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When working on a current transformer the secondary windings must be?
When working on a current transformer the secondary windings must be shorted. <<>> Properly loaded
What is a current transformer and how can a current transformer be used to extend the range of a wattmeter?
A current transformer (CT) works just like any other transformer. It is important to note it is not referenced to ground. It reflects current flow, not phase to ground voltage, so it attempts to hold current constant, not voltage. There is usually one primary "winding": one large cable through a donut CT, and multiple secondary windings. If a secondary winding is shorted (a wire placed across its terminals), the voltage built up in the secondary will be equivalent to V = I*Z, where I is the secondary current, and Z is the impedance of the secondary winding, usually specified as some flat value plus so many ohms per winding. If too big of an impedance is placed on the secondary, a voltage will build up to the point that the donut CT saturates, resulting in an output wave that has a large amount of harmonics. If the CT is open circuit\'d, voltage will build up until it is high enough to arc over (usually where it should have been shorted). CT\'s can be damaged by open circuiting them, so don\'t do it.
What is open circuit fault of transformer?
A fault causing a difference between the primary and secondary currents of a power transformer. Usually a leakage to earth from somewhere in a winding. Such errors are caught by a protective circuit comprising two current transformers for transformation of the primary and secondary currents respectively, in a common scale typically 0...1 or 0...5 A and bringing them together in a loop. A transverse branch in the middle of the loop collects the differential current set up in case of a fault, the so called 'spill' current. The differential branch uses this current to operate the operating winding of the protection relay and through this the alarm and trip contacts. The protection circuit thus simulates what is happening in the power circuit in an under graded and more direct issue.
Why can a CT be shorted and a Pt cannot?
A current transformer pushes a ratio of primary current in the secondary. A potential transformer pushes a ratio of primary voltage in the secondary. Shorting a CT allows the full current the CT wants to push to flow. Shorting a PT makes it very difficult for the PT to produce rated voltage. To keep the voltage at rated value, the PT would have to push ~infinite secondary current.
What multimeter readings set to measure resistance indicate an open circuit and short circuit in the primary and secondary windings of a mains power transformer. And what readings show good?
You cannot use an ordinary multimeter to assess the difference between a normal power transformer winding and a shorted power transformer winding. You need a Q meter and/or to power it up at a reduced voltage with limited current to see what happens. Note that a shorted power transformer is capable of exploding, so you need to take appropriate safety precautions.
When working on a current transformer the secondary windings must be?
When working on a current transformer the secondary windings must be shorted. <<>> Properly loaded
Why a 750 kw motor takes 17 amps starting current?
The starting current is high because the motor's rotor winding has very low resistance. It's similar to a transformer with a shorted secondary windings. As the motor accelerates,the back emf increases which resist the flow of current in the rotor winding. Hence,the current drop to the rated full-load value.
Why current transformers secondary is always short circuited?
Unless a burden (i.e. meters, relays, etc.) is connected to the CT, current transformers should always be shorted across the secondary terminals. The reason is very high voltages will be induced at the terminals. Think of the CT as a transformer, with a 1 turn primary and many turns on the secondary. When current is flowing through the primary, the resulting voltage induced in the secondary can be quite high, on the order of kilovolts. When a CT fails under open circuit conditions, the cause of failure is insulation breakdown, either at the shorting terminal strip, or at the feedthrough (in the case of oil filled apparatus), because the distances between terminals are not sufficient for the voltages present.
How does a current transformer work?
A current transformer is used in high voltage circuits where it is not possible to measure current directly. A CT is a step up transformer with only one turn in primary. There will be as many cores based on the purposes like metering, protection etc. The secondary of a CT should never be kept open circuited bcoz very high flux will be developed in the secondary and hence it may be damaged.
What is a current transformer and how can a current transformer be used to extend the range of a wattmeter?
A current transformer (CT) works just like any other transformer. It is important to note it is not referenced to ground. It reflects current flow, not phase to ground voltage, so it attempts to hold current constant, not voltage. There is usually one primary "winding": one large cable through a donut CT, and multiple secondary windings. If a secondary winding is shorted (a wire placed across its terminals), the voltage built up in the secondary will be equivalent to V = I*Z, where I is the secondary current, and Z is the impedance of the secondary winding, usually specified as some flat value plus so many ohms per winding. If too big of an impedance is placed on the secondary, a voltage will build up to the point that the donut CT saturates, resulting in an output wave that has a large amount of harmonics. If the CT is open circuit\'d, voltage will build up until it is high enough to arc over (usually where it should have been shorted). CT\'s can be damaged by open circuiting them, so don\'t do it.
