Download Fundamental Electricity Student Study Notes

Motor Controls
Student Study Notes 2014
WARNING INSTRUCTOR OR LAB AIDE SUPERVISION IS ABSOLUTELY
REQUIRED FOR ALL LAB ACTIVITES SINCE EVEN THE LOW
VOLTAGES USED IN MOST OF THESE LABS CAN BE FATAL.
Note, if you have not completed Electrical
Troubleshooting you must complete the safety and lock
out tag out section. You should read through thestudent
study notes up until Motors & Controls (2014 revision).
The motor controls section of the SSNs starts there.
Electrical Safety
1. List at least 10 rules of electrical safety. Decide which two are the most important
and why you think these two are so important. (For Mechatronics students, make
this list neat and clear since you will be signing it as a contract to work safely in
the lab and later, on the job.)
2. How does low amperage kill? (HVAC, 2nd edition; HV p. 64)
3. What safety precautions will you take when you must work on live electrical
circuits for troubleshooting?
4. What happens when a person becomes grounded?
5. What is the function of a ground circuit? (IM Fig 4-19; HV p. 64-66)
6. What is the purpose of a ground fault circuit interrupter (CFCI) (HV p. 66) ?
7. What determines the type of electrical personal protective equipment will you
use on the job?
8. What national code regulates electrical safety?
9. What type of PPE protects against arc flash injuries?
10. Study Figure 2-21 & 2-22 p. 46 regarding safe approach distances.
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Learning Activities: Industrial Maintenance, 3rd edition (IM), ELECTRICAL SAFETY, p.
43 - 48
Lockout/Tagout
1. Why is it difficult to conduct electrical troubleshooting on de-energized electrical
circuits?
2. What is the title and purpose of the law that governs lockout and tagout?
3. What must be posted near each complex machine?
4. List the steps for applying and removing a lockout.
5. What must happen at shift change?
6. How do lockout and tag out rules apply to outside workers coming into the
facility?
7. State the rules regarding locks used for lockouts.
8. State the rules for tags used for tag outs.
9. List all the types of energy sources that must be locked out.
10. List the three groups of workers and describe their level of involvement with
logout and tag out.
11. State the rules regarding inspections of lockout and tag out procedures.
12. How are group lockouts managed?
Learning Activities: (IM), LOCKOUTS AND TAGOUTS, p. 48 - 49
Power Distribution
1. Briefly describe how electricity is distributed from the power generation plant to a
load in a commercial facility. (IM, Fig. 4-12)
2. What is the function of a service entrance switchboard?
3. What is the function of a distribution switchboard?
4. What is the function of a circuit breaker panel?
5. How is it decided what type of electrical enclosure to use?
6. Describe the construction of a plug-in busway.
7. Where do feeder circuits run?
8. Where do branch circuits run?
9. Define voltage, current, and resistance. (HV p. 340-341)
10. What is the total voltage drop allowed for a circuit?
11. Sketch and label one cycle of a 1Ø sine wave. (IM, Fig. 4-14; HV p. 345-346)
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12. What is RMS in relation to the sine wave?
13. Sketch and label one cycle of a 3Ø sine wave.(IM, Fig. 4-15)
14. Sketch and describe the various types of DC voltage. (IM, Fig. 4-16)
15. Sketch the voltage measurements between the wires of a 120 V circuit, a 208 V
1ø circuit, a 208 V 3ø circuit, and a 460 V 3ø circuit. (HV p. 365 -367)
16. Describe a residential120/220 V 1Ø service. (HV p. 365-366)
Learning Activities: IM, POWER DISTRIBUTION p. 120 -124
LAB: Test a 3 phase power supply
If a load like a 3ø motor is not energizing verify that the correct voltage is available. This
is called testing source voltage and it is the measuring of a voltage rise. A 3ø power
supply consists of three hot wires and a safety ground wire. Connecting the volt meter
probes between any two of the three wires should produce a reading of source voltage
+/- 5%
208 voltage source readings should be: between line 1 and line 2 = 208, between line 1
and line 3 = 208, and between line 2 and line 3 = 208, between line 1 and ground =
120, between line 2 and ground = 120, between line 3 and ground = 120.
480 voltage source readings should be: between line 1 and line 2 = 480, between line 1
and line 3 = 480, and between line 2 and line 3 = 480, between line 1 and ground =
277, between line 2 and ground = 277, between line 3 and ground = 277.
This test might be conducted at a fused disconnect, at a receptacle plug-in, at the motor
leads, or at a 3ø circuit breaker.
LAB: Test a high voltage 1 phase power supply
High voltage single phase motors have two hot wires and a ground wire. Some heating
element, welders, and AC units are also high voltage single phase (two phase).
Connecting the volt meter probes between any of the two hot wires should produce a
reading of source voltage +/- 5%
208 voltage source: between line 1 and line 2 = 208, between line, between line 1 and
ground = 120, between line 2 and ground = 120.
480 voltage source readings should be: between line 1 and line 2 = 480, between line 1
and line 2 = 480, between line 1 and ground = 277, between line 2 and ground = 277.
