Introduction — The DDZ-II series electric actuator is an electromechanical unit working in harsh environments with frequent operation, and in practice a rather failure-prone device. Its typical troubles — "hunting" and "stalled rotation" — usually stem from steps neglected during commissioning: the unit is put into service incompletely commissioned.
Excessive operation is a principal cause of damage. How to make the actuator move as little as possible while still meeting control quality?
The brake is a lever-type mechanism and must be adjusted with the right tension: free enough for flexible running, decisive enough to brake in time. This prevents overshoot by inertia followed by reversal — the oscillating state that heats and wears the mechanism — and also prevents an over-tight brake that adds torque, drags the motion and burns out an overloaded motor.
Mechanical position and feedback current must be synchronised: over the full 0–100% stroke the feedback should read 0–10 mA (II) or 4–20 mA (III) with no error. Otherwise the control valve never reaches the required opening — medium flow too large or too small — destabilising the whole loop and increasing actuator cycling.
Zeroing: at the 50% point of the amplifier input range, feed the calibrator constant-current signals I1,2 and I8,7 into the input and feedback terminals with I1,2 = I8,7 = 12 mA (III) [5 mA (II)], then adjust the zero potentiometer R8 anywhere within 10%–90% of its mechanical travel to bring the error below ±(fraction of) mA. After zeroing, stabilise the amplifier: feedback I8,7 = 12 mA [5 mA (II)], input = 12.18 mA [5.18 mA (II)]; carefully adjust stability potentiometer K13 until the forward lamp just lights. Slowly decrease I until the forward lamp goes out and the reverse lamp just lights — I is then about 11.82 mA [4.82 mA (II)]; the deviation of I from conventional true value must be within 64 μA (III), or 60 μA for types II1,2.
The servo amplifier needs a suitable sensitivity setpoint: too sensitive and the actuator hunts continuously; insensitive and action lags. Adjust on site while observing until satisfied, and tune the PID parameters so the loop modulates healthily, avoiding excessive actuator cycling.
The actuator's mechanical end positions are set by hard stops. If the motor keeps working at a mechanical end position, the motor is directly at risk — it can burn out. Two causes:
First, a faulty servo amplifier (one-sided output): in automatic the valve only opens or only closes.
Second, stalling can occur even when everything seems "normal". In automatic with the controller output at 0 mA, the actuator sits at its mechanical zero and feedback should be "0" mA — but the actual "0" mA feedback may be 20 or 30 μA off. A field ammeter cannot show this error, yet it fires the servo amplifier, keeping the motor energised and unable to turn (against the mechanical stop). Operators rarely notice; in time the motor burns.
Remedies: fit a hand operator with limit function, e.g. DFD-07. Or trim the current: test whether the amplifier still has output; if so, slightly adjust the controller output lower-limit potentiometer over the same 20–30 μA range — once the amplifier has no output, the actuator stays de-energised and safe. The same applies at the full-open mechanical position: change the iron core's axial position in the differential-transformer coil so the position transmitter output is truly 0 mA.
In practice, commissioning quality directly affects loop performance and actuator life — and safety and production. Do not neglect it.
