Trane RTWA 70-125 TON - Manual (Page 35)

RLC-SVD03A-EN 35
Compressor Capacity
Checkout Procedure for MCSP Load/Unload Outputs
ƽ WARNING
Live Electrical Components!
During installation, testing, servicing and troubleshooting of this
product, it may be necessary to work with live electrical
components. Have a qualified licensed electrician or other
individual who has been properly trained in handling live
electrical components perform these tasks. Failure to follow all
electrical safety precautions when exposed to live electrical
components could result in death or serious injury.
The MCSP controls the Load and Unload Solenoid valves on the respective
compressor with 115 V triacs (solid state relays). Unlike mechanical relays
however, a triac has a rather high leakage current when off, comparatively
speaking. While this leakage is not nearly enough to actuate a solenoid valve,
it may, under no load conditions (as would be experienced when a solenoid
coil failed open), look like it was stuck “on” when using a voltmeter to test it.
Thus it is important to verify that the solenoid coil is continuous and providing
a normal load or to connect a known good load, such as a low wattage 115
Volt lamp, to the terminals when testing the outputs. Refer to the Chiller
Control Wiring diagrams and Component Location Drawings for the following
procedure.
With the particular compressor running, the triacs may be checked (under
load as explained above) by measuring the voltage from terminals E7 or E8 to
115 neutral. The triacs operate in the high side and switch 115 Vac power from
J7-1 to either E7 (load solenoid) or E8 (unload solenoid) to move the slide
valve in the appropriate direction. Except during compressor starts and stops,
in normal operation, the solenoid valves can only be energized for a period of
between 40 and 400 milliseconds once every 10 seconds. Often, if the chilled
water setpoint is being met under steady state conditions, they may not
energize at all. To assure loading and unloading is occurring it may be
necessary to make slight adjustments to the chilled water setpoints to force
action. As the pulsed on-time is potentially short it may be difficult to see,
especially if using a meter movement type voltmeter. (Use of a low wattage
115 Vac test lamp may be of some help for a visual indication of output triac
operation.)
When a triac is off, about 0 Vac should be measured on its terminal with the
solenoid load connected. When it is on, the voltage should be close to 115
Vac (the drop across the triac is about 1-2 volts).
The best time to check the unload solenoid is immediately after a power-up
reset of the MCSP. For the first 30 seconds after applying power the unload
solenoid should be on continuously. The next best time to check it is after the
compressor starts. For the first 30 seconds after a start the unload solenoid
should be on continuously.
Checking the load solenoid is more difficult. 30 seconds after a start, the
compressor will usually start loading, until water temperatures are satisfied.
Remember however, that under certain limit conditions, the MCSP may