Synchronous Generator/Motor

Best if you raise your blast shields as quickly as possible.

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thanks

is car alternator can be called as synchronous generator?

No, not exactly the same. Old cars used DC generators, then when electrical demands in cars kept increasing, the size and cost of DC generators made them impractical, so they switched to "alternators". Similar, but not synchronous. The engine speed is changing all of the time, so the AC voltage and frequency that an alternator creates is constantly varying. With a car alternator, it is immediately rectified to DC anyway and a "Voltage Regulator" is doing its best to manipulate the rotor field strength to keep the DC voltage under control. With a true Synchronous Alternator, the output is intended to be AC, so frequency it produces is just as important as the voltage, therefore speed control of the prime mover is critical.

But a car alternator still has a rotating field coil, fed via sliprings, like a mains or standby synchronous alternator (these are usually brushless but same principle). Doesn't that mean it is synchronous? Obviously the output frequency before the rectifier varies with engine speed but it's still synchronous.

An asynchronous generator is just an induction motor driven at above synchronous speed, as Rixter in #3. And it can't be standalone, needs to feed into an external system, not available in a car.

But did OP mean (in #2) "....called as synchronous...." or "....called asynchronous...."?

Doesn't that mean it is synchronous?

It's semantics...

Synchronous to what? "Synchronous" means in synchronicity to the frequency you mean to create. With a car alternator, you are making DC, there is no frequency.

Both are ALTERNATORS. If used to make AC, it is an AC Synchronous Alternator. If used to make DC, the term "synchronous" becomes meaningless, it's just an alternator used as a DC generator.

Very True.

I suspect that the OP is not in a position/knowledge (looking at his suspect question!) to fully understand what you are writing, though you give really good accurate explanations. e.g. Its not your fault he is not understanding!!

Also, some people will have it (not me) that a car alternator is still a dynamo as DC comes out!! At least out of the modern ones where the diodes are integrated (for many years now!!).....some of them post on CR4 as well!!

I personally still see it as an alternator.

Interestingly, only last weekend I was working on a friend's old motorbike,1973 Yamaha which was not charging. Both the rectifier pack and the voltage regulator are mounted externally. The regulator is electromechanical (vibrating contact) type. Soon spotted there was a gap between the contacts so it clearly wouldn't work. Fairly basic piece of kit, adjustment by bending things, so bent the fixed contact till it touched its mate, and bent the tag for voltage adjustment, and it worked fine.

No, I don't think it is semantics. A car alternator works just like a mains or diesel (synchronous) alternator - rotating field coil fed with power, the field power also controlling main output. The fact that the output of a car alternator is rectified doesn't affect it. It wouldn't be too hard to test - run the alternator at a known constant speed and measure the frequency of any of the phases upstream of the rectifier. I believe it would show no slip.

An induction generator works differently, described by Rixter in #7. Output control is just via engine speed, increase that and power increases, at slightly higher slip.

I personally don't think that the OP was asking a fully valid question....which is leading to some confusion here.....

In a synchronous generator (or motor), the rotating field and the rotor rotate at the same speed. A car alternator falls in this category. (The AC is rectified to DC to charge the car battery.)

If there's a load on the shaft of s synchronous machine, the rotor falls slightly behind, still rotating at the same speed. If the shaft is driven rather than being loaded, it advances slightly ahead of the field (still rotating at the same speed. In the former case, it's a motor. In the latter, it's a generator. You can think of the magnetic field as a flexible coupling that connects the mechanical side to the electrical side. Power can flow either way.

In an induction generator (or an overdriven induction motor), there will be a slip between the rotor and the rotating field. Just as the rotor falls behind synchronous speed in a motor, it rotates faster in an induction generator. The relative motion between the rotor and the rotating field (difference in rpm) is what generates torque. It's a driving torque if it's a motor, and a load torque if it's a generator. If the induction machine runs at synchronous speed, the rotor "sees" a stationary field and no torque is developed.

'..., the rotor falls slightly behind, still rotating at the same speed. If the shaft is driven rather than being loaded, it advances slightly ahead of the field (still rotating at the same speed....'

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So, if the rotor falls behind (or advances) a little while still rotating at the same speed, how does the frequency anticipate changes in load in time to increase (or decrease) a little, to keep the magnetic field in the right place relative to the rotor?

...or is this an 'all children are winners' type effect, such that one comes out ahead without either going faster than the other?

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J/K

Great comment.

Think of the rotating magnetic field as a flexible coupling. It's either being driven from the electrical side or the mechanical side.

Two cars are traveling side by side at the same speed and direction. One car then, without a change in speed, moves ahead of the other, then maintaining a fixed distance ahead.

If you u want, you can image flexible couplings between the road and the vehicles. We can call those 'tires'.

