The 1-11 system.
On the 1-11 the CSD had a Rootes blower or air pump.
To start the engine APU air was bled in to the pump which turned and through the CSD gearing turned the g/box and consequently the N2 section of the engine.
When the Gen came on line the air bleed was cut off and the CSD started functioning as a CSD. At Idle/low power an HP Compressor bleed fed the blower which acted as a motor and added speed to the CSD input shaft through an epicyclic g/box keeping the Gen on speed.
At cruise power and above the bleed air was shut off and the blower acted as a brake by drawing ambient air through what had been the HP air bleed exhuast and compressing it before discharging the air back over-board. In this way CSD input shaft speed was removed to maintain Gen shaft speed at 8000 rpm.
In the very early days of the 1-11 however they found the blower breaking up after a very short while in service. The problem it was found, was that while while at cruise the blower was drawing in air at substantially sub-zero temperatures then at Top of Descent when the engine was throttled back the CSD input speed from the engine g/box went from an overspeed situation, for the generator, to and underspeed situation. As mentioned previously in an u/speed condition HP bleed air is used to add the the CSD input shaft speed. So in this situation the poor old blower had been nicely cold soaked for a couple of hours at -30 then all of a sudden it gets a blast of air at about 200 degs C, well the steel rotors of the blower and the aluminuim caseing expanded at different rates and graunched the whole thing up.
The fix was to have a small HP compressor bleed in the ambient air inlet that just kept the blower warm enough to cope with the sudden temp change.