Types
of loads in
Supply systems:
There are three basic electrical
load components effecting the electrical supply – Resistive, Capacitance and
inductance.
Resistive does exactly what it says
- it resists the potential applied causing a corresponding quantity of
current to flow, according to the famous Ohms law.
Then there are the sly ones:
Capacitive:
If the load is capacitive it not just causes a current flow component
similar to resistance it also resists the voltage
to flow thereby causing a displacement between voltage and current. The
voltage lags by the amount of capacitance in the circuit. It gets stranger
when one opens the circuit switch – The capacitor tries to maintain the
voltage and exactly the same graph applies but inverted as the voltage
decays.
Inductive:
When voltage applied to an inductor
it also causes a current similar to resistance to flow but also resists the
flow of currents causing the current to lag the voltage. The amount of lag
is a function of the amount of inductance. But it gets stranger at opening
the switch: The inductor have stored this “current energy” and when
opening circuit it tries to maintain the
current levels and the same graphs applies but inverted
as the current decays.
FYI: It is
inductance of a load that brings
about what we refer to as POWER FACTOR in electrical systems.
See here.
But it gets tricky - The next
question is:- What is the behavior between AC and DC supplies
Capacitance and inductance are sly
buggers, they only show themselves when there is change in the supply
system.
AC (Alternating current) always have
change referred to as frequency and DC (Direct current) has no change –
EXCEPT WHEN DC POWER IS APPLIED OR REMOVED from the load – or when the DC
level changes but that would normally be insignificant.
I guess that is why capacitive /
inductive loads are called reactive because they only appear when change is
present. We also say they have reactance – which would imply their own type
of “resistance” – capacitive reactance and inductive reactance.
Let’s cover DC first:
Let’s do the graphs here:
Just like the circle
mathematically is always defined as C =Pi *diameter (and if you physically
measure circumference and divided by diameter it will always be equal to the
value of Pi the below graphs will also always be the same when measured.
Capacitance:
Inductive:
As can be expected the formulae are
also inverse:
For capacitance RC time constant =
RC – exponentially up.
For inductance RL time constant =
L/R – exponentially down. They mean the same thing - the time it takes to
63% of the transient change to full voltage.
What did the electrical
fraternity derive from these graphs?
What is important is for 0 to 63% of
the supplied voltage the graph gradient will be virtually the
same - but thereafter up to 98% (which happens to
be at 5 * 63% time point) it flattens out proportionally again and after
that the time to full voltage becomes impractically long, so at this point
for all practical purposes we take it as the transient is complete – fully
charged or discharged.
If 63% is one time unit we can
now when we relate to real life conditions we simply say
1 time unit or 5 time units.
Sometimes switchgear is rated
against these time constants – like Lovato has done for their contactor DC
application ratings – they reference the IEC categories DC1 or L/R <= 1msec
and for inductive loads DC3 (i.e. x3) or
DC5 (i.e. x5) or L/R
=15msec
And that's it as an introduction...Similar topics
have been covers as blog discussion topics
here...
Now we delve into more specific topics related to rules
and regulations for supply systems.
If you wanted to know more about supply earthing
systems...see links on left of page