Some possibilities are:

A - the other car slowed down momentarily and then returned to the same speed

B - the path of the other car was not as direct

C - some differential between the two paths in time gradient

D - new math

E - I am lying. Cars are imaginary

F - Obama's fault

G - 42

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Which is the best choice?

B - one car went into a dip while the other one did go straight. Like a bridge and an exit lane down and the access lane up again.

Same speed, same direction but a longer way to travel for the car which then is behind.

Cool. Good answer.

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Now, can you help me locate the dips in the magnetic fields path of rotation?

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Perhaps that explains it! Could it be that the OP took similar wayward path to arrive at a similarly unsatisfying conclusion and wants to take the rotor out to search for the dips in the path of rotation?

H ?

So you are going with B/μ as the answer then. Interesting choice.

Frequency does not/cannot anticipate changes in load, it simply responds to it. The immediate source of the incremental change in generation to meet the incremental loading comes from the energy stored in the rotor, then the governor adjusts the power output of the prime mover to bring the frequency to its setpoint.

GA

You bring back some good knowledge and common sense into an argument that was simply getting stupid and had little to do with the way alternators work. Thanks.

Oh, come on. Just having a little fun around someone giving a great explanation yet including an impossibility.

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....so I asked in a joking manner (hence the 'J/K') about the little anomaly.

When I got a response that reiterated a small part of the original explanation, and ignored my question all together, it seemed silly

I responded in kind.

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Not to your liking, but it would be stupid in itself to proclaim it was stupid to ask.

If you are just grumpy because you have lost track of the question, I will pose it again

The original comment claimed that without changing speed, the rotor became advanced or was retarded relative to the rotating field. How is it possible?

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It isn't that the frequency anticipated the change and then slowed momentarily and once the leading or following was to the appropriate amount then returned to the previous frequency.

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It isn't that there is a shorter path for one or the other to rotate about such that the change in relative position occurs without a change in speed by the rotor or field..... this is a 3 dimension plus one time like dimension problem, so no short spins. And this is a macro system so no claiming 1/2 spins either.

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The answer is that the rotor does change speed. It slows briefly and then speeds up to follow or speeds up a little till it is ahead. There are changes in speed. The variation is not large but it is how the rotor and field end up at different relative positions.

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There is no sport in deflating a bit of fun. It is too easy to do to be considered the least bit clever. It is not typically beneficial unless the bit of fun is causing some actual disruption. It is stupid, and as such beneath a smart person such as yourself to practice.

Look at the designs of synchronous and induction motors. If you have a running motor and you connect some sort of engine to its shaft to drive it faster than it would normally run, it becomes a generator, supplying power to the external source.

" In a synchronous generator, the waveform of generated voltage is synchronized with (directly corresponds to) the rotor speed. The frequency of output can be given as f = N * P / 120 Hz. where N is speed of the rotor in rpm and P is number of poles.

• In case of inductions generators, the output voltage frequency is regulated by the power system to which the induction generator is connected. If induction generator is supplying a standalone load, the output frequency will be slightly lower (by 2 or 3%) that calculated from the formula f = N * P / 120.
• Separate DC excitation system is required in an alternator (synchronous generator).
  Induction generator takes reactive power from the power system for field excitation. If an induction generator is meant to supply a standalone load, a capacitor bank needs to be connected to supply reactive power.
• Construction of induction generator is less complicated as it does not require brushes and slip ring arrangement. Brushes are required in synchronous generator to supply DC voltage to the rotor for excitation."

http://www.electricaleasy.com/2014/12/synchronous-vs-induction-generator.html

If induction generator is supplying a standalone load, the output frequency will be slightly lower (by 2 or 3%) that calculated from the formula f = N * P / 120.

Why and how? Mechanically impossible if the prime mover is turning at a true constant speed....only a change in speed of the prime mover can effect a drop in frequency and 2-3% is way out of the allowed tolerances for frequency in ANY system I have heard of. Even a half of a cycl must be accounted for and added back to make sure that the frequency over any 24 hour period is as exact as possible.

To reduce load, voltage (brown out) may be reduced in some places.....frequency MUST be maintained as long as voltage is being supplied, or shut down completely.

Brushes are required in synchronous generator to supply DC voltage to the rotor for excitation.

I could easily believe that some modern alternators do not need to have brushes or slip rings in them at all, as they are a source of extra maintenance.

Even the alternators that I worked on 40 odd years ago only had brushes in the small (tiny!!) DC exciter that sat on the same shaft and fed DC though the shaft to the Rotor....if you meant those, all's well and good, but I could imagine that today even these are possibly not required....

