Onan Generator - no output
Barry Streets
fine.
The model number is 7nhmfa26106c serial # c920455304 and it has about 1300
hours run time on it.
About 20 hours ago it stopped producing output so I dropped it out of the Rv
and went through cleaned everything up, replaced spark plugs air filter. I
bought new brushes after talking with a local RV tech. When I went to
replace the old brushes, they were longer than the brand new ones so I first
cleaned the slip rings and put them back. Starting it up on the bench it ran
fine and I could load to 50% with the voltage at 118vac. So I reinstalled it
and it seemed to be fine even running both air conditioning. Now about 20
hours later it again stopped producing voltage. I checked the voltage at the
brushes and got less than a volt. I'm thinking that the voltage regulator
module need to be replaced.
Is there any thing else I should check first????
Barry Streets
Calif Bill
"Barry Streets" <berry...@comcast.net> wrote in message
news:ZcOdnSSIF5CyIVbb...@comcast.com...
Check to see if a brush is broken or stuck.
Alan Robinson
Barry,
The BGM /NHM Marquis, spec B onwards, is one of the few Onan rv gensets
that WILL run without producing output - this is because, as an
electronically governed set, it uses a signal from the governor to tell that
the genset is running, rather than the presence of output from the set.
From the information you've given, there's a pretty good possibility that
the regulator is ok and the problem is elsewhere. The genset passes 'field
flash' voltage (12v dc) from the control board, to the regulator, and then
to the brushes/slip rings and rotor windings when you are starting the set.
This 'field flash' establishes and intial magnetic field in the rotor, which
produces some output in both the 'main' windings and a special winding (that
Onan calls a quadrature winding) which provides power to the voltage
regulator. The regulator uses the power from the quadrature winding to
provide a higher dc voltage to the brushes/slip rings/rotor windings, which
produces higher voltage in the main and quadrature windings. Since the
regulator now has more power available from the quadrature winding, it can
send a yet higher voltage to the rotor winding, which produces more output,
round and round, etc. until the regulator senses that output from the main
windings has reached the desired level, at which point in time it stabilizes
the voltage it is sending to the rotor winding (and will adjust up or down
as needed to maintain output voltage despite load changes).
If you followed the above, the key point is that the field flash voltage
MUST make it thru to the brushes - so that the genset has some initial
output from which it can start building up.
Set your meter to read DC volts and hook one lead to each of the brushes.
(Positive brush to right, negative brush to left). Check voltage while
cranking - should be 10-11v dc, which NORMALLY would build to 30+ volts once
the regulator has something to work with.
If you DON'T see the 10-11v dc at the brushes, start checking at the
control board - field flash leaves the board on connector P1 pin 3, and goes
to connector P4 pin 7of the voltage regulator. Leaves the regulator on P4
pin 9, goes to brush F1 (+/right), thru the rotor winding, back thru brush
F2 (-/left), back to the voltage regulator at connector P4 pin 10, thru the
voltage regulator, and then to ground on P4 pin 8 (to the L0 terminal
section of the connection strip in the back of the control box, which is
grounded). Any connection problem anywhere along the way means no initial
magnetic field to build up from - and thus no output. Or the control board
may not be putting out field flash... On the other hand, if you DO have
field flash voltage to pin 7 and ground to pin 8 of the regulator and don't
see field flash leaving / returning to the regulator on 9 and 10, then the
regulator is bad.
If you DO see the 10-11v dc at the brushes while cranking, but drops to
near 0 when switch is released and genset running, set meter to ac and check
P4 pins 2/3 (main winding voltage) and P4 pins 11/12 (quadrature winding
voltage) when gen starts with switch still depressed - main winding should
be about 40-50v, quadrature should be about 15% higher than main winding. If
these voltages are correct and voltages die away as soon as switch is
released, then regulator is bad. If voltages are not correct (particularly
if quadrature is lower than main), regulator MAY be bad and loading down the
quadrature output, or there may be shorted turns in a winding. Easiest way
to tell is disconnect regulator, start genset, and provide dc power directly
to brushes (thru a 5a fuse, please).
Since it started working when you had everything apart, then stopped
working later, I'm guessing that you have a bad connection somewhere -
either in the field flash circuit or the quadrature circuit.
Alan
Barry Streets
hour. Allowing it to cool overnight didn't fix the problem.
Barry Streets
"Calif Bill" <bmcke...@ix.netcom.com> wrote in message
news:13cniai...@corp.supernews.com...
Barry Streets
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 Streets" <berry...@comcast.net> wrote in message
news:ZcOdnSSIF5CyIVbb...@comcast.com...
Alan Robinson
> 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
Barry Streets
back. In general the generator has worked so well up to now that it a bit
frustrating.
Barry Streets
"Alan Robinson" <a...@bmi.net> wrote in message
news:5jc52rF...@mid.individual.net...
Barry Streets
the field resistance at the plug going to the voltage regulator. Across pins
9 and 10 I got 67 ohms, then at the brushes I got 67 ohms. I then removed
the brushes and measured between slp rings and got 25 ohms. Bingo I then
went ahead and replaced the brushes even though the new ones were a bit
shorter than the original ones. I also cleaned the connections at the
brushes until they were shiny bright. After reassembling the brushes back in
place; I then measured at pins 9and 10 and got 26 ohms. I'm not sure why
changing brushes would have such a change in values but I can't argue with
success. The generator seems to work fine even under a load of both ac units
running. Again Thanks Alan and the rest for all your valuable advice.
Barry Streets
"Barry Streets" <berry...@comcast.net> wrote in message
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