GEM Drive field controller analysis

In this blog I am analyzing the waveform of a supposed faulty "GEM Drive Single Phase Field Control Module". The device is fine but it has an interesting way of controlling the flyback voltage of the field coil. It is a 4 SCR bridge and can supply up to 15A. Normally you would have a high power diode reverse biased across the + and - of the coil to absorb the collapsing field current. In this case the design uses the negative current of the coil to trigger the top half SCR's and short the coil, effectively acting as a diode. The waveform on the CRO shows some slight negative voltage that is required to turn on the SCR's but apart from that it is the same as having a diode.
Why would you do that? Well all I can think of is that it would mean one less high power component that could fail. The negative current control would not fail unless both of the SCR(s) failed. For the expense of a little negative spike in the waveform that's a good idea.