The ones I looked after ran for years without much more than a dust blow out and a visual check every 6 months or so and not really much of that as loads were relatively small and many brushes were working in parallel......I do believe that brush replacement was around every 36 months or dockyard refit....

What I am trying (badly!) to say is that the big alternator part, even in those days, does not have bushes or slip rings in it....maybe I am simply being too picky!!

The DC exciter was "outside" of the main box then and therefore easily accessible....

Today, I could well imagine that the exciter could be a small multi pole alternator, with maybe a higher frequency (lots of rotor poles!). Exciter Stator field windings getting a controlled and variable external DC. In the shaft diodes between exciter and alternator supplying the main alternator rotor with DC power, then there would not be any need for brushes or slip rings, or should I say, that is how I would design it if asked.....

I am not saying that all modern alternators are brush-less, I would expect even today that such designs are still in use somewhere....but the need for brushes nowadays must surely be heavily reduced....But I am guessing!!!

If induction generator is supplying a standalone load, the output frequency will be slightly lower (by 2 or 3%) that calculated from the formula f = N * P / 120. Why and how?

I don't see your problem. It's quite similar to an induction motor running at less than synchronous speed when under load, slip depending on load, but 2 - 3% typical at full load.

Brushes are required in synchronous generator to supply DC voltage to the rotor for excitation.

Excitation is required, but via brushes or brushless is irrelevant. I believe most modern alternators are brushless, it just means there is a rectifier mounted on the shaft, field current controlled without contact (I don't know all the details). It doesn't affect the principle.

All of the above is negated when discussing a class of induction generator known as a Doubly Fed Induction Generator (DFIG).

A DFIG is separately excited, has a field winding, slip rings, outputs at synchronous speed, has voltage regulation, and is more complex than a normal synchronous generator.

PS- any induction generator that can be self-excited by capacitors (not all can and choosing the values can be tricky) and is not connected to another source will output at synchronous speed (defined as the AC output running at exactly the rotor speed adjusted for the number of poles) including above, below, and at powerline frequency if the prime mover can be adjusted to that speed.

Squirrel cage induction motors are self excited, correct?

All of the devices written about hear so far are induction devices. A motor can synchronous or not. The AC portion of a generator/alternator must be synchronous.

The AC portion of a generator/alternator must be synchronous.

Not if it's an induction generator. i.e. an ordinary induction motor driven at higher than synchronous speed, when it outputs power to the mains.

What has been missing is what it means to be "synchronous", as JRaef and others have commented, "...Synchronous to what...".

A synchronous machine outputs usable power if and only if the rotor's magnetic field rotates at exactly the same speed as the stator's rotating magnetic field. In contrast, an induction machine has no output under the above conditions, and only has output if there is a difference between the two rotating magnetic fields. In fact the same physical machine can operate in both modes, that's how some machines are started DOL (Direct On Line).

A wound rotor machine at zero speed has the rotor windings shorted (or through resistance for speed/torque control), then the stator is energized which in turn induces a magnetic field in the rotor winding, the rotor then starts rotating to "catch up" with the stator's field. As the line frequency (synchronous) speed is approached the field winding is switched from the short to a source of DC power, the poles are now separately excited, and the machine pulls into synchronous operation. Same machine, same rotor, same physical arrangement, same slip rings, just two different modes of operation, asynchronous and then synchronous.

Some machines meant for this type of duty have an amortisseur winding that is identical to the squirrel cage of a pure induction machine. During startup the squirrel cage does all the work, and during synchronous operation it is used as the damper winding to smooth out all those incremental ripples in the rotor speed once the rotor field is energized.

I don't see a problem defining synchronous. There must be 2 things, speed and frequency, to be synchronous (to each other). In a synchronous machine the speed and frequency have a fixed relationship -

N = 120*f/P where N = rpm, f = Hz, P = no. of poles.

In an asynchronous machine that doesn't apply. In an asynchronous motor the rotor speed is (typically) 2 - 3% lower than synchronous, in an asynchronous (induction) generator the speed is higher. As I'm sure you know. Rixter put more detail in #7.

Induction generator is nothing but an induction motor which runs in the positive slip. Whereas synchronous generator has separate exciter which is a conventional generator. Separate excitation is not required in induction generator. In synchronous generator brushes are required to excite the rotor.

In synchronous generator brushes are required to excite the rotor.

Except when it's brushless (but that's a minor point, it still has a rotating field winding supplied with current).

Induction generator is nothing but an induction motor which runs in the positive slip. Whereas synchronous generator has separate exciter which is a conventional generator. Separate excitation is not required in induction generator. In synchronous generator brushes are required to excite the rotor.

In synchronous generator brushes are required to excite the rotor.

Except when it's brushless (but that's a minor point, it still has a rotating field winding supplied with current).

This could go on for some time.