Calculating Armature Current from Active & Reactive Power & Voltage

In summary, to calculate the armature current for a single-phase, full-wave controlled bridge rectifier supplying a permanent magnet DC motor, you can use the formulas RMS Active Current = Active Power / (RMS Voltage) and RMS Reactive Current = Reactive Power / (RMS Voltage). Assuming negligible power loss and a large sized capacitor, the armature current can be calculated as Armature Current = 12000 / (339) = 3.53A. However, since DC motors do not consume reactive power, this calculation may not be entirely accurate. You can also use the formula S^2 = root P^2 + Q^2 to find the apparent power and then solve for current using I = S/V. Keep in mind
  • #1
ramox3
12
0
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?
 
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  • #2


RMS Active Current = Active Power / (RMS Voltage)
RMS Reactive Current = Reactive Power / (RMS Voltage)
 
  • #3


so what can I say about the value of the current in the armature?
 
  • #4


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
 
  • #5


the motor windings are coils they consume reactive power to develop magnetic fields
 
  • #6


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 ?
 
  • #7


FOIWATER said:
the motor windings are coils they consume reactive power to develop magnetic fields
I don't think that's 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.
 
  • #8


FOIWATER said:
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.
 
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1. How do I calculate armature current from active and reactive power and voltage?

To calculate armature current, you can use the formula I = (P^2 + Q^2)^(1/2) / V, where P is the active power, Q is the reactive power, and V is the voltage. This formula is known as the power triangle and is commonly used in electrical engineering.

2. What is the difference between active and reactive power?

Active power, also known as real power, is the actual power consumed by a device or system. It is measured in watts and is responsible for producing useful work. Reactive power, on the other hand, is the power that is used to establish and maintain magnetic fields in inductive loads. It is measured in volt-amperes reactive (VAR) and does not produce useful work.

3. Why is armature current important to calculate?

Armature current is important to calculate because it is a measure of the current flowing through the armature winding of an electric motor or generator. This current is responsible for producing the magnetic fields that drive the motor or generator and is directly related to the amount of power that the device can produce.

4. Can armature current be negative?

Yes, armature current can be negative. This occurs when the motor or generator is in a regenerative state, meaning that it is generating power instead of consuming it. In this case, the direction of the current flow is reversed, resulting in a negative value.

5. Are there any limitations to using the power triangle formula for calculating armature current?

Yes, there are some limitations to using the power triangle formula. It assumes that the power factor is known and does not take into account any losses in the system. Additionally, it may not be accurate for systems with non-linear loads or those with high levels of harmonics. In these cases, more advanced calculations may be necessary to accurately determine armature current.

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