[latex]I=\frac{E}{R}[/latex]

• E = Voltage in volts
• I = Current in amps
• R = Resistance in ohms

[latex]1A=\frac{1V}{1\text{ ohm}}[/latex]

[latex]?=\frac{1V}{2\text{ ohms}}[/latex]

[latex]0.5A=\frac{1V}{2\text{ ohms}}[/latex]

Watt's Law

Overload

Overcurrent

Effects of short circuits

• Thermal energy: High values of current will create lots of heat, which can damage equipment and wires. Thermal energy can be expressed by I²t (current squared times time)—the longer the fault persists, the greater the potential thermal damage.
• Mechanical forces: High-fault currents can create powerful magnetic fields and exert huge magnetic stress on busbars and equipment, sometimes warping them out of shape and creating other problems.

Fuses

Common ratings

Voltage rating

Continuous-duty rating

Interrupting capacity

Pushbuttons

Switches

Single-pole, single-throw (SPST)

Single-pole, double-throw (SPDT)

Double-pole, double-throw (DPDT)

Double-pole, single-throw (DPST)

Triple-pole, single-throw (3PST)

Isolating switch

Float Switches

Temperature Switches

Pressure Switches

Flow Switches

Mechanical-Limit Switches

Proximity Switches

Inductive-proximity switch

Capacitive-proximity switch

Photoelectric switch

Pushbutton

Drum Switch

On-Delay Timers

Off-Delay Timers

Questions

1. Which letter identifies the component that normally provides overcurrent protection for the motor branch circuit?
2. Which letter identifies the component that normally provides overload protection for the motor branch circuit?
3. When energized by the control circuit, which letter identifies the component that provides normal starting and stopping actions?
4. Between which two letters is the control circuit power normally taken from?
5. Which letter(s) identify devices that must be horsepower rated?

6. A melting alloy type of thermal overload is called a:
   1. Solder-pot relay
   2. Dash-pot relay
   3. Thermal relay
   4. Bimetallic relay
7. The part of a manual AC motor starter which senses motor overload current is the:
   1. High-limit switch
   2. Solder-pot assembly
   3. Overload contact
   4. Heater element
8. If automatic starting after a power failure represents a safety hazard for a motor drive, it should be equipped with:
   1. Low voltage release
   2. A flashing red light
   3. Low-voltage protection
   4. A warning sign
9. An overload relay that uses a strip of dissimilar metals is called a _______ relay
   1. Melting alloy
   2. Thermistor
   3. Solder pot
   4. Bimetallic
10. Referring to the drawing, assuming that fuse C is blown, which pair of points will result in a reading of zero volts when the voltmeter is connected across them?
11. To connect a motor solely for jogging operation, the control circuit will:
    1. Use holding contacts
    2. Connect holding contacts in series with the start button
    3. Connect holding contacts in parallel with the start button
    4. Not use holding contacts
12. Timed relays are the best method to provide plugging for a motor. True or false?
13. The electrical interlocks in a reversing magnetic starter are:
    1. Controlled by the timing relays necessary for circuit operation
    2. In parallel with the forward and reverse pushbuttons
    3. Normally closed contacts
    4. Normally open contacts
14. Overload protection for motors is achieved by connecting:
    1. Thermal sensing elements in series with the motor
    2. A reset relay in series with the motor
    3. Cartridge fuses in series with the motor
    4. A set of N.C contacts in series with the motor
15. With a three-wire control circuit, when power is restored after a low voltage situation:
    1. An operator must restart the motor
    2. The motor will polyphase
    3. The motor will automatically restart
    4. The motor will automatically restart after a time delay

Answers

1. B
2. D
3. C
4. B and C
5. A and C
6. A
7. D
8. C
9. A
10. 3 and 6
11. D
12. False
13. C
14. A
15. A

Pilot Devices - Self Test 2

Questions

1. In a motor control circuit with multiple start/start pushbutton stations, the stop buttons would be connected in ______ and the start buttons would be connected in _______.
2. To troubleshoot an electrical control circuit, the best drawing to be used would be:
   1. Schematic
   2. Wiring
   3. Pictorial
   4. RIser
3. A double contact pushbutton is illustrated below. When connected as a start button in the control circuit of a magnetic starter, which pair of terminals are normally utilized?
4. Timed relays are the best method to provide plugging for a motor. True or false?

