Motor Flicker
Motor Flicker
(OP)
I've been calculating motor starting flicker using the basic formula: LRA x (R cos0 + X sin0)
where cos0 = starting pf and R & X values are in ohms
I've run across a different method of calculation that I'm not familiar with and was wondering if anyone could provide some insight. Is this method more accurate?
The two methods do NOT give the same result.
The formula is:
%Flicker= 1-(sqroot(Zcable*2)/sqroot(Ztransf*2 + Zcable*2))
where *2 represents the value squared
replace Z with R+jX values for cable and transformer
An infinite system bus is assumed.
where cos0 = starting pf and R & X values are in ohms
I've run across a different method of calculation that I'm not familiar with and was wondering if anyone could provide some insight. Is this method more accurate?
The two methods do NOT give the same result.
The formula is:
%Flicker= 1-(sqroot(Zcable*2)/sqroot(Ztransf*2 + Zcable*2))
where *2 represents the value squared
replace Z with R+jX values for cable and transformer
An infinite system bus is assumed.






RE: Motor Flicker
RE: Motor Flicker
The rest of the impedance for cable, transformer and system are calculated on the same base. EBASCO's method get the same results as I've described above. The answers are several percentage points less flicker than using the
LRA x (R cos0 + X sin0) method. I just can't quite picture it in my mind.
RE: Motor Flicker
I want to understand the term "zero sequence current".
Can anyone elaborate or point me to a website that will explain.
Thanks,
Jim
RE: Motor Flicker
RE: Motor Flicker
William
RE: Motor Flicker
RE: Motor Flicker
Ideally you want to include the source impedance Xs in the calcs, or if you have a service transformer use infinite but ahead of it.
You use the same method as if you are calculating SCC, with
the follwing in series (use per unit method and convert all to correct bases). You can use Z=R+jX if you know R, but all practical purpose, only R to be considered will be that of the cable.
Xs+Xt+(Rc+jXc) +Xm. Place a short at end of Xm and calculate starting I = 1/(Xs+Xt+(Rc+jXc) +Xm) where Vpu=1
Total voltage drop will be across each element can now be found by voltage devider rule.
If you use LRA as known current you need not include Xm and just find VD=IZ across known component.
If you ignore immediate trasnformer imepdacen upstream you will have very small VD.
Hope this helps.
RE: Motor Flicker
It all depends on which impedances you use: locked rotor, transient, subtransient, etc. These will give you different results at different times. For the overall picture, I would use the LRA from the NEC.
William