McQuay RAH 047C Installation And Maintenance Manual page 114

Maintenance
Recommended method to "lock off" power while the
3
disconnect is off:
Rotate the handle to the "Reset Lock" position.
a
Manually push in the lockout mechanism into the slot
b
on the faceplate.
Insert a padlock into the lockout hole located on the
c
disconnect handle.
Test rotate the handle to insure that power "lockout"
d
is provided.
Recommended normal method to "restore" power to a
4
unit that is locked out:
Unlock and remove the padlock when it is safe (doors
a
are shut, no personnel are within reach of the
condensing unit or are inside the air handler).
Shut the control panel door and ensure the interlock
b
mechanism is operable.
Rotate the handle to the "On" position.
c
Figure 129: Through-the-Door Handle Disconnect
Pressure Sensors
The MicroTech III controller uses 0 to 5" W.C. static pressure
transducers for measuring duct static pressure. As the duct
static pressure varies from 0-5" W.C., the transducer output
will vary from 4-20mA. The transducer output signal is 4-
20mA however the signal entering the VFD is converted to a
DC signal via a 500 Ohm resistor across the output signal at
the transducer.
If building static pressure control is provided, a -0.25" W.C. to
0.25" W.C. static pressure transducer is used. As the building
static pressure varies from -0.25" W.C. to 0.25" W.C., the
transducer output will vary from 4-20mA. The transducer
114
Release
Mechanism
output signal is 4-20mA however the signal entering the VFD
is converted to a DC signal via a 500 Ohm resistor across the
output signal at the transducer.
Troubleshooting Pressure Transducers
Use the following procedure to troubleshoot a suspect sensor:
If the duct static pressure always reads 0" WC on the unit
keypad/display and the VFD speed is continuously ramping to
100%, check the following:
If the unit has two duct static pressure sensors (SPS1 and
SPS2), verify that they both function properly per the
following procedure. Also check for faulty wiring connections
at the VFD analog inputs.
The controller displays and controls to the lower of the two
readings. If a sensor is defective and inputs 0 volts to the VFD,
the static pressure reading on the keypad/display reads 0 and
the controller attempts to increase the 0 value to set point by
ramping the VFD up.
If a second sensor (SPS2) is not installed or the pressure tubing
to it is not connected, make sure the 2nd DSP Sensor=
parameter in the Unit Configuration menu of the keypad/
display is set to "No" so that the controller ignores the second
static pressure analog input.
If a second sensor (SPS2) is installed, make sure the 2nd DSP
Sensor= parameter in the Unit Configuration menu of the
keypad/display is set to "Yes".
Check the 24 V (dc) power supply to the sensor, verify that
there is 24 V (dc) between the suspect transducer "+" and "-"
terminals.
Using an accurate manometer or gauge, measure the same
pressure that the suspect transducer is sensing. To do this, tap
into the transducer high and low pressure tubing or locate the
measurement device taps next to the transducer taps.
If the suspect sensor is measuring duct static pressure, verify
that the high and low pressure taps are properly installed. An
improper pressure tap installation can cause severe fluctuations
in the sensed pressure. Refer to the model-specific installation
manual for pressure tap installation guidelines.
Measure the DC voltage output from the transducer across the
sensor "S" and "-" terminals.
If the measured voltage and pressure do not match, there may
be a wiring problem, the factory 500 ohm resistor across "S"
and "-" or the transducer may be defective. Check the
transducer input circuit wiring and connections for defects. If
the measured voltage and pressure match, the VFD parameters
and/or ModBus communication between the controller and the
VFD will need to be verified.
Remove powers from the controller by opening system switch
S1. If available, swap a similar good transducer with the suspect
transducer or try installing a new transducer. Restore power by
closing S1 and verify whether the suspect transducer is
defective.
McQuay IM 987