Two full converters, one feeding the armature circuit and other feeding the field circuit of a separately excited dc motor, are shown in fig 12.9 (a).

1) This scheme offers two quadrant drive, Fig 12.9 (b) and its use is limited to about 15 kW.

2) For regenerative braking of the motor, the power must flow from motor to the ac source and this is feasible only if motor counter emf is reversed because then eaia would be negative.

3) Note that direction of current cannot be reversed as SCRs are unidirectional devices. So, for regenerative braking , the polarity of ea must be reversed which is possible by reversing the direction of motor field current by making delay angle of full converter 2 more than 90°.

4) In order that current in field can be reversed, the field winding must be energized through single phase full converter as in Fig

5) For the armature converter 1,

$V_0=V_t=\frac{2V_m}{\pi}cos\alpha_1 \ \ \ for 0 \lt \alpha_1\lt\pi$

6) For the field converter 2,

$V_f=\frac{2V_m}{\pi}cos\alpha_2 \ \ \ for 0 \lt \alpha_2\lt\pi$

7) From the waveforms in Fig 12.9 (c) , it is seen that

Rms value of source current,

$I_{sr}=\sqrt{I_a^2.\frac{\pi}{\pi}}=I_a$

8) Rms value of thyristor current

$I_{tr}=\big[I_\alpha^2 . \frac{\pi}{ 2 \pi}\big]^{1/2}=\frac{I_a}{\sqrt 2}$

9) Input supply pf =

$$pf=\frac{V_t.I_a}{V_a.I_{sr}}=\frac{2V_m}{\pi}cos\alpha_1.\frac{I_a\sqrt 2}{V_m.I_a} \\ =\frac{2 \sqrt 2}{\pi}cos \alpha_1 $$