Electrical Engineering

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"?

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

hi,
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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.

a ds is a nintendo product and is very famous.Some of the ds games are Pokemon black,white,white2,black2,mario,mario kart, Pokemon diamond,pearl,and platinum Pokemon heart-gold, soul-silver,a bunch of Pokemon mystery dungeon,etc.

the improved version is the 3ds a nintendo product and can turn 3ds games to 3D

some are Pokemon x and y(comeing october 3rd),mario kart 7,lego city undercover the chase begins, pilotwing resort,and etc.

The rotating blades form an air screw and screws itself into or out of the air.

3 Ph: tres fases, en un sistema eléctrico trifásico. Esto se refiere a la forma como "llegan" las ondas de voltaje y corriente a la carga. pp: "pico pico", se refiere a la medida tomada estre dos valores extremos próximos de una onda (ususamente senoidal) cualquiera. RMS: valor "Raiz cuadrada media" de una onda cualquiera.

Measurement of flow rate in a pipe

Load Shedding is the action taken to reduce the load on something. It mainly refers to the electricity supply in order to avoid too much load on a generating plant.

Off hand no but your explanations are not clear as to who is doing what to whom,,

If you don't allow the conductors to sag then, when they contract in cooler weather, they could (a) snap, (b) break their insulators, (c) pull poles over, or (d) pull intermediate poles out of the ground.

As far as I know.

Boost voltage or start voltage is a voltage increase that we use at low frequencies to compensate for the voltage lost. With no boost there would arise a flux which is to small. We would not be able to keep the demand of a constant magnetic flux, which result in losing the torque. The boost is responsible for maintaining the max torque at low frequency.

The electric cooker usually has an internal compartment that is double-shielded to prevent heat from escaping. The cooker should let very little air escape in order to keep the temperature well regulated.

The wave in amplitude modulation must be a sine wave. A sine wave represents smooth repetitive oscillation, which is necessary for this process.