How Dc Output > Ac Input At Bridge Rectfire
Posted 19 September 2011 - 07:47 AM
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,
Input to bridge rectifier = 110VAC
Output = ((110 x 1.414)-1.4) x 0.636) = 98VDC OK
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?
Posted 19 September 2011 - 09:54 PM
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.
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