circuits, the control will most often start the A1 or B1 compres- sor first, especially after long off periods. The MBB controls fan stages to maintain the head pressure set point and will auto- matically adjust unit capacity as required to keep compressors from operating outside of the specified envelope. There are no pumpout or pumpdown sequences on these chillers.
For all units, if temperature reset is being used, the unit con- trols to a higher
SERVICE
WARNING
Electrical shock can cause personal injury and death. Shut off all power to this equipment during service. There may be more than one disconnect switch. Tag all disconnect locations to alert others not to restore power until work is completed.
Electronic Components
CONTROL COMPONENTS — Unit uses an advanced elec- tronic control system that normally does not require service. For details on controls refer to Operating Data section.
Access to the compressors is through latched panels from beneath the control box on all models or from opposite the coil side (sizes
Electronic Expansion Valve (EXV) — See Fig. 24 for a cutaway view of the EXV.
The motor directly operates the spindle, which has rotating movements that are transformed into linear motion by the transmission in the cage assembly. The valve includes a posi- tive
There are two different EXVs. See Table 30 for number of steps. The EXV motor moves at 200 steps per second. Com- manding the valve to either 0% or 100% will add extra steps to the move, to ensure the value is open or closed completely.
Table 30 — EXV Steps
UNIT SIZE 30RAP | EXV STEPS |
1596 | |
025,030 | 2500 |
1596 | |
2500 |
The EXV board controls the valve. Each circuit has a thermistor located in a well in the suction manifold before the compressor. Suction pressure as measured by the suction pres- sure transducer is converted to a saturated suction temperature. The thermistor measures the temperature of the superheated gas entering the compressor and the pressure transducer
determines the saturated temperature of suction gas. The differ- ence between the temperature of the superheated gas and the saturated suction temperature is the superheat. The EXV board controls the position of the electronic expansion valve stepper motor to maintain superheat set point.
The MBB controls the superheat leaving cooler to approxi- mately 9° F (5° C). Because EXV status is communicated to the main base board (MBB) and is controlled by the EXV boards, it is possible to track the valve position. The unit is then protected against loss of charge and a faulty valve. Just prior to compressor start, the EXV will open. At low ambient tempera- tures the EXV is closed at start up. After initialization period, valve position is tracked by the EXV board by constantly mon- itoring the amount of valve movement.
The EXV is also used to limit cooler saturated suction tem- perature to 50 F (10 C). This makes it possible for the chiller to start at higher cooler fluid temperatures without overloading the compressor. This is commonly referred to as MOP (maxi- mum operating pressure). At ambient temperatures above 110 F, MOP is bypassed at
If it appears that the EXV module is not properly control- ling circuit operation to maintain correct superheat, there are a number of checks that can be made using test functions and initialization features built into the microprocessor control. See the EXV Troubleshooting Procedure section to test EXVs.
EXV Troubleshooting Procedure — Follow steps below to diagnose and correct EXV/economizer problems.
Check EXV motor operation first. Switch the Enable/Off/ Remote Contact (EOR) switch to the Off position. Press ESCAPE on the Navigator™ display until ‘Select a menu item’ appears on the display. Use the arrow keys to select the Service Test mode. Press ENTER . The display will be:
|
|
| OFF |
> TEST | |||
| OUTS |
| |
| COMP |
| |
Press ENTER (password entry may be required) and use 
to change ‘OFF’ to ‘ON’. Switch the EOR switch to Enable. The Service Test mode is now enabled. Move the pointer down to the OUTS
the valve position will flash. Use 
to select 100% valve position (hold 
for quick movement) and press ENTER .
The technician should be able to feel the actuator moving by placing a hand on the EXV. A sight glass is located on the valve body to verify that the sleeve is moving to expose/cover slots in the orifice. A hard knocking should be felt from the actuator when it reaches the top of its stroke (can be heard if surround- ings are relatively quiet). Press ENTER again twice if necessary to confirm this. To close the valve, press ENTER , select 0% with 
and press ENTER . The actuator should knock when it reaches the bottom of its stroke. If it is believed that the valve is not working properly, continue with the checkout procedure below:
Check the EXV output signals at appropriate terminals on the EXV module (see Fig. 25). Connect positive test lead to red wire
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