Amps cut in half?

[Stephen Trapani]

An electrical guy once explained to me why the amp readings are cut in half, but I forgot what he said. Does anyone here know?

Last wknd when I hooked up my new alternator (rated at 60amps) the in line amp meter I have installed read 60amps while the Fluke clamp meter read 30amps right there on the wire coming out of the alternator. Why is that?

 

[Mark Maulden]

This is sorta wide open.... Assuming things are connected correctly, first thing I'd do is verify your "instruments" are reading correctly. If your batteries are charged (via shore power etc), the alternator won't charge that high for any length of time. You probably know the wattage of your cabin lights. Add them all up off of one circuit and divide by the voltage and that's what the "amps" should be going through the breaker. Your clamp on should be able to measure that at the breaker for verification. What if you moved the clamp-on toward the battery? Alternators have magnetic fields which might possibly be interfering with the reading... Then again, your batteries would have to be somewhat discharged...Depends on your battery size. Does your alternator ammeter have an external shunt?

 

[JohnShannon]

If its right its a Fluke. Ammeters should read amps, you shouldn't have to do math on the result. Think about it, the guy who made the meter could have put any set of numbers he wanted to on the card. What would have motivated him to put the wrong numbers?

Now as Mark hints at perhaps you have an external shunt (resistor) that is twice the value it should be, and this is causing your meter to read high.

 

[Stephen Trapani]

Until I can get back on the boat (5 hr drive from here), deduction is best I've got. The 60/30 readings were immediately upon startup, when I know the battery banks were near dead. I just assumed the ammeter was giving me the right reading and the Fluke was doing some mathematical trick, or the electricity was. I could have sworn an electrical guy once explained this phenomenon to me, but maybe I misremember.

As the batteries charged, the readings dropped, but the Fluke on the wire coming off the alternator was always half the ammeter. And yes, the Fluke was hanging within 12 inches of the alternator, so maybe there was some interference, however you think it would have varied as I handled it. Occasionally I would unclamp the Fluke and reclamp it so it wasn't always in the exact same position, but it kept reading half the ammeter.

Of course, I WANT the Fluke to be wrong and the ammeter to be the right reading, since it was saying my alternator was putting out close to it's rated charge initially.

 

[kloudie1]

Steve, some of the older Fluke clamp meters are not capable of measuring DC amperes except by connecion of a special unit to ports in the instrument.. The alternator puts out a DC voltage with a "ripple" in it (not pure DC volts/amperes) which can kinda confuse clamp meters.. Ya might want to e-mail the Fluke folks and tell 'em the model of the meter and tell them what you are seeing.. Those guys will have some good info.. but if your particular fluke does not have a setting for measuring DC amperes, (it would be a capital A with a straight line over it ) it can't measure them correctly and is giving you a best guess based on the ripple it is seeing from the alternator..

 

[Stephen Trapani]

Steve, some of the older Fluke clamp meters are not capable of measuring DC amperes except by connecion of a special unit to ports in the instrument.. The alternator puts out a DC voltage with a "ripple" in it (not pure DC volts/amperes) which can kinda confuse clamp meters.. Ya might want to e-mail the Fluke folks and tell 'em the model of the meter and tell them what you are seeing.. Those guys will have some good info.. but if your particular fluke does not have a setting for measuring DC amperes, (it would be a capital A with a straight line over it ) it can't measure them correctly and is giving you a best guess based on the ripple it is seeing from the alternator..

I do have the Fluke in hand. The setting I used is a captital A under a solid line with a dashed line underneath it.

 

[Maine Sail]

I do have the Fluke in hand. The setting I used is a captital A under a solid line with a dashed line underneath it.

Which Fluke model # is it?

