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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
Is there any power consumed in pure inductor and pure capacitor?
In an AC circuit energy flows into an inductor in one quarter-cycle and flows out on the next. Ditto for a capacitor. Therefore over a cycle, no net power or energy is consumed in a pure capacitor or inductor.
Another Answer
'Power' is the rate at which energy is transferred. In the case of 'pure' inductance or capacitance, energy flows from the supply to the device during the first quarter-cycle of current, where it is temporarily stored within the magnetic (inductors) or electric (capacitors) field, only to be returned to the supply during the following quarter-cycle. You could describe this action as 'energy sloshing back and forth between the supply and the device'. The rate at which this energy transfer takes place is called 'reactive power' and is expressed in 'reactive volt amperes'. None of this energy is lost or, to use your expression, 'consumed'.
'True power', expressed in watts, is the rate at which energy is expended -i.e. energy taken from the supply and then dispersed through work (e.g. by motors) or heat transfer (losses). This does NOT occur in purely-inductive or in purely-capacitive loads. So, in answer to your question, 'power' (in the sense of 'true power') is NOT 'consumed' by a pure inductor or capacitor (remember, technically, it's energy that gets consumed, not power -power is simply a rate).
You should bear in mind, however, that there is no such thing as a 'pure inductor' or 'pure capacitor' as each will exhibit some degree of resistance. Resistance results in the expenditure of energy, the rate at which it occurs being 'true power'. So 'real' inductors and capacitors combine 'reactive power' with 'true power', the vector-sum of which we call 'apparent power', expressed in volt amperes.
Why squirrel cage rotor is also known as short circuited rotor?
A squirrel-cage rotor resembles a hamster or gerbil exercise wheel, and consists of a pair of ring-shaped conductors, connected together with numerous bar-shaped conductors. These bars are termed 'active conductors' because it is into these that voltages are induced by the machine's rotating magnetic field. The resulting currents circulate along the bars and return via the ring-shaped conductors which act to interconnect, or short-circuit, the common ends of the bar conductors.
Which is the result of using a more powerful motor to run an elevator?
The same amount of work is done in less time.
true
Apparent power is the vectorial sum of the true power and reactive power. In this case, the total reactive power is the difference between 7200 var and 3600 var -i.e. 3600 var.
So you can now use the equation,
(apparent power)2 = (true power)2 + (total reactive power)2,
to determine your answer.
Why telephone line has negative voltage?
A negative live voltage is used to prevent electroytic corrosion on the copper wires.
The triplen harmonics are defined as the odd multiples of the 3rd harmonic (ex. 3rd, 9th, 15th, 21st etc.). Triplen harmonics are of particular concern because they are zero sequence harmonics, unlike the fundamental, which is positive sequence. The consequence of this fact is that the magnitude of these currents on the 3 phases are additive in the neutral. This can lead to very large currents circulating in the neutral, and unless the neutral is sufficiently oversized this can present a fire hazard. These currents can also circulate in the transformer causing significant overheating there too. Single-phase power supplies for equipment such as electronic ballasts and PCs are the most significant source of Triplen harmonics
What type of current flows in stator and rotor winding of a dc shunt generator?
The field winding in the stator is exposed to the full current generated by the rotor's winding.
Why is residual magnetism is necessary?
'Residual magnetism' isn't something that's 'necessary'; rather, it's something you're stuck with, whether you want it or not! Residual magnetism is due to a phenomenon called 'hysteresis', which is derived from a Greek word, meaning 'to lag'.
A bit of background first. If we were to wind an insulated coil around the sample of ferromagnetic material, and pass a current through that wire, we would create and apply magnetic field strength (symbol: H), expressed in amperes per metre, to that sample. This results in a magnetic field being set up within the sample, the intensity of which we call its flux density (symbol: B) expressed in teslas.
If we gradually increase the magnetic field strength, the resulting flux density would also increase until a point, called 'saturation' is reached -at this point any further increase in magnetic field strength will NOT increase the flux density. If we were to graph this behaviour, then the result would look something like an elongated 'S', rather than a straight line. This graph is known as a B-H curve.
Now, if we were to reduce the magnetic field strength to zero, the magnetic flux density would also reduce towards zero (following a slightly-different curve) but would not reach zero when the magnetic field strength reaches zero -in other words, when we remove the magnetic field strength, the sample 'retains' some flux density -and we call this 'residual magnetism' or, more accurately, 'residual flux density' or 'remanance'. This is what we mean by 'hysteresis' -i.e. changes in magnetic flux density lag behind changes in magnetic field strength.
To remove this residual flux density, we would actually need to reverse the direction of the magnetic field strength (by reversing the direction of the current through the coil) until the flux density falls to zero.
Different ferromagnetic materials have different values of residual flux density. For example materials that make good permanent magnets have very high values of residual flux density while others, such as metals used to make transformers, electromagnets, etc., have very low values of residual flux density.
