This system is very suitable for Fail-safe systems. You will mostly use it, if you use your actuator as a regulating device with high-speed shut or emergency shut. This is necessary, if you don't want to destroy the valve seat.

pros:

• a very safe system in opposition to frequency converted systems.
• there are two different rotations-speeds.
• if one of the two engine-coilings fail, the engine will work at all.
  

contras:

• This kind of engine is very expensive
• With the frequency-converted engine you got a speed range
• You got a very lager torque-wake.