Electrical Engineering

If a three light circuit is connected in series with two 6V batteries, and the three lights are identical, the voltage across each of the lights is 12V divided by 3, or 4V.

ac transmits the voltage over long distances. so v use ac in trains

series circuit

In that case, the current will also be doubled. This follows from Ohm's Law (current = voltage / resitance)

Sustained overload is overload that is continuous, or permanent. A resistive load is a good example. This is the kind of overload you get from simply plugging too many items into a circuit.

Transient overload is overload generated by a reactive load, a momentary short circuit, or other temporary condition. An example of a reactive load is the compressor of an air conditioner starting. This kind of overload can be caused by a reactive load being added to an already moderately loaded circuit.

Ohm's law says V=IR, where V is voltage, I is current and R is resistance. To get resistance, divide both sides by I:

V/I = R. If you check the units, you'll see that they work out this way, but would not work out correctly if you tried I/V.

(but remember, this only applies to systems which obey ohm's law, which is usually not quite true in real systems, but is generally true in textbook problems)

Be it three phase or single phase, larger motors should be started with a magnetic contactor.

The sex of the engineer has no meaning. What matters is the training, desire, and aptitude. It does not matter if a girl chooses electrical engineering or electronics engineering - it is her choice - based on her will to excel in one field or the other.

Watt rating is the amount of real power that is absorbed (or supplied). VA is normally the units given to the total complex power which is made up of the real power and reactive power. The formula for complex power is as follows.

S = P + jQ

P is the real power which is used by the resistive elements of the component while the reactive power (imaginary power) is that power which the capacitive and inductive parts of the circuit cause to flow.

Watt ratings are always lower (or equal) to the VA rating due to the fact that it neglects imaginary power that is flowing, because of this both ratings should be taken into account.

A good quality analogue instrument is probably as accurate as a digital instrument, but the digital instrument is more robust, has no movement so is not affected by gravity, and is easier to read.

Copper losses are energy losses from the windings, due to the currents passing through them. During an open-circuit test, there is no secondary current (so no secondary copper losses) and the primary current is very low (so the primary copper losses are minimum).

No, they are two different concepts.

The start capacitor is used in conjuction with a starting winding on a single-phase induction motor to produce a small component of rotating field inside the motor. This ensures it rotates the desired way.

The run capacitor is used to correct the power-factor of the motor, which reduces the current drawn from the supply for the same amount of power.

It depends on the resistance of each resistor. If each resistor, for example, is 0.333 ohm, then you could connect them in series. If each resistor, for example, is 3 ohms, then you could connect them in parallel.

according to the ohm's low if we increase the resistance the current decreases. As ohm's low states voltage is directly proportional to the current and resistance. So current can defined as the ratio of voltage to the resistance, as large the resistance, as low the current.

Yes, by electromagnetic induction.

Inductors are low pass devices, they conduct most easily at low frequencies. DC is the limiting case for low frequency AC: i.e. DC is the lowest possible AC frequency, zero Hz and thus conducts best through an inductor.

Capacitors are high pass devices, they conduct most easily at high frequencies. Infinite frequency AC is the limiting case for high frequency AC. Infinity Hz would conduct best through a capacitor.

In an alternating current circuit the voltage can be stepped up ordown efficiently with a transformer.

1,000,000V is not a measure of power. You need the amperage in the equation to figure out the power (wattage).

W = A x V

Yes a 208 voltage motor will operate on 220 volts. You have to increase the motor overload protection by 10% of the motor's full load amperage to protect the motor.

That is called a fuse.

A fuse is a protective device, which will interrupt the flow of electricity if it is subjected to a current that is greater than its rating.

Some fuses are designed to handle a large inrush of current and are used in circuits such as air conditioners, where a large amount of current is needed at startup, for a short duration and then current drops to a lower level.

These are sometimes called time-lag fuses.

negative tempareture

It depends on the reverse voltage. Up to about 5.6 volts, the zener has a negative temperature coefficient. Beyond 5.6 volts it begins to show a positive temperature coefficient.

A continuity test is a simple test, used to determine whether there is a break in a circuit. This can be done, for example, using an ohmmeter which passes current through the circuit from its internal battery -a deflection (analogue meter) or a '000' reading (digital meter) indicates that there is continuity. A continuity test MUST NOT be performed on an energised circuit, as this will likely damage the ohmmeter and present a shock hazard to the user.

