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Electrical Engineering
Electrical engineering is a field of engineering that deals with the study and application of electricity, electronics and electromagnetism.
23,056 Questions
Which wire has the lowest resistance?
Resistance is directly-proportional to the length and resistivity of a conductor, and inversely-proportional to its cross-sectional area.
So a shorter wire would have less resistance than a longer wire made from the same material, and a wire with a greater cross-sectional area would have less resistance than one with a smaller cross-sectional area made from the same material.
Resistivity depends on the material from which the wire is made, with some materials being better conductors than others. For example, silver has the lowest resistance compared with other metal conductors having identical dimensions. Similarly, a copper wire will have a lower resistance than an aluminium wire of identical dimensions.
Why the polarity of transformer is important?
a polarity test is a test which use to check the polarity of tranformer.the rision of this test to running the two or more transformer in parallel.like bettry situation or to fin the terminals
There are many causes of power failures in an electricity network. Examples of these causes include, faults at power stations, damage to power lines, substations or other parts of the distribution system, a short circuit, or the overloading of electricity mains.
How you convert Amps into MVA?
1mva = 1000kva so you simply divide by 1000.
10000KVA = 10MVA
K = kilo = 1000
M = mega = 1000000
What voltage would you expect to measure across closed switch?
Original Correct Answer:
The voltage across an open switch should equal the supply voltage.
More Detailed Answer:
The above answer is basically correct. However, it may not be EXACTLY the supply voltage.
This is counter-intuitive at first glance and confuses a lot of people, including electrical engineers. The reason is this. Voltage, Current and Resistance are all interrelated by Ohms Law. Voltage equals Current multiplied by Resistance.
It is easy to think that since a switch is open, then you do not have current flow through the circuit. Thus, current times any Resistance is equal to zero volts because the current is equal to zero. Thus, by this logic, you would expect to find zero volts across the switch. In actuality this is true.
But, when you insert you meter, you change the conditions of the circuit, and the following is the result.
Let's say that you have a circuit with a resistance load like a heater, and a inductive load like a motor. The switch that powers these devices is open, thus their is no current flowing through the resistance or inductive loads.
Now you put the meter across the open switch. When you do, you insert a very large resistance in parallel to the open switch. Why? Because to get volts, the meter measures current flow through a known resistance, and then calculates voltage. To keep the resistance from impacting the circuit performance, the resistance is very large. Therefore, when you insert the meter, you will get a flow of current through the meter.
Because of this large resistance, the current trough the resistance load, inductive load, and wires is very small. Thus, the voltage drop across the loads and wiring is very small. Therefore, it appears that the entire voltage in the circuit is across the huge resistance in the meter. The result is a voltage reading that is very near the source voltage.
Let's do the math. Let's assume you have 120 volts. You also have a resistance of 500 ohms, and a motor winding that has 0 ohms resistance when DC is applied (This is true for motors). The meter has a 10 million ohm resistance.
If these loads are in series, the total resistance is 10Million 500 ohms. The 11.9 microamps. By multiplying the current flow to each resistance, you get 6 millivolts across the resistance, no voltage across the motor winding, and 119.994 across the open switch or meter. Since a meter rounds it reading, you would get 120.
If the loads are in parellel, you would get the same thing, becuase the switch is in series with both loads. In this example, the motor winding would have all the current flow through it since it is zero ohms, and the parallel resitance load is 500 ohms. Thus, the total resistance is the 10 million of the meter, and this resistance drive the current, and thus the largest voltage drop is at the swtich/meter. You could decide to remove the motor from this parellel circuit. If you did then the then the result is the series circuit above.
What is Difference Between Current Source and Voltage Source?
A current source varies the output voltage to maintain the desired current. A voltage source has a constant output regardless of the current draw (up to the capacity of the supply, of course).
Capacity in Ah is not directly related to voltage. If you have 3 AA batteries, each with different mAh ratings, you should be able to connect them in parallel or series to your hearts content. In series, the voltage will be additive: 3*(1.5volts) = 4.5 volts total voltage across all three batteries in series, assuming the battery voltage is 1.5 volts. In parallel, the voltage will be equivalent to 1 battery. In parallel, the three batteries are able to provide 3 times more current at 1.5 volts than if all three are in series at 4.5 volts.
Be careful when parallelling batteries of different voltages though. This is not a good idea, as they will try to force each other to match terminal voltage (voltage at the outputs of the batteries). An example: 1.5 volt AA battery, and a 12 volt car battery can be put in series - the total voltage will be 13.5 volts. The total current that can be sourced will be limitted by the AA's limit (much less than the car battery's limit). If put in parallel, the AA will try to force the voltage of the car battery down to 1.5 volts by drawing current into itself from the car battery. Alternately, the car battery will try to force the AA to 12 volts by pushing current into the AA battery. The AA battery will overheat, and likely catastrophically fail (blow up).
Why speed control of DC shunt motor is necessary?
The total voltage of the batteries or DC power source connected to a shunt motor affects how first it moves. Increasing the DC voltage will make the shunt motor run faster.
Why do you calibrate and recalibrate measurement equipment?
Usually, when you measure something that requires calibration, that work is generally considered important either monetarily or socially. So, you want to make sure you are sure because things are riding on your measurements. The short answer is so you don't screw up :)
Why are glass insulators used on power lines?
