3 Phase Synchronous Motors For Learning

I wonder if such a thing actually exists.....3 phase is mainly used where high power is needed......which to me implies that small 3 Ph. motors probably do not exist.

For lower power applications, single phase motors are generally used......as they are usually cheaper. With a split field they follow many of the electrical laws that a 3 Phase motor follows.....

You could have a serious problem finding what you are looking for!!!

I hope I am wrong for your sake.....

Buy a alternator also known as AC Generator.Lowest KVA available is 2.5KVA single phase and Three phase 5kVA.When supplied power it can work as motor.Ensure that poles have Damper windings which will enable starting as synchronous motors have no starting torque.Or use a small motor to rev it up to near syn speed and then switch in the field when it will pull in.U can either mechanically disconnect the pony motor or disconnect supply and allow rotor to keep running.

I cannot see for the life of me how that will help him.....perhaps you could explain it better for us all, thanks in advance....

He wants a lower capacity synchronous motor.Basically synch.Motors are used on large ratings like motors of ratings 9MW etc.However an ac generator which is used in DG sets can be run as syn.motor by supplying power .For his study purpose this can be used.

I think he was hoping for something really small, like he could hold it in his hand (or a tick bigger) for example......!

I am interested in seeing the spaceing of the windings inside the motor. I understand how the motor works in principle however not well enough to make my own. Utilmately I would like to produce a scale model of a power plant in which a synchronous motor would be used as an AC generator (because then the frequency can remain constant for a constant rotation speed). The 3 phase motors I have found are all induction motors. I know they make a fractional horse synchronous machine because we used one in one of our labs, I don't have the manufacturer information however so I was looking for ideas for where I could find one. The local rebuilding shop was nice enough to give me one but when I took it apart it ended up being an induction motor.

Is there any way of knowing if a AC machine is synchronous or induction? Are there any applications where small synchronous motors are used?

Synchronous does not necessarily imply 3-phase. If I understand correctly, Slo-Syn motors are essentially stepper motors that use a capacitor to produce a second phase from single-phase power. I see them regularly in size 23 (2.3inch) and 34 motors.

They have the advantage of producing low rpms from standard line frequency, which might be good for your model.

Fractional horsepower induction motors are 99.999% single phase as far as I am aware.....I have personally never heard of a 3 phase fractional horsepower motor. It is of course possible, but I feel highly unlikely.....

Be careful with regards AC 3 Phase generation, it is possible to use a 3 phase induction motor for this but it gets quite complicated......its not simple, you need to get it to the speed required (according to the Hz needed) as a motor first and then switch over, output voltage control is also not simple!!!

Most 3 phase generators have a DC exciter generator that not only produces the magnetic field power, but also plays a part in output voltage control!!!

Its a LOT more complicated than you seem to think!!!!! I suggest that you learn a LOT more before proceeding further, it may put you off completely!!

You need to buy a generator of the size I have mentioned which can be dismantled for study.All generators are all synchronous except in steam turbine applications where non salient syn. machines are used.The syn.machine will have a DC winding on the Rotor normally and a ac distributed winding on the stator.The DC winding on rotor can be seen as salient poles.I have seen manufacturers producing machines with DC supply to stator and ac winding on Rotor.Easiest way to distinguish between Syn and induction motor is normal induction motors of the size u are looking are all cage type i no winding on the rotor.All syn machines will have winding on stator and rotor.(Though slip ring induction motors or wound rotor motors also have windings which normally are not used on sizes required by u).Your assumption that u will get constant frequency is not correct.The prime mover has to give a constant speed to get constant frequency when generating.Secondly if incoming supply frequency changes speed changes will be there when operating as a motor.All generators available in the market are mostly synchronous .

Thanks so much for your help. I am aware of the need to keep the input shaft rotational speed constant. I will look for a defunct generator to take apart.

There are some simple things to look for Slip rings and a pole faces on the rotor for 3 phase machines. If it is running it will run at a synchronous sub multiple of your supply frequency say 1000 rpm for a 6 pole 50hz machine or 1500 for 4 pole. 60hz expect 1200 and 1800 for the same poles.

There are single phase synchronous machines used for clocks but since you have specifically asked for 3 phase I wont go there.

Pl. specify rating and voltage.

We used something along this line in a practical experiment this year at the tech college that I go to.

It was a three phase syncro motor/generator set designed for education.

I know they were made for Australia as they ran on 41.5VAC & 24.0VAC either as variable speed motor or fixed speed generator.

This was a "Set" of motor\generators that could be interchanged on a slide.

Having said all this, we finished for the year two weeks ago so I wont be able to get you details until next year.

Please email me at Sapper1@iinet.net.au if you would like me to get more details of Make\Model and supplier.

Sapper

There are 2 sorts I recall one is Liebotek and the other Jay- something could even have been Jay - vac the mob that make vacuum pumps.

With the diodes removed a car alternator is a 14volt synchronous motor and as one or more posts have stated will need to be spun up to speed and then excitation applied to the slip rings.

The only problem you now face is getting a 14volt 3 phase supply. If you are using this apparatus to learn about motors and alternators like you would using the Liebotek gear used in technical colleges rather than to actually drive a real load it isn't too hard.

1. Obtain a three phase transformer and suitable 12vdc supply for the field supply.
2. Modify 2 old alternators (preferably the same type) and source a motor or engine of any sort as the prime mover. Drive one alternator and apply excitation to produce the required 3 phase extra low voltage output. Spin the other alternator (now synch motor) up to the same speed (Synchronous speed). Connect the 3 phase power you are producing to the stator windings of your synch motor then apply excitation to the fields and cease driving the motor. It will the run at synchronous speed (the speed your prime mover is doing).
3. Look around to see if you can purchase Liebotek or similar educational equipment.

You can learn heaps from this set up.

The winding detail is easy to see. The synchronous nature is easy to demonstrate. If you have access to a CRO then the relationship between phase angle and excitation can be explored too.

If you get enough bits you can rewind stators and all measures of good things too.