Number of Poles in Hitachi LR180-03C Alternator

[dkinzer]

After swapping out the V-belt for a serpentine belt on the Yanmar 4JH4E on our 2007 Hunter 41AC, the tachometer reads about 25% higher than actual RPM. Consequently, I'm planning to re-program the VDO tachometer to get it to indicate closer reality. As a check on my calculations, I'd like to know how many pulses the alternator generates on the tach sensor line per revolution of the alternator - this number is dependant on the number of poles in the field windings. I'm sure that there are at least 3 poles but there could be 4 or even 6 poles.

Does anyone know what is typical on alternators like this?

 

[dlochner]

It may be different on your Yanmar, however some Yanmars, like my 3JH2e do not pick up the engine RPMs from the alternator. Instead, there is a device located on the aft end of the engine that counts revolutions of the ring gear.

If your engine is using the alternator, then the model of the alternator and the belt ratio will likely be more relevant.

 

[Davidasailor26]

@Maine Sail is the only one around here I can think of who might know the design of the alternator. In any case I agree with dlochner that the belt ratio is going to be an important factor. I think the only way to really make sure you’re dialed in to the right calibration will be empirically, using an optical tach to compare to the VDO.

 

[dkinzer]

@dlochner : I'm certain that the alternator pulse output (W lead, I think) is connected to the VDO tachometer. I have a diesel driven genset (non-marine application) that has a magnetic pickup mounted near the flywheel that senses the passing of each tooth of the ring gear on the flywheel. I wish that my 4JH4E had the flywheel sensor like that.

 

[dkinzer]

@Davidasailor26 I have measured the diameters of the original crankshaft pulley and alternator pulley: 4-3/4" and 2-7/8" respectively with a belt that is 3/8" thick. The new pulley diameters for the serpentine belt are 5-1/4" and 2-1/2" with a belt thickness of about 1/8". From this I calculated the ratio of the speed of the alternator pulley to the crankshaft pulley to be 1.75 in the V-belt configuration and 2.16 in the serpentine belt configuration. The difference between these is a factor of about 1.23 which is pretty close to the approximate 1.25 ratio between the VDO indicated RPM and the measured RPM.

 

[dlochner]

@Davidasailor26 I have measured the diameters of the original crankshaft pulley and alternator pulley: 4-3/4" and 2-7/8" respectively with a belt that is 3/8" thick. The new pulley diameters for the serpentine belt are 5-1/4" and 2-1/2" with a belt thickness of about 1/8". From this I calculated the ratio of the speed of the alternator pulley to the crankshaft pulley to be 1.75 in the V-belt configuration and 2.16 in the serpentine belt configuration. The difference between these is a factor of about 1.23 which is pretty close to the approximate 1.25 ratio between the VDO indicated RPM and the measured RPM.

The belt thickness doesn't matter, the ratio of the pulley diameters does. The faster the alternator spins, the higher the tach will read. The numbers you are getting seem close enough. Analog tachs can not be read with great precision, so if you are off a few percentage points I wouldn't worry. The dials are not accurate enough to measure 2 percentage points. Follow the tach's instructions to calibrate the tach, it really doesn't matter if the tach reads 850 rpm and the actual RPM is 860.

 

[Ralph Johnstone]

Unless you enjoy doing calcs. for alternators, skip it and invest in a cheap hand held optical tachometer.

All of $14.00. You'll want to check your VDO for accuracy every few years so you'll still need it. My VDO drifts slightly with age.

I'm sure that there are at least 3 poles but there could be 4 or even 6 poles.

With three poles, that would be a collector's item.

 

[danooxnard]

I found an app for the iphone that is an optical tach. It uses the flash as a strobe, you put a white chalk mark on the pulley and adjust the flashing until the mark stops and stays still. Then you read the RPM. It worked pretty well. Search for strobe tachometer in the app store.
Dano

 

[Ralph Johnstone]

I found an app for the iphone that is an optical tach.

I remember hearing about that. Incredible.
Is there anything an iphone CAN'T do ? If they come out with an app that cooks dinner, washes the dishes, and turns down the sheets, I'd buy it regardless of any price.

 

[LloydB]

''I'd buy it regardless of any price." you can find it but if you wanna delete it you have to pay a monthly subscription about 17% of the original price for seven years or half of your retirement fund.
Sorry I just couldn't wait till Friday

 

[dkinzer]

it really doesn't matter if the tach reads 850 rpm and the actual RPM is 860.

I agree but mine is much farther off. For example, when my optical tachometer indicates that the crankshaft pulley is turning at 1060 RPM the VDO tachometer indicates 1400 RPM.

After further research, I have discovered that the W lead on the alternator is connected to only one of the stator windings so there will be one pulse per revolution of the alternator. With the original crankshaft pulley diameter of 4-3/4" and the original alternator pulley diameter of 2-7/8" the W lead would have about 1.65 pulses per crankshaft revolution.

So, with the serpentine pulley diameters of 5-1/4" and 2-1/2", I need to change the VDO setting to 2.10 and re-check it.

 

[Ralph Johnstone]

You must have a very old VDO tach or just not following the installation instructions for a current model VDO.

 

[dkinzer]

I've concluded that the Hitachi LR180-03C alternator has six poles. I brought the tachometer home and powered it up in configuration mode, finding that it was set for 10.29 alternator pulses per crankshaft revolution. This points to a ratio of crankshaft pulley to alternator pulley diameter (with the stock V-belts) of 1.715 which compares favorably to my measurements.

I was able to change the pulse count to what I have calculated it should be for the serpentine belt setup so the next step will be to re-install the tach and see how close the indicated RPM is to that measured using a photo tachometer.

 

[Ralph Johnstone]

And to think, you could have finished this job days ago if you had just followed section three on page four of the instructions