Voltage drop across slipring brushes

[roydm]

Hi,
I ride old motorcycles one of which has a Bosch alternator 3 phase wound rotor type similar to most older cars.
For some treason I am seeing excessive Voltage drop across the positive slipring brush ~700mV while the other ring is less than 300 this is enough to significantly reduce the alternator output.
If I touch up the sliprings the Voltage will drop down to 100 mV range and the output is back to normal for a week or so but it slowly builds up again.
The brushes are fairly new, replaced about a year ago, the common coppery graphite looking ones.

I can't explain why it's always the positive brush that drops most Voltage.
Any thoughts?
Thanks
Roy

 

[Skogsgurra]

Not so long ago, low voltage rectifiers were built from copper with a thin oxide layer. And ordinary brushes in DC motors are known to have a slight rectifying property. That is probably what you are seeing.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[roydm]

Thanks Gunnar,
That might explain the discrepancy from one brush to the other (current direction, brush to Brass / Brass to brush), perhaps I should try a set of carbon brushes but they may have a higher resistance. The current through the brushes is only about 3 Amps.
The difference between a just cleaned slipring and a couple of weeks use is enough to let the battery go flat with heated gear in use, won't start again, I usually unplug a couple of miles before stopping to let the Voltage recover.
You would think the alternator spinning would be enough to keep the rings clean but it's not.
The joys of riding old motorbikes

 

[waross]

Does this employ permanent magnets?
The voltage output with fixed field strength should be roughly proportional the the speed. Go a little faster and 1 Volt loss acros the brushes should be a non issue. I suspect that your field is weak.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

A commutator or slipring shouldn't have a bright metal surface, it should have an even and shiny patina a few shades darker than the parent metal. Black pitted rings or segments is bad news, as is bright raw metal. Don't keep polishing the sliprings to remove the patina!

Mersen publish an excellent guide to brushgear. There are some photos of various problems with explanations starting on page 24 of

https://www.mersen.com/fileadmin/user_upload/pdf/ea/5-carbon-brush-technical-guide-mersen.pdf

.

 

[waross]

Misdiagnosing a problem will lead to frustration.

Voltage drop across the positive slipring brush ~700mV while the other ring is less than 300 this is enough to significantly reduce the alternator output.

I have problems with this statement.
Does this alternator have a voltage regulator?
If the regulator is the Zener diode type is it the correct rating? It should be around 13.8 to 15 Volts.
Does the bike have the original headlight or has the light been replaced with a higher wattage light?
Do you typically ride at very low RPMs?
What is the actual current output of the alternator? Is the 3 Amps the actual current or the rated current?
let the battery go flat with heated gear in use,
What do you mean by "heated gear"? If you have battery powered hand warmers or something similar you are probably exceeding the alternator capacity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

Waross,
Yes the Bosch alternator has an after-market electronic Voltage regulator, it's very similar to a car alternator with a wound rotor and 3 phase stator feeding a 3 phase bridge except the bridge is in the regulator not the alternator itself.
Original headlight 60/55 Watt, the total lighting load would be ~70 Watts
Heated Jacket liner 77 Watts, gloves ~27 Watts for the pair
The Ignition would probably average ~ 50 Watts
Total 220 Watts

3000 - 5000 RPM is where I mostly run
Output 14 Volts 20 Amps according to the manual but doesn't say at what revs.

Yes, I'm riding the ragged edge, I understand that.

It's not really a big issue, it is a 30 year old Moto Guzzi after all, what I was really puzzled about was the uneven Voltage drop in the brushes 0.7 vs 0.3. With about one Volt missing I assume the rotor is not fully excited. Weak charging is common to most old motorbikes I was simply trying to figure out why.

Gunnar's explanation for the un-even drop kind of makes sense, I thought I might try a different brush material rather than the coppery looking graphite ones

ScottyUK
Yes I was taught not to polish the commutator also but It did drop the Voltage across the brushes and boost the output for a while.

 

[waross]

Well if this is like automotive alternators, forget about the brushes.
The brushes excite the field. If the output voltage is below 14 Volts, the regulator increases the voltage to the brushes until the output voltage comes up to 14 Volts. If the voltage drop across the brushes increases, the output will drop slightly and the regulator will increase the voltage to the brushes to compensate for the drop.
Generally you should get 14 Volts at fast idle with no heater load. I would expect full 14 Volts at 3000 RPM.
Hook an ammeter in series with the battery lead. Turn on all your heaters and accelerate the engine until the battery starts to charge.
You may have a faulty alternator or you may have too much load for your riding speed profile.
If the battery doesn't charge at a reasonable RPM, you may have a faulty alternator.
Hook an ammeter in series with the alternator lead. Turn on all your heaters and accelerate the engine the output reaches 20 Amps. If the output won't reach 20 Amps the alternator is suspect.
Hook an ammeter in series with the battery lead. Turn on all your heaters and accelerate the engine until the battery starts to charge.
If the battery will not charge until the RPMs are quite high you may have too much load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

Thanks Bill, that's given me some homework.