Hot answers tagged

37

In addition to PlasmaHH's answer, industry uses almost exclusively three phase power since an induction motor needs at least a three phase supply to start and run in a known direction. Single phase induction motors require lossy, unreliable, and expensive tricks to do the same (extra windings, lossy windings, speed sensitive switch, capacitors, etc). The ...


30

Why not three lines all in the same phase? Because then there is no return path. Because single phase has no "rotation". Three phase makes it very simple to make a rotating motor with phase sequence determining the direction of rotation. Swap two phases and the direction is reversed. Is there less loss when the phases of the three lines are all different?...


25

When you have single phase power distribution, you need one phase and one return, both carrying the same current. If you now instead use symmetrical three phase power, you use three phases with a third of the current carrying capability, and you can get rid of the neutral. This simply saves some money in copper. If you now add more phases, you can not save ...


18

Three is the lowest number of phases which are equally spaced around the circle, and which can be used to create a rotating magnetic field in a given direction. Any more phases just require more wires, and more windings in an induction motor. Two phases can set up a rotating magnetic field if they are 90 degrees apart ("quadrature"). Quadrature-generating ...


18

Before connecting a generator to the grid, they spin it up to more or less the right speed. Then they hook what is basically a voltmeter between a generator phase, and the corresponding line phase. They adjust the generator drive until the observed voltage is a) very slowly changing (frequency difference below some threshold) and b) drops below some low ...


16

You're confusing an accurate number of cycles over a 24 hour period with very rigid instantaneous frequency control. That's not how it's done in most places. The frequency is maintained at around its nominal frequency by matching generation to load - all the time that the load is greater than the generation, the frequency will be (very) gradually falling, ...


15

Addition to other answers: The main purpose is that having at least three phases allows your motor to start in expected direction. For one-phase induction motors some workarounds are necessary (like putting additional wiring with a capacitor used during the start-up). It was correctly explained in previous answers. Why not more? Simply - it is not ...


12

Three phase has a very important property: if you look at power (V^2/R) across all three phases and sum them, that power is CONSTANT across the entire cycle. This means 3 phase motors can drive at a constant power and the generators see a constant load. 2 phase is insufficient to get this relationship. One could use higher phase counts, but it costs more ...


12

If the voltage across a capacitor is described as sin(x), then the current through the capacitor (being 90 degrees out of phase) can be described as cos(x). The power is given by the product of voltage and current. For a capacitor, this is proportional to sin (x)*cos(x). This product is given by sin(2x)/2 which is a sine wave. Thus it is positive for half ...


11

Why only 3 phases? Well if we need more phases we can convert 3 phase easily into 6 phase/12 phase etc using a transformer wired to do so. The main application of more phases is for less ripple voltage into a full bridge rectified capacitor bank. I've never seen one but learned about them from an ancient lecturer at university while doing electrical ...


10

Good answer from @Transistor. To add a bit more: - Three phase is inherently current and voltage balanced from a standpoint of it generating interference. At any one point in time (and a reasonably balanced load) the magnetic emission is low because all the magnetic fields cancel out due to the currents being balanced. There is net voltage balance in the ...


10

It adds or subtracts multiples of \$2\pi\$ (360 degrees) to each point in the phase plot so that the phase is plotted continuously rather than having jumps in it. We know that physically the phase is going to be continuous from frequency to frequency. However as the output of the analysis is a vector gain at each frequency, all angles are reduced to the ...


10

When there is a 90° phase shift between two 5MHz clocks, that means there is a 50ns skew between them. If you divide those two clocks by 5000000 using two instances of the same counter structure, you'll get two 1Hz signals with 50ns skew between them (which is roughly 0° at 1Hz). I doubt you can see this with bare eyes. So to me, this test gives you no ...


10

The paper is about using the EV motor windings as a transformer or inductors as part of the charger when the vehicle is charging instead of using a separate component. A common problem of doing so is that the current flowing through the motor windings will cause it to produce unwanted rotation or vibration(torque) while the car is being charged. The study ...


9

Many of the other answers erroneously state that you need 3 phases for a motor to start reliably or turn in a specific direction, and to use constant power. Actually, this could be done with two phases, 90° off from each other. You still get defined direction and constant power draw over a cycle. However, such a two phase system would require a ...


