I needed to test a Delta wound generator today from a vfr800 but was unsure how to test it correctly. I tested it with a multimeter across the yellow wires and found no more than 5ACV across any wires at 5k RPM with generator disconnected. The generator was replaced and showing around 70ACV at 5k rpm. This fixed the problem but got me thinking.

My question is - I have hantek DSO3064. How would you connected the scope to the windings. Would you go earth with the ref clamp and probe each of the yellow wires or would you put the ref to one of the yellows and probe another yellow which is effectively what your doing with the multimeter.

I was unsure so didnt want to risk damage to the scope. Head says ref to earth but logic says across the yelllows with the ref and probe. Any help gratefully received.

Regards

Mark

does the scope have differential input channels or is the ground shared across all ?

Looking at the specs- 2X channels ref to batt- with a math ......

I was under the impression it's differential input channels but I could be wrong - I usually am. its a 4 channel version.

On the Pico UK site they have used a vfr800 as a test and they used ref to battery - and probe to yellow generator wire while it was connected to the bikes regulator/rectifier but I needed to test with it unplugged as the regulator caps the ac to 15-17acv pk-pk if it's plugged in. The test for the generator should be done while disconnected from the bikes loom as per Hondas test instructions.

https://www.picoauto.com/library/automotive-guided-tests/alternator-phase-wires

EDITED due to incorrect picture

Hello Mark152152!
The photo you show there is a wye or y wound unit. It is possible to ohm test each leg of the winding. (you actually test 2 legs in series between any 2 connections).
  A delta wound is like a triangle. If you try to ohm test a delta winding, the other 2 windings will be in parallel with the one being tested.
-Ward-

My mistake. Good old google. I tested the dead generator before removal and found I had 0.2ohm between any of the 3 yellow wires. I had infinite resistance to earth but very little AC voltage coming out at 5k RPM

The New generator was tested and had 0.2ohm between any of the 3 yellows, Infinite resistance to earth but was about 22AC at idle increasing to 70AC @5k RPM.

I would have expected to see either a short to earth or resistance in the circuit. The dead generator looked darkened at the bottom but no issues were evident other than discolouration. I could see no major burning - just discolouration. Thankfully the new generator works fine so it was definitely at fault SEE PIC

http://i1334.photobucket.com/albums/w643/marks152152/Mobile%20Uploads/2015-10/IMAG2115_zpsinabaav7.jpg

Thanks for any input. Just curious what is the best way to test it with a scope. Head says ref to bikes negative and probe each output on the generator.

I have a megohmeter I used to today to measure the windings in form of a wye wound motor. This was a three phase old 10 horsepower motor intended to be used on a lathe.
In the schematic you see in this post,in the center of the schematic, one end of these windings each is connected to the other two; three together at one end.
With the megohmeter we measure the resistance of the windings and /or whether they are grounded to the case of the motor (shorted to the case).
The megohmeter measure ohms as does your dvom but with a great deal more voltage. I tried this:
Probe one dvom with another; have one set to read ohms resistance and the other to read volts. As you know, the ohms reading is due to a voltage sent out by the dvom measuring ohms.
One dvom will show about a half volt going in and the other shows the 10 megohm impedance, unless you have a damaged or innacurate meter. Mine were all showing about .6 V and 11megohms; they are all normal.
   The megohmeter will measure the resistance of my DVOMs and it shows the 10-11 megohm impedance on it's scale but the meter set to read volts shows the 250, 500 or 1000 volts the megohmeter tests with. Lastly,  I tried measuring the amp draw of the tester using the Amperage function of the DVOM , in series with the load placed by the megger.  The spike seems to blow fuses, I had read a 1.39 mA draw but only once; two meters are now goofed from this I suppose.
   In testing the motor with this megohmeter, I found the analog scale shew ALL the windings not shorting to the case, no opens in them either.
So you can say that you could hook up your mains power because of this and at try to run it which we will.
That is; each winding is measured across it to check for resistance - like measuring the resistance of an injector - and measured to see if it is shorted to it's case , again like measuring a coil to see if it is shorted to ground - the case of the motor. It measures 400Mohms max.

It's actually got me wondering if its Wye or delta. Can anyone confirm

Ive just had it confirmed that it is indeed a delta wound generator.

The photo may be Delta wound but your original schematic showed three ends connected together, that happens on a wye or 'star' wound motor.

fisher wrote on Oct 12^th, 2015 at 5:58pm:

The photo may be Delta wound but your original schematic showed three ends connected together, that happens on a wye or 'star' wound motor.

You are fully correct. My stator is Delta and the original diagram showed a Wye stator. I just googled "Delta stator" and posted the diagram. I didnt check it.