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Electric Circuits
1. What is the purpose of any electrical circuit?
2. What happens to the electrons in the wire when a voltage source is applied to
the wire? (IM, Fig. 4-1 & 4-2; HV p. 337-338)
3. Are electrons consumed like firewood or gasoline?
4. What are the characteristics of a good insulator, a good conductor, and a
semiconductor? Give an example of each type of material. (HV p. 339)
5. What must every electric circuit contain?
6. Sketch and describe an open circuit and a closed circuit. (IM, Fig. 4-36)
7. Sketch and describe how to test a set of contacts (a switch). (IM, Fig. 4-40)
8. What is the function of a voltmeter, ammeter, and ohmmeter. (HV p. 347-348)
9. Sketch how a voltmeter is connected into a circuit.
10. Why is a voltmeter a high resistance instrument?
11. Sketch and describe a voltage drop and a voltage rise. (IM, Fig. 4-19)
12. What are the uses of a bar graph on a DMM?
13. How are ghost voltages produced?
14. What is a practical application of using ghost voltages when troubleshooting
circuits? (IM Fig 4-36)
Check-off Method
As you troubleshoot circuits you will work from a wiring diagram or schematic. To check
to see if the circuit is wired correctly, use the check off method. Working from the
schematic find and trace each wire. When the wire is checked and verified to be
correctly placed and securely connected, check mark the wire on the schematic. Work
through the circuit in a logical order and make sure to check every wire. This method
works for most electrical and electronics circuits and even hydraulic and pneumatic
circuits. Make sure the circuit is de-energized when using the check off method since
you are likely to come into contact with potentially live components, or you could pull
loose a live wire that might cause a dangerous short circuit.
Learning Activities: Use the Check-off Method on all lab activities.
LAB: Describe how you will find an operational short or a wiring problem.
Use the check-off method when you have a malfunctioning circuit, but the fuse or circuit
breaker does not blow. For example, two lights might light when only one should light.
Make sure the circuit is completely de-energized. Study the system prints and locate
the part of the circuit that is likely to contain the fault then use the check-off method to
locate the problem. Look for crossed wires connecting two branches of a circuit,
incorrectly installed wiring, or incorrectly numbered wires or swapped wires, 6 for 9 for
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example.
LAB: Sketch and describe how to locate an open circuit and test contacts using a
voltmeter.
The circuit will be de-energized if there is a series open, for example a light switch that
turns off all the lights.
If the switch you are testing with your voltmeter and switch is closed you should read
nearly .000 volts. This means that no voltage is being used “dropped” as the current
flows through the closed contacts. You do not want contacts using voltage or they will
heat up and eventually fail. Depending on the sensitivity of the meter you might actually
see a very small reading, for example 136 mV instead of .000V. When you read the
meter display make sure you read not just the numbers but the units that the numbers
represent. The trick is to watch the meter readout since it will change as the contact
change position. When switch is open you should read source or ghost voltage, more
on ghost voltage later. When the contacts close the meter screen will go blank the read
nearly zero volts. Individual lengths of wire can be tested using a voltmeter with the
same readings.
If a switch is open and you use a jumper wire to bridge the switch the equipment should
start. Use extreme caution since the circuit will energize using this test. If the circuit
energizes properly, then the component that is jumpered is defective. Be sure that you
do not jumper around a load since this will cause a short circuit. Make sure the jumper
is removed after the circuit is repaired. Use jumper wires of an odd colored wire so that
you will notice it if it is left on the circuit.
LAB: Sketch and describe how to test contacts using an ohmmeter.
Always remove the component being tested from the circuit to get an accurate
measurement and to avoid damaging your meter, or worse. Contacts will read nearly
.000 ohms when closed, OL when open. Contacts that do not operate in this fashion
are not operating correctly. The trick is to watch the meter readout since it will change
as the contacts change position. If the closed contacts offer higher than expected
resistance they are dirty or worn and must be replaced, or they are not closing
completely and need adjustment.
IM, TESTING FUSES AND CIRCUIT BREAKERS, Fig. 4-36 p. 139
LAB: Sketch and describe how to locate multiple opens in series.
If there are several opens in series a high quality digital meter will register ghost voltage
when the probes bridge one of the opens. Ghost voltage is usually a low voltage that
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has no logical explanation in terms of circuit operation. It results from the increased
sensitivity of digital meters which are probably measuring the capacitive energy across
the multiple opens.
The voltmeter will register near source voltage when the probes bridge all of the opens.
IM, TESTING CONTACTS, p. 144 Fig. 4-40.
Fuses
1.
2.
3.
4.
5.
6.
7.
Sketch and describe the operation of a fuse. (HV p. 362)
What is the purpose of a fuse?
What type of fuse is used to protect electric motors? How does it operate?
Sketch and describe the ohmmeter test of a fuse. (IM, Fig. 4-35)
What does an ohmmeter measure? How does one work?
Why do you never use an ohmmeter on a live circuit?
What are the values of the following symbols: K = kilo =
, M = mega =
m = mille =
8. Sketch and describe the voltmeter test of a single fuse.
9. What is the purpose of a fused disconnect?
10. How will you test fuses in a three phase fused disconnect? (IM, Fig. 4-35)
11. Why do you not put in a higher rated fuse if a lower rated fuse keeps blowing?
12. What is the purpose of a circuit breaker? (IM, Fig. 4-35; HV p. 363-364)
13. How will you test a circuit breaker? (IM, Fig. 4-35)
14. What maintenance do circuit breakers or any seldom used switches require?
Learning Activities: IM, Over Current Protection Devices p. 136-138, Fig. 4-34 & 4-35
LAB: Test fuses and a circuit breaker using a voltmeter and an ohmmeter.
Single Fuse: An individual (single phase) fuse is tested using the same method as
testing a switch or a set of contacts on a relay using a voltmeter. The probes are placed
across the fuse, one on each end of the fuse. A good fuse receiving voltage will register
near zero volts, a bad fuse will register source voltage or ghost voltage. If a fuse is not
receiving voltage it will read 0 volts so you must determine that the correct voltage is
being applied to the circuit as part of a fuse test.
