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Old 03-07-2011, 09:27 PM   #1
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Alternator AC Voltage

What should the alternator AC voltage at idle speed be at the wire that goes from the alternator to the tachometer?* The alternator is a 65 Amp. Leece-Neville.* Does this voltage increase as the RPM increases?* Mine was reading a steady 16.00 volts AC at about 700 RPM.
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Old 03-08-2011, 04:55 AM   #2
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RE: Alternator AC Voltage

You have an AC alternator, or do you mean DC. If DC, 16 volts seems a little high to me. Mine puts out +/- 14.5 volts by my dash gauge. Voltage should not increase with RPM, but of course your amperage output will.
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Old 03-08-2011, 06:58 AM   #3
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RE: Alternator AC Voltage

Alternators make AC voltage that is converted to DC.* However, the terminal that gets connected to the tachometer is measured in AC.* This terminal produces the pulses that are measured as RPM on the tachometer.* I am having a tachometer problem and the shop said to measure the AC voltage at the alternator terminal that has the wire that goes to the tachometer.* I am not sure what voltage I should expect to see at this terminal.* When I measured the voltage yesterday on the AC range I was getting 16.00.
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Old 03-08-2011, 08:29 AM   #4
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RE: Alternator AC Voltage

I had no idea that alternators make AC power. That's the best thing about this forum, the learning. Now I have to relearn everything I thought I knew about alternators.
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Old 03-08-2011, 11:39 AM   #5
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RE: Alternator AC Voltage

I call it an AC cruise generators that the main engine powers.* We have a DC alternator at one end and a 5 KW* 120 volts AC cruise generator with an electric clutch at the other of the DD 671.* *When cruising, I shut the main gen set down and switch over to the cruise gen.* Cruise gen. are an option with the air conditioner packer on smaller boats, and some what popular on sail boats.* I put a 17 inch pulley on the 671 so at idle, 750 rpm, it does not brown out.*

An AC 120 volt cruise gen made more sense to me than altenator, batteries, and invert.* We also have inverter and battery, but very seldom have used it.*
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Old 03-08-2011, 06:30 PM   #6
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RE: Alternator AC Voltage

Oh boy did this post ever go sideways !!

Reefdrifter - you are correct with the assumption that the "TACH" terminal of your alternator is AC but is actually more a pulsed voltage than an alternating one. It is years since I last worked on such a system and the old brain has very little recall back that far as to what a normal voltage would be however you should be able, with a decent AC voltmeter see some increase in voltage when you increase the engine speed.
What is actually happening is the alternator is producing these voltage "pulses" in relation to the speed it is rotating but not actually changing the voltage at the same time. Most AC voltmeters will read a slightly higher voltage when the frequency goes up and that is what you should see when you increase the engine RPM's
Hope this helps you out and I am currently looking in some of my old manuals to see if I can locate the relative "TACH" voltage for your series of alternator.

Good luck
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Old 03-08-2011, 10:13 PM   #7
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RE: Alternator AC Voltage

Sideways slips do happen..........no big deal unless we hit something...LOL
I guess at this time I am wondering if my reading of 16 at the tach terminal at an idle speed of about 700 RPM is what should be expected. My fairly new VDO tach at the upper helm station quit working until I shut down the engine. When I started it up again the tach started to work but was not reading the correct RPM. I tried to recalibrate it but with no luck. The tach shop in Ft. Lauderdale said to check the AC voltage at the tach terminal so now I am wondering what the correct voltage would be. As far as I can tell, all the wiring is ok all the way up to the helm station with the 16 volt reading. I sent the tach into the shop to be bench tested so I will know if something went bad with the electronics.
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Old 03-12-2011, 07:07 PM   #8
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RE: Alternator AC Voltage

Just checked the voltage on my alt. 7.26 @700rpm and 7.26 @1000rpm. My alt. is an Autolite (Ford) but they all have their differences. The voltage will vary depending on the number of lobes on the rotating assembly. There is a formula to figure what the voltage should be. If you have a steady voltage at the tach output you should have an indication at your tach.

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Old 03-13-2011, 12:42 PM   #9
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RE: Alternator AC Voltage

I thought I'd posted this before but the netherworld gremlin seems to have prevailed. Maybe I chickened out since I'm no eggspert either.

The tach out put terminal frequently produces a lower voltage, ~6-8 volts at low current capacity. It is usually/often run by a small diode trio set, separate from the main power output, that yields a 1/2 wave current meaning only 1/2 of the AC is presented which also shows as a lower voltage than the main output power terminal. The other half of the sine wave is completely blocked. That produces a highly pulsing signal to the tach. As your alternator changes speed the frequency changes and it is that frequency change that the tach reads, not the voltage.

That signal can be degraded by poor wiring or terminals including at the tach. The tach itself must be able to be set to read or scaled to the alternator frequency. The frequency may be different from mfgr to mfgr depending on how many poles are on the rotor. If this is a recent change then I suspect the diode trio. If this is an incompatibilty between a NEW alternator and old tach then you may have to try scaling changes if possible which may mean going to the tach mfgr..

You seem to have a higher than expected voltage at the tach terminal. Did you measure it with the tach attached or unattached? Without ANY load at all the voltage presented may be higher than expected with the tach attached. A DMM loading is so tiny it is effectively no load in places like this. It also could mean one or more of those trio diodes is shorted and allowing to many pulses through which effectively raises the voltage seen by the meter and at the same time reduces the pulsing effect meaning the tach no longer has a clean, clear frequency to read.