To summarise, residual magnetism is something that occurs naturally and the amount of residual magnetism depends on the type of magnetic material involved. It's not a matter of being 'necessary', it's simply a characteristic of ALL magnetic materials.
Half a volt, because the energy is the charge times the potential difference.
Another AnswerYou should be asking what the 'potential difference' is between the two terminals, not the 'potential'. Note that 'voltage' is another term for potential difference.
What is the recommended safe voltage for electrical equipment on a building site?
It depends on the legislation of the country you are in.
What will happen if we slip rings are used in motors intead of commutator?
If slip rings are used, a dc field current fed to the rotor through the slip rings causes a magnetic field that rotates with the rotor. This is used in synchronous motors and also in alternators in cars and power plants.
In a car alternator the dc current supplied to the rotor is used to control the votage generated in the stator, which is connected through diodes directly to the battery.
Why the speed regulation does not apply to series motor?
Series motors have poor speed regulation because a reduced load causes the speed to rise, which causes the current to drop, which causes the speed to rise more because there is less field.
Series motors are usually used to drive massive loads such as trams, where their high starting torque is a big advantage, and the speed cannot go out of control.
#6 copper wire has a resistance of 0.3951 ohms per 1000 ft (from wire tables). A 750 ft circuit contains 1500 ft of wire so the resistance is 0.5926 ohms. With 65 amps flowing the voltage drop is 38.5 volts.
The power lost in the cable is 2½ kW.
Why is not feasible to operate bulbs a low voltages although it amount to high efficiency?
Lamps (not 'bulbs' ; we plant bulbs!) will only produce their rated power at their rated voltage. These values are printed on the glass envelope. Reducing the voltage applied to a lamp will, therefore, reduce the power it produces. In fact, there's not a simple, linear, relationship between a lamp's voltage and its power, and this means that even small reductions in voltage will result in large reductions in power.
Operating lamps, or any devices, come to than, at lower voltages really has nothing to do with efficiency. Certainly, operating a lamp below its rated voltage will most-certainly NOT result in high efficiency. Efficiency is defined as the ratio of output power to input power and has nothing to do with voltage.
What does Incoming and running generators are in same phase while paralleling two generators mean?
When two generators are to be run in parallel, one generator is first started and some load is given. Then second generator is brought in parallel with the running generator (subject few conditions, like voltage, frequency, phase angle). First generator which is running is known as running generator, the second generator which gets connected is known as incoming generator
What is the difference between simple dc motor and servo motor?
DC motor :
-DC (Direct Current) Motors are two wire (power & ground), continuous rotation motors.
-The speed of DC motors is controlled using pulse width modulation (PWM), a technique of rapidly pulsing the power on and off. The percentage of time spent cycling the on/off ratio determines the speed of the motor, e.g. if the power is cycled at 50% (half on, half off), then the motor will spin at half the speed of 100% (fully on). Each pulse is so rapid that the motor appears to be continuously spinning with no stuttering.
Servo Motor :
-Servo motors are generally an assembly of four things: a DC motor, a gearing set, a control circuit and a position-sensor (usually a potentiometer).
-The position of servo motors can be controlled more precisely than those of standard DC motors, and they usually have three wires (power, ground &). Power to servo motors is constantly applied, with the servo control circuit regulating the draw to drive the motor.
-PWM is used for the control signal of servo motors. However, unlike DC motors it's the duration of the positive pulse that determines the position, rather than speed, of the servo shaft.
Is it true that a low voltage system cannot harm you?
Yes... unless you opened your chest and applied the low voltage directly to your heart (which would be totally stupid).
kV is kilovolts, kW is kilowatts, kVA is kilovolt amps and kVAR is kilovolt-amps reactive.
A common formula is kVA-squared = kW-squared + kVAR-squared.
Yes because the transformer heating (power losses) depend on the load current and the load voltage.
It can be assumed that the voltage stays more or less constant, therefore the iron loss is also constant. The copper loss depends on the square of the load current.
So it is the VA of the load that determines the power loss and any heating.
How many amps does half hp motor single phase?
Presumably you mean 'rated full-load current'? The thing to remember is that motors, whether rated in horsepower (North America) or watts (practically everywhere else), are rated according to their output power. Because motors are not 100% efficient, the input power is always somewhat higherthan the output power. 0.5 horsepower is equivalent to about 373 W output power. If we assume that the motor runs at, say, 90% efficiency at its rated output, this means its input power will be around 414 W. To determine its input current you must then divide its input power by the rated supply voltage.
What is the term for the transformer winding to which the load is connected?
The load is connected to the transformer's secondarywinding, while the primary winding is connected to the supply. The terms, 'primary' and 'secondary', do NOT relate to voltage levels.
Why single phase motor has low starting torque.?
The starting torque is low because this type of motor requires an additional way of ensuring that it starts rotating the right way. It is not economical to give this additional mechanism the capacity to deliver the full load torque.
What is the function of electric generator in terms of electric power and motion?
It changes mechanical energy to electricity.