If you think the fuse is bad (open), de energize and lock out the circuit then remove the
fuse from its holder using a fuse puller, not any other tool! Place the fuse on a non
conductive surface like wood and place the probes of the ohmmeter on either side of
the fuse. A good fuse will register a very small resistance; a bad fuse will register OL.
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Three phase fuses: After ensuring that there is power available to all three phases,
fuses in a 3 phase fused disconnect are tested by placing one probe of the voltmeter on
the line side (top) of one fuse, and the other on the load side (bottom) of the fuse next
to it. The test proceeds as if you were testing a three phase power supply with the
following readings.
208 voltage source: between line side fuse 1 and load side fuse 2 = 208, between line
side fuse 1 and load side fuse 3 = 208, and between line side fuse 2 and load side fuse
3 = 208.
480 voltage source: between line side fuse 1 and load side fuse 2 = 480, between line
side fuse 1 and load side fuse 3 = 480, and between line side fuse 2 and load side fuse
3 = 480.
If you think one of the fuses is bad, remove it from the circuit as described above and
test it using an ohm meter.
Circuit breaker: A circuit breaker is tested by looking at the position of its operator
(toggle switch). A toggle in the middle position is tripped. Move it to the off position then
into the on position. You should feel the resistance of the contacts moving into the off
then the on position. If the operator will not stay in the on position but jumps
immediately back to the center position there is likely a short circuit.
Testing the contacts of the circuit breaker is accomplished using the same method as
for testing a set of contacts. But extreme caution must be taken since the breaker must
be removed from the circuit breaker panel. This will require two people wearing the
correct PPE.
Current Measurement
1.
2.
3.
4.
5.
6.
7.
What is the unit of measurement for current?
Sketch and describe how a clamp-on ammeter is used.
How does a clamp-on ammeter measure current?
When might a clamp-on ammeter be used?
Sketch and describe how an in-line ammeter is used.
When might an in-line ammeter be used?
What safety concern will you have when using a volt-meter in the in-line
ammeter mode?
Learning Activities: IM p. 125 -126, Fig 4-20
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LAB: Sketch and describe how to test a heat element using an ohmmeter, voltmeter
and ammeter.
Place the probes off your volt meter across the connections to the heat element. Before
the heat element is energized it should read near zero volts. If it reads source voltage or
ghost voltage the element is likely bad. When the heating element is energized the
meter face should change to a reading of slightly less than source voltage. The change
of the meter display reading is part of the test.
It is also possible to test the element using a clamp-on ammeter on the wires feeding it.
Before the element is energized the meter should read zero amps. When the element is
energized the meter should read amperage. The element is good if the current is at the
manufacturer’s specifications or similar to a known good circuit. If there is no current
flow when the element is energized, check the element using an ohm meter since there
is likely and open in the element.
To test the element using an ohm meter, completely disconnect the wires leading to the
element and ensure that power to the electrical equipment is de energized using proper
lock out tag out. Place the probes of the ohmmeter on the two connection points for the
element. A good element will read some resistance (compare to manufacturer’s
specification or a known good element). A bad element will read OL (infinite resistance)
since there is an open in the element.
IM, HEAT ELEMENT p.147 – 148 Fig. 4-44
Series Circuits
1.
2.
3.
4.
5.
6.
7.
Sketch and describe a series circuit. (HV p. 351-354)
What happens if there is a break in a series circuit?
Describe the rule of current flow in a series circuit.
Sketch and describe what we mean by the term "in series with".
Describe the rule of voltage drops in a series circuit.
Describe the rule of total resistance in a series circuit.
Why are fuses and circuit breakers connected in series with the components
being protected?
8. What can occur when loads are mistakenly wired up in series with other loads,
for example two lights?
9. What problem could occur if a high resistance like a loose connection was wired
in series with a load?
Learning Activities: IM Series Circuits p. 114
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Parallel Circuits
1. Sketch and describe a parallel circuit. (HV p. 353-357)
2. What happens in a parallel circuit if there is an open in one of the circuits?
3. Sketch and describe what we mean by the term "in parallel with".
4. Describe the rule for total current in a parallel circuit.
5. What happens to current flow if you add more loads to a parallel circuit?
6. Describe the rule for total resistance in a parallel circuit.
7. Sketch and describe a short circuit.
8. What happens if you have a short circuit in a parallel circuit?
9. Why is a short circuit dangerous?
10. What is a dead short?
11. Describe the rule of total voltage drop across each branch load in a parallel
circuit.
12. Describe the rule for current in any branch of a parallel circuit.
Learning Activities: IM, Parallel Circuits, p. 115
LAB: Sketch and describe how to locate a dead short circuit.
In a circuit with a dead short the circuit breaker or fuse will be open. If you reset the
circuit breaker or replace the fuse it will blow open as soon as the circuit is reenergized. This occurs because total resistance for the circuit is very near zero ohms.
This allows extremely high amperage that trips the breaker or blows the fuse.
If you find a tripped circuit breaker or open fuse and you suspect a short circuit, look for
signs of overheating, burn marks etc in the wiring and switches to find the obvious
location of the short. The overheating is caused by high amp flow. Look for frayed wires
or wires touching each other or metal.
