Synchronous Motor as Traction Motor

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

Three phase synchronous motor cannot start at its rated frequency or also cannot work at speed other than synchronous speed. Three phase stator winding develops rotating field (N-S pole) rotating at synchronous speed equal to 120f/P. Rotor generates fixed field (N-S pole) in the air gap. Let at any instant, N-pole of stater aligns with S-pole of rotor and force of attraction will try to carry the rotor along with rotating synchronous field. In view of the inertia of the rotor, it takes time for it start moving  but in the mean time S-pole of rotor now encounters S-pole of stater developing force of repulsion instead of attraction reversing the direction of rotation. And finally rotor does not rotate and only experiencing force of attraction and repulsion at double the frequency of stater applied voltage.   Starting of Synchronous Motor With the development of variable frequency supply, it is possible to start with frequency of half a cycle making the rotor to start before pole is changed. The rotor now remains in electro-magnetically locked condition and follows the rotating field at synchronous speed.

• δ is called the torque angle representation the phase angle between the applied voltage and back emf. This represents the angle by which the rotor field lags stator field in the air gap even though mechanically rotor is rotating at synchronous speed.
• Torque is maximum when δ=90⁰ and power  is given by

P_in = 3*E_f V_T Sinδ/Xs; T=3*E_f V_T Sinδ/(Xs*ω_s)

Putting the values as E_f=kf_s∅_f; ;X_s ∝ L_sf_s; ω_s=2πf_s we get

T∝V_T/f_s

By maintaining  the ratio of V_T/f_s constant up to rated values of V_T and f_s, output torque is maintained constant.

Speed of Synchronous Motor

Speed of synchronous motor remains constant at synchronous speed except during dynamic conditions of load change.

Equivalent Circuit of Synchronous Motor 

Simplified Equivalent circuit of Synchronous Motor

The variables for controlling the motors are applied Voltage and frequency. Stator inductance and induced e.m.f will have a status of variable effect and the equivalent circuit and phasor is drawn as follows:

 When E_f, V_T and frequency is kept constant, and load is varied, this results in change of load angle δ , Load current and power factor angle. Locus of E_f is a circle as shown in the diagram above. Maximum value of δ is 90⁰ thereafter, rotor pulls out of synchronism. On similar lines, if Ef is varied and load is kept constant, the load current varies on the locucs of red line as shown below. Locus of current with variation E_f

 Constant Load V_T

Locus of Ef is shown above when load is kept constant. It may be seen that  Power factor changes from lagging to leading and at one location it is unity.

Application of Synchronous Motor as Traction Motor

It is only by SNCF that synchronous motor has been put into Traction Motor application for the very advantage of constant speed and speed signal for control application can be derived from the input frequency. The only problem had been complex control system and if for whatever reason the motor goes out of synchronism, it has to re-synchronosied again. Emf is present even with zero stator current when machine is rotating, and therefore, could be controlled by naturally commuted converter. Synchronous motor is thus a self commutated machine. Synchronous Traction motor has a salient pole structure and provided with damper cage. It functions like synchronous, Induction as well as reluctance motor as per the situational demand.

Variable available are stator voltage, current, frequency and rotor excitation. For control, rotor position feedback is necessary to sense the load angle, air gap flux and power factor. The torque speed characteristics of synchronous motors is having two speed control regions namely constant torque and constant power operation with no reduced power zone which was available in case of Induction traction motor. At each speed, torque is variable between zero to pull out value. Motoring Maximum Torque Pull Out torque limit