Many people read about no load and blocked rotor test. It is easy to know what happens in these test by there names. But the question is that what is the concept behind these test.
NO LOAD TEST
By the name no load test it means that there is no load- that is load is zero. But it is exact opposite. No load means infinite load test. It is because in no load there is NO load , and no load means it is open circuit . Open circuit means inifinte resistance.
But that was the case in transformers, where you can open circuit the low voltage of transfomer and obtain readings. But how will you do that in Induction motor? How can you make infinite resistance at load side?
That is fairly easy. According to equation
If slip=0, then Rl' will be infinite. And you can make slip zero by making synchronuous speed Ns equal to actual speed N,
So slip will be zero.Load resistance will be infinite.
BLOCKED ROTOR TEST
In blocked rotor test, the load resistance is made zero. How can you make load resistance zero?
Again according to equation
If slip is made 1, then Rl' would be zero. In order to make slip 1, the actual speed must be 0.
In order to make actual speed zero, you have to stop rotor from rotating.This is called blocked rotor test as rotor is blocked from rotating.
I have added a small video to explain the concept better.