Electrical Engineering

Usually, dielectric materials have permanent dipoles. As temperature increases, the molecules in the dielectric have more thermal energy and therefore, the amplitude of random motion is greater. This means that the molecules are less closely aligned with each other (even in the presence of an electric field). Hence, the dielectric constant reduces.

Capacitance is measured in farads (F), after Michael Faraday. A 1 farad cap is really big; capacitors are usually measured in microfarads (µF), or thousandths of a farad. A power supply in a big power amplifier might have two caps of maybe 12,000 microfarads each

we can't use dc everywhere because dc is hazardous for living beings plus we can't transform mechanical energy into dc power directly.

you can see that we use basically ac appliances at home,also dc but with very low voltages. ac is alternating in nature has zero and negative so if any one get in contact with ac it will result in hazard for the half positive cycle and repelling of body for the negative cycle. but in case of dc, the supply is continuous so it'll hazard the body continuously.

another aspect is generation. as i told earlier that we can't transform mechanical energy into dc power directly means we are generating ac supply in most generating stations where we are dealing with mechanical energy conversion. even dc generator generates ac power but by means of commutation process we convert it into the dc.

so it is not possible to use dc or ac everywhere. we have to deal with the both :)

Because in an AC circuit, a capacitor opposes the change of voltage in at. At the peak of the voltage waveform, there is no change (it is nearly flat), so at this same point in time, the current waveform is at zero.

Likewise, when the voltage waveform crosses zero, it is changing at its fastest rate, so the current is at its peak.

If you draw these two waveforms next to one another, you will see the voltage has the appearance of being behind the current - hence the term 'lag'.

Converting voltages of course. Rather like a transformer, early rotary converters were always mechanical devices i.e. motor generators, though these days electronic solid state versions are available. Put DC in get AC (single or three phase) out. OR put AC in and get DC out. They were mainly used to power trams and subway trains back in the early days before they'd invented the technology to control the speed of AC motors. The power station produced very high three phase AC voltage (it travels better) and this was converted to much lower voltages to power the DC traction motors in strategically placed substations

proper choice of coil pitch and distribution of coils while designing the winding, the harmonic flux in the air gap is reduced to very low value , which eliminates the crawling effect.......................

In Australia most power is generated in 3 phase the split down to single phase wen it reaches individual residences 3 phase power is more efficient to produce.

most single phase motors have switching gear and capacitors to help kick a motor on when it starts because single phase power doesn't have enough power to turn over a motor. 3 phase motors are generally simpler and require no starting gear ( normally cheaper to build)

Most workshops have 3 phase machinery for the extra power it prvides

Some motors can be 2 phase (rare) as well.

Give a brief idea of principle of hysteresis motor,application.

HERE'S AN EXAMPLE:

IF THE POLARITY IS BACKWARDS AT A LIGHT FIXTURE THEN THE HOT (120VOLTS) WILL BE THE OUTSIDE OF THE LIGHT BULB SCREW SHELL AND THE MIDDLE PRONG INSIDE THE SOCKET WILL BE THE NEUTRAL -- THEREFORE WHEN YOU CHANGE THE BURNT OUT LIGHT BULB YOU WILL GET SHOCKED BECAUSE THE SCREW SHELL PART IS HOT!! IF WIRED CORRECTLY YOU WILL NOT GET SHOCKED BECAUSE THE HOT (120VOLTS) WILL BE INSIDE THE SCREW SHELL ON THE PRONG AND YOU WON'T BE TOUCHING THAT PART WHEN YOU CHANGE THE LIGHT BULB!!

The ball bearings in the motor probably need to be changed. If the motor does not have ball bearings then the bushings have worn out and the motor armature is dragging on the stator. Don't run this motor until the bearing have been repaired as excessive wear on the stator from armature rubbing will introduce metal filings into the stator windings which could short out and destroy the motor.

If it's the normal operating noise that you're asking about, a three phase motor has a smoothly spinning magnetic field driving it's rotor that has almost no uneven motion, and produces almost no vibration in the iron parts of the machine. A single phase motor runs with a pulsating field that swaps polarity twice per cycle, but DOESN'T actually rotate. This causes the iron in the motor to vibrate slightly whenever the AC is applied, and this vibration causes the noisier operation of a single phase motor compared to a three phase motor.

Single phase motor running noise can be reduced if the start winding li left partly energised during normal running. For instance, some room air conditioners leave a small capacitor connecting the starting winding so it delivers partial power during running to give a 'sort-of' rotating field in the single phase compressor motor, which helps to reduce the 'pulsating' component and enhance it with a 'rotating' field that quietens the compressor down while you're trying to sleep.....

The split rings cause the direction of the magnetic field in the armature to change direction as the motor rotates. If this didn't happen the motor would not rotate.

Slip rings can be used for an AC motor which causes the direction of the magnetic field to change with the AC waveform.

there are wristbands available that ground you, so any static electricity you generate is transferred away from you. Also, touching anything metal will discharge the electricity just like the wristband, however, and more you generate will be transferred to the computer component you are working with (which is where i believe you were going with this question)

To offset the magnetic field so the motor can rotate without being caught in between the two poles where they resist rotation. Example, is where a motor is humming, but there is no rotation. However, if you kick start the rotation with a slight push then the motor rotates. This is caused by the two poles that are equal in resistance for pull and push.