If you need to use a meter to find the general location of the short, completely isolate
the circuit from any electrical source. Connect the ohmmeter to the circuit to read total
resistance. If there is a dead short it total resistance should read near .000 ohms. Make
sure that all switches are closed manually or with jumper wires when taking this
measurement so that you are reading total circuit resistance
With the meter connected to read total resistance, isolate one branch of the circuit. If
the resistance does not change from near .000 when you isolate a branch that branch
does not contain the short circuit . Reconnect the branch then move on to test the next
branch.
When the resistance reading jumps for near .000 to some resistance after you isolate a
branch, that branch contains the short circuit. Inspect that branch for signs of
overheating to locate the short circuit to find the exact location of the short.
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IM, Short Circuit p.140 Fig. 4-37
LAB: Sketch and describe how to locate a high resistance short to ground.
A high resistance short to ground might not blow the fuse but it will likely create
localized heating at the sight of the short. It is a ground fault meaning that a hot wire is
touching grounded metal through a high resistance pathway. The high resistance limits
current flow and prevents the fuse from blowing. The ground fault will act like an
additional parallel load increasing total current draw.
A high resistance short to ground is caused when a hot wire touches grounded metal
through a section of high resistance causing current to flow in the ground circuit. For
example oily, damp dirt or wood chips. It is very dangerous because you could touch
grounded metal and being an easier electric path to ground current could flow through
your body.
Look for signs of heating, cracked wire, or a build-up of dirt or debris around a wire or in
a junction or switch box. If the circuit has a ground fault interrupter (GFI) the GFI should
open, de-energizing the circuit.
If you see nothing obvious, attach an ammeter to the grounded metal or the ground
circuit. Note the amount of current flow in a good ground circuit should be .000 Amps. If
there is current flow in the ground circuit there is a ground fault. If this is the case, start
at the far end of the circuit and disconnect one parallel branch at a time. If the current
flow does not drop when the branch is disconnected, reconnect that circuit and move
on to the next circuit. When the amperage reading takes a sudden drop to near zero
after a branch is removed the branch just removed contains the high resistance short to
ground. Inspect that section for the exact location of the short.
Series/Parallel (Compound) Circuits
1. Sketch a series/parallel (compound) circuit.
2. What happens when a load is placed in series with another load in a parallel
circuit?
3. What problem does this cause?
4. How would you troubleshoot this problem?
5. What happens when a number of small loads (dirty contacts corroded
connections) are placed in series with a load in a parallel circuit?
Learning Activities: IM, Series/Parallel Circuits p. 116
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Ohm's Law
1.
2.
3.
4.
5.
6.
7.
8.
Define amperage.
List two differences between AC and DC? (HV p. 340- 346)
Amps are represented by the two letters ____.
What is the electromotive force (voltage)?
Define resistance.
Ohm is represented by the Greek letter ______.
What is the purpose of an electrical circuit?
How do you determine wattage (power)? How would you use this formula? (IM
Figure 4-4; HV p. 343)
9. The word definition of ohm's law is: Current in a circuit is proportional to voltage
and inversely proportional to resistance. Proportional means as one goes up or
down the other goes up or down (as E rises so does I if R remains constant).
Inversely proportional means as one goes up the other goes down, or as one
goes down the other goes up (as R goes down I goes up as long as E remains
constant). Amperage is controlled by resistance and voltage.
11. What happens to its resistance when an electrical device or conductor is
heated?
12. Why do we need a potential difference between electrical conductors to make
current flow?
Learning Activities: IM, Ohm’s Law p. 112, Fig 4-3
Battery Operation
1.
2.
3.
4.
5.
6.
7.
How does a battery work?
Which terminal is larger, the negative or the positive?
Red is positive, black is negative.
If you receive an acid burn wash the wound and get immediate first aid.
Charge a battery at low rates to prolong battery life.
How does a battery short out?
What basic maintenance will a battery require?
Battery Voltage vdc
12.7-12.9
12.5-12.6
12.3-12.4
12.1-12.2
11.9-12.0
State of Charge
100%
80%
60%
40%
20%
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Resistors
1.
2.
3.
4.
Why is resistance added to circuits?
In what two values are resistors rated?
What is a variable resistor?
All carbon resistors marked with a rating or color coded:
The 1st band is
The 2nd band is
The 3rd band is
The 4th band is
5.
1000 ohms =
1 000 000 ohms =
Lab: Determine the values of various resistors using a DMM.
Ampacity
1.
2.
4.
5.
6.
7.
8.
9.
10.
Define ampacity.
What effect does wire diameter have on ampacity?
What effect does length of the wire have on ampacity?
Describe the American wire gage AWG ratings. (HV p. 360-361)
What is the common sense way to select the correct wire size?
What % voltage drop is allowed for branch and feeder circuits?
What three key ratings will you consider when selecting an electrical wire?
Why must you be concerned about the insulation quality of the wire? Where will
you look to determine the quality of the wire insulation?
What is the NEC code ampacity for AWG 10, AWG 12, and AWG 14? (HV Fig
14-56, p. 361)
Learning Activities: IM, Wire Size, p. 124
Lab: Sketch and describe how you will find the cause(s) of low voltage at a load
The reason for taking these tests is that a load is malfunctioning for no obvious reason,
for example, not working, working erratically, or overheating. Low voltage could be
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causing these types of problems
If you suspect low voltage at a load it could be caused by overall circuit low voltage,
small voltage drops in series with the load (bad connections) or by wire runs that are
too long or wire gages that are too small for the load.
Determine if the voltage at a load is low by using this formula.
Take and record a full-load voltage drop test across the load that is furthest from the
source for the branch circuit. All loads in the branch circuit should be energized.