For an insulating material dielectric strength and dielectric loss should be respectively

In order to pass DC current through a transformer you would need some basic knowledge of what a transformer actually is.

A transformer is simply put a coil of copper wire around metal plates of iron.

When you put current onto the coil, the metal plates will build up a magnetic field.

During the build up of this magnetic field, the current will be 'low'

When the magnetic field is maximized, the coil will work as a shortcut on a DC power source.

In order to use a transformer, ie transform voltage up or down, we need a fluctuating magnetic field. This can not be done without electronics of some variety, and the resulting voltage will be AC, not DC.

The easiest way to use a transformer with a DC power source is to make a circuit that electronically change polarization of voltage back and forth. Depending on transformer, this can be as low as 50 times per second (EU standard 50 Hz AC 220 Volt, US standard 60 Hz AC 115 Volt) or higher like 100.000 times per second (100 Khz) as is in many high speed switch power supplies. It all depends on the transformer you use and what you want to use it for.

If say you want to make a power source for an electric drill and you want to use a car battery as the source, then I would forget it. An electric drill need a lot of juice and in order to do so via a transformer, this transformer would need to be very very large.

Every ampere of 12 Volt would only give you approx 0.07 Ampere of 115 Volt.

A drill easily drain 800 to 1000 Watts. Approx 8 Ampere at 115 Volt.

Following this, an ideal transformer with hardly any loss would eat away 100 Ampere every 8 ampere on output 115 Vac.

With the loss factor (some of the transformed voltage will be given away as heat) and you would probably need approx 110 Ampere 12 Volt in order to generate 8 Ampere 115 Vac. A normal 60-65 Ampere car battery would then last a mere half hour fully charged. Another problem is that most batteries are not designed for such a load over any prolonged time (more than 30-60 seconds).

Systems have been and are buildt that can withstand such a drain but they consist of many batteries connected in paralell in order to even the drain between all batteries.

Currently, the transmission of electrical power from bulk power generators all the way to household, commercial and industrial consumers is made through high voltage alternating current (AC) transmission and distribution systems, and through a series of voltage transformations to reduce losses and minimize cost. Voltage levels used are of standard values.

AC system is used in the transmission of bulk power, instead of DC (Direct Current), because of its ability to transform voltage to various levels by using a transformer. The voltage transformation follows the faradays Law which states;The emf induced in a circuit is directly proportional to the time rate of change of magnetic flux through the circuit.

Note that, at higher voltage, the current was reduced thus line losses was also reduced.

Ability to transform voltage and to flow power in two opposite directions (bidirectional) are the only advantages of AC system over DC system.

DC transmission system on the other hand has more advantages over AC transmission system:

1. Reactance.

DC system does not introduce a reactance in the line.

2. Resistance

DC system reduces the amount of resistance in the line.

3. Power

In DC system, the power is just the real component.

4. Frequency

In DC system, the frequency is zero, thus no frequency variation to monitor. Connection will not also require synchronization.

5. Susceptance

DC system does not introduce susceptance in the line thus removing the effect of charging current.

6. Analysis

Analysis of AC system always involved complex numbers, while DC is only a real number, thus simplifying the analysis.

7.Number of conductors.

A DC system requires only two conductrs as compare to three for ac system.

8.Absence of inductance.

A DC system has no inductance as a result the voltage drop in DC system is less than that of AC system for same load and sending end voltages and hence a better voltage regulation.

9.NO skin effect.

A DC system has no skin effect so we can utilize entire cross section area of line conductor.

10.Less insulation require.

A DC system require less insulation than an AC system because of less potential stress for same working voltage.

11.absence of capacitance.

Absence of capacitance in DC system leads to less power loss because there will not be charging and discharging of capacitance.

If DC system has the ability to transform voltage into various levels, efficiently, the DC transmission system will the more efficient, stable, easy to monitor, and most importantly, easy to analyze than AC system.

Currently, the transformation of DC voltage into another level involves 3 steps;

1. AC to DC Rectification

2. AC to AC transformation, and

3. DC to AC Conversion

If compared to AC system voltage transformation, steps 1 and 3 are the only added steps. Unfortunately, these steps incur additional heavy losses. However, modern technology converter/inverter has attempted to reduce the amount of this loss. A 1000 MW, 230 KV AC - 450 KV DC/ 450 KV DC - 230 kV AC converter/inverter, for example, has a combine power loss of around 1.6 %.

The use of DC system in the transmission of power will be more economical if the price from its benefits is higher that the cost incurred from the addition of rectifier and converter.