If a technician measures the voltage across the open switch, a voltage of 230 V would also be read on the voltmeter. This happens because Line 1 ends at the left side of the open switch and Line 2 simply bleeds through the run winding of the motor, through the closed overload, and ends at the right side of the switch.

Since the motor is not running or consuming power, the windings are nothing but conductive wire for Line 2 to bleed through. If a technician were to measure the voltage across the run winding (between R and C) of the PSC motor, the voltage would be 0 V because the motor winding is simply passing Line 2 when it is not running. Line 2 would be at both the R and C terminals of the motor, and the potential difference or voltage difference between Line 2 and Line 2 is 0 V. However, if the technician references either the R or C terminal to ground, the voltage would be 115 V.

In Figure 2, the switch is closed and the motor is running. The motor is now consuming power and will drop the entire line voltage of 230 V across its run winding (terminals R and C) while it is running.

If the technician measures voltage across terminals R and C of the motor, he will measure 230 V. A voltage measurement across the closed switch will read 0 V. This is because Line 1 ends at the C terminal of the motor when it is running. The switch experiences Line 1 at both its terminals. The potential difference or voltage difference between Line 1 and Line 1 is 0 V. If a technician measures from one terminal of the switch to ground, the voltage reading will be 115 V.

In the above examples with switches and motors, the voltmeter across the open switch reads 230 V and the closed switch reads 0 V. If one concludes that open switches always read voltage and closed switches always read 0 voltage, the conclusion would be wrong. The next example, with parallel switches, will clarify this concept.

PARALLEL ELECTRICAL SWITCHES

A voltmeter across terminals A and B of the top switch will read 0 V because it is measuring a potential difference between Line l and Line 1. Since the motor is running, it is dropping 230 V across the run winding (C and R terminals) of the PSC motor.

However, a voltmeter across terminals C and D of the bottom switch will also read 0 V. This happens because Line 1 actually extends to the common terminal of the motor when it is running. This would make terminals C and D of the bottom switch both Line 1, and the voltage difference between Line 1 and Line 1 is 0 V. Actually points A, B, C, and D are all Line 1. Figure 3 is a scenario where both an open switch and a closed switch read 0 V.

When performing electrical troubleshooting, technicians need to determine where Line 1 and Line 2 are, not whether the switch is open or closed. Measuring across the same line will always give 0 V. Measuring across Line 1 to Line 2 will always give the total circuit voltage, which in these examples is 230 V.

Tomczyk is a professor of hvacr at Ferris State University, Big Rapids, MI.

Publication date: 03/01/2001