Answers

1. Series, parallel
2. A
3. 3 and 4
4. False

Motor Starters and Contactors - Self Test 3

Questions

1. The minimum number of control circuit conductors to a stop/forward/reverse pushbutton station that provides low-voltage protection for a three-phase reversible motor is:
   1. 2 wires
   2. 3 wires
   3. 4 wires
   4. 6 wires
2. Which of the following type of motor starter does not normally provide running protection for the motor?
   1. A magnetic starter
   2. A pushbutton starter
   3. A toggle-switch starter
   4. A drum-switch controller
3. If the circuit in the drawing was operating normally and an overload occurred, then:
   1. Both lights would come on
   2. The green light would come on, and the red light would go out
   3. Both lights would go out
   4. The red light would come on, and the green light would go out
4. With a three-wire control circuit, when power is restored after a low voltage situation:
   1. The motor will automatically restart after a time delay
   2. The motor will automatically restart
   3. An operator must restart the motor
   4. The motor will polyphase
5. A melting alloy type of thermal overload is called a:
   1. Solder-pot relay
   2. Thermal relay
   3. Bimetallic relay
   4. Dash-pot relay
6. A starter rated at 10HP, 600 V, if used on a 120V motor would most likely be rated at:
   1. 2 HP
   2. 3 HP
   3. 10 HP
   4. 2.5 HP
7. A N.C.T.C contact will be open immediately after the coil is de-energized. True or false?
8. An overload relay that uses a strip of dissimilar metals is called a _______ relay
   1. Thermistor
   2. Melting alloy
   3. Solder pot
   4. Bimetallic
9. A shading coil is not required for a DC voltage coil. True or false?
10. Overload protection for motors is achieved by connecting:
    1. A set of N.C contacts in series with the motor
    2. Thermal sensing elements in series with the motor
    3. Cartridge fuses in series with the motor
    4. A reset relay in series with the motor
11. The part of a manual AC motor starter which senses motor overload current is the:
    1. Overload contact
    2. Solder-pot assembly
    3. Heater element
    4. High-limit switch
12. The essential difference between a magnetic motor starter and a magnetic contactor is that the contactor does not contain:
    1. Shading coils
    2. Holding contacts
    3. Overload relay protection
    4. Horsepower-rated contacts
13. The rotation of an AC three-phase induction motor may be reversed by interchanging:
    1. The control circuit
    2. The forward/reverse interlocks
    3. The forward reverse coils
    4. Any two power lines
14. If an AC magnetic contactor with a 480V coil was energized with 120V, then most likely the:
    1. Overload relay would trip
    2. Contactor would not pick up
    3. Control circuit fuses would blow
    4. Coil would overheat during normal operation
15. In a control schematic, a dotted line between two coils generally means that the two coils:
    1. Are operated together
    2. Are mechanically interlocked
    3. Are electrically interlocked
    4. Have a common set of contacts
16. During normal operation, a loud chattering noise is heard from the enclosure of an AC magnetic starter. What is the most likely cause?
    1. Broken shading coil
    2. Open in the seal-in circuit
    3. Rust on pole faces
    4. Power contact is not making good contact due to poor pressure
17. The purpose of electrical interlocking in a three-phase reversing AC magnetic motor starter is to:
    1. Ensure that the forward direction of rotation is selected first
    2. Allow both coils to be energized together
    3. Prevent both coils from being energized simultaneously
    4. Maintain the coil circuit after the stop button is released
18. A double-contact pushbutton is illustrated below. When connected as a start button in the control circuit of a magnetic starter, which pair of terminals are normally used?
19. A time-delay relay includes both timed and instantaneous contacts. True or false?
20. Fill in the missing words. The electrical interlocks on a reversing starter are normally _______ contacts
21. A N.O.T.O contact will be closed while the coil of the relay is energized. True or false?
22. A N.C.T.O designation refers to an off-delay timer. True or false?
23. The electrical interlocks in a reversing magnetic starter are:
    1. Normally closed contacts
    2. Normally open contacts
    3. In parallel with the forward and reverse pushbuttons
    4. Controlled by the timing relays necessary for circuit operation

Answers

1. D
2. C
3. D
4. C
5. A
6. A
7. False
8. D
9. True
10. B
11. C
12. C
13. D
14. B
15. B
16. A
17. C
18. 3 and 4
19. True
20. Closed
21. True
22. False
23. A

Diagrams - Self Test 4

Questions

1. Which letter identifies the component that normally provides overcurrent protection for the motor branch circuit?
2. Which letter identifies the component that normally provides overload protection for the motor branch circuit?
3. When energized by the control circuit, which letter identifies the component that provides normal starting and stopping actions?
4. Between which two letters is the control circuit power normally taken from?
5. Which letter(s) identify devices that must be horsepower rated?

