# Determining R & X from 1 AC Measurement

• strokebow
In summary, the question is asking how many values of R and X can be determined from just 1 A.C. measurement of the current, voltage and power at the device's terminals. If the current, voltage and power are measured at the terminals of a device containing only resistance and inductive reactance, then the answer is three values: XL, R, and X.
strokebow
If the current, voltage and power are measured at the terminals of a device containing only resistance and inductive reactance, how many values of R and X can be determined from just 1 A.C. measurement?

(P.S. This is not homework but an elec engineer who has been stumped)

You mean one measurement of each voltage, current and power? What do you mean by how many values? As in: XL = 150 ohms, R = 150 ohms for instance?

Averagesupernova said:
You mean one measurement of each voltage, current and power? What do you mean by how many values? As in: XL = 150 ohms, R = 150 ohms for instance?

Yes. One measurement of each, how many values can you get like you have given examples

strokebow said:
If the current, voltage and power are measured at the terminals of a device containing only resistance and inductive reactance, how many values of R and X can be determined from just 1 A.C. measurement?

(P.S. This is not homework but an elec engineer who has been stumped)

It may not be homework, but it's awfully homework/coursework-like. So many of the same rules apply. Tell us what you think, and we'll try to help guide you as YOU figure it out.

As for some hints... If you measure the voltage and current, then measuring the "power" gives you no new information. If you have a known source voltage and source impedance (like from an impedance analyzer instrument), then you can measure the complex impedance of the device at its input terminals.

What you measure as a complex impedance can be built up infinitely many ways with resistors and inductors. You are measuring the total combined impedance. Oops, maybe I just gave the answer (bad Mentor).

Why are you asking this question? What is the context?

berkeman said:
If you measure the voltage and current, then measuring the "power" gives you no new information.

Actually, it does. By measuring power I think the OP means measuring actual power, and NOT VA.

berkeman said:
It may not be homework, but it's awfully homework/coursework-like. So many of the same rules apply. Tell us what you think, and we'll try to help guide you as YOU figure it out.

As for some hints... If you measure the voltage and current, then measuring the "power" gives you no new information. If you have a known source voltage and source impedance (like from an impedance analyzer instrument), then you can measure the complex impedance of the device at its input terminals.

What you measure as a complex impedance can be built up infinitely many ways with resistors and inductors. You are measuring the total combined impedance. Oops, maybe I just gave the answer (bad Mentor).

Why are you asking this question? What is the context?
The context is I was just browsing through an electric machines textbook on transformers (you may have seen my other post) and this question was at the end of the section. It had me puzzled that's all.

Averagesupernova said:
Actually, it does. By measuring power I think the OP means measuring actual power, and NOT VA.
I believe it refers to actual power also.

My thinking is this...

You have the actual power so you can obtain a resistive component value.
From the voltage and current measurement you can obtain an impedance and using this value in conjunction with the resistive value already obtained you can obtain a reactive component.
Thus, 2 components: 1 reactive and 1 resistive

? Agree ??

Since you are an electrical engineer you know what "Thevenin" means. If you measure the open circuit voltage at the terminals and you measure the short circuit current you can determine Z, the thevenin source impedance from Z=V(open circuit)/I(short circuit). That Z can be formed by many different circuit configurations of resistive and reactive components internal to the device.
It is not clear to me what you mean by "Power" measurement. Power of what?

## 1. What is the purpose of determining R & X from 1 AC measurement?

The purpose of determining R & X from 1 AC measurement is to calculate the resistance and reactance of a circuit or component. This information is important for understanding the behavior and characteristics of an electrical system.

## 2. What is the process for determining R & X from 1 AC measurement?

The process for determining R & X from 1 AC measurement involves measuring the voltage and current of the circuit using an AC voltmeter and ammeter, respectively. Then, using Ohm's law and the phase difference between the voltage and current, the resistance and reactance can be calculated.

## 3. What equipment is needed for determining R & X from 1 AC measurement?

The equipment needed for determining R & X from 1 AC measurement includes an AC voltmeter, an AC ammeter, and a phase angle meter. These instruments are used to measure the voltage, current, and phase difference of the circuit, respectively.

## 4. What factors can affect the accuracy of determining R & X from 1 AC measurement?

The accuracy of determining R & X from 1 AC measurement can be affected by factors such as the quality and calibration of the measuring instruments, the presence of noise or interference in the circuit, and the complexity of the circuit which may require more advanced measurement techniques.

## 5. What are the applications of determining R & X from 1 AC measurement?

Determining R & X from 1 AC measurement has various applications in electrical engineering, such as in designing and analyzing circuits, troubleshooting faulty components, and predicting the performance of electrical systems. It is also important in fields such as power systems, telecommunications, and electronics.

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