Making an alternator that charges ‘properly’

[Tim StevensParticipant @timstevens64731]

Can I add a few words to the interesting piece by Bob Hayter, pp 140 – 143, ME 4721?

We are told that it is important to position the magnets alternately N up and S up. It is just as important, though, to do the same with the coils. When doing the continuity test, use the compass to check that alternate coils produce a (feeble) N and S as the current flows. Get this wrong and the coil outputs will tend to cancel each other out.

It would also be helpful to know a little about the wire for the coils. What to ask for, and what gauge – and for those in distant parts of the ex-empire, what the gauge means in millimetres. And incidentally, the output contacts should not be marked red and black – this is a convention for DC output.

And finally, please add a note that the output will vary a lot with the driven speed, and will be no use for many electrical tasks. In fact, it would help if a few words on rectification and voltage control could be added – just in case anyone wanted to use it for charging batteries, or driving DC motors, or small LEDs.

Regards

Tim

[Tim StevensParticipant @timstevens64731]

[EmgeeParticipant @emgee]

Hi Tim

Did you make a mistake with the keyboard re marking the output red and black, your statement advises against this actiion ?

Emgee

[FulmenParticipant @fulmen]

No, he's right. An alternator produces alternating current, red/blue indicates direct current.

[Dave HalfordParticipant @davehalford22513]

Black red is now only for the US

In the UK Dc comes in various mixes of blue, brown and grey depending on +ve or -ve earthed

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Dave Halford on 10/07/2023 21:03:55:

Black red is now only for the US

In the UK Dc comes in various mixes of blue, brown and grey depending on +ve or -ve earthed

No, that is for AC.

[EmgeeParticipant @emgee]

Yes now I read the post title and see alternator I agree I was mistaken, however don't vehicle alternators have integrated rectification so the output is DC + and – ?

Emgee

[Chris Pearson 1Participant @chrispearson1]

If we are talking about automotive alternators, I think that brown is for the (rectified) live conductor and black is for the connexion to earth.

[Ady1Participant @ady1]

I opened up one of those baylis crank torches once and it looked like it was a 3 phase alternator to charge the battery

[Robert Atkinson 2Participant @robertatkinson2]

I've not seen the original article, but if this is an alternator for a steam engine you could make it linear and get rid of the crank and flywheel…..

Robert

Edited By Robert Atkinson 2 on 11/07/2023 07:50:12

[Martin ConnellyParticipant @martinconnelly55370]

Car alternators are also based on a 3 phase AC system rectified to DC by a diode pack (6 diodes in the pack). This reduces the AC ripple on the DC output compared to a single phase alternator. When controlled by the inbuilt regulator and with the output connected to the battery there is no need for large smoothing capacitors on the output.

When I worked on gas turbine powered alternator packages we often used brushless alternators. These had a small alternator on the shaft with the external magnetic field controlled by a regulator. The induced current in the rotating alternator coils was then used to power the rotating coils of the main alternator's electromagnet and so produce the output of the alternator in the main stator coils. I don't know for certain but I expect the small alternator's output was rectified. It was probably multiphase as well but I don't know if it was limited to 3 phases or if more were used for better smoothing. I suspect the impedance of the main rotor coils was taken into account to avoid harmonic issues.

Martin C

[Robert Atkinson 2Participant @robertatkinson2]

Martin,
Yes the aviation brushless alternator use 3 phase excitation alternators with rectifiers mounted on the shaft.
Engines with electronic control often have small unregulated permanent magnetalternators to provide power to the engine computers if all other power fails.

The world of aviation power generation is undergoing a lot of change. Many aircraft now use wide frequency range alternators (typically still called generators in the industry) rather than the tradition 400Hz tharequired heavy constant speed drives. There are some that use higher frequencies and a AC-AC converter to generate 400Hz. The converters use the same technolgy as our VFDs.
There is also a move to more DC power. The standard is currently 270 V but ther are moves to higer voltages. Semiconductor based devices are replacing heavy iron transformers and where you do need a motor a local inverter taliored to the task gives all the advantages of a VFD. In particular starting load control and for applications like pumps pressure or flow control without wasteful regulators and bypasses. It's also lighter and safer than hydraulics or bleed air.