Can a current transformer be used as a voltage transformer?
Current Transformers cannot be used as a voltage transformers, they are used to measure large electrical currents. They are manufactured in the shape of a dough nut, round with a single hole in the middle. The conductor carrying the current passes thru this hole. the CT has two terminals, these would be connected to a ammeter or can be shorted together. When they are shorted together you can use a standard clamp on ammeter by passing the wire shorting the CT terminals together thru the meter jaws. The CT is sized by ratio of turns for the current is measuring, standard ratios of 200 to 5 or 500 to 5 .So if you were meassuring a current of 200 amps, your clamp on meter would read 5amps. A panel ammeter would be set up to display the amps as 200 amps. Current transformers, when installed should always have their ouput termianls shorted together or attached to an ammeter, to do so otherwise will damage the CT.No. A transformer is simply windings on a core, and simply changes the voltage of a signal. It doesn't add power, so as the voltage goes up, the amperage goes down, maintaining constant power (minus losses).In my mind, an amplifier adds power.
Why potential transformers are open circuited?
Beacause CT is shorted so it is kept opened
What fails on AC electric motors?
Bad bearings, shorted or open windings and perhaps a bad starting capacitor.
Why is the transformer turns ratio test needed?
Turns ratio test is very important in order to find out that the transformer has the right ratio corresponding on its rated voltage in primary and secondary. For example, three phase transformer is subjected to turns ratio test when each phase has equal turns ratio then the transformer is balance.The test is performed to ensure that overheating or overcurrent conditions have not shorted turns in the transformer windings, which would distort the desired output or input voltage. This test is particularly important for instrument transformers, i.e. CTs and PTs which are connected to protective relays. These relays are extremely sensitive, and operate on very small fluctuations in secondary voltage or current. Shorted turns in these transformers can lead to big problems if they cause the relay to operate abnormally, or not at all.
What is open circuit fault of transformer?
A fault causing a difference between the primary and secondary currents of a power transformer. Usually a leakage to earth from somewhere in a winding. Such errors are caught by a protective circuit comprising two current transformers for transformation of the primary and secondary currents respectively, in a common scale typically 0...1 or 0...5 A and bringing them together in a loop. A transverse branch in the middle of the loop collects the differential current set up in case of a fault, the so called 'spill' current. The differential branch uses this current to operate the operating winding of the protection relay and through this the alarm and trip contacts. The protection circuit thus simulates what is happening in the power circuit in an under graded and more direct issue.
Why can a CT be shorted and a Pt cannot?
A current transformer pushes a ratio of primary current in the secondary. A potential transformer pushes a ratio of primary voltage in the secondary. Shorting a CT allows the full current the CT wants to push to flow. Shorting a PT makes it very difficult for the PT to produce rated voltage. To keep the voltage at rated value, the PT would have to push ~infinite secondary current.
Why should current transformers be shorted off load?
If a current transformer (CT) has its secondary winding left open-circuit whilst there is current in the primary winding, the magnetic core will saturate, large eddy currents will occur in the core causing the core to heat up rapidly, the voltage in the secondary will rise to dangerously high values and the current transformer will fail catastrophically. To further describe why this happens, consider an "ideal" transformer (which has no losses at all - a reasonable assumption because transformers are generally very efficient), therefore, power in equals power out. Power can be calculated by multiplying the voltage by the current (amps using the symbol I) in each case. Therefore: Pin = Pout, or Vin x Iin = Vout x Iout, or again Vprim x Iprim = Vsec x Isec (where prim refers to the primary or input winding and sec refers to the secondary or output winding). The primary winding of current transformer is connected in series with the load, therefore the current in the primary winding is fixed by the load. Thus if the secondary winding is open circuit, the secondary current is zero, and the voltage will rise to a very high value in trying to balance this equation. It will exceed the breakdown value of the insulation and will cause an arc because the insulation will never withstand an infinite voltage! In addition to this if the secondary is open circuit, there is no magnetic flux from secondary winding current, to balance and negate any of the magnetic flux from the primary winding current as occurs in power and voltage transformers. This is the reason for magnetic saturation of the core, and the rapid heating effect of the current transformer which affects the ability of the insulation to withstand the high voltage described above. The failure of current transformers under these conditions can be explosive and highly dangerous.
