Electrical Engineering

friction.

A simple AC generator consists of:Figure 3 Simple AC Generator(a) a strong magnetic field, (b)conductors that rotate through thatmagnetic field, and (c) a means bywhich a continuous connection isprovided to the conductors as theyare rotating . The strongmagnetic field is produced by acurrent flow through the field coilof the rotor. The field coil in therotor receives excitation throughthe use of slip rings and brushes.Two brushes are spring-held incontact with the slip rings toprovide the continuous connectionbetween the field coil and theexternal excitation circuit. The armature is contained within the windings of the stator and isconnected to the output. Each time the rotor makes one complete revolution, one complete cycleof AC is developed. A generator has many turns of wire wound into the slots of the rotor.

The magnitude of AC voltage generated by an AC generator is dependent on the field strengthand speed of the rotor. Most generators are operated at a constant speed; therefore, the generatedvoltage depends on field excitation, or strength.

'I' stands for current. It comes from the French word intensité , used by French scientist André-Marie Ampère to describe the 'intensity of the electric current.' This has been absorbed into international usage.

Example: V=IR. (Voltage=Current*Resistance)

The units of current are amperes, named after André-Marie Ampère, and symbolized by 'A'.

Example: This battery has a current of 0.25A.

It is define as twice the maximum symetrical short circuit current.

one way is to use RTD sensor put inside the motor windings. The RTD sensor is connected to an electronic thermal overload relay. This type of relay can provide running temperature of the motor, on the electronic thermal over load relay display panel. RTD sensor is a wire wound resistor that changes resistance depending on the temperature

because of the surge voltage, end coils of the winding have lesser turns and hence the mmf distribution of the h.v windings is not uniform along the axial direction....

A linear motor or linear induction motor is essentially a multi-phase alternating current (AC) electric motor that has had its stator "unrolled" so that instead of producing a torque (rotation) it produces a linear force along its length. The most common mode of operation is as a Lorentz-type actuator, in which the applied force is linearly proportional to the current and the magnetic field (F = qv × B). Many designs have been put forward for linear motors, falling into two major categories, low-acceleration and high-acceleration linear motors. Low-acceleration linear motors are suitable for maglev trains and other ground-based transportation applications. High-acceleration linear motors are normally quite short, and are designed to accelerate an object up to a very high speed and then release the object, like roller coasters. They are usually used for studies of hypervelocity collisions, as weapons, or as mass drivers for spacecraft propulsion. The high-acceleration motors are usually of the linear induction design (LIM) with an active three-phase winding on one side of the air-gap and a passive conductor plate on the other side. The low-acceleration, high speed and high power motors are usually of the linear synchronous design (LSM), with an active winding on one side of the air-gap and an array of alternate-pole magnets on the other side. These magnets can be permanent magnets or energized magnets. The Transrapid Shanghai motor is an LSM.

so they could cross over the sea

check out http://www.managementscientist.org for answers

The number of poles on stator and rotor is always the same. If they are not equal?æno torque will be produced, thus, the machine will not function.?æ

Ben Franklin proved that lightning was static electricity by means of his kite experiment. He believed that lighting was a form of static electricity on a huge scale.

less current flowing through them

If ther is a resistive load we got curent and voltage in phase. If the load is inductive curent lags behind the voltage. IN THIS CASE THER IS BOTH LOAD THAT MEANS CURENT WILL LAG BEHIND THE VOLTAGE

An obliquely incident EM wave cannot transmit or penetrate into a PEC when it is incident.So , you'll only have an incident and a reflected wave.The electric reflection coefficient is -1.

"INSULATION resistance is not about standard nd safe range because resistance value depend upon the application for which we are providing it in what range the current flows."

The answer above is incorrect. Application and current rating & range have nothing to do with insulation resistance. There are standards available for insulation resistance testing both in terms of the test procedure and the values. IEEE Standard 43 provides a basis for insulation resistance testing. Both NETA ATS (Acceptance Testing Specification) and NFPA 70B gives criteria for insulation resistance testing.

Insulation Resistance (IR) testing, also known by the slang terms meggering or megging, is a procedure where the quality of the electrical insulation is evaluated to determine if it is acceptable for service. It is also used to compare against previous measured values to determine if there has been any degradation to the equipment being tested. In this case we are talking about motor insulation. The question deals with two comparative readings and wants to know if the value of 30 megohms is acceptable.

