Option 1: N ∝ Ï•
Option 2: N ∝ Ia
Option 3: N ∝ Eb/Ï•
Option 4: N ∝ Ï•/Eb
Correct Option: 3
Explanation
Speed of DC motor is directly proportional to back EMF and inversely proportional to flux. From Eb = V − IaRa and N ∝ Eb/Ï•, increasing Eb increases speed and increasing flux decreases speed.