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Discussion Starter · #1 ·
I have a 480v 3 phase to 230v 6 phase transformer feeding a 24 diode (4 per phase) rectifier supply for some DC motors that puts out 300v; my question is can I add 2 diodes per phase making it a 36 diode rectifier (6 per phase) and would that increase my DC voltage output to the motors.
 

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The answer is pretty much no.

Adding diodes would only increase the current handling capacity per phase. You might get about 1-3 volts increase due to offsetting the voltage drop of the diodes.

And most importantly, the diode MUST be of the same type and specification or the current will not divide equally amongst the diodes in each phase, which will result in the diodes failing.

IF the diodes have sufficient voltage ratings, you could probably add a boost transformer on each phase before the diodes to increase the voltage.
 

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mxslick said:
The answer is pretty much no. Adding diodes would only increase the current handling capacity per phase. You might get about 1-3 volts increase due to offsetting the voltage drop of the diodes. And most importantly, the diode MUST be of the same type and specification or the current will not divide equally amongst the diodes in each phase, which will result in the diodes failing. IF the diodes have sufficient voltage ratings, you could probably add a boost transformer on each phase before the diodes to increase the voltage.
You are right about the increased current, but voltage will not change at all. The new diodes will have the same break through voltage as the rest and since they are in parallel, the voltage will remain the same.
 

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Discussion Starter · #5 ·
Reason why I asked is because I have a 36 diode supply that puts out around 330 to 340 v dc with the same 480v/230v ac transformer supply.
 

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Discussion Starter · #6 ·
In theory I agree with you Tsmil but the 1-3 volts offset seems to solve the mystery for me if I'm adding 2 diodes per phase.
 

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If input voltage is the same, and transformers are the same, Check diodes that are there for an open, that will lower voltage. I'm assuming that the diodes used are silicon diodes. Typical forward voltage drop for a silicon rectifier is 0.7V. 230 x 1.414 x .995 - 0.7 = 322.9V. That would be the non filtered voltage. Filtering would reduce ripple, but on a 12 half phase bridge, there is very little ripple to start with.
 

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Reason why I asked is because I have a 36 diode supply that puts out around 330 to 340 v dc with the same 480v/230v ac transformer supply.
The voltage has nothing to do with the number of diodes, if that's what you were thinking. The voltage has to do with the transformer feeding it. All the diodes do is rectify the AC to DC, and because the diodes only conduct at the peak of each sine wave, the resulting DC voltage is based on the PEAK of each AC sine wave, not the RMS voltage that we use when describing it. The only reason for that special transformer that you are referring to as "6 phase" (it really isn't) is to create phase shifts in time for when those diodes conduct so as to reduce the harmonics they create, and to provide a smoother DC, less ripple for the capacitors to deal with (if any).

So nothing you do with regard to diodes is going to change the voltage output. If you need a higher voltage output from your rectifier, you need a higher input voltage, either to the primary of your existing 24 pulse transformer (easier) or a new 24 pulse transformer (more difficult). But start with why you think you need higher voltage DC? Standard industrial DC motors designed to be run from drives connected to 3 phase supplies are either 300V for 230V supplies, or 500V for 480V supplies. So if you have 500V motors, you need a 480V 3 phase input. If what you are doing is trying to make up for voltage drop between the drive and motor, you should have a little head room of adjustment in the drive, but if not, you need to move the drive closer and deal with the VD on the AC side.
 
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Discussion Starter · #13 ·
Actually, I need a lower voltage for the contactor control coils in the 36 diode setup (330-340v dc) but I have since adjusted that problem with resistors in series with the coils which are only rated 230v dc but when I noticed the 24 diode setup in another area only had 300v dc, thought maybe I was missing something about the function of the rectifiers. Our system is supposed to be for a 300v dc design motor and control but I have rectifiers putting out 330 to 350 vdc using 36 diodes. This is shortening the life of the control coils unless I add resistors in series but would like to get to the bottom of this. Also 230v ac supply is same to all rectifiers.
 

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Actually, I need a lower voltage for the contactor control coils in the 36 diode setup (330-340v dc) but I have since adjusted that problem with resistors in series with the coils which are only rated 230v dc but when I noticed the 24 diode setup in another area only had 300v dc, thought maybe I was missing something about the function of the rectifiers. Our system is supposed to be for a 300v dc design motor and control but I have rectifiers putting out 330 to 350 vdc using 36 diodes. This is shortening the life of the control coils unless I add resistors in series but would like to get to the bottom of this. Also 230v ac supply is same to all rectifiers.
The bottom of this is to get coils rated to handle the voltage the rectifiers are outputting.:rolleyes::laughing:

Resistors are a band-aid approach that will eventually fail...and serve no good except adding heat (which may be a good thing if the contactors are in a damp or cold environment.)

As for your question on the offsetting I was referring to, it was best explained here:

Tsmil said:
I'm assuming that the diodes used are silicon diodes. Typical forward voltage drop for a silicon rectifier is 0.7V.
I was thinking that extra diodes would help make up that 0.7 volts per diode by adding more, but Tsmil and JRaef are right it won't help.
 

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Discussion Starter · #15 ·
Actually it is kinda funny cause I'm over thinking my solution to correct the problem, but this is kinda of a learning experience for me. There is only the uv coils that need to be changed since they're on all the time, they were designed with a resistor in series, all I did was increase the resistor size. All the other contactor coils are on and off for a short time. I appreciate the responses that affirm what I knew in theory but haven't applied in the real world before so didn't want to act like I knew what I was talking about so I think my next step is to reconfirm my ac feed and transformer taps and verify good diodes and filtering. Any other suggestions of what to check ?
 
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