First we should clarify the test methods and results. NETA ATS 2007. Section 7.15.1 covers AC induction motors and generators. The testing involved is broken down into those motors 200 hp (150 kw) and less, and those > 200 hp (150 kw). The test voltage value is based upon the voltage rating of the motor's winding and is found in table 100.1:

Rating = 250V; Test Voltage = 500 VDC; Minimum Resistance = 25 megohms

Rating = 600V; Test Voltage = 1000 VDC; Minimum Resistance = 100 megohms

Rating = 1000V; Test Voltage = 1000 VDC; Minimum Resistance = 100 megohms

Rating = 2500V; Test Voltage = 1000 VDC; Minimum Resistance = 500 megohms

Rating = 5000V; Test Voltage = 2500 VDC; Minimum Resistance = 1000 megohms

Rating = 8000V; Test Voltage = 2500 VDC; Minimum Resistance = 2000 megohms

Rating = 15000V; Test Voltage = 2500 VDC; Minimum Resistance = 5000 megohms

Rating = 25000V; Test Voltage = 5000 VDC; Minimum Resistance = 20,000 megohms

Rating = 34500V; Test Voltage = 15000 VDC; Minimum Resistance = 100,000 megohms

It is important to note that the values given are based on a standard temperature of 40C (or sometimes 20C depending on the engineer's specification). You must correct your readings to a standard temperature as the value on the insulation's resistance is going to vary inversely with temperature. That is as temperature increases the resistance will decrease. The rule of thumb is that the measured value will halve for every 15C above standard and will double for every 15C below standard. As an example let us say that we have a 25 hp induction motor rated 480 VAC. The ambient temperature is 15C. Using our table we would set the tester to 1000 VDC and take a reading for one minute. At the end of the minute we get a reading of 450 megohms. Per NETA ATS Table 100.14 the correction factor is 0.31 so IR = 450 megohms x .031 = 139.5 megohms. The minimum acceptable value is 100 so this motor is acceptable. On the flip side if the motor is in a very warm process area, say 50C, then temperature correction factor is 1.59 thus IR = 450 megohms x 1.59 = 716 megohms. As you can see the temperature makes a very large difference in the results! This discussion up to this point has been about a spot-reading check. However for a true check we want to know the Dielectric Absorption value.

There are two different standard tests for Dielectric Absorption. The first is the Dielectric Absorption Ratio (DAR) and the second is the Polarization Index.

DAR = Reading @ 60 sec / Reading @ 30 sec. Let us say that the 30 sec reading = 325 megohms and the 60 sec = 450 megohms. Thus:

DAR = 450 megohms / 325 megohms = 1.38 The minimum DAR per NETA is 1.4 so this particular motor is borderline at best and should be investigated further.

PI = Reading @ 10 min / Reading @ 1 min. Let us say that the 1 min reading = 450 megohms and the 10 min = 1100 megohms. Thus:

PI = 1100 megohms / 450 megohms = 2.44 The minimum PI per NETA is 2.0 so in this case the motors is acceptable.

One final factor that should be taken into consideration is Relative Humidity (RH). The amount of moisture present in the air also affects the measured test values. The more moisture then the lower the reading. There is no published standard correction factor for RH however when NETA Techs perform these tests then they always record the RH for baseline comparison. The bottom line is that the readings will vary based upon temperature and humidity. A reading on warm humid day may be acceptable whereas the same reading on a cold dry day may not.

So the question asked here is unanswerable as there is not enough information given. What were the temperatures at the time of the readings? Was it dry during one and raining during the other? What does the person asking mean by "normal conditions"?

I highly recommend that anyone interested in this subject get the free book "A Stitch in Time" by Biddle at:

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Please note that I also changed the category from Health/pregnancy to Electrical Engineering.

hi,
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its vet one , but with little modification it can be for human buy buying a oxygen mask or make your own like this one

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hope this helps
Hytham Nabil

Avalanche voltage is the voltage value that causes a reverse-biased semiconductor junction to break down and conduct. Sometimes this is undesirable. A rectifier diode in a power supply that avalanches has failed. It can also be desirable. A Zener diode used as a regulator works by avalaching at a specific voltage, thus limiting the circuit voltage at that level.

A patented water purification system.

It's the RMS value. A 120 volt lamp (light bulb) is rated according to its RMS voltage. Just like appliances in the home are rated at 120 volts (like your fridge, microwave and toaster), or 220 volts (like your clothes dryer). Note that these appliances will have to stand up to the peak voltage on the AC line. Naturally. And the peak voltage on an AC line is 1.414 times the RMS value of voltage. That means in a 120 volt AC line (120 volts RMS), the peak value of the voltage will be 1.414 times the 120 volts, or right at about 170 voltspeak for each cycle.

Robots help engineers by lifting heavy weights or substances. Robots also help in construction or design of certain delicate things.

Restricted earth fault protection is used to protect a specific zone, and should not trip for a fault outside of that zone (usually limited to a transformer, and possibly extending to lowside, highside, and tertiary breakers).

There are at least six seas that border or surrounded by continental Europe. These include: the Aegean Sea, the Ionian Sea, the Black Sea, the Mediterranean Sea, the North Sea, and the Atlantic Ocean.