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Air Bender
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Joined: 25/01/2011
Location: Australia
Posts: 206

Posted: 06:55am 22 Feb 2011

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While checking out the output of my mill today which has a couple of bridge rectifies wired in I switched the multi meter from DC volts to ac volts and the voltage jumped from 20 to 50 volts. Is this normal or have i wired my rectifyer wrong. And if it is normal which voltage output do i take notice of.

All the best Dean.

Tinker

Guru

Joined: 07/11/2007
Location: Australia
Posts: 1904

Posted: 02:12pm 22 Feb 2011

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Dean, I think a simple sketch what you measured where would help a great deal to make sense of your question.
An answer based on guesses does not help, does it

Klaus

Alasdair
Regular Member

Joined: 12/01/2011
Location: Australia
Posts: 62

Posted: 04:26am 23 Feb 2011

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Hi Dean
if your mill has an alternator, the rectifiers need to suit the winding
layout, ie single phase windings can use bridge rectifier, multi phase
windings need diode configurations to suit. Your multimeter may still
not give correct output voltage readings with dc, because the voltage
will have significant ripple which as the meter grabs samples it will
appear to continuously show different readings, a filter capacitor will
smooth it out, but show an off load peak voltage (peak of ripple cycle)
an older analogue meter can give a better value, but you still need
to allow for a drop under load. It's probably best to rectify nearer your
batteries or inverter, as dc enhances corrosion at any exposed point,
especially slip rings and connections, ac doesn't, plus it's easier to
service stuff at ground level.
Cheers Alasdair.

Amc-elec

Air Bender
Senior Member

Joined: 25/01/2011
Location: Australia
Posts: 206

Posted: 10:20am 23 Feb 2011

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i think the wording of my question may not have been very good. I think the problem is proberly my ignoance in using a multi meter. I had the multi meter conected to the two coming from the rectifiers that will nornaly be conected to a battery and when i switched the multi meter from dc volts to ac volts it showed twice as many volts i was just presumed that both voltages should have read the same and that is where i am probebly wrong. i took my multi meter and conected it to a battery and it done the same thing reading the same twice as many volts when i switch the multi meter from dc volts to ac volts. So now i dont think it is problem with my rectifing. it is proably just my ignorance on how a multi meter works i shoudnt de trying to measure dc volts with the multi meter switched to ac volts.

Thanks Dean

Alasdair
Regular Member

Joined: 12/01/2011
Location: Australia
Posts: 62

Posted: 10:36am 23 Feb 2011

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Hi Dean
that is an easy mistake, digitall meters will show weird numbers
when in the wrong range especially AC over DC
Cheers Alasdair.

Amc-elec

Greenbelt

Guru

Joined: 11/01/2009
Location: United States
Posts: 566

Posted: 03:04am 02 Mar 2011

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Air Bender,
I have a little information that may help.

The sine wave in AC voltage is a graphical indicator of Voltage value at some instant in time.you will notice the Wave is imposed on a straight line, this line represents
(time)as the cycle progresses. The sine wave is considered a circle that the rotor must travel in one revolution.
As a generator, the rotor Magnet approaches a field coil, the closer it gets the higher
the voltage in the coil. the magnet passes the coil, the voltage begins to drop, at half the cycle 1/2 turn, the Voltage is (0) at the center line.

The next field coil has the winding direction reversed so the voltage polarity is reversed, this negative half turn mimics the conditions of the first half.
the cycle begins at (0) voltage on the center line. you can draw several lines parallel to the center line, above it and below. right through the sine wave.

Where each line crosses the sine, this is a different Voltage value.
The TOP an BOTTOM of the loop is the (PEAK) Voltage.

It was discovered long ago that a DC voltage from a battery is at PEAK Voltage continuously.and current flow was steady through the resistance. It was figgered out that the AC Voltage of 100, would only give the power equivalent of 70.7 volts from a battery. So the Engineers devised a system to make the energy in kilowatts the same. 240 volts off the grid transformer is actually 1.415 times higher, that is, The PEAK Voltage is higher.giving a averaged out power equivalent to DC 240.

All the lip above is to give a reason for the difference in the way that a Meter reads the Voltage. An AC Meter measures RMS VOLTS, translated to (Root Mean Square)
so the numbers are placed at 70.7% of the true Peak Volts.
When you connect an AC meter to DC it may only have a half wave clipper inside,
The meter would expect a pulse from the negative half cycle but DC is steady.those Digital meters are Hyper anyway. Hope you're not to confused now. Cheers.

Time has proven that I am blind to the Obvious, some of the above may be True?

Alasdair
Regular Member

Joined: 12/01/2011
Location: Australia
Posts: 62

Posted: 02:02pm 02 Mar 2011

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Man that's using a sledgehammer on a birds egg. Just use the
basic math for rms conversion. For peak efficiency your magnetic
flux lines should synchronously sweep through full N/S cycle
per winding / per phase multiplied by the number of poles per
revolution for each phase. You need the magnetic field to
achieve maximum transition to achieve maximum peak to
peak voltage in a single cycle. Three phase, as Nicola Tesla
found out does the most efficient job in both motor and
generator pole configuration, as near to perfect as can be
made.
Regards Alasdair.

Amc-elec

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