 

[Bill Roosa]

First off the clamp on meter will only be effected by the magnetic field of the wire(s) that are clamped. Any other magnetic fields cannot effect them due to Maxwells equation of flux linkage. The has been demonstrated on numerous occasions. So it is not interferance. the meter is set to DC so any AC component (ripple) will be subtracted out. Normally that is on the order of the voltage ripple and is small compaired to the total current. In any case both ammeters would be effected in the same fashion and to the same extent.
The only thing left is the shunt's calibration or the gages calibration. Since it is always 1/2 I'm thinking you have the wrong shunt for the gage or there is a calibration resistor that has been left out (installed wrong) in the shunt wiring. That is easy to fix BTW. The gage is just a volt meter and the shunt is just a known resistance (the part that is attached to the gage at least) So when a current flows through the shunt it experiences a voltage drop which the gage measures. If you mess with the resistance of the shunt you WILL get different readings on the gage. If memory serves there are only two different resistance rating for shunts. If should be stamped on the shunt and also on the back of the gage. compair and see if they are the same rate. It should be milivolts/amp (mV/A) or just milivolts

 

[Bill Roosa]

What is the alternators max output?

 

[Stu Jackson]

Also, what is the "in-line ampmenter" you have?

 

[Stephen Trapani]

Fluke model #336. The Max output of the alternator is 82, tested. I'm sorry the in line meter on the boat is 5 hrs away and I don't know the make offhand.

 

[Stephen Trapani]

Bill if you tell me where the "shunt" and "gauge" are that you are referring to I think I can figure out what you were saying, but go easy on me, electricity is not my field (no pun intended). ;-)

 

[JohnShannon]

The shunt is a resistor, it might look like two brass blocks with wires screwed into them, one heavy (carrying current) and one small (measuring the voltage and going to your meter) mounted on some plastic and connected by a strip of metal (that is the calibrated resistance.

digikey.com and type in shunt you will find some pictures.

Alternatively you have an ammeter with onboard shunt, in which case heavy wires atttach to the back of your ammeter.

The good news is stuff is charging.

 

[Stephen Trapani]

The shunt is a resistor, it might look like two brass blocks with wires screwed into them, one heavy (carrying current) and one small (measuring the voltage and going to your meter) mounted on some plastic and connected by a strip of metal (that is the calibrated resistance.

digikey.com and type in shunt you will find some pictures.

Alternatively you have an ammeter with onboard shunt, in which case heavy wires atttach to the back of your ammeter.

The good news is stuff is charging.

This shunt is on the Fluke? On the alternator? I finally talked to the guy who loaned me the Fluke. I think he's going to have it tested.

 

[kloudie1]

Steve, you are talking about a normal panel meter that has its shunt internally.. the one ya just hook by itself into the panel, right?? No shunt on the alternator lead somewhere that the meter is hooked to?? On the Fluke, it sounds like you have it all hooked up correctly.. .. I know that there are two distinct circuits for reading DC amperes (Hall effect circuit) and AC amperes (simple current transformer) on a clamp meter.. I also know that some of the DC reading circuits can be fooled by ripple riding on the DC.. My gut feel is that the panel meter is telling you the truth .. (unless there is a shunt in the circuit and the meter has been changed to one that reads higher , without changing the shunt..)

 

[Stephen Trapani]

Steve, you are talking about a normal panel meter that has its shunt internally.. the one ya just hook by itself into the panel, right?? No shunt on the alternator lead somewhere that the meter is hooked to?? On the Fluke, it sounds like you have it all hooked up correctly.. .. I know that there are two distinct circuits for reading DC amperes (Hall effect circuit) and AC amperes (simple current transformer) on a clamp meter.. I also know that some of the DC reading circuits can be fooled by ripple riding on the DC.. My gut feel is that the panel meter is telling you the truth .. (unless there is a shunt in the circuit and the meter has been changed to one that reads higher , without changing the shunt..)

Yeah, the meter is a run of the mill panel meter from West Marine. I did have the alternator tested before hooking it up and it was putting out in the 70amp range on the bench, so my gut feeling is the same as yours, that the panel meter was correct and the Fluke was a fluke. But I thought there was some ordinary principle that would explain the 1/2 discrepancy. -I guess not.

