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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
Why do engineers put power lines so that they are slack in hot weather?
in the winter season materials such as rubber insulators and copper wire shrink and in the summer they expand just like wood. Therefore slack is needed on powerlines in relation to the weather so there is a little give to them. Engineers have nothing to do with the slack in the lines, it occurs naturally. In the sunlight objects expand a little due to the heat. This goes for roads, bridges, sidewalks, shingled roofs and hanging power lines. They sag more in the summer than in the winter. Engineers actually have to find ways so that they wont get slack. This expansion and contraction of any given material will actually shorten the life of it over time, and it will have to be replaced more often.
What are the advantages and disadvantages of sequential circuits?
A An IC has only limited points to probe troubleshooting, a circuit which is discrete in essence have all the nodes available, however the IC has the advantage of small real estate requirement for the same function
What are the electrical appliance that use AC?
Neither.
AC is alternating current
DC is direct current
Either can be large or small
The transmission losses are greater for (non superconducting) DC power so in most applications the absolute value of DC current will usually be lower than those in AC systems.
How do you change the direction of rotation of synchronous motor?
A synchronous motor aways runs at the synchronous speed, which is the supply frequency divided by the number of pairs of poles in the motor.
For a 4-pole motor the speed is 1500 rpm (50 Hz supply) or 1800 rpm (60 Hz supply).
The mechanical phase of the rotor is liable to be a little unstable under changing load conditions.
How many 14 gauge electrical wires can you put in a 2x4 electrical box?
Answer varies. Is it a deep or shallow box? is there a receptical, a switch in the box as well? is it just a junction box? are they single wires or NM cable?
What is diversity factor in electrical calculation?
Try to think of a diversity factor as similar to a noncoincidental load factor. If your thermostat phyically limits you to only using either heat or AC at one time, then you can simply use the largest of the two as your load factor. With diversity, think of a college campus environment. If you are in class then the odds are your TV, oven, microwave, etc back in your dorm are probably not being used. R, Bill
How do you know the voltage of a 3 phase panel box?
A 277/480V box should be clearly marked, but not always. The only way to be certain of a box's voltage is to use a voltmeter on it. This type of voltage is very dangerous. When voltages start to get this high, at best you may loose a finger or limb, at worst, you die. I recommend calling an electrician, who will be able to safely check this out.
In AC current, its a quality measurement of voltage. If voltage is harmonious or "clean" you will see an equal wave length on both sides of your baseline. You can test and see this using an electrical scope.
What is effect of an unbypassed resistor on the common emitter amplifier circuit?
An emitter resistor in a common emitter circuit will cause the stage to experience the effects of degenerative feedback if it is unbypassed. The degenerative feedback reduces gain. This is probably the primary effect in the described circuit.
Why most of the practical loads are inductive?
Most loads are actually resistive, such as an incandescent (normal) light bulb or electric heat or cooking equipment. Other loads are mostly inductive because they incorporate either transformers or motors, which are both inductive. Off hand I cannot think of a normally capacitive circuit, which would be the opposite of inductive.
How do you reduce rpm of an electric motor from 1200 rpm to 240 rpm?
You would use a worm drive meshed with a gear that has 130 teeth.
Does a ac generator have a brush?
AC generators (alternators) usually have a dc supply to the rotor coils, supplied through slip-rings with carbon brushes. Because the slip-rings can be made very smooth the life of the brushes is greatly extended when compared with brushes on the commutator of a dc motor or an ac commutator motor.
Cars generate electrical power with a three-phase AC alternator, connected directly to the battery through rectifier diodes. The alternator voltage is controlled by the voltage regulator, which works by controlling the dc supply to the rotor.
How can calculate maximum demand load?
maximum demand load can be calculated as:
# maximum demand=demand factor * Connected load or by
# maximum demand = connected load * Diversity Factor
Note: Demand factor and diversity factor are NOT same
What is the kva for 7.5 hp 3 phase motor for 1 hour?
1 hp is generally around 750 watts, so you can expect a 7.5hp motor at full load to be ~5.6kW.
If lightly loaded, the motor will draw less current, thus less wattage. Also, you cannot calculate current directly from the kW rating and voltage (so full load current will not be 5600/230 = 24.3Amps), as motors are not 100% efficient, and do not have 1.0 power factor. I'd expect the full load amperes to be greater than the kw/voltage value by 20-30%.
