13
product of the volts and amperes, the power factor of the circuit under test is
the ratio of the power reading on the meter to the product of the volts and
amperes. For example, if the line voltage is 120, current 2.5 amperes, and
power indicated by the meter is 225 watts, the power factor is equal to
100 x 225 = 75%
120 x 2.5
4.10
Use Of Wattmeter With Two & Three Phase Circuits
Figure 4-4 shows the connections for measuring power in a single phase, two or
three phase, three-wire circuit using two Model 390-2 wattmeters. If a reading is
obtained on both meters, the true power is the sum of the two readings. If the
pointer of one meter deflects to the left of zero, it means that the power factor is
less than 50%, and the connection to the voltage terminals for that meter should
be reversed. The reading then obtained should be subtracted from the reading of
the other meter. If the power factor is exactly 50%, one meter will indicate zero. If
only one Model 390-2 is available, connect it successively in the two positions as
shown in Figure 4-4.
Figure 4-4. Connections for Measurement in Single,
Two and Three Phase AC Three-Wire Circuits
4.11
Troubleshooting
You will find many uses for the Model 390-2 in checking the three simple tests
which diagnose most cases of electrical trouble: line voltage, current drain and
power consumption, . High current drain and high power consumption generally
indicate shorted turns in a motor, too heavy a load or shorted sections in ele-
ments of heaters, irons, etc. This also might be indicated by an excessive drop in
line voltage when the device is connected to a power outlet. A drop in line
voltage may also indicate inadequate or overloaded supply lines. No current
would indicate an open cord, switch or circuit in the device being checked. These
15A
V
3A
A
VOLTS
NORMAL
PRESS
FOR
MODEL 390 2
300W
3000W
150V
300V
1500W
600W
±
±
VOL
TS-AMPERES
WATT
S
50
75
100
125
15
0
100
150
200
250
300
1000
500
200
100
2000
1000
400
200
30
00
15
00
60
0
30
0
0
15A
V
3A
A
VOLTS
NORMAL
PRESS
FOR
MODEL 390 2
300W
3000W
150V
300V
1500W
600W
±
±
VOL
TS-AMPERES
WATT
S
50
75
100
125
15
0
100
150
200
250
300
1000
500
200
100
2000
1000
400
200
30
00
15
00
60
0
30
0
0
3-PHASE
SOURCE
LOAD
NEUTRAL,
IF 1 OR 2 PHASE
L
L
L
1
2
3