cable insulation in relation to C and L 2

[alwaink]

Hi,

I'm not in power engineering and am having a little trouble putting the fundamentals in the big picture.

1. How does a power cable's insulation (rubber) thickness affect the capacitance and inductance of the cable?
ex: if insulation thickness goes up, L goes up and C goes down?

2. "Delta" vs. "Wye" motors..."delta" doesn't have a neutral/ground, so where's the return path for voltage/current?

thanks

 

[rbulsara]

Do a google search on "cable capacitance" , "cable inductance" and "cable impedance" and you will find some relavent info for your question no. 1.


As for question no.2: You need to refer to some basic graphs of 3 phase sinewaves. Do a google for that too.
But in nut-shell the current in delta reutrns through other phases. At any given instance the net current in each phase is zero. The polarity of the current is constantly changing for each phase, but net value comes to zero.

 

[jghrist]

Capacitance decreases as the conductors are further apart. Inductance increases as the conductors are further apart. The distance between conductors increases with insulation thickness if they are touching.

 

[cuky2000]

1. How does a power cable's insulation (rubber) thickness affect the capacitance and inductance of the cable? If insulation goes up with same cable diameter, the capacitance goes down. Recall that Xc=1/?C, so as C goes up, reactance goes down.

Reactance is different; primarily depend of the physical configuration of the conductor.
_{NOTE: For trefoil arrangement, it is obvious that conductor separation will increase with the increase of the insulation thickness. For larger separation, the effect of the insulation for the inductance is not a major factor.}


2. "Delta" vs. "Wye" motors..."delta" doesn't have a neutral/ground, so where's the return path for voltage/current? The return path is by the distributed capacitance of the system

An ungrounded system such as delta connection described above is one in which there is no intentional connection to ground. However, in any system, a capacitive coupling exists between the system current carry conductors and neighboring grounded surfaces.

 

[rbulsara]

cuky:

I would take exception to your answer to question no. 2. That is not the correct answer, for the return path for normal currents.

awaink:

Even most star connceted motors neutal are not grounded and the 'return' current is thru the phases.. as I said in my first response.

 

[rbulsara]

and even if the neutral of a star motor is grounded, in a "balanced" loaded motor, no current will flow thru the neutral as net current will add to zero.

 

[Skogsgurra]

cuky,

You may be thinking of HF components in a PWM VFD fed motor. There is a good deal of high-frequency currents going back via motor and cable capacitance. But no power frequencies. Never. It is as rb says.

Gunnar Englund

http://www.gke.org

 

[alwaink]

Thank you all. This is making more sense now.