r.m.s values of voltage and current?

Question

I'm an electrician and when at work we always use I=P/V to find the current in a curcuit. However we use V=230 volts. Since 230 is an r.m.s value would the value of I obtained also be an r.m.s value, if so shouldn't we then find the average value of current for a more accurate reading.

I know that r.m.s is 0.7071*peak, but the average is 0.637*peak, so if this is the average value for half a sine wave shouldn't this be the average current.

Answer 1

RMS stands for "root mean square" . In some sense, it is an average. Sinusoidal waveforms can be tricky. If the voltage goes between -220 and +220, then the "average" is zero, right? But that "average" zero voltage powers our homes and businesses.

Answer 2

The alternating current is in the form of a sine curve.

The average value of current over a positive half period is
[2/π] or 0.637 of the maximum current value.
The average value of current over a negative half period is ─ [2/π] or 0.637 of the maximum current value.
The average value of current over a whole cycle is zero.


The average value of the SQUARE OF THE current over a positive half period is 1/2 of the maximum current
The average value of the SQUARE OF THE current over a negative half period is also 1/2 of the maximum current
The average value of the SQUARE OF THE current over whole cycle is also 1/2 of the maximum current.

The average value for the half periods are equal and alternatively +ve and -ve and the sum for any cycle being zero the impulses to the moving part of the galvanometer are equal and opposite resulting no motion of the moving part.

If we use a hot wire ammeter the meter works because of the heating effect in the wire which depends on the square of the current.
The meter reads the square of the current and we mark the square root in the scale. Thus the meter reads the RMS value.


Thus depending upon the circumstance and the quantity we want to measure we use either average or Rms values.

Both give the correct value and we are not measuring instantaneous voltage or current.

Answer 3

If you're dealing with ac, phase comes into the equation. It's no good going for the arithmetic average, you need to use calculus to find out the power used over a half-cycle. Provided the phase difference between voltage and current is small, r.m.s. values are the ones that give the right answers.

Answer 4

Using AC r.m.s. values for voltage is that same as using DC voltage for a simple circuit involving only resistors and heating effects. A 230 r.m.s. AC voltage provides the same power consumption as a 230 DC voltage in a resistor. However many applications (transformers, radios, etc.) require AC to operate and capacitance and reactance (from capacitors and coils of wire) affect the current produced.

Answer 5

The RMS (root mean square) value of an AC cct gives the same power calculation results as if it was DC.
This is why RMS is used. It is 0.7071 of the peak value.

Answer 6

I guess it depends on what you are going to use your calculation for.
If it's calculating the max running current then you are correct to do what you are doing...
However, if it's the average value current then yes it will be lower.... and assuming it is 230V peak to earth then the average to earth will be 115V.... so that's a good start.

Answer 7

Hi Lee, there is an excellent answer to your question in. www.practicalphysics.org/go/guidance_107html.
or quicker than that, just type rms value of voltage.
Its quicker than me explaining it.

Answer 8

RMS is more accurate than average. That's why it's used.

Answer 9

r m s = root mean square