DC Motor Characteristics

Motor Characteristics

Torque and armature current

• \(T_a/I_a \Rightarrow\) Electrical Char.

Speed and armature current

• \(N/I_a\)

Speed and Torque

• \(N/T_a \Rightarrow\) Mechanical Characteristics

Remember:

\[\begin{array}{ccc} T_{a}\propto\Phi I_{a} & \mbox{and} & N\propto\dfrac{E_{b}}{\Phi}\end{array}\]

Characteristics of Series Motor

\[T_{a}\propto\Phi I_{a}\propto I_{a}^{2}~\left(\because\Phi\propto I_{a}\right)\]

• Before magnetic saturation \(T_a \propto I_a^2 \Rightarrow\) Parabola

• After magnetic saturation \(T_a \propto I_a \Rightarrow\) Straight Line

• \(T_{sh} < T_a\) due to Stray losses

• Motors are used where high \(T_{st}\) required

• Small load \(\Rightarrow I_a \downarrow \Rightarrow E_b\) is small and neglected
  \(N\propto E_{b}/\Phi\propto1/\Phi\propto1/I_{a}\)

• \(I_a \downarrow \downarrow \Rightarrow N \uparrow \uparrow \Rightarrow\) don’t start no mechanical load\(^\ast\)

• \(N \uparrow T_a \downarrow\)

Characteristics of Shunt Motor

• \(\Phi =\) constant. At heavy load \(\Phi \downarrow\) slightly due to armature reaction

• \(T_a \propto I_a \Rightarrow\) Straight line

• Since, heavy starting load needs \(I_{st} \uparrow\uparrow\), shunt motors never be started on heavy load

• \(N\propto E_b \Rightarrow N =\) constant as \(E_b\) is constant

• With load, \(E_b \downarrow\) slightly more than \(\Phi \Rightarrow N \downarrow\) slightly

• Therefore, shunt motors are constant speed motors

Characteristics of Compound Motor

Cumulative compound motor

• develops large \(T\) at low \(N\) just like series motor.

• No disadvantages of series motor at light or no load.

• \(\Phi_{se}\) helps \(\Phi_{sh}\) to \(\uparrow\) \(\Phi_T\). Hence runs at reasonable speed.

Differential compound motor

• As two fluxes opposes, resultant flux \(\downarrow\) as load increases, machine runs at a higher speed with increase in the load.

• Try to run with dangerously high speed at full load.

• Generally not used in practice.