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Synchronous Motor as Traction Motor

By on May 12, 2013
Synchronous machines are used as Generator or Condenser motor and never as traction motor before advent of variable frequency drives. The motor consists of stater equipped with three phase stater winding and rotor with DC field excitation. DC field may be with salient or non-salient poles or with permanent magnet.

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

Pin = 3*Ef VT Sinδ/Xs; T=3*Ef VT Sinδ/(Xs*ωs)

Putting the values as Ef=kfsf; ;Xs ∝ Lsfs; ωs=2πfs we get


By maintaining  the ratio of VT/fs constant up to rated values of VT and fs, 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 Ef, VT 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 Ef 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 Ef

 Constant Load VT


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


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