Why field winding on stator for dc machines and on rotor for synchronous machines

[atifmwali]

hello everyone
i have a question
i was studying about machines and as i was going though different machines i realized that for dc machines the field winding is on the stator while in synchronous machines the field winding is on the rotor

could anyone explain the reason for this setup

thank you in advance

 

[crshears]

The rotating armature of DC machines has alternating current generated in its windings; this is then rectified to DC by the commutator. The necessary field is thus provided on the stator, or stationary portion, of the machine. Incidentally it is the need of brush gear to transfer the power generated from the armature that limits the capacity of dynamos.

AC generators [alternators] on the other hand produce an alternating current in their stationary armature by means of the rotating field in their centre; the absence of brush gear in the main power leads allows for such machines to be of far greater capacity, since the slip rings do not need to transfer the entire output of the machine but only the amount of electrical power needed to excite the field windings.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

AC generators may be either. At one time it was common for small alternators to be built with stationery fields. The alternating current was passed via slip rings.
And there is the case of the brushless exciter, common on the vast majority of alternators from about 10 kW to over 2000 MW.
There is a stationery DC field coil. This exites AC into the exciter rotor. This AC is rectified to DC by the rectifiers on the diode plate. The resulting DC is fed to the main rotating field.
Probably the most common application for developing DC is the alternator on millions of automobiles on the road today.
The rotating field is either powered by slip rings or by a clever induction scheme by a stationery field coil concentric with the shaft. The slight decrease in efficiency due to the introduction of two additional air gaps is compensated for by the reduced maintanance and greater reliability in damp and/or corrosive atmospheres.(The rotating field with a stationery coil is used in marine and large truck applications to avoid issues with sticking brushes.)
The brushless automotive alternator is unique in that a stationery coil induces a rotating magnetic field.
The stationery windings develop AC which is rectified to DC by the diodes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

Actually both types can be either way. The issue with DC machines is that the communitator converts the DC to a reversing DC for half of the rotation. (inverting a DC signel for a 50% duty is like having AC).

Also there is a big issue is with the use of brushes, because they wear, drag, and fail. So if the designer can extend the life of the brushes, then the motor lasts longer.