Suppose that I am using a digital multimeter to measure the line current in a balanced three-phase, delta connected system. If I know the phase voltages and impedances, I can calculate the line current, and it will be represented by a complex number, which may be in either polar or complex form. Will the multimeter display the real part of the number (in rectangular form) or the modulus (in polar form)?
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How does a multimeter work?
- Thread starter Abraham V
- Start date
It will give the modulus.
It is worth thinking about why we are expressing electrical systems as complex numbers and the limitations of the meter. The only reason we have to use complex numbers in electrical equations is because we are describing the whole system in relation to itself. We arbitrarily selected a phase and called it zero degrees, and then described the relative position of the other phases using complex numbers. We could have chosen any other phase, or any other angle and the relative positions of the other phases would stay the same, but the math would get messier. Since your multimeter cannot see the rest of the system, it has no use for complex numbers or rectangular or polar notation because its reading is only pertaining to what it is measuring at the time. The meter is not looking at any of the other phases, so it doesn't need to describe what it is currently metering with respect to anything other than itself.
It is worth thinking about why we are expressing electrical systems as complex numbers and the limitations of the meter. The only reason we have to use complex numbers in electrical equations is because we are describing the whole system in relation to itself. We arbitrarily selected a phase and called it zero degrees, and then described the relative position of the other phases using complex numbers. We could have chosen any other phase, or any other angle and the relative positions of the other phases would stay the same, but the math would get messier. Since your multimeter cannot see the rest of the system, it has no use for complex numbers or rectangular or polar notation because its reading is only pertaining to what it is measuring at the time. The meter is not looking at any of the other phases, so it doesn't need to describe what it is currently metering with respect to anything other than itself.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
A multimeter displays a single magnitude value. I know of no three phase multimeter.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
230703-1143 EDT
A multi meter has two input terminals. Virtually all multi meters in AC mode read a voltage that is converted to a small current and the current is displayed using some scalling value.
A Fluke 27 or Simpson 260 uses a diode rectifier to convert the AC to DC. Each of these meters takes an average value of the average value over a number of cycles, and displays that average value, but scaled to RMS. However these meters are calibrated in an RMS value, and thus only correctly reads for something that produces an average equivalent to what a sine wave would produce.
A Fluke 87 is a true RMS meter, meaning it calculates the RMS value in its conversion from AC to DC and therefore reads the RMS of any shape waveform within some limitations.
.
A multi meter has two input terminals. Virtually all multi meters in AC mode read a voltage that is converted to a small current and the current is displayed using some scalling value.
A Fluke 27 or Simpson 260 uses a diode rectifier to convert the AC to DC. Each of these meters takes an average value of the average value over a number of cycles, and displays that average value, but scaled to RMS. However these meters are calibrated in an RMS value, and thus only correctly reads for something that produces an average equivalent to what a sine wave would produce.
A Fluke 87 is a true RMS meter, meaning it calculates the RMS value in its conversion from AC to DC and therefore reads the RMS of any shape waveform within some limitations.
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