# 3 phase current measurement transulation to RMS current

Discussion in 'General Electronics Chat' started by riscy00, Apr 18, 2018.

1. ### riscy00 Thread Starter New Member

Jun 23, 2010
4
0
I have current clamp probe on 3 wire which provides the reading of each wire, I like to estimate current based on 3 current readouts from the RMS meter. How to put this number into summed current correctly, below the example based on what I read but the Isum is too high as the efficiency of AC-DC PSU is 80% (based on DC PSU input to the circuit after the diode rectifier). I going to fix phase 3 problem since it not balanced, please treat as balance.

Ø Phase 1= 1.27A (AC)

Ø Phase 2 = 1.22A (AC)

Ø Phase 3 = 0.57A (AC) This need checking

Ø ISum = (1.27+1.22+0.57) / 3 = 1.02 Amp (RMS) (1st guess method)

Ø PGENI = 43V * 1.02Amp = 43W This cannot be right, much too high.

POUT = 30V * 0.93Amp = 28W

2. ### MisterBill2 Well-Known Member

Jan 23, 2018
1,984
369
finding the average current tells you nothing useful here. Are you feeding a 3-phase motor, a DC power supply with a 3-phase input, or something else? If the load is a motor then there is a problem., because all three phases should be similar. Anything else and the phases may not be the same. AND, is there a neutral connection to the load? There is a lot of needed information missing here.

3. ### riscy00 Thread Starter New Member

Jun 23, 2010
4
0
I fixed the issue, it now 0.9A, 0.93A and 0.92A (I changed the setting a bit).

What is RMS total current of 3 phase current measurement?, is there an equation for this?

Jan 15, 2015
3,912
1,779
While you can add the 3 currents and get a sum, divide by 3 and get an average it tells you nothing. In a three phase circuit the current is measured and listed by individual phase. What the concern is would be the total power.
V = Voltage
I = Current
W = Watts
PF = Power Factor

W = V * I * PF (Single Phase Circuit)
W = V * I * PF * 1.732 (Three Phase Circuit) The 1.732 is a constant and the square root of 3.

The PF or Power Factor is a function of the load. Just as an example a purely resistive load will have a power factor of 1.0. Loads like motors will generally have the PF included in the name plate data or manufacturer's data sheet.

Anyway, there is no single number for 3 phase current. The above equations also assume a balanced or close to balanced load.

Ron

5. ### MisterBill2 Well-Known Member

Jan 23, 2018
1,984
369
If the goal is to determine the efficiency of a DC power supply, or any power supply, that is determined by finding the output power and the input power, and then dividing the output power by the input power. This is useful because most of the input power that does not become output power is instead converted to heat, which then must be removed from the power supply. You have not mentioned any details about the PSU that you are checking, but in many cases there are cooling fans connected at the power source connection and they cause the currents at the different phases to be a bit different.