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AC Metrology
11 questions By Tony R. Kuphaldt
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Question 4 of 11
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.
Reveal answerAn open-circuited secondary winding on a current transformer is capable of generating extremely high voltages.
Notes:There are a couple of different ways to explain why current transformers pose this safety hazard. One could explain it in terms of the winding ratio, or perhaps in terms of its function as an AC “current source”. Do not be surprised if your students present multiple explanations for this behavior.
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Question 5 of 11
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.
Reveal answerPerhaps the easiest solution is to wrap the power conductor so it goes through the CT toroid several times, thus changing its effective ratio.
Notes:The “multiple wrap” solution is a neat trick I’ve used more than once to measure current with an oversized current transformer. Discuss the effect of multiple “turns” of primary conductor on the ratio of a CT with your students, calculating the new ratios formed by doing so.
Although the “multiple wrap” solution is simple, it is not the only possible solution to this problem. Another solution would be to use multiple current transformers, but I’ll leave that up to you and your students to figure out!
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Question 6 of 11
Describe the purpose of a current transformer in an AC power circuit.
Reveal answerCurrent transformers, or CTs, are used to measure high currents in potentially high voltage AC circuits, providing precise current reduction and galvanic isolation between the measuring circuit and the power circuit.
Notes:Ask your students to describe the physical appearance and construction of a CT. If possible, bring one in to your classroom for viewing and discussion.
