How do calculate the current when I have the active power and reactive power and volt


by ramox3
Tags: active, current, power, reactive, volt
ramox3
ramox3 is offline
#1
Aug15-11, 02:51 PM
P: 12
How do calculate the current when I have the active power and reactive power and voltage?

A single-phase, full-wave controlled bridge rectifier supplies a permanent magnet dc motor. The rectifier is connected to a 240V ac voltage source and absorbs 1200W of active power and 1000VAR of reactive power from the source. If power loss in the rectifier is negligible how do i calucalte the armature current?
Phys.Org News Partner Engineering news on Phys.org
PsiKick's batteryless sensors poised for coming 'Internet of things'
Researcher launches successful tech start-up to help the blind
Researchers propose network-based evaluation tool to assess relief operations feasibility
I_am_learning
I_am_learning is offline
#2
Aug16-11, 08:22 AM
I_am_learning's Avatar
P: 669
RMS Active Current = Active Power / (RMS Voltage)
RMS Reactive Current = Reactive Power / (RMS Voltage)
ramox3
ramox3 is offline
#3
Aug16-11, 11:11 AM
P: 12
so what can I say about the value of the current in the armature?

I_am_learning
I_am_learning is offline
#4
Aug16-11, 11:50 AM
I_am_learning's Avatar
P: 669

How do calculate the current when I have the active power and reactive power and volt


I am unsure of what might be consuming the reactive power, perhaps the capacitor Filter you might be using. I don't think DC motor Consume any reactive power during Steady State. So, going with that, all the active power is being consumed by the DC motor.
And the Current Going into it (i.e. into the armature) Depends on whether you are using Capacitor and what size. Assuming you are using Large Sized Capacitor so that the Terminal Voltage Across the Motor is almost constant near the peak of 240*Sqrt(2), then the Current into the armature is
Armature Current = 12000 / (339) = 3.53 A
FOIWATER
FOIWATER is offline
#5
Aug16-11, 12:17 PM
PF Gold
FOIWATER's Avatar
P: 369
the motor windings are coils they consume reactive power to develop magnetic fields
FOIWATER
FOIWATER is offline
#6
Aug16-11, 12:19 PM
PF Gold
FOIWATER's Avatar
P: 369
find apparent power with real and reactive then use that to solve for current.

S^2 = root P^2 + Q^2

I = S/root3 V ?
I_am_learning
I_am_learning is offline
#7
Aug16-11, 12:23 PM
I_am_learning's Avatar
P: 669
Quote Quote by FOIWATER View Post
the motor windings are coils they consume reactive power to develop magnetic fields
I don't think thats the case for DC motor. Since, they can even overate on Pure DC, there is no way they can consume reactive Power, which occurs only in AC.
uart
uart is offline
#8
Aug16-11, 01:24 PM
Sci Advisor
P: 2,751
Quote Quote by FOIWATER View Post
find apparent power with real and reactive then use that to solve for current.

S^2 = root P^2 + Q^2

I = S/root3 V ?
There is no "sqrt3" as this is a single phase bridge, but otherwise you are correct. But remember that that will only calculate the RMS current on the AC side. If you know the shape of the current waveform however, then from this you can deduce the average current on the DC side.

Lacking any other details, this type of question usually assumes that the current on the DC side is approximately constant due to the motor inductance, so that the line current is a square wave. Given this assumption you can actually calculate both the DC current and the (phase control) firing angle from the given data.


Register to reply

Related Discussions
Reactive Power Apparent Power Issue Advanced Physics Homework 1
How to calculate power loss due to eddy current formation? Advanced Physics Homework 0
Calculate the Current, Lost Volt and P.d across parallel resistors? Introductory Physics Homework 4
Electric Power measurement Current Transformers and reactive power Electrical Engineering 8
AC Power and Combining Volt/Current Introductory Physics Homework 6