What is the difference between a three phase induction motor and a DC motor?

Answer 1

There are several.

DC motors have numerous different connections, but probably the most common is the shunt-wound. In this design, two heavy leads go to the rotor windings, and two light leads go to the field windings of the stator.

In an induction motor, the only "windings" are in the stator, and there are always some multiple of three connections for those windings. The rotor has a "winding" that consists of aluminum or copper bars shorted by rings at their ends resembling an exercise wheel for a rodent cage, thus the "squirrel-cage" nomenclature. This cage is assembled around iron laminations, making a solid rotor with no electrical connections to the outside world.

DC motor are inherently variable-speed machines. By varying the voltage applied to the armature and/or to the field the motor speed and torque can be controlled. AC induction motors are not inherently variable speed. Their speed is directly dependent on the frequency of the AC power applied to the stator windings, and this is normally fixed by the utility providing the power. Then again, AC motors can be connected directly to utility power with a switch, DC motors require some type of electronic controller between them and the utility, unless the motors are small enough to be run from batteries.

DC motors require much more maintenance, as the carbon brushes and armature commutators require regular cleaning and service. Build up of carbon dust can be detrimental to the operation of a DC motor. AC motors may occasionally need to have their bearings greased or oil changed.

With the advent of high-performance variable frequency drives, DC motors are no longer the first choice for variable speed applications. The higher reliability of AC induction motors and VFDs has resulted in DC falling in popularity. There's still a lot of installed DC equipment that end users cannot justify replacing, but there are very few new installations of DC systems on modern equipment.