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MT Series SSC Electronic Valve Actuator, 24 Vac Floating (3-Position) Control
Service Kits
Operation
Siemens Building Technologies, Inc.
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 spring 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.
Document Number 155-314P25
411555648 (031215F): Kit contains:
- Terminal cover
- Cover screws-2 (not shown)
- Tailpiece
- Tailpiece screw-1 (not shown)
- Cable lock
- Terminal block-3
411656088 (031215F or later): Kit
contains:
- Terminal Cover
- Cable lock
- Cover screw- 1 (not shown)
TIME TO
NORMAL
CHARGE
ACTUATOR
OPERATION
Max
0
t 0
t c
TIME
MAX
180s.
Figure 1. SSC81.5U Electronic Spring
Return Mechanism.
Technical Instructions
August 1, 2007
TIME TO
DISCHARGE
t n
t d
MAX
30s.
Page 3
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