9

It is just a matter of manipulating complex numbers. $$ \angle H(\omega) = \tan^{-1} \left( \frac{\Im\{H(\omega)\}}{\Re\{H(\omega)\}} \right)$$ Where \$\Re \{ \cdot \} \$ is the real part and \$ \Im \{ \cdot \} \$ is the imaginary part. (NOTE: this equality is not always strictly true depending on the signs of the real and imaginary parts of \$H(\omega)\$. ...


9

The power feed to a North American residence is normally provided by a center-tapped transformer, with the center tap of the transformer secondary grounded. the full secondary winding produces 240 Volts. the connection is like so: simulate this circuit – Schematic created using CircuitLab Arghhh! The shcematic editor won't let me draw what I want! ...


8

Additionally, it seems to me which function leads which is rather relative as I can shift any function 360 deg, any easy reconciliation? Absolutely. What is leading or lagging is up for interpretation/definition. Personally, the only definition that would make sense to me would be that any lag by more than 180° is actually a lead. Maybe that book defines it ...


7

Winding from top to bottom or bottom to top does not change the "dot" (which represents the direction of the magnetic north/south pole for a given current direction). You can use the "right hand rule" to keep this straight. Place your right hand around the coil with your fingers in the direction of current flow. Your thumb points to the North magnetic ...


7

They use a Synchroscope. I have seen this done in power plant control rooms. https://en.wikipedia.org/wiki/Synchroscope


7

Figure 1. Waveform amplifier. The resistors form a half-supply reference for the single-rail powered op-amps. With a 5 V supply this reference voltage will be 2.5 V. The 100 uF capacitor stabilises this voltage. As @Trevor points out in the comments, it also prevents any AC input signal from (3) affecting (2). The non-inverting input is held at 2.5 V. With ...


7

The author is talking about charging an electric vehicle by re-using its drive train electronics and the motor windings as parts of the charging circuit. This means that during charging, currents flow through the motor's windings, which could cause the motor to create mechanical torque and potentially cause the car to move. It therefore has to be locked in ...


6

Why bother with phase at all? The original question poses the problem of phase disambiguation over the entire unit circle for a (sine-driven) frequency modulated laser. The core approach, as proposed, will never achieve the stated aim because you need more information to disambiguate phase over 360deg than you have in a single frequency reflection scheme (...


6

A voltage is, definitionally, between two conductors. If you have one conductor, you have no voltages. No voltage, no power, nothing happens. Not terribly useful. If you have two conductors, you have one pair (2C2), which allows for one voltage. We call this single-phase. Now we can actually make things happen, which is a substantial advantage over having ...


6

EMP from solar winds aka Carrington effect happens every hundred years or less (due to solar cyclic wobble from planet alignment with Jupiter which gives rise to an 11 year cycle) and there are longer cycles too, recorded by the Mayans in their calendar. Ground induction effect is inductive and suppresses low frequency more than surge pulses. That's why ...


5

When you say "winding from top to bottom", I say this does not change the convention if you wind from "bottom to top" instead. The dot convention ensures this: - If an AC voltage is applied to a coil, the other coil(s) will be in phase with that AC voltage when (for instance) you use an O-scope with the probe at the dot ends. BTW, it looks like you are ...


5

I'll focus my answer on transmission alone, without explaining why 3 phase is useful in general because other answers did that. Transmission of power is a compromise. A compromise between transmission efficiency and ease of conversion. The most efficient way of transmitting electric power is DC. This is why most superlong lines are HVDC (high voltage direct ...


5

Having parts of an individual power system run at different phase angles from other parts is routine and unavoidable. This is not a problem until it is necessary to re-connect parts. In the Utility where I worked, the service people at the site would connect a phase-meter to each of the parts. Due to the difference in phase, the phase-meter would run like ...


5

Very simply, the snubber provides a load to dissipate energy from rapid changes in voltage associated with reactive components. In your diagram, consider what happens when the triac experiences a voltage spike. The triac itself has capacitance between the line and the gate, and the spike may provide enough current through this capacitance to turn it "on." ...


5

The group delay is a function of frequency, so you cannot really say it's the same as the delay of a certain frequency -- that would be a particular case. Also, the step response is an infinite sum of sines plus DC, which means that the Heaviside function is not really proper to determine the group delay (since you are measuring a combined effect of group ...


Only top voted, non community-wiki answers of a minimum length are eligible