Guys have you ever noticed that these kinds of strips are only built over frames of the induction motors. However, if we see a DC motor of the same rating, we didn't find these strips.
Let me tell you the reason. the first thing is that the stator of "induction motor". gets more heated as compared to the stator of "DC motor".
As we know we give "AC current" to the stator winding of "induction motor". whether it is a "single phase" or "three Phase". And ac current makes "Changing Magnetic Field"
Any metal comes in the vicinity of this "Changing Magnetic Field. Like, the "winding" of the rotor, an EMF get induced in that. and due to the closed circuit, AC current starts flowing in that.
This also applies to the "stator core" and the "frame". And, cyclones of electric current get produced in both. this cause excessive loss of energy in form of heat and this current is known as "Eddy Current".
To reduce this current as much as possible, we make the "stator core" from thin layers so that the "Effective Resistance" of the core is increased. But it is not possible to make the frame in layers, it should be strong. "Eddy Current" does not contribute to the function of the motor. Only, It does energy loss in the form of heat.
That's why to dissipate this heat, the strips are placed on the frame of induction motor, this increases the surface area of the motor. and the temperature of the motor remains in the limit. so that insulation of motor doesn't get melt and not cause "short circuit" between turns of the "winding".
Remember that in the "stator winding" of DC motors we give Direct Current, which produces stationary magnetic poles. until the magnetic field associated with the "stator Core" and the "frame" does not changes, no EMF can be induced in them. So, there will be no such "Eddy Currents". .And, no such production of heat in the stator Core and the "frame" of DC motor.
That's why the "Pole shoes" of DC motors are made solid, they do not need to "laminate" means no need to make them in layers, and there is no need to do this type of special cooling arrangement in its frame.
But in its rotor Eddy current will produce. Even if the magnetic field of the stator is stationary but the rotor rotates then the rotor will feel like a magnetic field change. Therefore it is necessary to laminate the rotor of the DC motor.
in the winding where we give AC supply or generate, the core must be laminated. Such as in a transformer, in Choke of Tubelight. The stator core of "alternator" is often laminated as AC voltage is produced in its stator winding. whereas the rotor consists of the solid core because it is energized by a DC supply to produce fixed poles.
Hope, now you have understood why these stripes are built only over the frames of induction motors.