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How Dc Output > Ac Input At Bridge Rectfire


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#1 AB2005

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Posted 19 September 2011 - 07:47 AM

Dear Marke;

I was testing a DC motor 6.5Kw via a variable out put transformer where the out put of transformer was fed into a bridge rectifier and then to Armature (no any capacitor was connected). Field was fed via separate transformer. Summery was as following,

Field

Input to bridge rectifier = 110VAC

Output = ((110 x 1.414)-1.4) x 0.636) = 98VDC OK

Armature

Input to bridge rectifier = 102VAC

Output to Arm = 126VDC while it should be;

((102 x 1.414)-1.4) x 0.636) = 91VDC

I also measured the voltage via analog meter and result was same. I increased the AC input voltage to bridge and found the same results that output voltage at bridge were always > than input voltage.

Can you please explain why the voltage were 126VDC instead of 91?
"Don't assume any thing, always check/ask and clear yourself".

#2 marke

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Posted 19 September 2011 - 09:54 PM

Hello AB2005

If you look at the armature voltage with a scope, you will see that the waveform does not look like a normal rectified wavewform which your calculations are based on.
The voltage that you are measureing is a combination of the voltage from the rectifier plus the voltage generated by the armature spinning in the field.
If you apply load to the motor, it will slow and the voltage willl reduce.

Best regards,
Mark




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