AC Metrology

AC Electric Circuits

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  • Question 1

    In power distribution systems, it is very important to be able to measure line voltage. You cannot control what you cannot measure, and it is important to control power line voltage so as to not exceed the insulators’ ratings.

    But how do you safely measure the voltage of a 750 kV power line? Obviously, no voltmeter small enough to be located on a control panel could safely handle 750,000 volts applied to it, as a voltage that high is capable of arcing several feet through the air (not to mention the safety hazards of having wires behind the meter panel connecting straight to the power line!).

    In industry, specialized transformers are used to safely measure the high voltages on power lines. Describe what is special about these “potential transformers,” and how they are implemented to measure dangerous voltages.

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  • Question 2

    A common instrument used for measuring high AC currents in power systems is a current transformer, abbreviated “CT”. Current transformers usually take the form of a “donut,” through which the current-carrying conductor passes:

    The purpose of a current transformer is to create a secondary current that is a precise fraction of the primary current, for easier measurement of current in the power conductor.

    Given this function, would current transformers be considered a “step-up” or “step-down” transformer? Also, draw how the secondary windings of a current transformer are arranged around its toroidal core.

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  • Question 3

    How much current will be output by a current transformer if the load current is 350 amps and the CT ratio is 600:5?

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  • Question 4

    It is very important that the secondary winding of a current transformer (CT) never be open-circuited while in operation! Explain why this is an important safety consideration.

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  • Question 5

    Suppose you wished to measure the current of an AC motor, the full-load current of which should be about 150 amps. This current is much too great to measure directly with your only AC ammeter (rated 0 to 5 amps), and the only current transformers you have available are rated at 1200:5, which would not produce enough output current to drive the 0-5 amp meter movement very far at a load current of 150 amps. Sure, you would get a measurement, but it wouldn’t be very accurate.

    Figure out a way to overcome this measurement problem, so that a motor current of 150 amps will produce a more substantial deflection on the 0-5 amp meter movement scale.

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  • Question 6

    Describe the purpose of a current transformer in an AC power circuit.

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  • Question 7

    What does it mean if a meter movement is described as being RMS indicating, average responding?

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  • Question 8

    Electrostatic meter movements use the physical attraction between metal plates caused by a voltage to deflect a pointer, instead of using electromagnetism as is common with most other meter movement designs. Although electrostatic meter movements are not as sensitive as PMMC mechanisms, they have the advantage of being able to measure both AC and DC with equal ease.

    Suppose you calibrated an electrostatic meter movement from 0 volts to 500 volts DC. Then, you connected this meter to a sinusoidal AC source and watched it register a voltage of 216 volts. What is the voltage of this AC source, in volts RMS?

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  • Question 9

    Most electromechanical meter movements are inherently average-responding. They display their indications in units of volts or amps “RMS” only because they have been calibrated to do so for sinusoidal waveforms.

    Some electromechanical meter movements, though, are true-RMS responding. For example, electrodynamometer movements, when connected as either voltmeters or ammeters (not as wattmeters), naturally provide indications proportional to the voltage’s or current’s true RMS value.

    Based on the inherent differences between these meter movements, describe how you could use electromechanical meter movements to perform qualitative assessments of waveform distortion. In other words, how could you use electromechanical meters to tell whether an AC waveform was sinusoidal or not?

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  • Question 10

    An electromechanical meter movement commonly used to measure AC power is called the electrodynamometer movement. Describe how this meter works, and draw a schematic diagram showing how it is used to measure the AC power delivered to a load.

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  • Question 11

    There is a lot of incorrect terminology and information in the world of high-fidelity audio equipment, due primarily to a large consumer base lacking technical knowledge. One of the common mis-statements of audio amplifier performance is power, expressed in “watts peak” and “watts RMS”. While the term “peak power” is not necessarily incorrect, “RMS power” most definitely is. What is wrong with the latter term, and what do you think audio equipment manufacturers mean when they specify an amplifier’s power rating in “watts RMS”?

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