I looked up a Baldor motor stated to be 7.5hp, and it was ~7kVA, 5.6kW at full load.
Another Answer
You don't specify whether you want to know the machine's output power or its input power.
The rated power of a motor is always its output power. In North America, this is expressed in horsepower whereas, in the rest of the world, this is expressed in watts (in practice, kilowatts). As 746 W is equivalent to 1 horsepower, then a 7.5 horsepower motor is equivalent to 5.6 kW.
If you wish to determine the rated input power to a machine, which is always larger than its output power, then you must know its efficiency at its rated output power. Divide the output power by the efficienty (expressed as a per unit value).
How do you increase and decrease the speed of an AC motor?
You really can't really "control" the speed of a the commonly available home-use 120 VAC fans.
The problem is that most of the motors on the "regular" fans are synchronous motors, and they want to sync with the line by virtue of frequency. Fans with different speeds have different windings in them that are specifically set up for the different speeds. A switch allows the different windings to be selected. Simply reducing the line voltage will only reduce the torque the motor will supply, but will not directly affect its speed. The lower torques will cause the motor to slow down, and that's true, but this is not necessarily a good thing as the fan was not designed to run like this.
Sophisticated electronic variable frequency drives (VFDs) are designed to provide AC power but at a frequency you can control. Your ability to control the frequency gives you the ability to control the speed of the fan directly. But these units cost. For the smaller non-commercial units, it is smarter (and cheaper) to just get another fan with multiple speeds than to try to fool around and control the speed by some "gizmo" or something else.
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I beg to differ. Most home fans are inexpensive single phase AC motors, either with a capacitor (PSC) or shaded pole. These are not synchronous motors. A simple light switch dimmer (aka phase controller or triac controller) can be used to control the fan. Caution: be careful to avoid fan stalling when setting at low speeds. The dimmer controls the speed by reducing the effective voltage to the fan. At low voltages (and resulting speeds) the fan torque is also reduced. Thus the likelihood of stalling is higher. If the fan stalls with the power on, the motor will overheat and possibly catch on fire.
VFDs are definitely not the right choice for a simple fan. These require motors (almost always 3 phase) designed for use with VFDs (aka, inverters) or a special output filter for the controller. Otherwise the bearings will pit within months to a year. Besides, VFDs with a single phase output is very rare.
Increasing the armature voltage would increase the speed. In a separately excited dc motor the speed adjusts so that the back emf generated by the armature is a little less than the supply voltage. The difference, divided by the resistance, gives the current drawn, which is also proportional to the shaft torque supplied to the load.
What is acceptable megger reading for 440volts?
If it is a motor connection that is being tested, engineers like to see it above 5 meg. In reality above 2 meg is fine.
What is meant by alternative current?
Alternating current is more efficinent. It is wireless and more efficient than direct current. And it is safer. Plus Tesla is better than Edison
What does a circuit breaker do in a circuit?
Creates an open circuit in the event that the circuit becomes overloaded. It's a protection feature, preventing melting wires, damage to electrical components, and fires.
Why is the voltage drop large in a high voltage transmission line?
The length of the wire causes resistive load to be distributed along line. This accumulated resistance causes the voltage drop.
AnswerThe voltage drop along a high-voltage transmission line isn't large in relation to the operating voltage of the line. One of the advantages (there are many) of using high voltages is that, for a given load, the line current is relatively low -so the voltage drop along the line is also relatively low.
Why you step up or step down voltage why not current?
The current is automatically stepped up or down when the voltge is stepped down or up by a transformer.
Most transformers are designed for a constant voltage, and the current varies depending on the load. Current transformers are also made to detect the current in a power cable so that it can be measured.
Another Answer
It's a misconception that transformers step current up or down (with the exception of current transformers, that is!). Transformers step voltages up or down. The secondary current is determined by the load. If there is no load, then there is no secondary current; if there is a heavy load, then there will be a heavy secondary current.
The secondary current then determines the primary current, because the primary power must match the secondary power (ignoring losses), so if the secondary current increases, so must the primary current. The current itself isn't stepped up or down.
<<>> Normal transformers work with constant-voltage supplies and are designed to step the voltage up or down, and the current is incidentally stepped down or up in the same ratio as the voltage.
Current transformers are designed to step current up or down (usually down), and are used most commonly to sample the current in power conductors.
