Many times service technicians will encounter switches in series or parallel with electrical loads. Keeping the electrical power on and using a voltmeter to voltage troubleshoot is the fastest and most reliable method. However, there will be times when a technician must switch to an ohmmeter and shut off the electrical power in order to get to the root of the problem.
Figure 1 shows us that a voltmeter across the R and C terminals of the PSC motor would read 230 V, because of it measuring Line 1 and Line 2. However, what would be the voltage between R and C if the run winding between R and C opened, causing the motor to stall and draw locked rotor amps (LRA)? This is, of course, before the overload has opened. Figure 2 illustrates this scenario. Notice that a voltmeter placed across the R and C terminals of the motor (the opened winding) will again read 230 V. Figure 1 and Figure 2 both illustrate that whether the motor is running properly or if it has an opened run winding, the voltage will still read 230 volts across R and C. So, how does the service technician determine if the run winding is opened or not?
The answer is with an ohmmeter.
Figure 4 shows a feedback circuit from the ohmmeter’s internal voltage source if a wire was not disconnected from the motor terminals.
In this case, the ohmmeter reading would be 0 ohms. This would fool the technician into thinking the winding was still good.
Figure 5 is 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 simply shows 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 is in relation to Line 2 for ease of understanding the measured voltages.
Notice that anytime the voltmeter probes see both Line 1 and Line 2, 230 V will be read on the voltmeter. Anytime 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 a voltage difference between these measured points. So, if the service technician can determine where L1 and L2 are when voltage troubleshooting, the rest comes easy!
Tomczyk is a professor of hvacr at Ferris State University, Big Rapids, MI.
Publication date: 04/02/2001