To transmit a lot of electrical power, you either need a high voltage or a high current. But a higher current means your cables got hotter and you lose a lot of energy so is better to use higher voltage. To increase the voltage, we need to use a step up transformers.

Step up transformers are transformers which has ability to increase the voltage and they have more turns on the secondary coil on the primary coil.

In terms of seizure Yes otherwise No

Breakers are sized by the conductors that are connected to them. Conductors are sized by the amperage that the load draws.

The electrical code states that a 50 HP 460 volt three phase motor draws 65 amps. The ideal amperage would be taken from the motors nameplate as different motors of the same horsepower will have different amperage's depending on what they are designed to do.

The feeders for this motor have to be sized at 125% of the motors full load amperage. 65 x 125% = 81 amps. A #4 copper conductor with and insulation rating of 75 or 90 degrees C is rated at 85 amps.

The breaker for this motor is 150 amp or 175 amp fusing or 110 amp time delay fusing.

The phase relationship between the primary voltage and the secondary voltage of a transformer is 180 degrees (typically) on single phase transformers.

If working with three phase transformers, a zero phase shift is often used in Y/Y grounded transformers. If the transformer is wired Delta / Wye, the phase shift will be 30 degrees. I've seen transformers with a phase shift of 150 as well (quite abnormal, but it exists!) due to the starring and a delta / wye configuration.

If you are looking at a transformer phasor diagram, this will show the phase shift between primary or secondary (three phase transformers). For single phase, there should be a drawing showing polarity markings - what goes in the polarity marking on the primary comes out on the secondary polarity marking.

Yes , all conductors have some

amount of resistance although it

varies depending on the material

that is conducting the electricity .

Copper has a very low resistance,

and aluminum has a slightly

higher ( but still quite low )

resistance . This is why they are

used as electrical conductors.

Conductor resistance results in

voltage drop over the conductor

and power loss, manifested as

heating of the conductor .

Therefore , you want as low a

conductor loss as is reasonably

practical .

The reason of resistance is that,

no electron has a linear motion

when an electric field is applied to

it . e- move in random due to

apparent collision with other e -' s.

This motion is termed to as

Brownian motion . The random

movement however interrupts

with the electric field and creates

resistance sue to a negetive field

generated .

Kilo (symbol: k) is a prefix in the SI and other systems of units denoting 10e3 or 1,000 .It comes from the Greek ("khilioi"), meaning thousand.sometimes(usually in computer files)it is taken to be 1024.

The number of electrons coming out of the positive end of the loop must be the same as

the number of electrons going into the negative end. If it isn't, then electrons had to be

created, or destroyed, or were injected, or leaked out, somewhere along the series loop.

DC, or Direct Current, was not invented in the traditional sense as it is a very simple and common way to transfer power through a conductor. Far too common to pinpoint the first person who discovered it. Thomas Edison developed direct current, but it was only with Nikola Tesla's help that Edison ever developed any useful generators for direct current. It's odd that we credit Edison with inventing the light bulb AND direct current when Nikola Tesla is to direct current what Edison is to the light bulb.

In a 3 phase induction motor, particularly the larger units, hot start and cold start refer to the temperature of the motor when starting. Hot start is usually a restart after coasting to a stop. Cold start is an initial start, after not having run for a relatively long period of time. This is important because there is usually a limit on the number of hot starts that are allowed in a certain period of time. This is due to the fact that these motors pull more current starting than when they run, and if you start them too often they can overheat.

A stepper motor (also referred to as step or stepping motor) is an electromechanical device achieving mechanical movements through conversion of electrical pulses. Stepper motors are driven by digital pulses rather than by a continuous applied voltage. Unlike conventional electric motors which rotate continuously, stepper motors rotate or step in fixed angular increments. A stepper motor is most commonly used for position control. With a stepper motor/driver/controller system design, it is assumed the stepper motor will follow digital instructions. One important aspect of stepper motors is the lack of feedback to maintain control of position, which classifies stepper motors as open-loop systems

Anything that is in contact with the medium conducting the over-current. In an ideal situation a breaker should trip or a fuse should blow. If not wire insulation can melt, the wire itself can melt or various components can either melt or burn outright.

Sq D used a motor overload heater that was used on its motor overload contactor. It was designed as a ratchet on the end of a shaft that was inserted in a barrel on the body of the overload. A melting alloy held the shaft in the barrel. The ratchet on the end of the shaft held a spring loaded contact in the closed position. The body of the overload was bolted in series with the load terminal on the output side of the contactor. When the motor load current heated the overload protection device above its trip point, the alloy melted allowing the shaft to turn in the barrel portion of the overload heater. Being spring loaded the ratchet turned from the spring pressure and opened the normally closed contact thereby opening the magnetics coil circuit, dropping the contactor open and shutting the motor off.