Take and record the no-load voltage test across the contacts closest to the load with all
other loads on the circuit de-energized. If there are no contacts near the load then
disconnect a wire close to the load and measure the voltage across the two ends of the
opened wire.
The % voltage drop = the difference between the no-load voltage and the full-load
voltage divided by the no-load voltage.
Example
No- load voltage = = 120.3 V
Full-load voltage = 115.1 V
Difference = 5.2 V
5.2V
120.3 V = 0.043 x 100 = 4.3%
If the voltage drop is greater than 3% the source(s) of the voltage drop must be located
and corrected; inspect the wires and contacts or electronic devices controlling the load.
Look for signs of overheating (loose connections) or corrosion. If there are no obvious
symptoms, then take voltage readings across wire connections or contacts and lengths
of wire, or perhaps electronics switches in series with the load. Contacts and lengths of
wire should offer little or no voltage drop and electronic controls should offer no more
than their specified voltage drop. It is possible to place too many electronic switches in
series with a load causing low voltage at the load. Each electronic switch or sensor
uses a small amount of voltage when it is passing power then too many in series might
use enough voltage to cause operational problems.
For example if a set of contacts are corroded and are using 20 volts when closed then
the remaining voltage available to the load might not be enough to have it operate
correctly. These contact will continue to deteriorate cause more problems in the circuit.
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IM, Contacts, Fig. 4-39
Study the circuit and required wire sizes to determine if the wire run is too long or the
wire size is too small. It is possible that the wattage of the load has changed since the
original installation and the old circuit cannot handle the current draw of the changed
loads. For example a brighter bulb is placed in a parking lot light stand.
Electromagnetism
1.
2.
3.
4.
5.
6.
Make a sketch illustrating the magnetic field produced around a single wire and
around a coil. (HV p. 346-347, Fig. 16-27, p. 421)
Describe the operation of an electromagnet.
Can lines of electric flux touch or cross?
How do you insulate an object from magnetic flux lines?
Unlike poles attract (north & south), like poles repel.
Describe the basic operation of an alternator.
Inductive Circuits
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
In a pure resistive circuit the voltage and current sine waves are in phase. Sketch
an in-phase current and voltage sine wave. (HV Fig. 15-6, p. 383)
Give two examples of pure resistive ac circuits.
What device does any inductive circuit contain?
Sketch and describe the process of electrical inductance in a coil and a single
wire.
Define inductance and name its unit of measurement.
What is inductive reactance?
In a pure inductive circuit the voltage and current sine waves are out of phase by
90 electrical degrees. Sketch an out of phase-phase, inductive current and
voltage sine wave. (HV Fig. 15-7, p. 383)
What is impedance?
Describe the electrical problem created by the CEMF produced by inductive
devices.
How can poor power factor be corrected?
What problems are caused by poor power factor?
What might happen if a signal wire like a thermostat wire was run parallel to a
wire carrying a high current?
Transformers
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1. Sketch and describe the basic operation of a transformer. (HV p. 422-423)
2. What is the difference between a step up transformer and a step down
transformer?
3. What is the purpose of a multitap transformer? (HV Fig. 16-31, pl 423)
4. Transformers are almost 100% efficient so power in VxA = power out VxA.
5. What controls amp flow in the primary?
6. What is the purpose of a tapped transformer?
7. All transformers hum. How can transformer hum be reduced?
8. How are transformers damaged?
9. What happens if the installation of the transformer windings deteriorates?
10. Describe basic transformer maintenance.
11. How are transformers rated?
12. How is a transformer de-rated?
Learning Activities: IM Testing Source Voltage p. 134.
LAB: Testing a transformer
Test secondary voltage. If there is no voltage, test the primary voltage. If there is no
primary voltage test the supply voltage, for example at a circuit breaker box. If there is
the correct primary voltage but no secondary voltage test the transformer using an ohm
meter.
Inspect the transformer for signs of overheating. Remove the transformer from the
circuit by removing the primary and secondary leads.
Check the resistance of the primary and secondary coils at the transformer terminals.
There should be some resistance reading if the coils are good. If the coils read OL the
coils are bad and the transformer should be replaced.
Check the resistance of the primary and secondary coils to the metal of the
transformers. There should be an OL reading if the coils and the insulation around the
metal are good. If there is a resistance reading the coils are bad and the transformer
should be replaced.
Check the resistance between the primary and secondary coils. There should be an OL
reading if the coils are good. If there is a resistance reading the coils are bad and the
transformer should be replaced.
Always try to determine why the transformer failed when the new transformer is
installed.
IM, TESTING TRANSFORMERS, Fig. 4-32
15
Solenoid Operation
1.
2.
3.
4.
5.
6.
7.
8.
Describe the operation of a solenoid. (HV p. 421)
What happens to current flow when a solenoid is first energized? Why does this
happen?
What is the pick up voltage, the seal in voltage, and the drop out voltage for a
solenoid or any coil?
How will the solenoid be damaged if high voltage is applied to it?
How will the solenoid be damaged if low voltage is applied to it?
What produces excessive humming in a solenoid?
What will happen to a solenoid when the armature is prevented from moving or
closing completely? Why?
What care and maintenance will a solenoid require?
Learning Activities: IM p. Solenoids p. 147
LAB: Test a coil for a solenoid or a relay.
Testing a coil is similar to testing a heat element. Place the probes of your volt meter
across the connections to the coil. Before the coil is energized it should read near zero
volts. If it reads source voltage or ghost voltage the coil is likely bad. When the coil is
energized it should read slightly less than source voltage. Holding the probes on the coil
connections while the coil is being energized should result a change in the meter
reading from near zero to nearly source voltage if the coil is good.
