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Transition Methods in Electric Traction: Open Circuit Transition, Shunt Transition, and Bridge Transition
The transition methods are defined as the methods of changing over the connections from one grouping to another. Depending upon the change over from full series to first parallel, the transition methods employed in the electric traction system are of the following three types −
Open Circuit Transition
Let's discuss each of these transition methods in detail.
Open Circuit Transition
The steps involved in the open-circuit transition are shown in Figure-1. In case of the open-circuit transition, first the series connection between the motors is disconnected and power is switched off with full load current.
Then, some of the starting resistance is reinserted in the motor circuit, the two motors are connected in parallel and the supply is restored. The open-circuit transition causes interruption of torque. Therefore, the use of the open-circuit transition is limited to very small units.
The steps involved in the shunt transition are shown in Figure-2. In case of shunt transition method, the traction motors are run up and brought up to full series position by cutting out the starting resistance gradually. Then, some resistance is reinserted and one of the motor is short circuited, then one end of this short circuited motor is opened and finally connected in such a way that the motors are connected in first parallel.
The series resistance is now gradually reduced to zero and the motors are placed in full parallel. There is a jerk in this transition system as one motor is shorted and ceases to act and then other jerk when it reinserted. The shunt transition method is employed in tramways, industrial locomotives and main line locomotives, etc.
The steps involved in the bridge transition are shown in Figure-3.
In this method, the starting resistance is split into two equal parts. The motors and the starting resistances are connected in series. Then, the starting resistances are gradually cut out and the motors come in full series. Now, in the transition step, each motor has a part of the starting resistance in parallel with it and two combinations of this kind are connected in series, then the bridge link is removed from the circuit and finally the motors are placed in first parallel. Now, the external resistance is gradually decreased to zero and therefore, the traction motors come in full parallel.
The main advantages of the bridge transition are that the traction motors always remain connected to the supply line during the transition and as the external resistances are so adjusted that the value of current remains the same, therefore, the torque remains constant. Consequently, uniform acceleration is obtained without causing inconvenience to the passengers. The bridge transition is mainly used for railway traction.
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