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Hairyloon

Alternator or motor?

Physically, an alternator and a motor are much the same thing: a magnet spinning in a coil, the principle difference being the source of the motion.

So is it possible to use the alternator of a generator as a starter motor to start the generator?
Leaving aside the obvious point that most times that you want a generator it's because you don't have power.

I have a feeling this is a silly question, but I cannot think what is the problem with it...
dpack

iirc there is a difference in both the wiring and the speed of spin/torque between starter motors and alternators

so although both are a set of copper wire coils and both fasten to a battery/engine (in rather different ways)they have very different build characteristics

a bit like comparing a film camera and a projector
Hairyloon

iirc there is a difference in both the wiring and the speed of spin/torque between starter motors and alternators

Yes, there would be. A starter motor is normally a DC motor driven by a battery, whereas an alternator generates AC. One is a coil spinning in a magnet, the other is a magnet spinning in a coil.
However, an AC motor (of the right type), I think, is equivalent to an alternator: if you connect AC across it, then it should spin.
Quote:
so although both are a set of copper wire coils and both fasten to a battery/engine (in rather different ways)they have very different build characteristics

They do, but I think that is in the same way that an alternator is very different from a dynamo, and I know for certain that if you put power across a dynamo then it will spin.
dpack

will turn an engine til it fires and will make useful electric if spun are very different

i recon you need very different rigs for the two jobs and anything that might do both would not do either well and would need two very different sets of gearing/mechanical connections and switch gear to swap between modes

as both can be had from a scrapper for a few quid get both and have a rodless back would be my tactic Laughing

i recon that car manufacturers have a starter and a alternater for good reason

ps alternator,magneto and dynamo are all different as are the huge variety of drive motors .each have different strengths and weaknesses
Hairyloon

I think perhaps we are not on the same page: you appear to be thinking about cars where I am thinking about generators. On a car the alternator is only taking a tiny fraction of the power and is tiny compared to the engine. If you tried to use it as a motor to spin the engine, it would most likely melt.
On a generator the alternator takes pretty much all the available power, it is as big as the engine, if it can be used as a motor, then it would have no problem spinning the engine at all

Instinctively, I think the same as you: it is not a good idea, but logically, I have entirely failed to find any reason why this might be so.
RichardW

This has been done by the Lister company on its Start O Matic range. Its not just a simple swap the wires over job. The alternator has extra windings.
Hairyloon

The Startomatic is a DC system, so it'll be a dynamo, not an alternator.
It has a box full of windings for regulatiion and control, but I can't imagine what would be needed on the dynamo itself. Confused
vegplot

Doubt it will be able to generate sufficient torque to overcome inertia.
Hairyloon

Doubt it will be able to generate sufficient torque to overcome inertia.

I don't see why not. Consider a car. cranking current on the starter is what? 200A?
That would be 2.4kW at 12v to start a 50kW engine.
A 2.5kW alternator is clearly capable of handling 2.5kW and only has to start a 3kW engine. dpack

i was thinking cars and motorbikes and quarry scale diggers etc

with a genny i spose the start up forces are usually done with a cord pull so they dont need a lot of power to roll em up to speed

unless there is a good reason to use an electric start is using the cord the best idea? Hairyloon

unless there is a good reason to use an electric start is using the cord the best idea?
It might be, but I was looking at a genny with a knackered pull starter and it set me wondering.
The other advantage of a starter motor is that you could set it up to start automatically, or by remote. dpack

did camorra(spp?)have a rig to start his genny to run the hot water/heating so as things were hot when he arrived at his island?

im not sure how useful a remote system would be

i recon mend the pull start is the simple option Laughing vegplot

Doubt it will be able to generate sufficient torque to overcome inertia.
I don't see why not. Consider a car. cranking current on the starter is what? 200A?
That would be 2.4kW at 12v to start a 50kW engine.
A 2.5kW alternator is clearly capable of handling 2.5kW and only has to start a 3kW engine.

Torque is not the same as power. crofter

The Startomatic is a DC system, so it'll be a dynamo, not an alternator.
It has a box full of windings for regulatiion and control, but I can't imagine what would be needed on the dynamo itself. Confused

Lister also made Startomatics producing AC, we used to have one...

For example see here:

http://www.ebay.co.uk/itm/Vintage-1957-LISTER-START-O-MATIC-GENERATORS-Advertisement-STATIONARY-MOTOR-/200746724807?pt=AU_AdvertisingCollectables&hash=item2ebd6fedc7

Interesting page here:

Quote:
A DC generator can be made to give an output voltage that remains constant despite variations in load current, by means of a series field winding that boosts the field as the load increases, thereby compensating for the voltage drop. Such a machine is termed 'level-compounded', to distinguish it from one with a deliberate fall or rise in voltage with change in load. Although a constant voltage output would be desirable in a charging set for a floating battery, the compound generator is unsuitable for one important reason. When, as often happens during starting and stopping, the output voltage of the machine falls below that of the battery, its armature current reverses as the generator runs as a motor. If the armature current is heavy, the series field winding can reverse the residual magnetism in the iron frame of the machine, causing it to build up with reversed polarity at the next start.



http://www.electrokinetica.org/d1/1/3.php Hairyloon

Doubt it will be able to generate sufficient torque to overcome inertia.
I don't see why not. Consider a car. cranking current on the starter is what? 200A?
That would be 2.4kW at 12v to start a 50kW engine.
A 2.5kW alternator is clearly capable of handling 2.5kW and only has to start a 3kW engine.

