Re: 1981 XV750 Virago Charging Problem.
- From: "FB" <flying_booger@xxxxxxxxx>
- Date: 7 Apr 2006 10:51:00 -0700
John Hermann wrote:
One thing I should have elaborated on was the measured voltage
at the battery while the engine is running. I get 13+ VDC when
at idle, but when I rev the engine up, the voltage drops to as
low as 12.75VDC; I would have thought the opposite would happen;
strange, but maybe a clue to the problem.
The voltage sensing device in a shunt regulator is a zener diode. It
conducts when the voltage reaches a certain level, causing a silicon
control rectifier to ground out one phase of the stator.
This drops the output current by half, since each phase needs the other
two phases for a return path. The AC current just keeps rotating from A
to B to C and back to A phase and on and on at a variable frequency.
The diodes have internal resistance that provides a load on the AC
phases so they aren't shorted out if they are in good condition.
If all the diodes have the same resistance in one direction and read
infinity the other direction they are OK.
But the zener diode trigger voltage should be around 14.5 to 15.0
volts.
Your description tells me that the zener never triggers, so the Vreg
isn't regulating, but it's
still rectifying AC into DC.
If the zener doesn't trigger, the SCR is always in the non-conducting
state so it never *sinks* the power into the Vreg's heat sink.
But you say thatactually voltage *drops* with rising RPM. That suggests
to me that current is being sunk into a shorted cell in the battery or
you have a lot of I squared R loss in your connections and you're
burning up power there.
Power dissipated in a bad connection equals the current SQUARED times
the resistance of the connection.
So, if your system needs, say 120 watts to run the lights and ignition
and the alternator is putting out 12 volts, the current passing through
the alternator output wires will be 10 amps according to Ohm's law.
If a poor connection has only 1 ohm of resistance, the power dissipated
at the connection will be 10 X 10 X 1 = 100 watts. You know you can't
put your hand on a 100 watt light bulb, so the
bad connection has to be getting very hot.
Maybe the bad connection only has 1/4 of an ohm of resistance, but
that's still
10 X 10 X 0.25 = 25 watts at the bad connection.
So, check out the places where the alternator starter hooks up to the
motorbike's wire harness and where the Vreg output hooks to the
harness.
Look for brown or black insulation and melted plastic connectors.
.
- References:
- 1981 XV750 Virago Charging Problem.
- From: John Hermann
- Re: 1981 XV750 Virago Charging Problem.
- From: Wudsracer
- Re: 1981 XV750 Virago Charging Problem.
- From: John Hermann
- 1981 XV750 Virago Charging Problem.
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