To test the coil on resistance, completely disconnect the wires leading to the coil and
ensure that power to the electrical equipment is de energized. Place the probes of the
ohmmeter on the two connection points for the coil. A good coil will read some
resistance (compare to manufacturer specification or a known good coil). A bad coil will
read OL (infinite resistance) since there is an open in the coil.
IM, TESTING COILS, Fig. 4-41
Relays
1. Sketch and describe the operation of a general purpose relay. (HV p. 425-427)
2. Sketch how you can identify the coil and the normally open and normally closed
contacts using an ohmmeter.
3. Describe the operation of a normally open and a normally closed contact.
4. How do you test a relay contact using a voltmeter or ohmmeter?
16
5. How do you test a relay coil using a voltmeter or ohmmeter?
6. Describe the construction and operation of a contactor. (HV p. 427)
7. Where are knife blade switches still found?
8. Describe the construction and care of contacts.
9. Why should disconnects be operated occasionally?
10. Describe the maintenance for relays.
11. Pole, break, and throw:
A pole is the number of separate circuits that can be opened or closed by a
switch or relay. A single pole contact can carry current through only one circuit
at a time.
A double pole switch can carry two circuits at a time.
A break is the number of separate places on a contact that open or close an
electrical circuit. A single break contact breaks and electrical circuit in one place.
A double break (DB) contact breaks the electrical circuit in two places.
A throw is the number of closed contact positions per pole. A single throw
contact can control only one circuit. A double throw contact can control two
circuits.
When ordering a relay ensure that it has the same pole, through, break, coil
voltage, and contact voltage and amperage capacity as the relay being replaced.
Learning Activities:
1.
IM Relays p. 146
LAB: Test a relay coil and contacts.
Resistance Remove the coil from the circuit and test it using the resistance function. A
good coil will offer resistance (you probably will not know the exact resistance unless
you check manufacturer's specifications or the resistance of a known good coil). A bad
coil will read OL if open.
Voltage When a good coil is energized it will give a voltage drop reading, close to
voltage source. When a good coil is not energized it will read nearly .000 volts. If it
reads voltage or ghost voltage the coil is bad.
17
IM, Testing Coils, p.145
Relay contacts are tested like other contacts as described above.
18
Motors & Controls (2014 revision)
Motor Control: Line and Wiring Diagrams
1. Simple motors operations might still be controlled using specialized relays called
magnetic motor starters, also called mag starters. Programmable logic controls
are used for more complex circuits but their operation and programming is based
on line diagram logic.
2. Why are line diagrams also called ladder diagrams?
3. What is the purpose of a line diagram?
4. What is the purpose of a wiring diagram and how is that different from the
purpose of a line diagram?
5. How you read line diagrams?
6. Sketch the symbols for normally open (N.O.) and normally closed (N.C.)
contacts.
7. Manual control requires someone to operate the circuit. Automatic controls use a
device or devices to operate the circuit.
8. Line diagrams are drawn in the position before the signal that starts the
operation of the circuit has arrived at the circuit. (normal)
9. How will you use the check off method to check the wiring of a control circuit?
10. Sketch and describe the operation of AND, OR, NOT, NAND, and NOR logic.
11. Interpreting a line diagram simply means to completely understand of a circuit.
How does the circuit energize and de-energize? What is the complete operation
of the circuit? What happens to the circuit when the power goes off then comes
back on?
12. Sketch a standard start/stop line diagram and describe its operation both starting
and stopping. Define a wire group in a line diagram. (Fig 4-38, p. 142)
13. Sketch a two-wire control line diagram and describe its operation both starting
and stopping.
14. What do we mean by the signal, decision, and action as it relates to a line
diagram?
15. Describe the function of a motor control center.
Learning Activities:
1.
2.
Study IM, Diagrams and Drawings p. 116; Circuit Wiring p. 140.
Wire the following motor control circuits during the term:
Step-down transformer provides low voltage for three wire control
(Standard Stop Start Station with Transformer) (Analyze only)
Start-stop station with pilot light to indicate when starter is energized
Float switch controls starter
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Station with pilot light to indicate when starter is de-energized
More than one start-stop station used to control a single starter
Starters arranged for sequence control of a conveyor system
Each starter is operated by its own start-stop overload and when one drops out
all starters and a master stop can be provided
Three starters are operated from a single stop-start station and an overload on
any one of the motors will drop out all three starters
Separate start-stop-jog with standard push buttons and a jog relay
Combined start-jog with separate stop with selector switch jogging with a
selector switch
Forward reverse stations (analyze only, wiring is optional)
Timers
1. What is a timer?
2. Describe the construction and operation of a motor driven timer also called a
synchronous timer.
3. What result will a one shot timer produce?
4. What result will a recycling timer produce?
5. What results will a multiple function, programmable timer produce?
ON DELAY Timers
1.
2.
3.
4.
Describe the general operation of an ON DELAY timer.
Describe the operation of a normally open, timed closed contact.
Describe the operation of a normally closed, timed open contact.
Give an example of where you might use an ON DELAY timer.
OFF DELAY Timers
1.
2.
3.
4.
Describe the operation of an OFF DELAY timer.
Describe the operation of a normally closed, timed closed contact.
Describe the operation of a normally open, timed open contact.
Give an example of an OFF DELAY timer operation.
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ON DELAY
Timer off
Signal Arrives
↓
Timer on – timing
Timed Out
↓
Timer stops – off
Signal Removed
↓
Timer off
Contacts in the normal
position
Contacts stay in the normal
position
Contacts change position
Contacts return to the
normal position
N.O.
