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Under Voltage


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Dear sir, I have two Mitsubishi VFD Freqrol for 1 HP. It has an error fault Undervoltage during acceleration, constant speed and deceleration. In the manual said that I must check the power line, but it seems OK in there. Can you tell me what must I checked in the vfd. someone said that the fault is in the switching supply part for the controller, is it true? other said that I must change the power capacitor?which one is true and what can i do to repair this VFD


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Generally an undervoltage fault concerns the DC bus voltage so ensure that you hve the correct voltage at the input of the drive, the correct number of phases etc. If this is ok then check the input section of the drive rectifyers etc. The simplest method is to test the dc voltage during the times that it is running if you can get access to the bus.


Good luck



:excl: P.S. the voltage on the bus will be DEADLY so be careful!!! :excl:

An expert is one who knows more and more about less and less until he knows absolutely everything about nothing
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I had a similar problem on a 15Kw Mitsubishi drive you’re talking over

13 years ago. The drive was tripping out on under voltage, all measured voltages

appeared to be in acceptable limits….it would just trip out but nobody ever saw it trip out.

All you would see was the red led on. Anyway it transpired that it was a genuine under voltage error the drive would trip out in the early hours of the morning, as the world was awakening and draining the grid system.


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This could also be indicative that the soft charge circuit is not performing correctly.

Usually, there is a resistor in series with the bus capacitors that limits the charging current when the drive is powered up. This resistor is then shorted once the capacitors are fully charged.

Typically, the shorting is done by a relay. If the soft charge resistor does not get shorted due to a fulty relay or similar, then it will come up with under voltage trips as soon as current is drawn.


Best regards,

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Dear all, thanks for the answer before.

Now i have the same problem with another inverter with Sanken type, it is 10 hp 380V. it has undervoltage fault too.

For the first time, the inverter is working well, then it seems do a restart. then i checked the fault history, and said undervoltage. I have checked the line power it's ok. I have checked the rectifier, it's OK too.

I have checked the power capacitor after the rectifier. it seem drop in capacitance. It has 4 capacitor which was written 820 uF/400V. And the capacitor arrangement is 2 capacitor in paralel and then series with 2 paralel capacitor again. So from I know it will have equivalent capacitance 820 uF but has a rating voltage for 800 V. when I measured this capacitor with capacitance meter, mit has only about 700-720 uF. So the real total capacitance will be dropped to 700 uF. Did "120 uF" lost can make this inverter undervoltage?.

I have searched for the sense circuit for the undervoltage sensing, and i found nothing for this. but i have this in the circuit for power supply. So my opinion is the circuit know that undervoltage failure from its supply voltage (control supply about 5 and 24 V ) CMIIW...

And control supply is influenced by the switching PWM and the power supply from main capacitor.From my problem above, if I change the capacitor so it have its capacitance back to 820 uF again or I make it bigger, it will make my inverter being good. can anybody give some advice?



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