Using Voltmeters and Ohmmeters

March 28, 2001

Figure 1. An electric PSC motor in series with two switches that are parallel to one another.

Figure 2. The run winding between R and C is open, causing the motot to stall.

Our last article covered voltage troubleshooting in series with power-consuming devices (motors) [“Electrical Troubleshooting,” March 5, p. 16]. The article differentiated between an open and a closed switch, and revealed how service technicians must ask themselves where Line 1 and Line 2 are when voltage troubleshooting — not whether the switch is open or closed. This article will cover trouble-shooting the power-consuming device or load.

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 3. Disconnecting a wire will prevent electrical feedback from the ohmeter's internal voltage source through another parallel electrical circuit.

VOLTMETER

Figure 1 shows an electric PSC motor in series with two switches that are in parallel with one another. The voltage between points A and B (the open switch) in this case would be 0 V, because the voltmeter would be measuring between Line 1 and Line 1. The voltage between points C and D (the closed switch) would also be 0 V, because of the voltmeter measuring voltage between Line 1 and Line 1 again. The motor is running and dropping all of the 230 V while it is consuming power. A voltmeter across the R and C terminals of the PSC motor would read 230 V, because of the meter measuring the voltage between Line 1 and Line 2, which is 230 volts.

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. A feedback circuit from the ohmeter's internal voltage source results if a wire is not disconnected from the motor terminals.

OHMMETER

The service technician must shut the electrical power down and disconnect one wire either from the R or C terminal of the motor as shown in Figure 3.

Figure 5. A 230-V, single phase, electrical schematic of a typical commercial refrigeration system.

Disconnecting a wire will prevent electrical feedback from the ohmmeter’s internal voltage source through another parallel electrical circuit. The technician must then place an ohmmeter across the R and C terminals of the motor. The measurement will read “infinite ohms” if the winding is open. This is the only way the service technician can tell if the winding is opened or not.

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