- Torque and armature current (Ta/Ia characteristic). It is known as electrical characteristic.
- Speed and armature current (N/Ia characteristic)
- Speed and torque (N/Ta characteristic) It is also known as mechanical characteristic. It can be found from (1) and (2) above.
While discussing dc motor characteristics, the following two relations should always be kept in mind :
Ta ∝ ΦIa and
N ∝ Eb/Φ
Ta/Ia Characteristic (Electrical)
We have seen that in series motor Ta ∝ ΦIa. In a series motor, as field windings also carry the armature current, Φ ∝ Ia up to the point of magnetic saturation. Hence, before saturation,
Ta ∝ ΦIa and ∴ Ta ∝ Ia2
At light loads, Ia and hence Φ is small. But as Ia increases, Ta increases as the square of
the current. Hence, Ta/Ia curve is a parabola as shown in figure.
N/Ta Characteristic (Mechanical)