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Dual alternator question

KiloWhiskey1

Well Known Member
Asking for a friend. He has a dual alternator system with a JPI EDM 930 engine monitor. His A&P wired the backup alternator through the same ammeter shunt as the main alternator. The amp readings on the engine monitor are erratic and in the -100 amp range. Doing some troubleshooting, we disconnected the backup alternator and the engine monitor reads the correct amp draw.

So my questions are:

1. Is there a way to use only one ammeter shunt when using dual alternators?

2. If we added a second ammeter shunt for the backup alternator, how should we wire the engine monitor? In the current setup the JPI only has two ammeter wires, one for the positive and one for the negative side of the ammeter shunt.

Thanks for the help.

Keith
 
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I suspect you wired the standby alternator output wrong.

Also verify the standby alternator pick voltage (should be around 13.8vdc if your normal buss voltage is ~14.2vdc).

Carl
 
What kind of backup alternator is it?
Mine is wire exactly like you describe and monitored by an EI MVP-60.
To be clear, my Monkworktz generator only delivers power if the main PP fails or anytime voltage drops below 13V.
In other words, there is no scenario where both alternator and generator supply power at the same time.
I am not sure how your system is set up? You should be able to turn your alternator field switch off on your main alternator and see what the backup alternator shows on the ammeter.
You already mentioned "disconnecting the backup returns readings to normal.
Now try and shut down the main and see what happens.
 
Thanks Carl and Ernst,

The backup is a B&C which has a field switch on the panel. The manual says to leave it on during normal operation. Its own regulator brings the backup online only when a low voltage situation is sensed. The problem is that the amp readings are always wrong when both alternator field switches are on (normal way to operate).

We didn’t do the install so we’ll need to verify the backup alternator and regulator are wired properly.

Bob Nuckolls’ Z-12 shows a dual alternator single battery setup which uses two ammeter shunts, so that got me thinking that two shunts may be required.
 
It sounds to me like you have related two separate things to your problem that are not related. Two ammeter shunts are not going to correct your problem. They would normally be tied together on the battery side (per Z12) anyway. It sounds like the JPI only has one current measurement input (2 wires) so therefore the AP connected the two alternator outputs together.

You have a different problem. The ammeter shunts are simply low resistance parts where the voltage is measured across the shunt to determine the current being put through the shunt. Ohms law, Volts = Current x Resistance. Yes two shunts would allow you to see the current output from each alternator, but that change would not correct the erratic measurement.

It sounds like you have a separate wiring error. Since the primary alternator is working then I would look at the wiring for the backup alternator. As others have mentioned the voltage regulator for the primary and the backup alternators should be set with different voltage set points. The primary set higher and since it is above the voltage setting for the backup alternator then the backup alternator will normally not attempt to supply current.

The other thing to remember is current is only supplied when it is required by the load. So if you are seeing erratic current readings that could be because there is actually high currents being drawn or effectively a short of some sort when you switch on the backup alternator.
 
... His A&P wired the backup alternator through the same ammeter shunt as the main alternator. ...

Keith, wouldn't this introduce a single point of failure, negating some of the advantages of having two alternators?
 
Keith, tell us what the electrical system voltage is when only one alternator is on at a time.
Like Carl said, there should be at least 0.4 volts difference between the two.
 
which alternator? If you have the SD-8 permanent magnet alternator, it produces AC with no internal rectifier to make it DC, like most other alternators. Instead, the rectifier is in the regulator module. So if the A&P doesn't know what he is doing and wired the output into the shunt BEFORE the regulator, the indication will be very erratic as the shunt now has both AC and DC flowing through it (This short cut would make sense, as the shunt is near the engine and the regulator is near the panel). Shunts are polarity sensitive (AC is switching polarity many times per second) and the instrument is also seeing the rise and fall characteristics of AC, but doesn't account for it as it ASSUMES it will be DC. These instruments need different logic to process DC vs AC. If it is set up this way, I would fully expect the instrument to go wonky.

Larry
 
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I’ll have to wait until we’re both back at the airport to get the answers to these excellent questions. The backup is a B&C BC410 28v 20amp alternator. Not on an RV. I’ll post again after working through the suggestions in this thread. May be next weekend before we’re both at the airport again.

The RV relationship is that if my good friend breaks down in some remote place, I’ll have to go get him I’m my RV :D
 
I have a B&C primary and backup alternator and they both are wired through the same shunt. I have Dynon avionics with the Dynon EMS system. There must be something wrong with the wiring. Both B leads (power leads) should be on one side of the shunt with the other side going to the main bus. The two small leads from the shunt are to measure voltage drop across the shunt. The more the voltage difference, the greater the load.

In my instalation I can only select one alternator at a time to prevent both being selected at once. This also allows me to verify both alternators are in working order prior to flight.
 
Shunt

I've heard of a Hall Effect shunt failing but not the standard issue. I have two alternators and one shunt. I chose a different route than most and installed a 3-position locking toggle for the alternators labeled PRI OFF STBY. I also raised the voltage on the B&C STBY voltage regulator to match the primary at 14.3 volts. I'll set the low voltage warning on the G3X at 13 volts or so. If it goes off I'll know instantly an alternator failed. The shunt will read battery discharge amperage until I select the STBY Alternator.

It was just a choice...

JET
 
One shunt? or Hall Effect sensor for dual alternators?

Bob Nuckolls’ Z-12 shows a dual alternator single battery setup which uses two ammeter shunts, so that got me thinking that two shunts may be required...

Keith- Nuckolls' Z-12 shows dual alternators, and two "Hall Effect" sensors, not a shunt. http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z12N_SingleBatteryDualAlt.pdf
A clever arrangement allows monitoring the Aux Alt only, as well as current from both (main and aux) b-leads.

(Thanks for the correction, Scott.)
 
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Two BC Alternators

I have one BC alternator 60 A and one stand by 40 A.
They have separate shunts.
There is a manual switch so that it´s possible to read the Amps on the G3X, one at a time.
When there is a low volt alarm on G3X, I will have to select the standby shunt.

Good luck
 
Thanks for all the additional replies. I was working off an old edition of Z-12 but I still think the latest drawing shows two places were current draw is being sensed. In this installation, I think the suggestion of using two shunts with a switch to isolate individual shunt readings to the engine monitor would work best. The backup alternator manual calls for the backup field switch to be on all the time so that it will supply current automatically if the main isn’t providing enough output.

I still need to verify the complete wiring of the backup alt, but haven’t had an opportunity to get back to his plane. I’ll post as soon as I know more.
 
SNIP….
In this installation, I think the suggestion of using two shunts with a switch to isolate individual shunt readings to the engine monitor would work best.

Reminder - use of any current shunt is not required (I never put one in an any of my projects). If you must have one (and you still have a problem) then just use the shunt on your primary alternator.

I do recommend you install the standby alternator just like the instructions say, and to alway keep in on line.

Carl
 
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