DC Series Motor as Traction Motor

Last updated on April 5th, 2015 at 06:14 am

There are two basic principles which governs the working of DC machine

• A current carrying conductor placed in a magnetic field experiences a force demonstrated by Lorentz Law. Direction of the force is given by Fleming’s LH rule.

• A voltage is induced in the conductor moving in a magnetic field (Faraday’s Law). This voltage is induced so as to oppose the magnetic field.(Lenz’s Law).  Let the conductor moves with a velocity of V m/sec on account of Force experienced, then

• e=BxLxV  Derived from Faradays’s  law

• e=(–)Nd/dt   Derived from Lenz’s Law and Farday’s law

These two equations derives all equations of DC Traction Motor

Equivalent circuit of DC Series Motor

• V=E_b+I_a(R_a+R_f);  E_b=PZN/60A; or E_b I_axN or N E_b/I_a

• Speed(1/Armature current)

Speed  is inversely proportional to armature current; field current is same as that of armature unless partly diverted therefore can also be said to proportional to field

• Power=E_b*I_a=Torque*angular speed= T*2πN/60

(PφZN/60A)*I_a = T *2πN/60

or T = (PZ/2π) φ*I_a

T ∝ φI_a  or   T ∝  I_a² (Unsaturated field)

       ∝  I_a  (Saturated field)

Torque is directly proportional to the square of the armature current resulting high starting torque and directly proportional when field is saturated. Starting Armature current is high, and field is thus saturated early.

• By substituting value of I_a in torque equation we get

T   ∝ 1/N²                             : when field is unsaturated and

      ∝  1/N                               : when field is saturated

      HP ∝  Speed*√Torque      : before saturation means

              HP ∝  Speed * Torque      : after saturation

                It provides higher torque within rated HP up to saturation.

For deriving tractive effort and speed characteristics to match the load, variables are applied voltage and field flux.   This achieved by varying the Single Phase 25kV AC Voltage through on load Tap changer, rectification, smoothing and applied to Traction Motor. Voltage can be varied in 32 steps.  Field is varied by Resistance insertion in parallel to field though shunting contactor in 3 to 4 steps in WAG7 and WAP4 locos respectively.

Torque/flux/speed vs armature current characteristics

Deriving Tractive Effort Vs Speed characteristics

Limits are set on the torque speed characteristics of Series motor as T_max and S_max as per the design parameter. T_max is fixed on the basis of short term thermal rating of the motor and adhesion available where S_max depends on the mechanical design and the services the locomotive has to perform. Applied TE of tap changer locomotive is in steps due to tap changer control and peak has to be kept within adhesion limit therefore average TE applied is always less then adhesion limit.

Figure 4

Speed Torque Equation of DC series motorLimits are set on the torque speed characteristics of Series motor as T_max and S_max as per the design parameter. T_max is fixed on the basis of short term thermal rating of the motor and adhesion available where S_max depends on the mechanical design and the services the locomotive has to perform. Applied TE of tap changer locomotive is in steps due to tap changer control and peak has to be kept within adhesion limit therefore average TE applied is always less then adhesion limit.

V= E_b + I_a (R_a +R_f); E_b =k₁ΦN; T= k₂ΦI_a

Substituting value of E_b and T in voltage equation and solving for speed, we get

V= k₁ΦN + T (R_a +R_f)/ k₂Φ = k₁ΦN + k₃T/ Φ

N=[ V/ k₁Φ – k₄T/ Φ²]

When voltage and flux is kept constant, this equation is of the form,

N=A-BT where A determines the intersection on speed axis and B is slope of the curve

Working Principle of DC Motor

The field  is stationary and generated  through main poles on stator frame. The armature winding is a closed winding through the Commutator and current is supplied through the  brushes. Brushes are placed along the neutral axis of stator field.  The mmf produced by the field (F_f) along the magnetic axis while the current flowing through the armature produces an mmf (F_a) directed along the brush axis. The two mmfs are in space quadrature and occur simultaneously in the motor. They react with each other and develop a torque under whose action the armature rotates. A voltage induced in armature can be determined by the Fleming’s left-hand rule. The picture below demonstrates field and its interaction to produce torque.

mmf diagram of DC Motor