McQuay RDS 802C User Manual

McQuay IM 987 55
Sequences of Operation
Sequences of O peration
The following sequences of operation are for a typical “C”
vintage applied rooftop unit equipped with MicroTech III, an
economizer, 3 to 1 turn down burner, variable frequency drives
(VFD), a return air fan and an external time clock. These
sequences describe the ladder wiring diagram logic in detail;
refer to "Wiring Diagrams"‚ page 59 as you read them. Note
that your unit’s sequences of operation may vary from those
described here. Refer to the wiring diagrams supplied with the
unit for exact information.
For detailed description of operation information relating to
the MicroTech III controller's software, refer to the appropriate
operation manual (see Table 1, page 3). These manuals
describe the various setpoints, parameters, operating states,
and control algorithms that affect rooftop unit operation.
Power-up
When primary power is connected to the unit, 115 V (ac)
power is fed through control circuit transformer T1 and control
circuit fuse F1C (line 166, Figure 75, page 67).
When system switch S1 (line 203, Figure 78, page 70) is
closed, low voltage transformers T2 (line 203, Figure 78,
page 70), and T9 (line 802, not shown) energize, and 115 V
(ac) power is supplied to the following:
• The supply fan VFD (line 135-137, Figure 74, page 66)
• M40A to energize the return fan VFD (line 147-149,
Figure 75, page 67)
• Heating control panel (line 603, Figure 88, page 80)
• Economizer actuator (lines 256-257, Figure 79, page 71)
Transformer T2 supplies 24 V (ac) power to the following
terminals:
• 24V and COM on the main control board MCB (lines 207
and 208)
• Switch S7 On-Auto-Off (line 216)
• Enthalpy sensor OAE (line 247)
• External time clock contacts (line 215)
• Airflow interlock switch PC7 (line 228)
• Dirty filter switches PC5 and PC6 (lines 242 and 247, not
shown)
• Gas furnace alarm relay R24 (line 225, not shown)
• Freezestat switch FS1 (line 243, hot water or steam heat
only)
• Smoke detectors SD1 and SD2 (line 220)
The time clock, S7 switch, and emergency shutdown terminals
(lines 214-222) control fan operation.
Note: Unit ships with factory installed jumpers between TB2
101 and 105 and between 101 and 106.
Fan Operation
When the main control board (MCB) commands the supply
and return fans to start, the unit enters the Startup operating
state. As a result, a 3-minute timer is set, output MCB-BO3
(line 307) energizes, and relay R26 energizes (line 306, not
shown).
After the 3-minute timer expires, the unit enters the Recirc
operating state. As a result, output MCB-BO1 energizes relay
R67 (line 401). This gives a start signal to supply fan drive
AFD10 (line 445). Four seconds after MCB-BO1 is energized,
output MCB-BO2 energizes relay R68 (line 404). This gives a
start signal to return fan drive AFD20 (line 445).
Within 120 seconds after the fans start, the controller expects
airflow switch PC7 (line 228) to close and thus energize binary
input MCB-BI6. (If MCB-BI6 does not energize, the controller
assumes the fans did not start. It then shuts down the unit and
generates an alarm.)
During the Recirc operating state, the outside air damper is
held closed. The controller does this by energizing output
MCB-BO5 (line 318). On VAV units, output MCB-BO12, the
VAV box output, is also de-energized (line 309) during the
Recirc state.
The supply fan adjustable frequency drive (AFD10) is
modulated to maintain the duct static pressure setpoint. When
energized, output MCB-BO14 (line 407) drives AFD10 toward
increased capacity; MCB-BO13 (line 405) drives it toward
decreased capacity. On VAV units or CAV units equipped with
return fan capacity control, the adjustable frequency drive
(AFD20) is modulated to maintain an acceptable building
static pressure (using either VaneTrol logic or direct
measurement of building pressure; see the appropriate OM for
more information). When energized, output MCB-BO16 (line
409) drives AFD20 toward increased capacity; MCB-BO15
(line 411) drives them toward decreased capacity.
Note: If the inverter bypass switch S4 (lines426 and 430) is in
the bypass position, MMP30 and MMP40 (line 132 and
144) protect the fans from excessive current draw. If
either the supply or return fan is drawing excessive
current, one of the MMPs triggers an auxiliary contacts
(line 426) and open the circuit, causing both fans to stop.

McQuay RDS 802C Specifications

General

Brand	McQuay
Model	RDS 802C
Category	Air Handlers
Language	English