARs to the circuit so that the motor can get more torque from the available line power in the first instant when it is turned on. The problem is that the capacitors must be taken out quickly or they will be damaged, and that can prove to be tricky. In the old days this was not a big deal because there was little to be hurt if the caps failed. Now it is different because most facilities have very expensive electronic systems all over the place. Very few people are willing to take the risk of having a massive voltage spike on their system if anything goes wrong.
"He's not dead, he's just pinin' for the fjords!"
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