Robert.

[Bob WorsleyParticipant @bobworsley31976]

Three phase excitation? Really? Three phase would then need three coupling transformers between the stator and rotor. Single phase excitation only needs one transformer, and at the power levels needed works ok.

Car alternators are three phase simply because a three phase machine is more efficient than single phase, got nothing to do with a smoother output, it is feeding a battery after all.

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Bob Worsley on 11/07/2023 10:02:21:

Three phase excitation? Really? Three phase would then need three coupling transformers between the stator and rotor. Single phase excitation only needs one transformer, and at the power levels needed works ok.

Car alternators are three phase simply because a three phase machine is more efficient than single phase, got nothing to do with a smoother output, it is feeding a battery after all.

Hi Bob,

I think you have mis-read. I said "3 phase excitiion alternators" i.e. the excitation alternator is a 3 phase machine. It is a alternator not a transformer with a moving coil.

The excittion alternator has a single winding DC powered stationary field coil on the frame and a 3 winding rotating output coil assembly on the main shaft of the total machine. The 3 phase output of the windings is rectified by six diodes, also mounted on the shaft. and the resulting DC is connected to the rotating, single winding field coil of the main alternator.
Typically the excitation alternator has more poles than the main alternator so the ripple frequency of the rectified DC suppled to the main fiel coil is higher than the output frequency.
By varying the current in the stationary excittaion alternator coil the AC output of the main alterator is controlled.

Hope this is clearer.

Edit, output ripple is important in a car system. Apart from modern electronics, AC ripple is damaging to lead acid bateries.

Robert.

Edited By Robert Atkinson 2 on 11/07/2023 10:25:54

Edited By Robert Atkinson 2 on 11/07/2023 10:26:32

[Tim StevensParticipant @timstevens64731]

The article to which i responded shows the making of a simple alternator, which would produce single phase AC output (un-rectified, unregulated). Reference in any responses above to three-phase, car alternators, excitation, etc, do not apply to the article itself or to my comments.

When I refer to the colour coding of the output, i was relying on conventions established many years ago, and I have worked on several vehicles which have been re-wired (often by owners rather than by experts), and the use of red for live and positive, with black for earth and negative, is a common factor – always used on DC circuits.

In the real world, there care several distinct 'systems' for the wiring colours in vehicles, and where home-mechanics are involved they can be based on what was available from the corner shop or from a mate at work …

Regards – Tim

[Nick Clarke 3Participant @nickclarke3]

Posted by Tim Stevens on 11/07/2023 10:26:44:

In the real world, there care several distinct 'systems' for the wiring colours in vehicles, and where home-mechanics are involved they can be based on what was available from the corner shop or from a mate at work …

Regards – Tim

Many years ago I was driving my old VW Beetle into work when I heard strange noises from the rear as I drove past the bus garage. Stopping and opening the engine cover I was greeted by fire as the petrol pipe had burst. running into the garage a mechanic smashed the glass cover on a fire extinguisher on the nearest bus and ran out and dowsed the flames for me.

I was recovered by the RAC and rang into work saying I would not be in.

I got a call back saying that I had to be in before the end of the school day or I will have my pay stopped.'

Now the engine compartment of an air cooled Beetle is fairly simple so dashing to my nearest supplier, not far away fortunately, for some air hoses and petrol pipe I replaced these and rewired the engine – but as I only had one reel of cable every thing was rewired in yellow with a maroon stripe. Basically the colour code for everything was yellow with a maroon stripe.

– But I got into school 15 minutes before the end of the school day and got paid for all of it!

[Clive SteerParticipant @clivesteer55943]

The upper frequency limit of many VFD's is 400Hz and I've used one to test various aircraft devices that needed 110V/400hz. However I did need to exercise care as the VFD output is not isolated from the mains.

I can see that in the aircraft industry where weight costs money that powerful BLDC motors with electronic control would have improved efficiency and less weight that an equivalent induction motor. If a plane could go "all electric" it would simplify the RAT.

