# basic question about taking ohm readings of stator or field



## joerm (Mar 20, 2016)

I'm taking ohm readings on both the field and rotor windings of a generator that I'm trying to diagnose and I'm suppose to be getting .125 ohms but instead I'm getting .700 ohms. I understand that if I were to get less resistance readings it would suggest a short in the windings. My question is what kinds of malfunctions can cause resistance readings to be higher than spec?

btw it's a brushless, capacitor based 9500w Winco genset with almost no voltage output. I replaced the diodes and flashed the field and have been able to get half of the voltage out of the outlets. apprx 60 volts out of the 120v outlets and 110 volts out of the 220v outlets. My next step is to replace the 3 capacitors even though they test good with my chinese digital cap tester. 
Thanks,
Joe


----------



## Osviur (Sep 7, 2015)

*High resistance*

Hi joerm,

As you say a short circuit between the stator windings (I suppose the stator because of the low resistance reading), means a lower resistance in the stator circuit, but a higher resistance can be produced only for a weak contact in the coils connection, or (very rare) some parts of the windings that have lost part of their section (¿?).
Anyway, this very low resistance measure is difficult to get with precition. An increase in the stator resistance circuit will increase the internal voltage loss because the current (Amps) circulating in the windings, produces a voltage drop of:

Load amps. X internal resistance = Imternal voltage loss
8500 W (cont. max load) / 120 v = 70 Amp,
70 Amp. (max load) X .125 Ohms = 8.75 Volts int. V loss
70 Amp. (max load) X .700 Ohms = 49 Volts int. V loss 

This a simple approach as if we were talking about a DC circuit, AC ciurcuits are more complex. 

The normal internal voltage loss can be overcomed by the regulator system, compensating the internal voltage loss as the load varies. But a high abnormal internal resistance will cause that in some point, when the exciter field is at maximum, because of the load increment, the output voltage will start to decrease as the load continues incrementing. 

Regards.


----------



## joerm (Mar 20, 2016)

Thank you Osviur, Very interesting, you've made it clear to me how very likely high resistance can and does occur. (Or I need to re-read all the good info you shared again) Perhaps an overheating in the windings caused something less than a short ie: burnt wires that didn't sever or weld completely which might cause higher resistance accompanied with a voltage drop enough to stall the motor and thus limit the destruction of the windings (not to say they might not ever be serviceable again) Are my thoughts a conceivable scenario do you think? If so I suppose the only thing I could do is check for loose connections or excessive patina on the metal end plate where I mounded the new diodes? 
Could you confirm if I'm understanding your points correctly?
Thanks again for all your help!!


----------



## Osviur (Sep 7, 2015)

*High resistance.*

Hi joerm,

A high resistance point ( In ex, a loose conection) , will produce a drop in the output voltage if there is a load connected, necessary condition to have a current circulating through th stator.

A melted coil or some turns shorted between them, will work as a secundary transformer in short circuit, being the primary the remaining stator turns. This condition will allow a very large current circulating in the shorted turns, heating them and taking a lot of power from the stator, and this from the engine. In this condition there is not necessary to have a load connected to the generator output, the big load is inside the machine.

In the last condition, a short between turns means less resistance ( because there are more wires in parallel) and you have measured more resistance, so if you measured correctly, it seems more a poor connection, but in this case the voltage reading has been done without load, just with a electronic voltmeter with an internal resistance of 10 Mega Ohms, so the generator is running free of load, and there is not practically internal voltage loss. No current, no loss.

If the stator is OK, the exciter system must be checked. In a brushless gen, an AC voltage is generated inside the rotor, this AC is rectified (converted to DC) using a set of(rotating) diodes and sent to the rotating windings of the exciter field. There is not electrical contact from the exterior to the rotor. Only an electromagnetic static field, fed from the stator induces a voltage inside of the rotor. If you had an electrical diagram, I could help you a little more. 

Thank you very much for your expressions.


----------



## joerm (Mar 20, 2016)

Osviur, After reading your thoughts I went out and pulled the stator off the engine and thus having better light to see and more room to work I discovered (just as you suggested) I loose connection from the field winding spade connector to the diode. As I pushed the ohm meter electrodes around I noticed the ohms would come in to spec range and then out again while wiggling the wires. I tightened down the connections put everything back together and fired it up and now get 108v out of the 120v outlet. I suspect with the help of a tack and hertz meter I should be able to fine tune the engine to get good power. Thanks to you the mystery of the high resistance / low voltage genset is solved.
Thanks for all your help,
Joe


----------



## Osviur (Sep 7, 2015)

*Thanks.*

Hi joerm

Thanks to you, to read my lines. You are welcome.


----------

