Electrical Engineering

When measuring light, the lumen and the watt both measure power -i.e. the rate at which electromagnetic energy is emitted (for visible light, this 'power' is termed 'luminous flux'). Although they are both SI units, one (the lumen) is used in the photometric system of measurement, whereas the other (the watt) is used in the radiometric system of measurement. Whereas the watt is used to measure the total rate at which electromagnetic energy is emitted from a light source, the lumen is used to measure only the visible part of the spectrum and, then, corrected for the sensitivity of the human eye (the eye is most sensitive to the green part of the visible spectrum. The watt, of course, can also be used to measure the input power to a lamp.

In the radiometric system, the input and output powers of a light are each expressed in watts, but in the photometric system, the input power of a light is expressed in watts, but the output power (luminous flux) is expressed in lumens. The ratio of a light's output (in lumens) to input (in watts) power, is termed efficacy (NOT 'efficiency'), and it various according to the type of light. An incandescent lamp, for example, has an efficacy of up to about 15 lumens per watt, whereas a fluorescent lamp has an efficacy of around 50 or so lumens per watt.

The three steps of the perception process are selection, organization, and interpretation.

A capacitor does not and cannot work as a 'power saver' or, more-accurately, 'energy saver'.

However, a capacitor can be used to reduce the power factor of a load. Reducing power factor, though, has absolutely no effect whatsoever on the amount of energy consumed by that load and will not reduce the energy bill for a residential consumer such as you or me.

What reducing power factor does is to reduce the load current which, in turn, will reduce the amount of copper that must be provided by the utility company, in terms of its cables and transformers. So power factor 'improvement' can save the utility company money, by allowing it to use smaller cables and transformers to supply any given load.

With a delta connection, picture a triangle. Each side of the triangle is a transformer or motor winding. Call the 3 corners of the triangle A, B, and C. There are only 3 wires except for a safety ground, which has no connection to, nor is any part of, the power transmission service lines.

All 3 windings have the same voltage, say for example 208V. If you take a meter, you will measure 208V between A and B, between B and C, and between C and A.

With a wye connection (also called a star connection) take our triangle above and use your wire cutters (literally) to cut the triangle apart at the corners. rearrange the three sides (transformer or motor windings) to form a Y. Where the three sides join in the center, connect a fourth wire. Call this wire "neutral". Call the three ends A, B, and C. Connect the 3 supply service lines to A, B, and C.

Now if you measure with your meter you will see 120V between the neutral and any single phase (A, B, or C), but you will still measure 208V between A and B, B and C, and C and A. The 208V is used to power three-phase loads and the 120V is very handy if you need to power a 120V single-phase load, such as a toaster! In a wye system, the supply service line-to-line voltage is 1.73 times the voltage from any phase to neutral.

Delta power is used for motors, 3-phase heaters, anywhere you don't need a neutral.

The biggest use of delta is in power transmission. Way too expensive to run a fourth wire all those miles, especially since a 3-wire delta transmits the same amount of power. Look up at a transmission tower and you see three phases and a ground wire for lightning suppression. No neutral.

At the destination, (a distribution transformer outside the home or business), the primary of the transformer is wired delta and the secondary is wired wye. This creates the neutral that can be used to derive single-phase power where needed. The neutral is grounded at the service entrance. Wye systems must be used where you have single phase loads that you must feed.

Interestingly, if you hide the neutral, you really can't tell the difference between star and delta systems. The phase-to-phase voltage is exactly the same. Most of the time 3-phase motors (which are internally delta-connected and have no neutral) are fed from a wye-connected source, simply omitting the neutral.

The three phase windings in a generator /tranformer/motor can be interconnected in two ways. If the similar ends of the coils are connected together and the other ends are connected to the 3 incoming supply service lines, that is called "star connection". The internally-connected point is brought out as the neutral connection. The service line "line-to-line" voltage (between any 2 lines) is 1.732 times the voltage between any line and neutral. Thus we can get two voltages for distribution within a building or site to supply lighting and power. However the service line currents are the same as the phase winding currents.

If the dissimilar ends of each winding are labelled A and B and the three windings are then connected together B->A, B->A, B->A, and the 3 incomiing supply service lines are then connected to the junctions, it is called DELTA. Here the values of line voltage and phase voltage are the same while the service line currents are 1.732 times the phase winding currents.

When the windings of a 3-phase motor are connected in STAR:

• the voltage applied to each winding is reduced to only (1 /.'/'3) [1 divided by root three] of the voltage applied to the winding when it is connected directly across two incoming power service lines in DELTA.
• the current per winding is reduced to only (1 /.'/'3) [1 divided by root three] of the normal running current taken when it is connected in DELTA.
• so, because of the Power Law V [in volts] x I [in amps] = P [in watts],

  the total output power when the motor is connected in STAR is:

  PS = [VL x (1/.'/'3)] x [ID x (1/.'/'3)] = PD x (1/3) [one third of the power in DELTA]

  where:

  VL is the line-to-line voltage of the incoming 3-phase power service

  ID is the line current drawn in DELTA

  PS is the total power the motor can produce when running in STAR

  PD is the total power it can produce when running in DELTA.

• a further disadvantage when the motor is connected in STAR is that its total output torque is only 1/3 of the total torque it can produce when running in DELTA.

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For more information see the answers to the Related Questions shown below.

You appear to be referring to a 'swamping resistor' which is connected in series with a moving-coil ammeter's moving coil.

To minimise any errors due to temperature changes in the resistance of the instrument's moving coil, the coil is connected in series with what is termed a 'swamping resistor' whose resistance is large in comparison with that of the moving coil, and which is manufactured from an alloy with a very lowtemperature coefficient of resistance (i.e. a metal whose resistance is hardly effected by wide variations in temperature).

For example, if the resistance of a 1-Ω moving coil increases to, say, 1.1 Ω, when its rated current flows through it, this will introduce a 10% error in the instrument's reading.

But by calibrating the instrument with a series swamp resistor of, say, 49 Ω, the overall resistance at rated current will become 50.1 Ω. This will result in an error of just 0.2%.

A swamping resistor, then, is an integral part of all moving-coil instruments and, when we refer to an instrument's 'coil resistance', we actually mean the combined resistance of the moving coil itself and its swamping resistor.

When two 4-ohm resistors are connected in parallel across 12 volts:

-- Each resistor has 12V across it, so the current is I = E/R = 12/4 = 3 amperes.

-- The total current from the battery is 6 amperes.

-- The effective resistance as seen by the battery is R = E/I = 12/6 = 2 ohms.

-- If you didn't know the voltage of the battery, you would calculate the

effective resistance of the parallel resistors in advance like this:

1/R = 1/4 + 1/4 = 1/2

R = 2 ohms.

When you connected them to a battery, the battery would agree.