How to calculate the inductance of a stator winding

[electricalguy]

Hi there, i have a three phase synchronous generator that is powered by a 1/3hp electric motor. I am having difficulty calculating the inductance of each phase of the stator winding when it is at full operation. I will give you some of the specs of the alternator. The rotor has 12 poles, with an inductance of 12.90mh, a resistance of 3.3 ohms and the excitation current is 0.47 amps. Using an inductance meter phase A of the stator when not running has a measured inductance of 108.5uh on the high end and 84.3uh on the low end depending on rotor position, with a resistance of 0.2 ohm. Phase B when not running has a measured inductance of 107uh on the high end and 82uh on the low end with a resistance of 0.2 ohms. Phase C when not running has a measured inductance of 106uh on the high end and 80.9uh on the low end with a resistance of 0.2 ohms. The operating frequency is 242.9 hz. The motor specs are a 1/3hp induction motor with 115volt supply, 6.4 amp supply at 1725 rpm. When the alternator is running unloaded the rpm is 2509 rpm based on the pulley ratio from motor to alternator. When fully loaded it is running at 2429 rpm. With these given specs how can i calculate the inductance of each line on the stator winding phase A, B and C. Also the stator is connected in wye. Thanks!

 

[anorlunda]

@jim hardy can you help with this question?

 

[jim hardy]

@jim hardy can you help with this question?

Difficult because he says he's already measured it, so I'm confused as to what is the question.

Wye connected, he says... Where did he connect his inductance meter probes?
Line to Neutral he'll measure the inductance of a single phase
Line to Line he'll measure twice the inductance of a single phase,

Using an inductance meter phase A of the stator when not running has a measured inductance of 108.5uh on the high end and 84.3uh on the low end depending on rotor position, with a resistance of 0.2 ohm.

What's inductance? Flux per amp-turn. So of course it varies with rotor position because reluctance of the flux path changes with air gap. And it sounds as if his airgap changes with rotor position by ratio of his high/low readings.. All three of his sets have ratio about 1.3 suggesting that's how much airgap varies. Is it a salient pole machine ? or the effect of slots reducing area of iron at airgap surface of rotor? ?

I will give you some of the specs of the alternator. The rotor has 12 poles, with an inductance of 12.90mh,

12.9mh (from a spec sheet ?) is a lot different than 80-107 measured .

I suspect if he'll pull the rotor out that's about what he'd measure.

Now

The motor specs are a 1/3hp induction motor with 115volt supply, 6.4 amp supply at 1725 rpm.

six pole pairs at 2429RPM gives frequency of of 2429 X 6 = 14574 cycles per minute = 242.9 hz.
So he's got a belt drive frequency changer.
I wonder it it's perhaps a car alternator driven by a washing machine motor ? . Sounds like really interesting project. Matbe he'llpost a picture


So once again - if OP would tell us what it is he's tryng to accomplish he'll get responses from both the practical AND the academic sides of the PF aisle .

I am having difficulty calculating the inductance of each phase of the stator winding when it is at full operation.

Why would its inductance change with load ?
B-H curve does flatten a little
but if you're not near saturation you can consider permeability constant.

Accounting for BH curvature in my day we did graphically
Take control of armature current and make a plot of open circuit voltage versus armature current
Move armature current in increasing increments all same direction - if you overshoot a point don't back down to the intended current just plot that point where you landed and move on to next point.
When voltage curve flattens start back down.
Now you have the BH curve, with field hysteresis , for your alternator and you can see how close to the knee is your normal operating point.

If you're after Leakage reactance try this
short circuit the field so as to cancel flux that goes through the rotor or better yet, remove the rotor and just.. test the stator...
force maybe an amp of AC through one phase , measure voltage
From voltage drop Ohm's Law gives you Z
You know R, and X= √(Z² - R²) for first approximation.

We'd enjoy seeing a picture of your experiment.

 

[electricalguy]

Thank you for the responses! I measured the inductance of the stator winding with an inductance meter from line to neutral for each phase. So its the single phase inductance i measured. The alternator I am using is a car alternator with a furnace fan motor driving it through a set of pulleys and a belt. The 12.9 mh is the value of rotor inductance that i measured with an inductance meter. I believe the rotor is a salient pole with 6 poles sets. 6 north poles and 6 south poles. My main problem is I have been trying to calculate the inductance of the stator Line to Neutral windings by measuring its line to neutral voltage and line current. Dividing voltage by current to get impedance then putting that value in an online calculator at the operating frequency to get its inductance value. The other problem being i am using a capacitive load and slight changes in the frequency show noticeable changes in inductance. So i am looking for a method to calculate the inductance for each phase so i can properly match up a capacitive load to the alternator.

 

[jim hardy]

My main problem is I have been trying to calculate the inductance of the stator Line to Neutral windings by measuring its line to neutral voltage and line current.

That ratio gives you the impedance of the load that is connected to your alternator.

What is your perception of term "Inductance" ?
Why is it that you want a number for "Inductance" of your armature winding ? What will you calculate from that number ? What is controlling field current ?

The other problem being i am using a capacitive load and slight changes in the frequency show noticeable changes in inductance.

Are you aware that capacitive loading directly affects terminal voltage through something called "Armature Reaction" , which is often represented as an inductance in series with the armature and usually named "Synchronous Impedance" ? And that it varies with frequency ?

You have something in mind and i don't know what it is so cannot point you toward an answer.

"A question well stated is half answered"

and it's important we both use the same vocabulary.

 

[jim hardy]

Here's a thread that helped another PF'er with alternator theory

https://www.physicsforums.com/threads/synchronous-generator-excitation.829603/#post-5214090