Characteristic of DC Motor ( Shunt, Separately and Series)

 The following characteristics are plotted in case of DC Motors:

• Speed- Armature current characteristics ( N or Ꞷ  vs  Ia )
• Torque- Armature current characteristics ( T  vs  Ia )
• Speed- Torque characteristics ( N or Ꞷ  vs  T )

NOTE:- For Shunt Excited DC Motor all characteristics are same as that of Separately Excited DC Motor.

Shunt and Separately excited DC Motor

The set-up of a Separately Excited motor is shown in the figure below:

Shunt Excited DC Motor

Separately Excited DC Motor

Torque- Armature current characteristics

Developed Torque in a DC Machine depends on Armature Current as shown
below.
We assume that field excitation is kept constant and hence flux is constant.

T = K Ⲫ Ia  ∝  Ia    ,   if  Ⲫ = Constant 

   

Speed- Armature current characteristics

The performance equation of a separately excited DC Motor is given by,

Eb = (Vs - Ia Ra) = K Ⲫ N

⇒ N = (Vs - Ia Ra) /K Ⲫ

Speed- Torque characteristics

In the Speed- Current characteristics, Armature Current (Ia = T / KⲪ) can be replaced by Torque as shown below,

N = (Vs/K Ⲫ) - (Ra T/(K Ⲫ)^2)

Due to Armature Reaction, flux reduces and so drop in Ꞷm increases

Series Excited DC Motor

Series Excited DC Motor

Speed-Armature Current Characteristics

In a Series Excited DC Motor, field current is same as Armature Current and so flux in the machine is proportional to Armature Current.

So, flux  Ⲫ = K1 Ia

Eb = K Ⲫ N = Vs - Ia (Ra + Rse)

N = {Vs /KⲪ} - {Ia (Ra + Rse) /KⲪ}

N = {Vs /K K1 Ia} - {(Ra + Rse) /K K1}-------(1)

Armature and series field resistance being very low, (Ra + Rse) can be neglected and we can obtained:

N Ia = VS /K K1 --------(2)

Thus, at lower currents, the N vs Ia characteristics resembles rectangular hyperbola ( according to equation 2 ). In this region, the speed decreases abruptly with increase in input current.

with larger currents, the magnetic circuit gets saturated and flux Ⲫ tends to approach a constant value. In that case, speed and armature current can be related by using equation 1. 

This represents a slightly drooping straight line nature of N vs Ia characteristics for larger values of armature current. 

The speed becomes zero when the input current is equal to short-circuit current of the motor, i.e.

Ia = Isc = Vs /(Ra + Rse)

Torque-Armature current characteristics

In a series Excited DC Motor, flux is proportional to Armature Current as given below,

Ⲫ = K1 Ia

Torque,  T = K Ⲫ Ia = K K1 Ia^2 = K' Ia^2

Again for large armature current Ia, Ⲫ ≈ constant   so,  T ∝ Ia

Speed-Torque characteristics

Armature Current can be expressed as a function of Torque as shown below,

Ia = {T/K K1}^1/2

Substituting this value in Speed Armature Current Characteristics,

N = {Vt /(K K1 T)^1/2} - {(Ra + Rse) /K K1}

When saturation sets in , Ⲫ = constant

N ={Vt /(K Ⲫ} - {(Ra + Rse)T /K^2 Ⲫ}

Some points:-

• Ideally suited for traction load, locomotives
• Series motors are never belt loaded or chain loaded it must be directly coupled to load.
• Series Motors must never be run under No-Load conditions else speed can be dangerously high.