What is the slipring used for in ac motors?

Answer 1

synchronous motors have a slipring to apply the dc field....

Addition

...to apply the excitation field to the armature/rotor that establishes the reaction with the rotating field in the stator, causing the rotor to turn the output shaft.

Modern motors use a rotating transformer with stationary windings surrounding rotating windings mounted on the rotor shaft, thus eliminating the sliding contacts between the brushes and the sliprings. This improves efficiency and reliability.

------- efficiency and reliability ??? ---------

doing the transformer thing eliminates the need to check and change the brushes.

you will still need to watch the bearings there are usually a couple brushes on the slip rings for redundancy (reliability?)

There is no way possible the transformer arrangement will improve efficiency

the best tightly coupled Transformers can be about 98% efficient

since this transformer must have a rather large gap so it may rotate it cannot be as closely coupled and will probably be more like 89% efficient.

so at best the field with rotating transformer will be 10% less efficient than the slip rings.

For a large plant the air compressors >300hp are always synchronous motors

by varying the field you can change the motor load from leading to lagging and as such save the VAR charges the power company generally tags on , easy to do with slip rings maybe not accessible with the package non slip ring motors.

this of course requires monitoring the power at the service entrance and access to the exciter(s) on the large motor(s)

alternators on cars have slip rings and many have a 2 wire external connection, which means the voltage is regulated on these 2 terminals, loose terminals longer wires perhaps a bad engine mount may mean the alternator will not charge the battery. without a kelvin connection there is no way the alternator can know what voltage the battery has. without a temperature sensor on the battery theres no way for the voltage to be related to the charge state. (less wire is cheaper in the short run but always less efficient)