The large glass insulators are easy to mould, and are strong, well able to insulate the pylons from the high voltage the cables are designed to carry.
What is race problem in flip-flops?
When you have asynchronous logic (not clocked) each path has different delay times through the gates. Usually depending on the number of gates it passes. When you have logic that can go in a different state when signals enter in different order, you have a race problem. It means that the signal are "racing" and the fastest one wins.
Function of earth fault relay?
An "earth fault relay" is a bit ambiguous. A relay used in the power system to detect neutral or ground faults measure the vector difference of the three phase power, or measure the neutral current directly. If current is above a set trip point, the relay will operate.
If you are referring to GFCI's, they effectively measure the current flowing in and the current flowing out on the two "hot" wires, and if these do not cancel each other out, then the GFCI will trip. This is because if current in does not equal current out, then some current must be flowing out a different way (to ground!).
What are the advantages of DC machines?
DC engines were the first sort generally utilized, since they could be fueled from existing direct-current lighting force conveyance frameworks. A DC engine's velocity can be controlled over a wide range, changing so as to utilize either a variable supply voltage or the quality of current in its field windings. Little DC engines are utilized as a part of instruments, toys, and apparatuses. The widespread engine can work on direct current however is a lightweight engine utilized for compact force apparatuses and machines. Bigger DC engines are utilized as a part of impetus of electric vehicles, lift and raises, or in drives for steel moving plants. The appearance of force hardware has made supplanting of DC engines with AC engines conceivable in numerous applications.
What is the difference between commutator and rectifier?
commutator also acts as an mechanical rectifier in the case of dc machines.by deviding the commutator into two parts that will accept the +ve cycle & ignore the -ve cycle.so it act as a rectifier.
How do you calculate the transformer turns ratio?
For an ideal transformer, the voltage ratio is the same as its turns ratio.
What is a non polarized capacitor?
A polarized capacitor is one which has a polarity, positive on one terminal, negative on the other. This makes it superficially look like a battery. In use, the capacitor has its positive voltage always higher than that on the negative terminal, it matters that this is the case and this gives rise to the term polarized. This sort of capacitor is commonly found in power supply filters.
Why lights in classroom is connected in series or in parallel?
only two connections are possible.. parallel connection is always desired than series. .
What are two types of EMF induced in a double wound transformer?
1. self- induction2. mutual- induction
How is direction of rotation for single phase induction motor determined?
A single phase induction motor is not self starting; thus, it is necessary to provide a starting circuit and associated start windings to give the initial rotation in a single phase induction motor. The normal running windings within such a motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction.
Why neutral wire carry no current?
Usually yes, sometimes no. For instance:
The neutral in a single-phase, 120V (in the US) branch circuit, such as one feeding receptacles, does.
The neutral in a 120/240V circuit feeding a 240V appliance does not.
The neutral in a 480Y feeder feeding a balanced load does not.
A neutral is there because of the possibility that current flow could occur. For instance, in a US household, with 120/240V service, if you plugged in 5 100 watt lamps on one side of the line, and another 5 100 watt lamps on the other hot leg, there would be no neutral current in the service cable feeding the house. The loads are said to be 'balanced'. The 500 watts of power flowing into the first hot leg goes through the first set of lamps, then the second set, then out the other hot wire. Neutral current still flows in the individual branch circuits, of course.
Now, if you moved one of the lamps to the other side, 600 watts would be coming into that side, but only 400 would be going back out the other hot wire, so 200 watts would flow through the neutral.
What causes insulation breakdown in transformers?
The main cause of insulation breakdown is excessive temperature. However, it can also be caused by excessively-high electrical stress due to sharp corners or other protrusions in the conductor which act to intensify the electric field beyond the capability of the insulation.
A synchronous condenser (also known as a synchronous capacitor or synchronous compensator) is a DC-excited synchronous computer (large rotating generators) whose shaft is now not connected to any using equipment.
What is the difference between shunt generators and series generators?
1. In Shunt generators armature current is equal to sum of field current and load current whereas in series generators field current and load current is same.
2. Shunt generators field winding has high resistance and large no of turns as compared to series generators.
3. Shunt generator field winding has thin conductor and series generator has thick.
How many amps are in 400 horsepower with 480 volt three phase?
This might help you. First and foremost is the information the manufacturer puts on the nameplate of the motor. Second, the generic amperage can be found on motor charts that are on the Internet. Third you can use a formula to find the approximate amperage. HP = Amps x Volts x 1.73 x %Eff x pf/746. Transposed for Amperage, Amps = HP x 746/Volts x 1.73 x %Eff x pf. Use power factor = .9. Eff = 746 x Output HP/Input watts use 1.5. For estimating a 400 HP motor running on 480 volts. Amps = 400 x 746/ 480 x 1.73 x 1.5 x .9 = 298400/747.36 = 399.27 amps. See how much easier it is to read the nameplate.
Can you use 24 volt transformer to powers 12v lights?
You can if you want to connect two 12 volt lights in series with each other. Make sure that the 12 volt lights are rated for AC and not DC.
CommentFurther to the above answer, you must also ensure that the two lamps have identical power ratings, or the lamp with the lower power rating will be brighter than the one with the higher power rating!