Torque is not the same as power.
No, but last I checked power = torque x RPM and electric motors have maximum torque an zero revs... Hairyloon

The Startomatic is a DC system, so it'll be a dynamo, not an alternator.
It has a box full of windings for regulatiion and control, but I can't imagine what would be needed on the dynamo itself. Confused

Lister also made Startomatics producing AC, we used to have one...
In that case, I'd guess that the extra windings you mentioned were to make it a DC motor as well as an AC alternator. I don't think inverter technology was up to much in those days. RichardW

Check the winding wire size in an average starter & compare it to the windings in an alternator. Hairyloon

At a higher voltage you need less current, so don't need such a fat wire. RichardW

I would assume that at the lower speed it will be spinning at (unless you are going to gear it down & poss use a slip clutch) it will quickly a, over heat & b, act as a direct short due to the slow speed & burn out the cable just like a stalled / seized motor will.

Ask your self this.

In the ever advancing process of reduction in costs to seek more profits why has no modern manufacture gone down this road? Hairyloon

I would assume that at the lower speed it will be spinning at (unless you are going to gear it down & poss use a slip clutch) it will quickly a, over heat & b, act as a direct short due to the slow speed & burn out the cable just like a stalled / seized motor will.
Why do you suggest it would be spinning slowly? It would be a whacking great big motor compared to the engine.

Quote:
In the ever advancing process of reduction in costs to seek more profits why has no modern manufacture gone down this road?

I can think of a number of possibilities. The first, as already mentioned is that most times, generators tend to be used when there is no mains power available. RichardW

Engines tend to be started at slow speeds. IE tick over or less. vegplot

What benefits would there be in a universal DC starter motor / generator alternator? It would have to be complex both mechanically and electrically which would impact on cost and long term reliability and well as over efficiency. It probably explains why it's not been done before on a commercial scale.

As I said before torque is going to the major issue. Alternators are high speed low torque devices and therefore it would be impractical (but not impossible but at what cost?) to convert it to a high torque motor. Dee J

Dynamos can be made to motor and vica-versa quite easily. The Siba dynastart is not uncommon in the marine engine world and have featured on some older motorcycles and light cars. The advantage of a dc motor or dynamo is that the commutator changes the magnetic fields at the correct times. To achieve the same with an alternator is either to run an open loop system where you drive the alternator with a fixed frequency AC, and hope it picks up and synchronises with it... but you won't get a lot of starting torque... Or set up a closed loop system where the shaft position is detected (rotary encoder) and the drive voltage is sequenced/alternated synchronously... producing better starting torque and control. There are various commercial solutions available http://www.industrial-scientific.com/special_products_division/alternator_drive_technology_kit.htm
amongst others... vegplot

He's probably lost interest now. dpack

im still watching and learning

if i understand correctly it can be done but it isnt worth the bother as 2 units will each do their job well and a single unit will be awkward doing each as required .

is that right ? vegplot

im still watching and learning

if i understand correctly it can be done but it isnt worth the bother as 2 units will each do their job well and a single unit will be awkward doing each as required .

is that right ?

That's my understanding. dpack

good ,thanks . Nick

He's probably lost interest now.

A+, good work! onemanband

Honda ACG alternator/starter Hairyloon

if i understand correctly it can be done but it isnt worth the bother as 2 units will each do their job well and a single unit will be awkward doing each as required .

is that right ?
Possibly, but I think it may be easier to persuade the alternator to run as a motor than it is to fit a motor to an engine that was never designed to have one. Falstaff

So why don't you just rework the pull string ? crofter

https://www.youtube.com/watch?v=zcBE8HEAyi4 Hairyloon

So why don't you just rework the pull string ?
Boring question. Mistress Rose

But eminently practical. Very Happy Hairyloon

But eminently practical. Very Happy
Nobody has yet come back with a convincing reason why my idea is not equally practical... though I am not yet convinced enough to simply try it. Hairyloon

Engines tend to be started at slow speeds. IE tick over or less.
Tend to be, yes, but not necessarily so. I have been told of a UPS power system which is basically a damn great big flywheel connected to a big diesel generator.
The flywheel is kept spinning and if the power goes, it simply dumps the clutch and they go from 0-full in an instant. It is quite impressive I am told. vegplot

But eminently practical. Very Happy
Nobody has yet come back with a convincing reason why my idea is not equally practical... though I am not yet convinced enough to simply try it.

You'd like us to convince you your idea is practical because you're not convinced your self? Hairyloon

You'd like us to convince you your idea is practical because you're not convinced your self?
There is a risk that it will be an expensive bang, so, I want you to try and convince me that it isn't practical.
Once you have failed thoroughly enough, I will try it... vegplot

You'd like us to convince you your idea is practical because you're not convinced your self?
There is a risk that it will be an expensive bang, so, I want you to try and convince me that it isn't practical.
Once you have failed thoroughly enough, I will try it...

The choice is yours, you've had opinions convincing or otherwise. Hairyloon

There may be an error of tense in my earlier comment. Falstaff

I don't think it will go BANG ! Shocked

Unpleasant burning smells and a little smoke would be my expectation.
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