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Alternator or motor?
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Hairyloon



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Fri Feb 06, 15 11:51 pm    Post subject: Alternator or motor? Reply with quote
    

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



Joined: 02 Jul 2005
Posts: 45374
Location: yes
PostPosted: Sat Feb 07, 15 12:26 am    Post subject: Reply with quote
    

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



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Sat Feb 07, 15 1:30 am    Post subject: Reply with quote
    

dpack wrote:
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



Joined: 02 Jul 2005
Posts: 45374
Location: yes
PostPosted: Sat Feb 07, 15 1:45 am    Post subject: Reply with quote
    

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

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



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Sat Feb 07, 15 3:14 am    Post subject: Reply with quote
    

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



Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales
PostPosted: Sat Feb 07, 15 8:41 am    Post subject: Reply with quote
    

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



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Sat Feb 07, 15 10:02 am    Post subject: Reply with quote
    

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.

vegplot



Joined: 19 Apr 2007
Posts: 21301
Location: Bethesda, Gwynedd
PostPosted: Sat Feb 07, 15 10:57 am    Post subject: Reply with quote
    

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

Hairyloon



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Sat Feb 07, 15 11:40 am    Post subject: Reply with quote
    

vegplot wrote:
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



Joined: 02 Jul 2005
Posts: 45374
Location: yes
PostPosted: Sat Feb 07, 15 12:47 pm    Post subject: Reply with quote
    

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



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Sat Feb 07, 15 1:10 pm    Post subject: Reply with quote
    

dpack wrote:
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



Joined: 02 Jul 2005
Posts: 45374
Location: yes
PostPosted: Sat Feb 07, 15 1:42 pm    Post subject: Reply with quote
    

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

vegplot



Joined: 19 Apr 2007
Posts: 21301
Location: Bethesda, Gwynedd
PostPosted: Sat Feb 07, 15 2:47 pm    Post subject: Reply with quote
    

Hairyloon wrote:
vegplot wrote:
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



Joined: 11 Feb 2007
Posts: 2252

PostPosted: Sat Feb 07, 15 2:53 pm    Post subject: Reply with quote
    

Hairyloon wrote:
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.


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

For example see here:

https://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.



https://www.electrokinetica.org/d1/1/3.php

Hairyloon



Joined: 20 Nov 2008
Posts: 15425
Location: Today I are mostly being in Yorkshire.
PostPosted: Sat Feb 07, 15 3:06 pm    Post subject: Reply with quote
    

vegplot wrote:
Hairyloon wrote:
vegplot wrote:
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...

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