# If Ta be the armature torque and Ia be the armature current then which of the following relation is valid for DC series motor before saturation?

If Ta be the armature torque and Ia be the armature current then which of the following relation is valid for DC series motor before saturation?

### Right Answer is:

T_{a} ∝ I_{a}^{2}

#### SOLUTION

- We know that torque is directly proportional to the product of armature current and field flux, Ta ∝ ɸ.Ia.
- In DC series motors, the field winding is connected in series with the armature, i.e. Ia= If. Therefore, before the magnetic saturation of the field, flux ɸ is directly proportional to Ia.
- Hence, before magnetic saturation Ta ∝ Ia
^{2}. **After magnetic saturation**of the field poles, flux ɸ is independent of armature current Ia. Therefore, the torque varies proportionally to Ia only, T ∝ Ia.Therefore, after magnetic saturation, Ta-Ia curve becomes a straight line.