What are the possible reasons this current transformer doesn't work?
Since you have created a current transformer, you need to measure current, not voltage with your meter.  If the current drawn by the fan is (for example) 3A, you would ideally measure about 3A ÷ 30 turns = 0.1A on your meter.
If you don't have an ammeter, you will need to measure voltage across a very small shunt resistance.  Your homemade current transformer cannot likely handle a large resistance on the secondary.  (In US English, this is called &quot;burden&quot;.)