Generally, three characteristic curves are considered important for direct current motors which are: (i) Torque Vs. armature current, (ii) Velocity vs. armature current; and (iii) Velocity vs. pair. This is explained below for each type of Dc motor. These characteristics are determined taking into account the following two relationships.
Ta α ɸ.Ia and N α Eb / ɸ
These equations can be studied in - emf and the pair equation. For a DC motor, the magnitude of the return emf is given by the same emf equation of a DC generator, ie Eb = PɸNZ / 60A. For a machine, P, Z and A are constants, therefore, N α Eb / ɸ
Characteristics of DC series motors
Vs par. Armature Current (Ta-Ia) This characteristic is also known as electrical characteristic. We know that the torque is directly proportional to the product of the armature current and field flux, Taα ɸ.Ia. In DC series motors, the field winding is connected in series with the armature, ie Ia = If. Therefore, before the magnetic field saturation, the flux ɸ is directly proportional to Ia. Therefore, before the magnetic saturation Ta α Ia2. Therefore, the Ta-Ia curve is a parabola for smaller values of Ia.
After the magnetic saturation of the field poles, the flux ɸ is independent of the armature current Ia. Therefore, the torque varies proportionally to Ia only, Tα Ia. Therefore, after the magnetic saturation, the Ta-Ia curve becomes a straight line.
The axis pair (Tsh) is less than the armature pair (Ta) due to extravagant losses. Therefore, the Tsh vs Ia curve is slightly lower.
In DC series motors, (prior to magnetic saturation) the torque increases as the square of the armature current, these motors are used where high starting torque is required.