I hope this helps,
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Old 03-13-2011, 05:08 PM   #10
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RE: Alternator AC Voltage

Quote:
reefdrifter wrote:

Sideways slips do happen..........no big deal unless we hit something...LOL
I guess at this time I am wondering if my reading of 16 at the tach terminal at an idle speed of about 700 RPM is what should be expected. My fairly new VDO tach at the upper helm station quit working until I shut down the engine. When I started it up again the tach started to work but was not reading the correct RPM. I tried to recalibrate it but with no luck. The tach shop in Ft. Lauderdale said to check the AC voltage at the tach terminal so now I am wondering what the correct voltage would be. As far as I can tell, all the wiring is ok all the way up to the helm station with the 16 volt reading. I sent the tach into the shop to be bench tested so I will know if something went bad with the electronics.
Bob, Check the circuit breaker on the line going from the alternator to the batteries.* If it is tripped, I think it can cause voltage to read high.

*
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Old 03-13-2011, 06:44 PM   #11
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RE: Alternator AC Voltage

I took the alternator off and had it bench checked. The alternator guy said all is ok with the alternator voltage both DC and AC. According to the alternator shop if you check my alternator AC tap with a digital meter it reads 16, if you check the same AC tap with an analog meter it reads 8. Which is the voltage that should be read. Go figure.....I am wondering what difference does it make if you are reading with a digital or analog meter? He says there is no problem with the alternator.
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Old 03-13-2011, 07:12 PM   #12
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RE: Alternator AC Voltage

Quote:
reefdrifter wrote:

I took the alternator off and had it bench checked. The alternator guy said all is ok with the alternator voltage both DC and AC. According to the alternator shop if you check my alternator AC tap with a digital meter it reads 16, if you check the same AC tap with an analog meter it reads 8. Which is the voltage that should be read. Go figure.....I am wondering what difference does it make if you are reading with a digital or analog meter? He says there is no problem with the alternator.
Do you have an internal or external voltage regulator?* I still think you should look for a circuit breaker or fuse between the batteries and alternator.* It could be an open circuit.

*
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Old 03-14-2011, 07:14 AM   #13
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RE: Alternator AC Voltage

Don,

It is an external regulator. I will look for a circuit breaker or fuse today when I go to reinstall the alternator.
Thanks for the tip. I will see what I can find.
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Old 03-15-2011, 04:50 AM   #14
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RE: Alternator AC Voltage

"I still think you should look for a circuit breaker or fuse between the batteries and alternator. "

This is almost NEVER done as any momentary disconnect of the alt from the batt while operating ,will usually destroy the alt diodes.

There might be a fuse TO the V reg .
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Old 03-15-2011, 06:42 AM   #15
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RE: Alternator AC Voltage

Quote:
FF wrote:

"I still think you should look for a circuit breaker or fuse between the batteries and alternator. "

This is almost NEVER done as any momentary disconnect of the alt from the batt while operating ,will usually destroy the alt diodes.

There might be a fuse TO the V reg .
FF, I didn't mean for him to start disconnecting anything.* I have had the situation where the voltage meter on my port engine read 16 volts.* In running down the problem, I found the circuit breaker tripped.* It was probably tripped as I was leaning over to work on the air conditioners and pushed the button with my ankle.* Reset the circuit breaker.* Voltage went down to about 14.5 volts.

*
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Old 03-15-2011, 11:10 AM   #16
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RE: Alternator AC Voltage

I had a 16 V reading on my boat several months ago and the electrician traced it back to the internal regulator on my alternator. (Which was running very hot!) He told me that this condition could ultimately fry my batts. The alternator was rebuilt and an EXTERNAL regulator added. Problem went away.
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Old 03-15-2011, 06:43 PM   #17
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RE: Alternator AC Voltage

I have never had a problem with the output DC voltage of my alternator but rather I want to check the voltage at the AC tap that the tachometer gets attached to. The alternator puts out exactly 14.4 volts DC. My question is how do you check and what voltage would you expect to see at the AC tap for the tachometer. I have read that this voltage should be about half of the output DC voltage but do you read this voltage at the AC tap on the AC Volt scale or the DC volt scale of the multimeter. I have been reading from the AC volt scale at 16 volts but am I doing it right? Should I switch to the DC scale?
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Old 03-18-2011, 06:55 AM   #18
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RE: Alternator AC Voltage

Quote:
DCBD wrote:

I had no idea that alternators make AC power. That's the best thing about this forum, the learning. Now I have to relearn everything I thought I knew about alternators.

In technical terms, an "alternator" produces AC, a "generator" produces DC.* Remember back in the good old days, your car had a generator, not an alternator.

Enter advertising and common useage, and, just like the engine/motor debate, an "alternator" on a boat or vehicle engine produces DC because it has internal diodes to convert the AC into DC.

What about that "generator" on your boat that produces 120 volts AC?* It's really an "alternator" because it produces AC.

English is a pretty screwed up language.*
*
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Old 03-19-2011, 04:45 AM   #19
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RE: Alternator AC Voltage

Battery cooking can cause a fire , it is prudent to only monitor the bat voltage with a digital , not an analog meter.
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Old 03-23-2011, 07:38 AM   #20
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RE: Alternator AC Voltage

Here is what I found out about measuring the voltage at the tachometer tap on the alternator.

My alternator puts out 14.4 volts DC after the AC voltage has been rectified. The tachometer tap should read half of the output DC voltage. Even though it is an AC tap it has to be measured using the DC scale of your multimeter. Since my alternator puts out 14.4 volts DC, I set my meter on the 10 scale knowing I should see around 7 volts. Sure enough I am reading 7 volts DC using my analog multimeter. I was told by two different alternator shops that you can't get a reading using a digital multimeter. I decided I would check it anyway with my digital meter and I got a reading of 7.11 volts. I don't know why they would say you can't get a voltage reading by using a digital meter. Anyway, that is how you read your AC tap voltage.........use the DC scale..........go figure!!!!
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