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Electric Traction: Behn Eschenburg Scheme of Regenerative Braking
The method of electric braking of traction motor in which the motor remains connected to the supply line and return the braking energy to the supply system is known as regenerative braking.
Figure-1 shows the connection diagram of Behn Eschenburg scheme of regenerative braking. In this scheme, an auxiliary transformer is used to excite the field winding of the traction motor. The armature of the traction motor is connected to the main transformer through a tap changer.
A choke coil or iron cored reactor (Reactor-I) is inserted in between the armature of the traction motor and the tap changer as shown in the connection diagram. A commutating pole winding (denoted by C) in series with a resistor R is shunted with another iron-cored reactor (reactor-II) in order to obtain the correct phase (leading) of the commutating flux.
Here, the current flowing in the field winding (𝐼𝑓) will lag behind the voltage (V) at the tap changer approximately by 90°, and the EMF (E) generated in the armature of the motor will be in phase with the exciting current (𝐼𝑓). The voltage acting across the Reactor-I will be the phasor difference of voltage (V) and the EMF (E). The armature current (𝐼𝑎) lags behind the voltage across the Reactor-I by roughly 90°. The phasor diagram of the Behn Eschenburg scheme of regenerative braking is shown in Figure-2.
From the phasor diagram it is clear that for a given voltage across the tap changer and the losses are neglected, then
The braking torque produced is
$$\mathrm{Braking\: torque \: \propto \mathit{I_{a}\, }cos\, \psi}$$
And the power returned to the supply system is
$$\mathrm{Power\: returned\: \propto \mathit{I_{a}\, }cos\, \alpha }$$
Hence, for a constant excitation, from the phasor diagram, we can conclude that −
The braking toque is approximately constant at all speeds.
The apparent power (volt-amperes) absorbed by Reactor-I are greater than those generated by the armature of the traction motor.
The power factor of the generated power is about 0.7 which is considered as low power factor. However, this low power factor is not considered a serious drawback as the Behn Eschenburg arrangement provides simplicity of operation, reliability, stability and free from self-excitation.
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