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It is of utmost importance for service technicians to
understand voltage troubleshooting when servicing HVAC and refrigeration
equipment. The majority of service problems are electrical problems, which
usually cause mechanical problems. This article will illustrate how to voltage
troubleshoot using a voltmeter.
The figure here shows a 230-V, single-phase, electrical
schematic of a typical commercial refrigeration system. The diagram includes a
timer assembly with a defrost termination solenoid (DTS), evaporator fans,
defrost heaters, temperature-activated defrost termination/fan delay (DTFD)
switch, low-pressure control (LPC), high-pressure control (HPC), compressor
contactor assembly, and a compressor/potential relay assembly. The system is
drawn in the refrigeration mode.
This exercise will simply show what voltages would be
measured across certain points of the schematic if a voltmeter were used in
troubleshooting. The diagram will also show where Line #1 (L1) is in relation
to Line #2 (L2) for ease of understanding the measured voltages.
Notice that any time the voltmeter probes see both L1 and
L2, 230 V will be read on the voltmeter. Any time the voltmeter probes see the
same line (L1 to L1 or L2 to L2), 0 V will be read on the voltmeter because
there is no voltage difference between the same lines. So, if the service
technician can determine where L1 and L2 are in the circuit when voltage
troubleshooting, the technician can determine what the voltage should be when
using a voltmeter for troubleshooting a circuit.
Notice that sometimes a closed switch and an opened switch
will read the same voltage. This is illustrated here when measuring the
voltages across the DTFD switches. Again, the service technician has to ask
himself where L1 is relative to L2. The technician must also notice that
power-consuming devices may read 0 V with a voltmeter if they are not energized
and only one line is measured with the voltmeter. This is illustrated across
the defrost heater where L2 is measured relative to L2 (itself). The defrost
heater is not energized in this case, and is simply a conductor of electricity,
not a power consuming device.