3 Phase Alternator confusion

[electronicsman]

These are some doubts that i get, it is not related to the actual circuit, so please excuse if do not provide all the inputs, please let me know if i missed any inputs.
In a 3 phase system the voltages are induced in particular order say ABC if the rotor rotates in Anti clockwise direction. Two questions.
a. If i power off the rotor where it goes and settles i mean in which position. I assume it can go and settle at any position within the 360 Degrees. Is it correct?
b. If it settles at any position, every time the system starts how only the ABCABC sequence is repeated or the voltages are induced in that sequence. What factor makes always the sequence ABC possible?

 

[FvM]

a) sounds plausible
b) don't think so, why it should?

 

[betwixt]

a. in general it may settle in any position but if more load is placed on A, B or C than the others, there will be more braking effect at peak current in that phase so it may slow down according to load and tend to stop more in one position. Exactly where it stops will depend on the load and momentum.

b. the sequence is generated by the angle of magnetism and/or the angle the coils are set at. The sequence depends on how one interacts with the other so it will always be the same for that direction of rotation. So in short, it is down to the physical design and alignment of the generator. Spinning it the other way will reverse the phase sequence. Typically the coils will be set 120 degrees apart so the magnetic field sweeping them produces voltage peaks one third of a cycle apart.

Brian.

 

[electronicsman]

This is my thinking,

When Rotor position 1, i thought it will first start inducing voltage in winding C. And when rotor is in position 2, does the voltage not first induce in stator winding B?

 

[FvM]

The sequence depends only on the direction of rotation, but the starting point depends on the initial position. The original question is ambigious in this point, does ABC only refr to the direction or also starting point?

 

[KlausST]

When Rotor position 1, i thought it will first start inducing voltage in winding C. And when rotor is in position 2, does the voltage not first induce in stator winding B?

When the rotation starts, then immediately on every winding voltage is induced. Sine waveform, identical amplitude, but 120° phase shifted...in ideal case.

So the there is no ABC, or BCA, or CAB...all are available at the same time. But if you refer to a certain point like the "positive peak" then
you get an order like written before.

Klaus

 

[wwfeldman]

... identical amplitude, but 120° phase shifted...in ideal case.

i don't think that's correct
after rotation starts, the pattern is ABC because of the 120 degrees separation of the windings and the
connections of the 3 phase output
if you back it up toward turn on, the phases are always in 3 phase relative values
the only time they are all the same is when there is no rotation

on start up, any phase can be first to peak, with the others following suit, ABCABCABC ...
always in 3 phase sinusoidal pattern

if you cross wire two outputs, the pattern changes, say ACBACBACB...
that's why 3 phase motors not wired correctly rotate in the wrong direction

 

[c_mitra]

Your diagram in #4 is not realistic. For a three phase alternator, You will need three pairs of N-S poles. Of course you can have more than that but this is the simplest. In addition, the way the stator coils are wound is also important. But forget that for a moment.

Say you have only one N-S pole pair in the rotor and it is moving in a certain direction. The stator coils are distributed in such a way that one coil will produce a sine wave with zero phase (voltage starts from zero) and the two remaining coils will have the same sine wave but shifted by 120 and 240.

This is the case when the rotor is rotating at fixed velocity \( \omega \); in case you are interested in the start-up process, the rotor picks up speed from rest, but the sine waves are not sine waves if the rotor picks up speed fast. For a real alternator, there will be several N-S poles in the rotor and the stator coils will be wound in a complicated way (but the overall effect will be the same).

 

[KlausST]

Hi,

One N-S pair just gives one sine fullwave per revolution, or 50 Hz at 3000 RPM
Two N-S pairs give two sine fullwaves per revolution, or 50 Hz at 1500 RPM
Three N-S pairs give three sine fullwaves per revolution, or 50 Hz at 1000 RPM

You don't need three N-S pairs for a three phase system.

Mathematically I'd say (for 1 pole pair, 230V RMS star)
* Amplitude = 230V × sqrt(2) × RPM / 3000
* Volt_A = Amplitude × sin(phi - 0°)
* Volt_A = Amplitude × sin(phi - 120°)
* Volt_A = Amplitude × sin(phi - 240°)

All four formulas should be true a dedicated point of time.
Amplitue at a given point of time is equal for all three phases, just depends on RPM.

*****

Maybe some member has time and mood to do measurements on a BLDC motor.
Measurement results depend on purity of sine shape of the induced voltage.

Klaus

 

[c_mitra]

One N-S pair just gives one sine fullwave per revolution, or 50 Hz at 3000 RPM
Two N-S pairs give two sine fullwaves per revolution, or 50 Hz at 1500 RPM
Three N-S pairs give three sine fullwaves per revolution, or 50 Hz at 1000 RPM

You don't need three N-S pairs for a three phase system.

You are right. But using only one N-S pair is wasteful for the alternator. Simply because much of the copper and iron stays unused.

It is however ok to have only one N-S pair in the rotor for a very small alternator (say a cycle dynamo).

It is tough to get a decent sine wave with only one N-S rotor and a single pair of coils in the stator.

Your formulae are correct but if the coils span more than 60^o or so, then there will be considerable distortion.

With multiple poles in the rotor, you can complete multiple electrical cycles in one physical cycle and generate more energy.

 

[KlausST]

Hi,

But using only one N-S pair is wasteful for the alternator. Simply because much of the copper and iron stays unused.

You are right if you think the magnet as a simple rod. Then the output waveform looks more like a positive peak and a negative peak with a lot of "almost zero" between them.
But the rotor geometrics is optimized. it is more "T" shaped.
This makes the waveform amost pure sine (for a 3 phase AC machine), no copper is unused .... and as a further improvement is that the torque is uniform without ripple. (on a equallly loaded three phase system).
And for a car alternators - there is no need for a pure sinewave - they may be optimized for best other performances (cost, efficiency..).

Klaus