6. In a motor control circuit with multiple start/start pushbutton stations, the stop buttons would be connected in ______ and the start buttons would be connected in _______.
7. To troubleshoot an electrical control circuit, the best drawing to be used would be:
   1. Schematic
   2. Pictorial
   3. Wiring
   4. RIser
8. A drawing which shows the relative locations of various components is a:
   1. Wiring diagram
   2. Schematic diagram
   3. Elementary diagram
   4. Ladder diagram
9. Referencing the following drawing, what is the number of wires required where indicated?

Answers

1. B
2. D
3. C
4. B and C
5. A and C
6. Series, parallel
7. A
8. A
9. 3

Circuits - Self Test 5

Questions

1. Which letter identifies the component that normally provides overcurrent protection for the motor branch circuit?
2. Which letter identifies the component that normally provides overload protection for the motor branch circuit?
3. When energized by the control circuit, which letter identifies the component that provides normal starting and stopping actions?
4. Between which two letters is the control circuit power normally taken from?
5. Which letter(s) identify devices that must be horsepower rated?

6. The minimum number of control circuit conductors to a stop/forward/reverse pushbutton station that provides low-voltage protection for a three-phase reversible motor is:
   1. 2 wires
   2. 3 wires
   3. 4 wires
   4. 6 wires
7. Inching is another term used for:
   1. Jogging
   2. Plugging
   3. Shunting
   4. Hunting
8. The rotation of an AC three-phase induction motor may be reversed by interchanging:
   1. The control circuit
   2. The forward/reverse interlocks
   3. The forward reverse coils
   4. Any two power lines
9. In a control schematic, a dotted line between two coils generally means that the two coils:
   1. Are operated together
   2. Are mechanically interlocked
   3. Are electrically interlocked
   4. Have a common set of contacts
10. The purpose of electrical interlocking in a three-phase reversing AC magnetic motor starter is to:
    1. Ensure that the forward direction of rotation is selected first
    2. Allow both coils to be energized together
    3. Prevent both coils from being energized simultaneously
    4. Maintain the coil circuit after the stop button is released
11. To connect a motor solely for jogging operation, the control circuit will:
    1. Use holding contacts
    2. Connect holding contacts in series with the start button
    3. Connect holding contacts in parallel with the start button
    4. Not use holding contacts
12. In regards to the drawing, the best label for the pushbutton labelled "Z" would be:
    1. Stop
    2. Jog
    3. Run
    4. Reset
13. In regards to the drawing, the best label for pushbutton "Y" would be:
    1. Stop
    2. Jog
    3. Run
    4. Reset

14. What is the minimum number of control circuit conductors required in Conduit A?
    1. 2 wires
    2. 3 wires
    3. 4 wires
    4. 5 wires
15. What is the minimum number of control circuit conductors required in Conduit B?
    1. 2 wires
    2. 3 wires
    3. 4 wires
    4. 5 wires

16. Timed relays are the best method to provide plugging for a motor. True or false?
17. A time-delay relay includes both timed and instantaneous contacts. True or false?
18. A N.C.T.C contact will be open immediately after the coil is de-energized. True or false?
19. A N.O.T.O contact will be closed while the coil of the relay is energized. True or false?
20. A N.C.T.O designation refers to an off-delay timer. True or false?
21. The electrical interlocks in a reversing magnetic starter are:
    1. Normally closed contacts
    2. Normally open contacts
    3. In parallel with the forward and reverse pushbuttons
    4. Controlled by the timing relays necessary for circuit operation
22. Overload protection for motors is achieved by connecting:
    1. Thermal sensing elements in series with the motor
    2. A reset relay in series with the motor
    3. Cartridge fuses in series with the motor
    4. A set of N.C contacts in series with the motor
23. Jogging refers to a:
    1. Motor that is not capable of developing constant torque
    2. Motor that is polyphasing
    3. Motor that periodically starts and stops
    4. Method used to stop a motor for exact positioning

Answers

1. B
2. D
3. C
4. B and C
5. A and C
6. C
7. A
8. D
9. B
10. C
11. D
12. B
13. C
14. ?
15. C
16. False
17. True
18. False
19. True
20. False
21. A
22. A
23. D

