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AC motor control circuits AC motor control circuits
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1 Questions
Question 1
An alternative to the conventional schematic diagram in AC power control systems is the ladder diagram.
In this convention, the hot and neutral power conductors are drawn as vertical lines near the edges of
the page, with all loads and switch contacts drawn between those lines like rungs on a ladder:
"Hot"
To 480
VAC
120 VAC
"Neutral"
Toggle switch
Indicator light
Limit switch
Solenoid coil
Temperature switch
Fuse
As you can see, the symbolism in ladder diagrams is not always the same as in electrical schematic
diagrams. While some symbols are identical (the toggle switch, for instance), other symbols are not (the
solenoid coil, for instance).
Re-draw this ladder diagram as a schematic diagram, translating all the symbols into those correct for
schematic diagrams.
le 00832
2 Question 2
Perhaps the most challenging aspect of interpreting ladder diagrams, for people more familiar with
electronic schematic diagrams, is how electromechanical relays are represented. Compare these two equivalent
diagrams:
First, the ladder diagram:
To 480
VAC
120 VAC
Toggle switch
M1
M1
Motor
L1
L2
Next, the schematic diagram:
To 480
VAC
"Hot"
"Neutral"
Mtr
Relay (M1)
Based on your observations of these two diagrams, explain how electromechanical relays are represented
dierently between ladder and schematic diagrams.
le 00833
3 Question 3
Draw the necessary wire connections to build the circuit shown in this ladder diagram:
Ladder diagram:
Toggle switch
M1
M1
Motor
L1
L2
Illustration showing components:
Relay
Motor
Toggle switch
(SPDT)
L1
L2
(Dashed lines represent connections
between relay terminals and socket
screw lugs, hidden from sight)
Yes, the Run switch shown in the diagram is a SPST, but the switch shown in the illustration is a
SPDT. This is a realistic scenario, where the only type of switch you have available is a SPDT, but the
4 wiring diagram calls for something dierent. It is your job to improvise a solution!
le 00836
Question 4
Interpret this AC motor control circuit diagram, explaining the meaning of each symbol:
L
1
L
2
M1
Run
motor
To 3-phase
power source
M1
Also, explain the operation of this motor control circuit. What happens when someone actuates the
Run switch? What happens when they let go of the Run switch?
le 00834
Question 5
Interpret this AC motor control circuit diagram, explaining the meaning of each symbol:
L
1
L
2
M1
Run
motor
To 3-phase
power source
M1
M1
Also, explain the operation of this motor control circuit. What happens when someone actuates the
Run switch? What happens when they let go of the Run switch?
le 00835
5 Question 6
A very common form of latch circuit is the simple start-stop relay circuit used for motor controls,
whereby a pair of momentary-contact pushbutton switches control the operation of an electric motor. In
this particular case, I show a low-voltage control circuit and a 3-phase, higher voltage motor:
L
1
L
2
M1
M1
Start
Stop
M1
motor
To 3-phase
power source
F1
F2
Explain the operation of this circuit, from the time the Start switch is actuated to the time the Stop
switch is actuated. The normally-open M1 contact shown in the low-voltage control circuit is commonly
called a seal-in contact. Explain what this contact does, and why it might be called a seal-in contact.
le 01347
6 Question 7
Draw the necessary wire connections to build the circuit shown in this ladder diagram:
Ladder diagram:
M1
M1
Motor
L1
L2
M1
Start
Illustration showing components:
7 Relay
Motor
L1
L2
(Dashed lines represent connections
between relay terminals and socket
screw lugs, hidden from sight)
le 00838
8 Question 8
Identify at least one fault that would cause the motor to turn o immediately once the Start push-
button switch was released, instead of latch in the run mode as it should:
L
1
L
2
M1
M1
Start
Stop
M1
motor
To 3-phase
power source
F1
F2
T
1
For each of your proposed faults, explain why it will cause the described problem.
le 03896
9 Question 9
Examine this three-phase motor control circuit, where fuses protect against overcurrent and a three-pole
relay (called a contactor) turns power on and o to the motor:
Fuse
Motor
Shaft
Fuse
Fuse
1
2
3
1
2
3
Contactor
1
2
3
motor
Schematic
diagram
1
2
3
1
2
3
Fuses
After years of faithful service, one day this motor refuses to start. It makes a humming sound when the
contactor is energized (relay contacts close), but it does not turn. A mechanic checks it out and determines
that the shaft is not seized, but is free to turn. The problem must be electrical in nature!
You are called to investigate. Using a clamp-on ammeter, you measure the current through each of
the lines (immediately after each fuse) as another start is once again attempted. You then record the three
current measurements:
Line
Current
1
52.7 amps
2
51.9 amps
3
0 amps
Determine at least two possible faults which could account for the motors refusal to start and the
10 three current measurements taken. Then, decide what your next measurement(s) will be to isolate the exact
location and nature of the fault.
le 03623
Question 10
There is something wrong in this motor control circuit. When the start button is pressed, the contactor
energizes but the motor itself does not run:
Fuse
Motor
Shaft
Fuse
Fuse
1
2
3
1
2
3
Contactor
1
2
3
motor
Schematic
diagram
1
2
3
1
2
3
Fuses
To
control
circuit
. . .
. . .
Identify a good place to check with your multimeter to diagnose the nature of the fault, and explain
your reasoning.
le 03828
11 Question 11
Working on a job site with an experienced technician, you are tasked with trying to determine whether
the line currents going to a three-phase electric motor are balanced. If everything is okay with the motor
and the power circuitry, of course, the three line currents should be precisely equal to each other.
The problem is, neither of you brought a clamp-on ammeter for measuring the line currents. Your
multimeters are much too small to measure the large currents in this circuit, and connecting an ammeter
in series with such a large motor could be dangerous anyway. So, the experienced technician decides to try
something dierent he uses his multimeter as an AC milli-voltmeter to measure the small voltage drop
across each fuse, using the fuses as crude shunt resistors:
Fuse
Fuse
Fuse
OFF
COM
A
V
A
V
A
mV
He obtains the following measurements:
Line
Fuse voltage drop
1
24.3 mV
2
37.9 mV
3
15.4 mV
Do these voltage drop measurements suggest imbalanced motor line currents? Why or why not?
le 03624
12 Question 12
Identify at least three independent faults that could cause this motor not to start:
Motor
motor
Fuses
Contactor
Contactor
H1
H2
H3
H4
X1
X2
Transformer
X1
X2
H1
H2
H3
H4
Switch
Switch
Transformer
To 3- , 480 volt power source
Schematic diagram
L1
L2
L3
L1
L2
L3
T1
T2
T3
T1 T2 T3
A1
A2
A1
A2
For each of the proposed faults, explain why they would prevent the motor from starting.
le 03829
13 Question 13
A special type of overcurrent protection device used commonly in motor control circuits is the overload
heater
. These devices are connected in series with the motor conductors, and heat up slightly under normal
current conditions:
L
1
L
2
M1
Run
motor
To 3-phase
power source
M1
OL
OL
Although the heater elements are connected in series with the motor lines as fuses would be, they
are not fuses! In other words, it is not the purpose of an overload heater to burn open under an overcurrent
fault condition, although it is possible for them to do so.
The key to understanding the purpose of an overload heater is found by examining the single-phase (L1
/ L2) control circuit, where a normally-closed switch contact by the same name