STC Generator as Electric Motor.

[its-st]

Hello DIYer  ,

I have a Changfa Generator with 3-phase STC-12KW alternator.

My Engine has no electric start, so I want to use the the alternator for a short time as electric motor to start the engine. Will that be possible?

Best greetings from Germany,
Michael

[mobile_bob]

possible but likely expensive

you would need to purchase if available (unlikely) or...

build a controller so to speak that can apply current to the slip rings at the
precise time (pulse) so that the rotor would act like an armature.

then you would have to have some relays and switch gear to make it all come together
and work.

probably more effort and money than it is worth, can you get a flywheel and starter for your changfa?
that would be by far the least expensive way to get the job done.

bob g

[Doug]

Syncornous machines are not self starting as motors.

Most use an asmortis ( badly spelled ) winding that is esentialy a set of induction motor bars buried in the pole face ( this also serves a second function of reducing hunting ann ossicaltions betweem load/prime mover and machine. I've also seen some pony motors used to bring them up to speed but this is rarely done on anything other than very high interia loads.

There is also the "Comutatorless DC machine" that essentialy a sycronous motor and clever drive that self syncs the motor to its input power ( starts at 0 Hz and builds to speed as it monitors the shaft possitions and load ).

But all the machines I discribed above are 3 phase.

In all honesty I don't think I've seen a single phase syncronous machine ( excluding salient pole induction motors and these have no real use except for things like clocks ). There also the problem of hunting and vibration single phased. Induction motors deal with single phase by sort of generating a second phase in the rotor that lags the the line 90 degs. This means when the line voltage crosses the zero point every 1/2 cycle there's still some current flowing and residual magnetism in the rotor so when the power comes back this residual flux combined withthe slip allows the induction machine to acelerate again but up to speed ( and you can here and see this effect as ther 120 hz hum in single phase sa compared to the mucg smoother power flow in a 3 phase machine ). The net effect of all this means under load and without an osmortise winding the rotor will be snapping back and forth trying to catch up to the line frequency ( or phase would be a better discription ) and over shooting, or hunting as we would call it.....

Non of this answeres you question its just a good chance for me to try and explane a little motor theory.

To answere your question no a single phase syncronous machine is not self starting as a motor and cannot start a load.....

Oh yes and the part about aplying a DC power to the rotot  at specific times is one of the ways large machines have been synconized to the line in the past and its probably the smoothest and easiest way.

Time permitting I'll post a circut of how this was done in the old days all electro-mechanicaly ( the best way in my ludite opinion )

Doug

[mobile_bob]

in theory one should be able to make it start even as a single phase motor, because it is now a dc motor
but commutated electronically thru the slip rings
to get the triggering down one would have to have some form of signal from a photo pickup off a wheel attached to the motor shaft
or maybe an inductive pickup off a gear tooth or something.

basically all you would need is a contactor to energize the stator coils/poles to establish them as fixed north south north south

and then by pulsing power to the slip rings properly timed and switched in polarity  the motor would start, and likely with good torque

now haveing said that, i don't think it would make a particularly efficient motor, but might be capable of developing enough power to spin
up a listeroid, with it ~3:1 reduction drive.

i think it could be done, but probably not cheap

bob g

[Doug]

Bob, its a single phase machine on a single phase line. There is no rotation of any kind to grab onto.

There are two " Models " to explain single phase motors counter rotating field and cross field. Let us consider for a moment the counter roating field model because it best explains starting and aceleration in a single phase induction machine. And for what we are discussing it lends itself well.

Let us assume we have a syncronous machine that we are trying to start and lets just accept we have a way of turning on and off the field flux fast enough to match the line frequency. Because we have a single phase system with no rotation of the field in one direction or the other that the net torque generated by the flux is the sum of torque spinning in both directions. In a standard induction motor we counter this effect by giving the rotor a push ( by means of the AUX winding ). Because of induction we have a laging current in the rotor that adds flux trying to catch up in one direction, so the faster we spin one way the more torque we generate in that direction and the less in the other untill we reach the equalibrium of syncronous speed ( we can't but this is just a model ).

Am I making any sence?
I know what I want to explain, but I don't think i worded this too well.

Without that push ( and I can't see how we could even make a stock ST head behave as if it had an AUX winding ) I can't see how we could even get the rotor turning. We are also faced with the fact we need to generate maximum torque at a dead stop.

Again looking at the single phse induction motor model of a  " two phase " machine with the AUX winding now our focus we are trying to create a poly phase like machine using lead and lag of a highly inductive main winding and a less inductive ( split phase ) or slightly leading PF ( Cap start ). This is something we can't duplicate in an ST or STC head without a rewind and even if we did try and get it wound the math is above me to design and I can't clearly picture how we could adjust the PF of this AUX winding with the changing frequency required to hold the rotor in sync. This now leads us to build an inverter with a true polyphase output and a startor with at least a minimum 2 phase winding that we may not have room for even if its designed small and only for shrt duty cranking.

Doug

But thats just my opinion, been wrong before.....

Now if we could get Dorris to crank it by hand problem solved eh?

[Doug]

This has been in the back of my mind for a while......

I guess it might be possible to burry a Replusion start induction motor into the pole faces of the rotor.

This would add to system stability in generator mode by adding the Induction motor Osmortice winding effect of the RI windings and centrifugal.

I also like Repulsion motors, sadly I don;t think too many people even knw what they are anymore......

[buickanddeere]

  Try one of these with a four cylinder automotive cam timing belt & pulley.  COLUMBIA PAR CAR GOLF CART STARTER/GENERATOR (500-1)
   Item number: 180146451260   
   
     Aim for 4000-5000rpm gen/starter rpm with Lister at working speed. Not certain of what top rpms are for the Hitachi but the on on my garden tractor turns 7500rpm
      Go for a large ratio if required for more starting torque

[mobile_bob]

Doug:

sorry it took so long to reply, but the thread got lost in the shuffle and i just saw it pop up again.

what i was thinking was if one used a sense off one of the poles of the stator to not only sense when but what polarity the
pole is a a particular time, and use this info to trigger a dc pulse to the sliprings, in theory it should allow the st head to motor.

basically electronic commutation triggered by a sense circuit from one of the stator poles

as i stated before probably alot more trouble and expense than it is worth.

bob g

[Doug]

Food for thought Bob....

Doug

[rosietheriviter]

Bob, its a single phase machine on a single phase line. There is no rotation of any kind to grab onto.
Doug

Doug,
Am I missing something here?  Are you saying that the alternator in question is, in fact,  a single phase?

Hello DIYer,
I have a Changfa Generator with 3-phase STC-12KW alternator.

Or are you saying that "in spite of the fact that it is a three phase - it will function as a single phase" because of??

Rose is Rose

[Doug]

Hmm....

Starting from the top of the list this is three phase he has.

I apear to have wandered off into the relm of single phase machines again.
I am the one missing something here ( again lol )