# rectifying AC to DC Math



## DieselTwitch (May 19, 2009)

Im am trying to figure out how to calculate a few things...

1.AC Voltage as a function of RPM of a generator. or is just Hz affected?
ex. if i have a 20kW 3 Phase generator that normally runs at 1800 RPM to produce 480 VAC /60 Hz. if I slow it down or speed it up how does that effect my Voltage and Amperage? I know that Hz is directly proportional to RPM.

2. How do i calculate the rectified DC Voltage and Amperage based on the previous out come?
ex. If 240 volts at 60Hz and 24 Amps (Max for a 20Kw 3P) and i rectify with a bridge diode array what would my DC Voltage and Amperage be?

3. What can I do to get higher DC voltage, around 600VDC? and what would getting this voltage do for my DC Amperage.

Basically Im looking for all the math involving all those questions.

Thank you for any help you have.


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## azdeltawye (Dec 30, 2008)

DieselTwitch said:


> Im am trying to figure out how to calculate a few things...
> 
> 1.AC Voltage as a function of RPM of a generator. or is just Hz affected?
> ex. if i have a 20kW 3 Phase generator that normally runs at 1800 RPM to produce 480 VAC /60 Hz. if I slow it down or speed it up how does that effect my Voltage and Amperage? I know that Hz is directly proportional to RPM.


If your generator is self excited than changing the speed should have a proportional change in voltage as the generator tries to maintain a constant volts per hertz ratio. If the generator has external excitation than you can keep the frequency (speed) the same and change the field current. However I wouldn’t deviate more than +/- 10% from nameplate ratings on either frequency or voltage AND I would de-rate the output power by the same amount.




DieselTwitch said:


> 2. How do i calculate the rectified DC Voltage and Amperage based on the previous out come?
> ex. If 240 volts at 60Hz and 24 Amps (Max for a 20Kw 3P) and i rectify with a bridge diode array what would my DC Voltage and Amperage be?


The average DC voltage output of a three phase full wave bridge rectifier (unfiltered) is 3Vp/∏. Where Vp is Vrms * √2. For your 480V 3 phase generator above the average DC voltage out of a hex rectifier bridge would be approximately (3*480*√2)/ ∏ or 648V DC. However if your load was inductive than the voltage would be greater since the inductance would tend to smooth out the ripples on the rectified waveform. How much greater would depend on the value of the load inductance and resistance. The maximum DC voltage would approach 480*√2/ ∏ or 678.8V as the ripple was decreased due to filtering.










DieselTwitch said:


> 3. What can I do to get higher DC voltage, around 600VDC? and what would getting this voltage do for my DC Amperage.


If you wanted 600VDC then you should be able to reduce the speed or excitation of the generator enough to meet that goal. The amperage would depend on what the resistance of your load was. 

For 648V the maximum current would be 20kW/648V or 30.9A. 


Hope that answers some of your questions.


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