N.C.
N.O. stays open
N.C. stays closed
N.O. goes closed
N.C. goes open
N.O. goes open
N.C. goes closed
OFF DELAY
Timer off
Signal Arrives
↓
Timer off
Signal Removed
↓
Timer starts timing
Signal Removed
↓
Timer off
Contacts in the normal
position
Contacts change position
Contacts do not change
position
Contacts return to the
normal position
N.O.
N.C.
N.O. goes closed
N.C. goes open
N.O. stays closed
N.C. stays open
N.O. goes open
N.C. goes closed
Learning Activities:
1.
2.
IM Timers P. 182
Wire ON and OFF DELAY timers.
Simulated gas furnace
A second motor must start after the first motor has started (ON Delay)
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A second motor must start after the first motor has stopped (OFF Delay)
System provides backspin protection and surge protection on stopping – time delay
between pressure switch closing and motor starting
Capacitance
1. Describe the construction and operation of a capacitor in an AC circuit.
2. The unit of capacitance is a Farad but most capacitors used in motor starting
circuits are in the range of:
a. micro Farad µf = millionth of a Farad
b. nano Farad nf = billionth of a Farad
c. pico Farad pf = trillionth of a Farad
.
3. What will you do before working on any circuit containing a capacitor?
4. How will you test a capacitor?
5. How are run and start capacitors for motors identified?
6. What happens when a capacitor shorts out?
7. What can cause a capacitor to fail?
8. In a capacitive circuit the voltage and current sine waves are out of phase by up
to 90 electrical degrees.
9. Total opposition to current flow in an AC circuit is called impedance and its unit
of measurement is the ohm. It is caused by the resistance of the wire, and the
opposition to current flow caused by coils, and the opposition to voltage caused
by a capacitor.
10. A capacitor creates capacitive reactance which opposed the change in voltage in
AC.
11. What is poor power factor and how is it created?
12. How can poor power factor be corrected?
LAB: Sketch and describe how to discharge and test a capacitor.
Inspect the capacitor for signs of overheating or bulging. Replace immediately if it is
deformed or damaged.
Using a 20,000 ohm, 2 Watt rated resistor drain the capacitor by connecting the resistor
leads across the capacitor terminal. If this is not done you meter could be damaged.
Hold it in place for at least 30 seconds.
Place the capacitor checker across the capacitor leads. If the reading is +/ 10% of the
rating of the capacitor it is good. The capacitor rating is usually found on the side of the
capacitor and is often rated in pico or micro Farads. Replace the capacitor if it is bad.
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WYE Windings and DELTA Windings
1. Sketch a wye or star winding, marking in the voltage readings.
2. Why do we not need a neutral for three phase operation?
3. Sketch a delta winding marking in the voltage readings.
Learning Activities: IM, p. 154, Fig 4-51
Three Phase Squirrel Cage Motors
1.
2.
3.
4.
5.
6.
7.
8.
9.
Describe the operation and construction of a squirrel cage induction motor.
What is the synchronous speed of a motor?
Why is it best to hook up a motor to the highest voltage possible?
W hat is single-phasing and what will it do to a motor?
What causes a three phase motor to start? What specialized relays are not
needed to start a three phase motor?
Sketch and describe how test a three phase, single voltage motor like a
refrigeration compressor.
Sketch and describe how to test a dual voltage, wye wound, three phase squirrel
cage induction motor. (IM Fig. 51 p. 154)
List the information often found on a motor data plate?
How do you reverse the direction of a three phase motor?
Learning Activities:
1.
2
IM, Three-Phase Motors, p.151
Connect a wye wound motor for high and low voltage.
LAB: Test single and dual voltage three phase motors.
Single voltage 3 phase motors will have 3 terminal connections. A good motor will have
equal resistance between each of the three connections, and read OL to the metal
frame of the motor (ground).
Three Phase, dual voltage, Wye wound motors have 9 leads T1 – T9.
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T1-T4
T2-T5 Same resistance between paired leads, no continuity between individual coils
T3-T6
T7-T8-T9 All leads have continuity, no continuity between these three coils and the
three other coils
No continuity between any coil lead and the frame of the motor.
Motor Overload Protection
1. What devices are used in magnetic motor starters to protect electric motors?
2. What is locked rotor current?
3. Why do motors have inrush current at start-up?
4. How large can inrush current get?
5. What type of fuses compensate for high inrush current?
6. Describe the general operation of motor protection overloads.
7. What will you do before and after restarting tripped overloads?
8. Why are trip indicators built into starters?
9. Sketch and describe the basic operation of an overload current transformer.
10. Describe the basic operation of an inherent motor protector.
11. How will you select the type of enclosure needed for a specific location?
12. What is the full load current rating (FLC, FLA)?
13. What is the service factor of a motor? What does this mean?
14. What is the general process for selecting heaters?
15. Overload heaters have been largely replaced by electronic devices that serve the
same function.
Learning Activities:
1. IM Magnetic Motor Starter P. 153
2. Select the correct heaters for a three phase motor.
Lab: Selecting Magnetic Motor Starter Overload Heaters
Motors draw very high inrush current at start up when started “across the line” using a
standard mag starter. Therefore overloads must allow for temporary (several seconds)
current overloads at startup. In addition motors are often subject to temporary
overloads during normal operation so the overloads allow for a temporary overload, but
they open if the overload lasts for too long. If fuses protect a motor they must be time
delay fuses to allow for these types of overloads.