Troubleshooting - Self Test 6

1. A drawing which shows the relative locations of various components is a:
   1. Wiring diagram
   2. Schematic diagram
   3. Elementary diagram
   4. Ladder diagram
2. To ensure safety for maintenance, the motor disconnecting switch must be locked in the "OFF" position. After the maintenance work is completed, the lock is removed by:
   1. The supervisor
   2. The project manager
   3. The person who put the lock on
   4. The lead hand
3. With regard to workers safety, "isolation" refers to:
   1. Moving to a remote location
   2. Disconnect from all energy sources
   3. Turn off the electrical switch
   4. Fence off the work site
4. Referring to the drawing, assuming that fuse C is blown, which pair of points will result in a reading of zero volts when the voltmeter is connected across them?
5. The disconnecting means which is NOT intended for interrupting current flow is the:
   1. Motor switch
   2. General use switch
   3. Isolating switch
   4. Circuit breaker
6. If the circuit in the drawing was operating normally and an overload occurred, then:
   1. Both lights would come on
   2. The green light would come on, and the red light would go out
   3. Both lights would go out
   4. The red light would come on, and the green light would go out
7. During normal operation, a loud chattering noise is heard from the enclosure of an AC magnetic starter. What is the most likely cause?
   1. Broken shading coil
   2. Open in the seal-in circuit
   3. Rust on pole faces
   4. Power contact is not making good contact due to poor pressure
8. If the control circuit shown below tripped due to overload, then which of the voltmeter positions shown would indicate line voltage?
   1. VM A
   2. VM B
   3. VM C
   4. VM D
9. What is the minimum number of control circuit conductors required in Conduit A?
   1. 2 wires
   2. 3 wires
   3. 4 wires
   4. 5 wires
10. An isolating switch can be used as a motor power circuit disconnect switch. True or false?

11. If the motor starter in the drawing is energized and functioning properly, what should the voltage be across the normally open contact (M)?
    1. Line voltage
    2. Zero voltage
    3. Half of line voltage
    4. Twice line voltage
12. If the motor starter in the drawing is energized and functioning properly, what should be the voltage across the N.C contact (M)?
    1. Line voltage
    2. Zero voltage
    3. Half of line voltage
    4. Twice line voltage

Answers

1. A
2. C
3. B
4. 3 and 6
5. C
6. D
7. A
8. ?
9. ?
10. False
11. B
12. A

Contactor Construction and Operation

Combination Starters

Identifying Wye or Delta with an Ohmmeter

• T1 has continuity to T4 &T9
• T2 has continuity to T5 &T7
• T3 has continuity to T6 &T8

Wye Connections

Low-voltage Wye

High-voltage Wye

Delta Connections

Low-voltage Delta

High Voltage Delta

Direct from the Line

A Control Transformer

1. Wire all [pb_glossary id="230"]normally closed[/pb_glossary] stop buttons in [pb_glossary id="269"]series[/pb_glossary] with each other,
2. Wire all [pb_glossary id="229"]normally open[/pb_glossary] start buttons in [pb_glossary id="268"]parallel[/pb_glossary] with each other, and
3. Connect the holding contact in parallel with the start buttons.

Jog Circuit with Selector Switch

• The main advantage of this circuit is the ease of installation and the cheapness of equipment.
• The main disadvantage is that you must change the position of your selector switch to change the function of your button.[embed]https://video.bccampus.ca/id/0_kk8zsh4n?width=608&height=402&playerId=23448552[/embed]

Dangerous Jog Circuit

• This circuit has the advantage of being simple to install and has separate buttons dedicated to starting and jogging the motor.
• The main disadvantage is the hazard introduced by the quick return of the jog button to its normal state.

Jog Circuit with Control Relay

• The advantage of this circuit lies in its safety and reliability. Having two separate buttons for “run” and “jog” functions increases ease of control for the operator.
• The disadvantage is the additional cost and installation time associated with the control relay and additional current drawn in the [pb_glossary id="242"]control circuit[/pb_glossary] due to the second coil.

Reversing Magnetic Starters

Mechanical Interlocks

Electrical Interlocks

Reversing Control Circuit

Pushbutton Interlocks

Ohmmeter

1. Close-to-zero ohms: This means that the leads of the ohmmeter are connected across two points that are [pb_glossary id="247"]electrically common[/pb_glossary]. The two terminals of a closed switch or contact would give an ohmmeter reading close to zero [pb_glossary id="222"]ohms[/pb_glossary].
2. Very-high-to-infinite ohms: An open in the circuit would allow zero current to flow and so would read as infinite ohms. The terminals of open switches and contacts would give very high ohmic readings.
3. Some ohms: An ohmmeter measuring across a load such as a pilot light would read a very high (approximately megaohms), but not infinite, value of ohms. This is one way of confirming that a pilot light is in good working condition. The terminals of a coil should have continuity and a low (approximately tens to hundreds of ohms), but not zero value of resistance. If a coil is shorted and reads zero ohms, it needs to be replaced.

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Bimetallic Strip

Melting Solder Pot

Other Types of Overload Relay

• 1-4 = line voltage, therefor Fuse B is good
• 3-6 = zero volts, therefor Fuse C is blown
• 5-2 = line voltage, therefor Fuse A is good