Re: Onan Generator - no output

Thanks Alan, for the input and I will recheck my measurements and report
back. In general the generator has worked so well up to now that it a bit
frustrating.

Barry Streets

"Alan Robinson" <alr@xxxxxxx> wrote in message
news:5jc52rF3spiacU1@xxxxxxxxxxxxxxxxxxxxx

"Barry Streets" <berrys2552@xxxxxxxxxxx> wrote in message
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Update, today I started to do more testing first the field resistance
measures 62 ohms so I went ahead and reseated ther brushes and rechecked
the reading. Next I srated the unit and measured the voltage at the
brushes; I got 34 to 36 volts. Checking the output I now get 121VAC and
119 vAC with both AC units running. I'm wondering if there is a loose
connection somewhere, as it didn't produce output whenI tried it several
times before. Suggestions anyone?????

Barry Streets


Barry,
If that 62 ohms was measured from brush to brush, then that is the
prime suspect. The rotor windings themselves should measure 21 to 25
ohms - added resistance thru the slip rings and brushes should add no more
than another 2 ohms. Recheck your reading, then remove the brush block and
use your meter probes to read directly from one slip ring to the other. If
there is more than 2 ohms difference, then you need to clean the slip
rings and brush tips.
Keep in mind that building output voltage from nothing - when the
genset isn't running - to the final voltage is a multi-step process. While
you are cranking the genset and have the start switch pressed, the control
board passes 12v battery voltage to the voltage regulator, which passes it
on to the rotor thru the brushes and slip rings. This initial voltage
(about 11v across the brushes) is called 'field flash', and provides some
magnetism to the rotor - which produces some output in the main windings,
and also in a separate winding (which Onan calls a 'quadrature' winding)
that feeds its output to the voltage regulator. The voltage regulator
takes the output from the quadrature winding and turns it into a higher
voltage (than the 12v from the battery) that it feeds thru the brushes and
slip rings to the rotor windings. This produces a stronger magnetic field,
which produces a higher voltage in both the main and quadrature winding.
The higher voltage in the quadrature winding is used by the voltage
regulator to produce an even higher voltage which it feeds to the rotor,
which results in a stronger magnetic field, etc. etc., round and round
until the voltage regulator senses that the main winding is producing the
desired voltage, at which point it holds the voltage it is feeding to the
brushes more or less constant, increasing or decreasing as needed to
maintain the desired main winding voltage.
The point is - if the field flash magnetism is weak - which is
certainly the case with 62 ohms resistance - the output of the quadrature
winding may not be enough to allow voltage to build up. This will
certainly be the case if you release the start switch as soon as the
genset starts to fire, instead of waiting until it gets up to speed (the
voltage produced is a function of both the rotor field strength AND the
rotor speed...).

Alan




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