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As SZ mentions, an undriven or weakly-driven PM generator is a problem in simple series-connection. It will behave like a constant torque source, so if the drive is uncoupled it will motor in the reverse direction at unlimited speed. Thankfully, this problem can be solved by a parallel diode; back-biased when generating but forward-biased while motoring, which would reduce the available motoring voltage to a fraction of a volt and hence avoid power loss and damage.
Parallel connection has the inverse effect, behaving as a constant speed source that will drive its prime mover in the same direction if the torque falls. This is typically not harmful but is wasteful. A series diode here would cure it, but unlike the parallel diode may incur losses of a few percent under normal operation. Preventing the dynamo motoring was the reason for the 'cutout' relay in generator control boxes used for battery charging. I could go on at length about the Lucas RB340 at ths point...
Parallel connection has the inverse effect, behaving as a constant speed source that will drive its prime mover in the same direction if the torque falls. This is typically not harmful but is wasteful. A series diode here would cure it, but unlike the parallel diode may incur losses of a few percent under normal operation. Preventing the dynamo motoring was the reason for the 'cutout' relay in generator control boxes used for battery charging. I could go on at length about the Lucas RB340 at ths point...