Two PM motors in series will behave somewhat strangely. if you load one
then the loaded one will slow down and give very little torque and the
other will try to *accelerate*. If there is no damping then they will try
this at startup, i.e. one will accelerate quickly and the other will
follow slower. This is a useless configuration unless the two motors drive
two independent pumps, in which case they can be used to maintain a
certain ratio of the pumped substances when the density of one of them
changes (and slows down its pump). The concept can be extended to more
than two pumps. The system is prone to slow oscillations. There are other
examples of use. F.ex. 2 fans venting a case. If one stalls the other will
try to double rpm and take up the slack in the necessary air movement.

Peter

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