 

[Maine Sail]

I can say with pretty good certainty that your Fluke is correct. My guess is that you have something like a 100mV ammeter display with a 50mV shunt.. The meter needs to match the shunt to work correctly. If the meter is a 50mV scale then the shunt needs to be a 50mV shunt. You can have a 500 amp 50mV shunt or a 500 amp 100mV shunt but of your display is 50mV then you need to use a shunt that is also 50mV.

As has been said shunts read millivolts/mV and the display converts this know voltage drop to display amps. Having the wrong millivolt range for either the shunt or meter could potentially cause your doubling effect.

 

[Stephen Trapani]

I can say with pretty good certainty that your Fluke is correct. My guess is that you have something like a 100mV ammeter display with a 50mV shunt.. The meter needs to match the shunt to work correctly. If the meter is a 50mV scale then the shunt needs to be a 50mV shunt. You can have a 500 amp 50mV shunt or a 500 amp 100mV shunt but of your display is 50mV then you need to use a shunt that is also 50mV.

As has been said shunts read millivolts/mV and the display converts this know voltage drop to display amps. Having the wrong millivolt range for either the shunt or meter could potentially cause your doubling effect.

So is it expected for my new 60a alternator to be putting out only 30a when first charging two almost dead banks on "ALL"?

 

[Bill Roosa]

30 amps into dead batteries is not unheard of. It would depend on the regulator and how dead the batteries are. In any case in a very short time even with compleatly drained batteries (10.5 volts) you will see a dramatic reduction in current down to 25-30% of the AH capacity.
The shunt it the sensor that is connected to the large cables attached to the battery ground. All the current should go through it. It has an additional set of small wires that are very precisly attached to the center of the shunt. These run to the gage that actually measures the voltage drop across the shunt (known resistance). the voltage drop is then converted to amps by knowing that delta V = amps x resistance. You can reqrite the equation to amps = delta V / resistance. Since you know the resistance (shunt) all you need is to measure the delta V (voltage drop) across the shunt. The gage is really a miliVolt meter with the face changed to read amps. The face acturlly is the calculator that does the division to convert delta V to amps.
some amp meters have internal shunts and you do not see the extra set of wires to the gage.

 

[Stephen Trapani]

30 amps into dead batteries is not unheard of. It would depend on the regulator and how dead the batteries are. In any case in a very short time even with compleatly drained batteries (10.5 volts) you will see a dramatic reduction in current down to 25-30% of the AH capacity.
The shunt it the sensor that is connected to the large cables attached to the battery ground. All the current should go through it. It has an additional set of small wires that are very precisly attached to the center of the shunt. These run to the gage that actually measures the voltage drop across the shunt (known resistance). the voltage drop is then converted to amps by knowing that delta V = amps x resistance. You can reqrite the equation to amps = delta V / resistance. Since you know the resistance (shunt) all you need is to measure the delta V (voltage drop) across the shunt. The gage is really a miliVolt meter with the face changed to read amps. The face acturlly is the calculator that does the division to convert delta V to amps.
some amp meters have internal shunts and you do not see the extra set of wires to the gage.

My alternator has only one lead, it travels to the ammeter on the control panel and then back to a post on what I assume is the starter solenoid because it is attached to the starter motor. The connection post on the alternator appears to be directly on the alternator body.

My house bank was too dead to run a small bilge pump. The other battery (bank) was very low also, probably barely able to turn the starter motor. The initial reading of 60 on the panel meter was IMMEDIATELY upon start up, with the two banks connected via ALL on the battery switch. It stayed there for about 10-15 minutes before it dropped down. The alternator is 82a max charge rated. The panel ammeter has a zero at the top center of it's dial and also on the left side of the dial has a minus range down to minus 60 for some reason.