How to test a B7200D Dynamo Winding

[Tractor J]

I have a B7200d and replaced the dynamo (P/N- 15531-64013). All is working great. I disassembled the old Dynamo – P/N- 15531-64013. The housing had caked on dirt on the housing, The copper wiring looks good. Bearings all feel good. How can I test the winding to see if the winding is good. With the Dynamo off the tractor. Using a multimeter, what should the ohms be? Thanks

 

[North Idaho Wolfman]

 

[Tractor J]

Would a 0.3 ohm be good?
Thank You for the info. John

 

[Hugo Habicht]

Hello John,

measuring a resistance that low with a multimeter is challenging. Wire and contact resistance are significant. Normally a four wire measurement is used for that.

What you should do (if not done already) is put the probes together, memorise the resistance, then measure the stator and subtract the earlier measured resistance.

Kind regards,
Hugo

 

[North Idaho Wolfman]

Would a 0.3 ohm be good?
Thank You for the info. John

Most likely it's fine, they normally will measure 0 Or Infinite when they are bad.
Like @Hugo Habicht stated measuring that small a resistance is hard.

 

[Tractor J]

Hello John,

measuring a resistance that low with a multimeter is challenging. Wire and contact resistance are significant. Normally a four wire measurement is used for that.

What you should do (if not done already) is put the probes together, memorise the resistance, then measure the stator and subtract the earlier measured resistance.

Kind regards,
Hugo

Touching the leads together I have 0.1 and when I touch the Dynamo wire I get 0.3, so I have a 0.2 reading
Thanks for the input.

 

[GreensvilleJay]

yes, that's the proper way to take any 'ohms' reading.

 

[Hugo Habicht]

yes, that's the proper way to take any 'ohms' reading.

Well, with resistances that low Lord Kelvins method is better. Separating the current path from the voltage path eliminates contact / probe resistance. See also: Wikipedia four wire sensing

So if you own a regulated low voltage power supply you can measure low resistance with much better accuracy.

Wrt to stator measuring the resistance is only one parameter. Measuring the voltage on all three stator wire pairs with dynamo rotating will show better if the stator is good. Magnetic problems or internally shorted windings will show here too.

Often damaged stators (due to faulty rectifier diodes) will show signs of burnt laquer insulation. This burnt insulation then flies off when soda blasting the stator for example.