Professional
Certificate in
Total
Productive
Maintenance
IEM 114
Instructional
Materials
Can you ace this AC voltage reading for an “auto-stop control” circuit?
If you’re troubleshooting a motor starter, you
should have a fairly accurate estimate of the
voltages between certain points of the circuit.
With this in mind, let’s see if you can
anticipate “on paper” what the voltage
readings should be in this schematic diagram
of an “auto-stop control” circuit. Assume you
are using an AC voltmeter to make the
readings, and the points A, B, C, D, and E are
the reference points for the voltage readings.
Note: The control transformer
secondary voltage is
120VAC.
For each set of conditions, fill in the correct
voltage readings in the tables below. Each
table, 1 through 5, has a different set of
conditions. The readings will be either 480V,
120V, or 0V. Have fun, and don’t cheat!
3.
Power (480V) is on;
disconnect is closed; TR coil is off;
Conditions
1.
Power (480V) is on; disconnect is
open; and OL1 contact is closed.
L
1
L
3
T
1
T
3
H
1
H
4
X
2
X
1
X
2
A
X
2
B
X
2
C
and OL1 contact is closed.
X
2
D
X
2
E
C
D
T
1
T
3
H
1
H
4
X
2
X
1
T1T3
H1H4
X2X1
X
A
X2X1
X
A
4.
Power (480V) is on; and the
motor is running at full speed.
L1L3
2.
Power (480V) is on; disconnect is
closed; OL1 contact is opened, and no buttons
are pressed.
L
1
L
3
L1L3
X
2
A
X
2
B
X
2
C
X
2
D
X
2
E
C
D
T1T3
H1H4
5.
Power (480V) is on; disconnect is
closed; OL1 contact is opened; and you are
holding the start button closed.
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L1L3
T1T3
H1H4
X2X1
X2A
X2B
X2C
X2D