I know of some vintage aircraft, that were never fitted with electric, being fitted with a small permanent magnet alternators so they can use modern aircraft radio and other devices demanded by ATC.

Most light aircraft engines use "aircraft grade" car alternators and electronic voltage regulators.

CS

[Robert Atkinson 2Participant @robertatkinson2]

There are two problems with using a VFD to power aircraft avionics other than the safety of no mains isolation.

1/ The output is "delta"
2/ Harmonics from the PWM.

1/ means it is OK for balanced loads like fans but most avionics need a neutral return. It can be solved by a delta star transformer. This also fixes the isolation issue.
2/ needs a filter, maybe just series inductors, but may be specific to the VFD model.

Been there, done that.

Robert.

[not done it yetParticipant @notdoneityet]

Don’t forget that multi-phase generators produce a fraction of the power (eventually a DC current) in each phase winding – meaning economies on winding wire, easier dissipation of thermal energy, etc.

[Tim StevensParticipant @timstevens64731]

Has anyone else seen the actual article yet?

Tim

[Ian PParticipant @ianp]

Being a subscriber to MEW only the original post made no sense to me. I did read it but without seeing the article was confused by coils (more than one) and how the 'feeble' N-S was obtained.

It seems also that there are two? output connections so the coils must be connected in parallel. If the output is AC then if only two wires I would make them the same (any) colour.

I have a feeling that this forum is frequented by more MEW readers that ME ones so not many will see the article mentioned.

Ian P

[noel shelleyParticipant @noelshelley55608]

Hi Tim, Yes I've seen it and from the Stuart 10 engine there would be so little power that rectification and voltage control would be pointless. As a permag unit voltage control would be to spill excess to ground, rectification would be simple bridge. A nice model ! During the war a stuart twin sirus ? was used to recharge transceiver batteries for spies in france, in fact it was a whole package air dropped, an alloy boiler that would burn almost anything. I have seen one, a very neat little package, engine and dynamo all in a felt lined tin box, felt I guess to silence it when running. Noel.

[Tim StevensParticipant @timstevens64731]

Ian P is concerned about the term 'feeble output' I used for testing the magnetic effect of the coils. The clue is in the words 'when doing the continuity test …' Such a test applies a low voltage – usually from a single cell in the test box – and allows the user to ensure that a current flows, by giving a reading on the scale (or sometimes by sounding a buzzer). As this uses a very low voltage it only produces a feeble magnetic effect – which is all you need as a compass is very sensitive.

I hope that clarifies things. In the case here, all the coils are shown to be connected in series – but there is no real need for it to be so, they could all be in parallel, or some other combination, as long as care was taken to ensure that any pair of circuits included the same arrangement of coils, so they are not fighting each other.

Regards, Tim

Edited By Tim Stevens on 11/07/2023 18:26:08

[Mark RandParticipant @markrand96270]

Posted by Robert Atkinson 2 on 11/07/2023 12:28:08:

There are two problems with using a VFD to power aircraft avionics other than the safety of no mains isolation.

1/ The output is "delta"
2/ Harmonics from the PWM.

1/ means it is OK for balanced loads like fans but most avionics need a neutral return. It can be solved by a delta star transformer. This also fixes the isolation issue.
2/ needs a filter, maybe just series inductors, but may be specific to the VFD model.

Been there, done that.

Robert.

Point of order:-

If the inverter has balanced positive and negative DC busses, then it can produce star output as easily as delta. That's just a matter of a centre tap on the HF (DC-DC or AC-DC) converter transformer that generates the DC bus voltage in the first place.

Regards

Mark

[duncan webster 1Participant @duncanwebster1]

My copy of ME arrived this morning, only slightly shredded by the dog. My very limited knowledge of generators suggests that a cast iron rather than ally rotor would be an improvement, as would a magnetic rig instead of the plastic arrangement for the coils. Both changes to reduce the air gaps in the magnetic circuit. I suspect the coils are wired in series to get a meaningful output voltage.

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