RLC Circuits - Phasors

LearnerDave · Oct 31, 2011

Hi Guys,

Just wondering if anybody would mind explaining the main purpose of working out the lead/lag of current compared to voltage in RLC circuits?

Is it to do when selecting/derating cables for current carry capacity?
Is there any other things that happen? maybe extra heat build up in cables?
If there is a excessive lead or lag, can it affect any other circuits, if so what effects can it have?

Cheers Dave

needasparks · Oct 31, 2011

could go for unity, the customer will be well happy but may upset the supplier.

platty1005 · Oct 31, 2011

Hi L'Dave
The problem with phasors in RLC circuits is to do with power
At unity the power in a
single phase circuit = Volts x amps
If it is an RLC circuit with a lead or lag
current the formula is
Power = Volts x amps x Cos (phi) The angle between the Voltage and the current
So for a circuit with a Power factor of Cos (phi) = 0.8
The power = Volts x Amps x 0.8
This means the current must be higher to give the same power output
Higher current means bigger cables overheating etc.

This is the reason discharge lighting power values are multiplied by 1.8
So an allowance is made if the Power factor correction (PF) equipment inside the fitting or luminaire fails
EG 10 x 70 watt = 700 watts x 1.8 = 1260 watts
Max Current for circuit i= corrected Power/Volts = 1260/230 = 5.4 Amps
If the PF equipment corrects to unity the actual current will be 700/230 = 3 amps

malcoa · Oct 31, 2011

It's for a number of things, from working out the phase angle of each phase you can determine the current in the neutral, it also helps to work out the size of capacitors needed to bring power factor close to unity, if the power factor is low it increases the current which affects cable sizing etc, also the electricity bill provider puts additional reactive power charges on.

e.g using a 5KW motor and pf of 0.65 and 0.9
W/(1.73xVxpf) =A
5000/(1.73x400x0.65)=11A
5000/(1.73x400x0.9)=8A

You can see an increase in current from one motor, from a factory with 150 motors it becomes a little more critical, for the distribution network its a problem and i guess thats where the reactive power (KVar) charge comes from.