2 VFD's with differenct DC bus voltages

[philly]

I noticed the other day that there were two VFD's in our facility that were sitting right next to each other and both displayed different DC bus voltages when sitting idle. These VFD's are both the same manufacturer but are different sizes. The larger aprox 300hp one read 700V DC while sitting idle, while the smaller on aprox 75hp read about 675V DC. Both of these drives are fed from the same MCC.

Why when sitting idle would these drives have a different DC bus value? Is it a matter of the size capacitors used in the drive to hold up the DC bus?

 

[Jraef]

I noticed the other day that there were two VFD's in our facility that were sitting right next to each other and both displayed different DC bus voltages when sitting idle. These VFD's are both the same manufacturer but are different sizes. The larger aprox 300hp one read 700V DC while sitting idle, while the smaller on aprox 75hp read about 675V DC. Both of these drives are fed from the same MCC.

Why when sitting idle would these drives have a different DC bus value? Is it a matter of the size capacitors used in the drive to hold up the DC bus?

In a word, yes.

 

[Besoeker]

In a word, yes.

If there is no load the capacitors they would charge to the peak of the ac supply (less the B6U drop of couple of volts) in both cases.

 

[philly]

If there is no load the capacitors they would charge to the peak of the ac supply (less the B6U drop of couple of volts) in both cases.

When you say the capacitor would charge to the peak of the AC supply do you mean the peak of the waveform in which the case for 480V would be 480V * 1.4 = 672V. Would this be the peak you are referring to?


How would different capacitor sizes effect this peak value? (I'm rusty on capacitor peak charging values).


If it is simply a case of peak supply voltage then shouldn't these two drives have the same DC bus value if they are both connected to the same supply source?

 

[steve66]

When you say the capacitor would charge to the peak of the AC supply do you mean the peak of the waveform in which the case for 480V would be 480V * 1.4 = 672V. Would this be the peak you are referring to?


How would different capacitor sizes effect this peak value? (I'm rusty on capacitor peak charging values).


If it is simply a case of peak supply voltage then shouldn't these two drives have the same DC bus value if they are both connected to the same supply source?

Yes, 672 volts would be the voltage for a capacitor input filter. Changing the capacitor value wouldn't affect that voltage (assuming it doesn't get really small) while the output is off.

However, a line reactor on the input will reduce the voltage that the capacitor charges to. The exact reduction depends on the ratio of inductance to capacitance. For example, for a pure inductive filter, the output voltage is the average of the input voltage.

Steve

 

[mxslick]

I noticed the other day that there were two VFD's in our facility that were sitting right next to each other and both displayed different DC bus voltages when sitting idle. These VFD's are both the same manufacturer but are different sizes. The larger aprox 300hp one read 700V DC while sitting idle, while the smaller on aprox 75hp read about 675V DC. Both of these drives are fed from the same MCC.

Why when sitting idle would these drives have a different DC bus value? Is it a matter of the size capacitors used in the drive to hold up the DC bus?

Simple..the design of the larger drive has to maintain a higher bus voltage to offset losses in the drive circuitry.

The manufacturer can give you a more detailed/accurate answer than any of us here could.

 

[philly]

I found that these two drives actually are fed from two different power sources. So that could explain why the DC bus values differ from each other.

The one supply feeding the drive reading 700V DC was measured to be about 500V. This 500V * 1.4 comes out to be about 700V exactly.

So it appears that the DC bus value is soley a function of supply voltage and the DC link capacitor will charge to whatever the peak of the supply voltage is? It would appear then that capacitor size would not matter, and that the cap will charge to peak voltage of supply regardless of cap size. Is that correct?

How does a reactor limit the voltage that the capacitor charges too? Voltage drop across reactor?

 

[steve66]

How does a reactor limit the voltage that the capacitor charges too? Voltage drop across reactor?

I'm refering to a line reactor (inductor) that is placed between the diode bridge and the filter capacitors. Filters with a capacitor input have the peak input voltage at their output. Filters with an inductor input have the average input voltage at their output. This link seems to explain it pretty well:

http://www.tpub.com/neets/book7/27f.htm

For VFD's, I believe the inductors (if there are any) are sized realitivaly small, so the output voltage is higher than average voltage you would get with a true inductor input filter. So it would be closer to the peak input voltage, but not quite the full input voltage.