Motorised 3 way mid position valve


Hi,
Can anyone tell me how the actuator part of a mid position 3 way motrised valve work?
Is it by adjusting the motor torque against a spring till it comes to rest in a particular position?
I am asking this primarily because I am trying to understand why my newly replaced valve actuator hums away at 50 Hz (quite loudly). The valve itself is new also and it is a Danfoss Randall HSV3 and HSA3 combination.
I really would ike to understand the working of the valve actuator. Please point me to any resources on the web - if any. That wil be aan immense help too. I have searched without too much success.
Thanks Ramendra
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Google ***3 way valve*** These type of valves use a torque motor that nulls (balances) based on control pressure(s) and (in control systems) some form of feedback. This site might help: http://www.omega.com/techref/techprinc.html MLD
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