Dear Houssam,

The temperature is one of the reasons of changing electrical machines parameters.

For Resistance the the influence of the temperature is proportional, however for the inductances the influence of temperature is reversed.

R=L/(u*A)

L(m): length of the conductor

A(m2):cross-sectional area of the conductor

u(S.m): electrical conductivity

the electrical conductivity is a function of temperature. you can take a look in

• https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity
• https://fr.wikipedia.org/wiki/Conductivit%C3%A9_%C3%A9lectrique

There are other phenomena that can change the electrical resistance (skin effect, and coronna effect)

For the inductance 

L=mu*L/A

where mu is the magnetic permeability which is inversly proportional to the temperature.

Thank you very much for your valuable answer Mr El Moutaz Billah.

Besides temperature, could power system faults contribute in changing electrical machines parameters.!?

Dear Houssam,

I couldn't understand the meaning of power system fault, however the fluctuation in current, voltage and frequency lead to parameters changes to.

Try to see the two phenomena which I mentioned before: 

https://en.wikipedia.org/wiki/Skin_effect

https://en.wikipedia.org/wiki/Corona_discharge

Regards.

okay, that what i wanted to say, variation of grid voltage and frequency values. 

Thank you very much!

Variations in temperature, frequency and voltage each will cause variations in the lumped parameter values.  Changes in temperature cause electrical conductivity and permeability to vary (note, skin depth will also change); changes in frequency, for a given voltage, change the volts-secs in the ferromagnetic material potentially leading to saturation (and change the skin depth); changes in voltage, for a given frequency, also change the volt-secs in the ferromagnetic material and may lead to magnetic saturation; furthermore increased current resulting from greater voltage may, itself, lead to saturation.  Also, dynamic changes in the load may lead to parameter changes. 

The external knobs are voltage, frequency, and load, but what matters most are flux and current.  The machine operation changes whenever flux and current vary .

See, for example, P. Krause, Analysis of Electric Machinery, IEEE Press, NY, 1995; or J. Chasson, Modeling and High Performance Control of Electric Machines, IEEE Press, 2005.  There are many similar excellent books.