Characteristics of DC shunt and series motors

Two main characteristics are drawn at rated (constant) supply voltage: torque vs armature current ($T$–$I_a$) and speed vs torque ($N$–$T$ mechanical characteristic).

Basic relations: $T\propto\Phi I_a$, $\;N\propto\dfrac{E_b}{\Phi} = \dfrac{V - I_aR_a}{\Phi}$.

DC shunt motor

The field is across the supply, so $\Phi$ is practically constant.

• $T$–$I_a$: $T\propto I_a$, a straight line through the origin (it curves down slightly at heavy load because of armature reaction). Starting torque is moderate because starting current is limited.
• $N$–$T$: $N = \dfrac{V - I_aR_a}{K\Phi}$ falls only slightly (about 3–5%) from no load to full load. The speed is nearly constant.

DC series motor

The field carries the armature current, so $\Phi\propto I_a$ (before saturation).

• $T$–$I_a$: $T\propto I_a^2$ for low current (a parabola); after saturation $T\propto I_a$ (a straight line). It gives a very high starting torque.
• $N$–$T$: $N\propto\dfrac{V - I_a(R_a + R_{se})}{I_a}$, so speed falls rapidly as load torque increases. At light load the speed becomes dangerously high, so the motor must never run unloaded.

  T                         N
  |         series          |\  series
  |       _.-'              | \
  |     ,'    shunt         |  \____
  |   ,'    _.-'            |-----------__ shunt
  | ,'  _.-'                |
  +------------- Ia         +------------- T

Comparison