Not my area is what I meant to post.
Separately excited shunt wound DC motors very much are.
DC converter's power factor.
- Thread starter Chelny
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Separately excited shunt wound DC motors very much are.
yes, I get that.
AC drive:
line voltage =400V;
rated input current (HO)= 39,9 A;
let's suppose it has the coolest pf in the world for drives =0,92; // as I've drawn in my sketch above.
Chelny
from previous post
S dc = 108.8 + j67.5
S ac = sqrt3 400 39.9 = 27.6 kva
pf = 0.92, ang = arccos 0.92 = 23.1 deg
S ac = 0.92 x 27.6 + j sin23.1 x 27.6 = 25.4 + j10.8
S total = S dc + S ac =108.8 + j67.5 + 25.4 + j10.8 = 134.2 + j78.3 = 155.4 / 30.3 deg
ang = arctan (78.3/134.2) = 30.3 deg
pf = cos 30.3 = 0.86
let's take it one step further
say you want to improve system pf from old 0.86 to new 0.90
S old = 134.2 + j78.3
ang = 30.3 deg
pf = 0.86
new pf = 0.90, ang = arccos 0.90 = 25.8 deg
pf = 0.90 = cos(arctan(new Q / P)), P stays the same since mech load is constant
solving;
new Q = tan(arccos0.9) x 134.2 = 65 kva
new S = 134.2 + j65
cap kvar required = 78.3 - 65 = 13.3 kvar (use 15)
S = 134.2 + j78.3 - j15 = 134.2 + j63.3 = 148.4 / 25.3 deg
new pf = 0.904
Q = V^2 / Xc so Xc = 400^2 / 15 kvar = 10.67 Ohm
Xc = 1/(2 Pi f C) or C = 1/(2 Pi 50 10.67) = 300 uF
say you want to improve system pf from old 0.86 to new 0.90
S old = 134.2 + j78.3
ang = 30.3 deg
pf = 0.86
new pf = 0.90, ang = arccos 0.90 = 25.8 deg
pf = 0.90 = cos(arctan(new Q / P)), P stays the same since mech load is constant
solving;
new Q = tan(arccos0.9) x 134.2 = 65 kva
new S = 134.2 + j65
cap kvar required = 78.3 - 65 = 13.3 kvar (use 15)
S = 134.2 + j78.3 - j15 = 134.2 + j63.3 = 148.4 / 25.3 deg
new pf = 0.904
Q = V^2 / Xc so Xc = 400^2 / 15 kvar = 10.67 Ohm
Xc = 1/(2 Pi f C) or C = 1/(2 Pi 50 10.67) = 300 uF
Last edited:
jumper
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S= Apparent Power=VA
https://www.allaboutcircuits.com/te...ent/chpt-11/true-reactive-and-apparent-power/
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S = total power (or apparent) in VA
P = real or active power in W, does work
Q = reactive or imaginary power in VAr (volt amp reactive), no work, does C charging/L magnetizing
j = sqrt(-1) a mathematical convention
S = P + jQ in rectangular coordinates
arctan(Q/P) = ph ang in deg
cos (ph ang) = cos (arctan(Q/P)) = pf
P = S x cos(ph ang) = S x pf
Q = S x sin(ph ang) = S x sin(arccos(pf))
|S| = sqrt(P^2 + Q^2)
S = magnitude / ph ang in deg= |S| /arctan(Q/P) in polar, and in this case phasor notation
by convention
positive Q = lagging pf or absorbing reactive power, L
negative Q = leading pf or suppling reactive power, C
P = real or active power in W, does work
Q = reactive or imaginary power in VAr (volt amp reactive), no work, does C charging/L magnetizing
j = sqrt(-1) a mathematical convention
S = P + jQ in rectangular coordinates
arctan(Q/P) = ph ang in deg
cos (ph ang) = cos (arctan(Q/P)) = pf
P = S x cos(ph ang) = S x pf
Q = S x sin(ph ang) = S x sin(arccos(pf))
|S| = sqrt(P^2 + Q^2)
S = magnitude / ph ang in deg= |S| /arctan(Q/P) in polar, and in this case phasor notation
by convention
positive Q = lagging pf or absorbing reactive power, L
negative Q = leading pf or suppling reactive power, C
Last edited:
@ Engenieur,
there are couple of theoretical questions:
Please your opinion.
regards,
Chelny
there are couple of theoretical questions:
- that calculated pf =0.85, it was calculated for max rated current. So I would like to know if it impacts so badly on pf within whole speed range. I other words is it true that ABB's guys work says that pf drops so critically at low speed range?
- Also I have some doubts that considered by us (e.g. size 75) has so bad pf value at low speed range (reffering to that work). This is Siemens product, after all, and this is 21th century.
Please your opinion.
regards,
Chelny
You simply can't do that.
All of us posting here, yourself included, have agreed that PF for DC motor control varies with speed.
You can't assign fixed values.
What would be the point of doing so with a system designed for variable speed operation and we have all agreed that PF varies with speed?
who said it was designed for variable speed operation?
and over what range?
what if the range is 90-100%?
he asked
pf?
how it varies with speed?
and for docs
NONE of which you have addressed
you have only addressed me, lol
if we know the max is 0.85 at rated speed
and is 0 at 0 speed
and linear between two points
anything can be calculated
but unless you calc the max, you can't do anything
Tell me something understandable please.Stop bickering.
His point has been addressed.
The 0.85 would relate to speed, not loading. Critically low? Just low. I don't know why that would be critical.
It is low at low speed. That's what the usual phase control configuration gets you.
We, Cortina*, designed, built, and installed what we designated as UPF drives. Essentially it was a fixed DC buss from a 24-pulse uncontrolled rectifier and DC-DC choppers. These were mostly in the 100-400kW range but we used the same technique on slip recovery drives up to several MW. Yes, mods this strays off the original topic but it illustrates what can be done to mitigate poor power factor issues that arise from phase control drive systems.
*this not promoting a product. Cortina is no longer.
that's the pot calling the kettle black :lol:
it looks linear 0-0.85 from 0-100% speed
contact the supplier to see if they can give you more data, they likely have it
we'll, that is inherent in the design of the system
what speed range will you operate? what turn down ratio? eg speed = 33%-100% = 3:1 turndown
See my example for paper machines.
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