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To select heaters for a magnetic motor (mag) starter, use the heater selection chart for
the magnetic motor starter being used. Select the overload heater closest to the full
load amp rating of the motor if the temperature of the motor and the mag starter are at
the same temperature. This protects the motor for 110% to 120% of the motor FLA for
temporary overloads during normal operation.
If the temperature at the mag starter is higher than at the motor a higher rated overload
is selected. If the temperature at the mag is lower than at the motor a lower rated
overload is selected. The exact overload compensation must be calculated from a
heater ambient temperature correction chart for the particular mag starter being used. It
is common that such temperature differences exist because motors and mag starters
are often in different locations.
Some mag starters will allow for adjustable trip levels to allow for variations in the
service factor of a motor or for temperature differences between the mag starter and
motor.
Single Phase Motors
1.
2.
3.
4.
5.
6.
7.
8.
How is a single phase motor different from a three phase motor?
Sketch and describe the construction and operation of a split phase motor.
Sketch and describe the construction and operation of a capacitor start motor.
Sketch and describe the construction and operation of a cap run and cap start
motor.
Sketch and describe how to test a single phase refrigeration compressor.
What is the purpose of a potential relay found on single phase compressors and
how does it work?
What is the purpose of a current relay found on single phase compressors and
how does it work?
What is the purpose of a positive temperature coefficient start device and how
does it work?
Learning Activities:
1.
2.
IM, Single Phase Motors, p. 154.
Test single phase motors for bad windings.
Electrical Maintenance
1. List five basic items of electrical maintenance.
2. Within what % of rated voltage should you find the incoming power to the motor?
3. Why must you try to find out why the motor fails in addition to simply replacing
25
the motor?
4. What factors will contribute to motor control failure?
5. What conditions can be monitored for any three phase electric motor?
6. How can you test the quality of winding insulation of a motor to detect problems?
(IM, p. 160)
7. How can you determine if the trouble is in the control circuit or in the power
circuit for a motor that is controlled by a magnetic motor starter?
8. What is the first test for any magnetic motor starter if the motor is not starting?
9. How do you determine if a three phase circuit has voltage or current unbalance?
(IM Voltage Unbalance, p. 130 & IM Workbook Exercise 4.4 & 4.5)
10. What are harmonics and how are they caused? (IM, Harmonic Distortion, p.183)
11. How can harmonics be prevented and corrected?
12. What causes “dirty” voltage? (IM, Voltage Variations, Fig 5-2)
13. What devices correct for dirty voltage?
14. How should PC boards be handled? (IM, PC Board Handling, p. 165)
15. How should PC boards be cleaned?
16. How do you use the check-off method to check for loose connections? What
safety precautions must you take when checking for loose connections?
Control Devices
1. Describe the operation of a mechanical limit switch, a pressure switch, a flow
control switch, and a float switch.
2. How else can the liquid level be controlled?
3. Describe the operation of a bi-metal switch.
4. Describe the operation of a thermocouple.
5. Describe the operation of a thermistor.
6. Describe the general operation of any transducer.
Reversing and Reduced Voltage Starting for Motors
1. How do you reverse a one phase motor?
2. How do you reverse a three phase motor?
3. Describe the basic construction of a reversing starter and explain why a
reversing starter must be interlocked.
4. Describe two types of interlocks.
5. Frequent reversing is hard on a motor and will require a special motor, high
service factor.
6. Why must large motors be started with reduced voltage?
7. In what two ways can an AC motor's speed can be changed?
8. Describe the operation of a variable frequency drive.
9. What operational features might be included in a VFD?
10. Describe the general operation of “soft start” electronic motor drives.
26
11. As voltage is reduced, torque is reduced how is this problem corrected by
modern AC speed controls?
12. How does a multi speed magnetic motor starter change the speed of a motor?
13. Describe the operation of a friction brake, plugging and electric braking.
Learning Activities:
1. IM, Variable Frequency Drives p. 155
2. IM, Inverters, p. 184 -185, Fig. 5-18
Power Quality
1. What qualities of electrical power can cause problems?
2. Describe voltage sag, under voltage, voltage surges, and transients.
3. What is the difference between a momentary, temporary and sustained power
interruption?
4. What problems can be caused by voltage or current imbalance?
5. Describe the process for determining voltage or current imbalance?
6. Describe phase unbalance and improper phase sequence.
Learning Activities:
1. IM Power Quality
2. IM Workbook Activity 4-4 & 4-5
Making Line and Wiring Diagrams
1.




2.





How do you create wiring diagrams?
Draw all loads in their exact location.
Draw in all control devices in their exact location.
Trace all wires from each load to L1 and L2, including all input devices.
All wires should have numbers and colors to match the actual wiring.
How do you create line diagrams from actual wiring?
Draw in each load.
Using the check-off method trace each wire back from the load.
Sketch in each input that operate each load.
Move from the load back to L1.
Make sure there is a complete pathway from the source to all control devices and
27
the loads.
3.
You do you figure out how a circuit works?
Obtain the line diagram or schematics and manuals from the manufacturer. Use
industry standard wiring and line diagrams.




4.






Study the equipment operation when it is working normally.
Make a written description of how the machine works.
Study the wiring and line diagrams and written descriptions.
Study the machine while doing maintenance.
How do you create line diagrams from word descriptions?
Write out a word description of machine operation.
List and name all loads.
List and name all inputs.
Start line diagram by placing all loads on the right side.
Sketch the inputs that will energize and de-energize each load.
Line diagram interpretation should match the word description.
28