Data Sheet for Product

MT Series SSC Electronic Valve Actuator, 24 Vac Floating Control Technical Instructions
Document Number 155-314P25
May 8, 2019
Siemens Industry, Inc. Page 3
Operation
A 24 Vac control signal to terminal Y1 extends the actuator shaft proportionately to the
length of time the signal is applied.
A 24 Vac control signal to terminal Y2 retracts the actuator shaft proportionately to the
length of time the signal is applied.
In the event of a power failure with no control voltage, the non-spring return SSC81U
will hold its last position.
In the event of a power failure, the SSC81.5U returns to the stem up 0 position. The
SSC81.5U includes an electronic return mechanism that functions as follows. See
Figure 1.
• At power-up (t
0
), a capacitor must
charge to its maximum capacity
(Max, t
c
). This will take a maximum
of 180 sec, during which time no
actuator movement occurs.
• Once the capacitor is fully charged
(t
c
), normal actuator operation
occurs.
• If a subsequent power failure occurs
(tn) of greater than 5 seconds, the
capacitor discharges (t
d
) and the
actuator spring returns to stem up 0
position.
Figure 1. SSC81.5U Electronic Spring
Return Mechanism.
CAUTION:
A valve must be connected to the actuator before applying power.
If applying power to the actuator when a valve is not connected, the
actuator will respond to a control signal and the shaft will extend until it
reaches its maximum end stop. Thereafter, it will not respond to any
signal. If this occurs:
1. Disconnect power.
2. Turn the manual position indicator (See Figure 9 and Figure 10)
on the top of the actuator to the 0 position and verify the actuator
shaft completely retracts.
3. Connect a valve to the actuator, reapply power. The actuator will
return to normal operation.