Line H Flashcards
List three functions, other than stopping and starting a motor, that a motor controller can
provide.
Provides running protection for the motor.
• Reverses rotation of the motor.
• Varies the speed of the motor.
• Provides quick stopping of the motor in an emergency.
• Varies starting voltage to the motor.
• Allows sequencing of several motors.
• Provides automatic and remote control of the motor.
List four components found in the power circuit of an across-the-line magnetic starter circuit.
- Disconnect means
- Overcurrent protection device
- Magnetic starter power contacts
- Overload relay heater
- Motor and conductors
List three components found in the control circuit of a magnetic starter.
- OLR auxiliary contacts
- Magnetic starter coil
- Magnetic starter seal-in contact (if used)
- Pilot-initiating devices (such as push-buttons)
What is the fundamental difference between a magnetic contactor and a magnetic motor
starter?
A magnetic motor starter consists of a magnetic contactor plus overload relay protection.
A two-wire control circuit provides under-voltage _____.
release
What is the difference between the power circuit and the control circuit of a motor starter?
Power circuit = main line contacts
Control circuit = electromagnetic coil and auxiliary contacts
What are the two basic parts of a thermal overload relay?
the heater element and the normally closed auxiliary contact
A three-wire control circuit provides under-voltage _____.
protection
Two-wire control circuits usually include ______-contact initiating devices.
maintained
A three-phase motor requires an overload ______ in series with each of the line
conductors to the motor.
heater
Three-wire control circuits usually include _____-contact devices.
momentary
What is the purpose of a pilot light?
to indicate the operating status of a motor in a remote location
Selection of the heater is based on the full-load ______ of the motor, which is easily
determined from the ______ of the motor.
current, nameplate
Before an OLR can be reset, time must be allowed for it to ______
cool down
When operating a motor from two or more locations, the STOP buttons are connected in ______, and the START buttons are connected in ______.
series, parallel
What is meant by full voltage control?
The control circuit is energized with the same value of voltage as the motor (power) circuit.
How does a wiring diagram differ from a schematic diagram?
A schematic diagram uses symbols and interconnecting lines to display the electrical
operation of a circuit.
A wiring diagram shows the physical relationships of all the devices and the actual
number of conductors and terminations between devices.
What is meant by a wire numbering system?
The wire numbering system is a numeric marking method applied to a schematic diagram to
help identify conductor terminations in a wiring diagram.
How does a numbering system help in the development of a wiring diagram?
Like-numbered terminals are simply interconnected with conductors. For example, 2 to 2; 3
to 3; and so on.
Why is it sometimes necessary to use control relays in conjunction with magnetic motor
starters?
Whenever a magnetic starter does not have enough auxiliary contacts for the control circuit
operations, then one (or more) control relays is used.
What is meant by the term jogging?
Jogging is the quickly repeated closure of a circuit. It starts a motor from rest and causes
small movements of the motor-driven machine.
When a normally open contact on a control relay is rated at 10 amperes AC, does this mean
that it can make and break all values of current up to and including 10 A? Explain your answer.
No, the value of 10 A is the continuous-current-carrying rating. For making or breaking
inductive types of circuits, the continuous current rating must be greatly reduced (see
Table 1 in Learning Task 2) or a contactor must be used.
What hazard can occur when using only a simple JOG button with a STOP-START control
circuit?
The motor may continue to run if the JOG button closes before the seal-in contact has a
chance to open.
How do you overcome a motor that continues to run if the JOG button closes before the seal-in contact has a
chance to open?
by using a definite break device, such as a selector switch or a control relay, in the jog control
(seal-in) circuit
What is a latching relay?
A latching relay is a control relay that is activated electrically but held mechanically. The coil
needs only to be energized momentarily for picking up (latching) or dropping out the relay.
What are the constructional differences between a three-phase, reversing magnetic starter
and a non-reversing one?
The reversing starter contains an additional three-pole contactor and a mechanical interlock
between the two contactors.
How does a reversing starter reverse the rotation of a three-phase induction motor?
When the second reversing contactor is energized, two of the supply lines to the three-
phase motor are interchanged.
Why is it necessary to prevent the forward and reverse contactors from being energized
simultaneously?
If both contactors are energized simultaneously, a line-to-line short circuit results.
With regard to a worker’s safety, the term isolate, means to:
a. remove to a remote location
b. turn off the switch
c. disconnect from all energy sources
d. leave the worker alone
disconnect from all energy sources
What is electrical interlocking on a reversing starter?
The use of auxiliary normally closed contacts to ensure that only one contactor can be
energized at one time. These n.c. contacts for one contactor are wired in series with the
control circuit of the other contactor, and vice versa.
With regard to a worker’s safety, what does the term lockout mean?
Lock the energy controls in the OFF position.
Why is there only one set of OLR contacts in a three-phase reversing starter?
A three-phase motor draws the same current whether running in the forward or reverse
direction of rotation, so only one set of overload heaters is needed to protect the lines to
the motor.
What types of work require lockout procedures?
any work that must be done on equipment that can become energized
If an electrical interlock is provided in a reversing starter, is it also necessary to provide a
mechanical interlock?
yes
Whose responsibility are the lockout procedures for a job?
a. Supervisor
b. Person doing the job
c. The apprentice or assistant on the job
person doing the job
What type of lock is used for a lockout procedure?
a keyed padlock
How does the OHS Regulation of WCB specify that your lock be identified?
marked or tagged with your name
Develop a control schematic illustrating push-button interlocking for a reversing magnetic
motor starter. *(H-1 LT6 Question 7)
Which locks are workers authorized to remove?
only their own locks
Develop a control schematic from the wiring diagram shown in Figure 1.
*(H-1 LT6 Question 8)
A voltmeter is used to test for a blown fuse in a three-phase disconnect supplying a
motor starter. In Figure 1, if fuse C is blown, determine the reading on the voltmeter when
connected across the following pairs of points:
a. 1 and 4 ______
b. 2 and 3 ______
c. 4 and 5 ______
d. 5 and 6 ______
e. 3 and 5 ______
f. 2 and 5 ______
g. 1 and 6 ______
h. 1 and 2 ______
i. 3 and 6 ______
a. 230 V
b. 230 V
c. 230 V
d. 0 V
e. 230 V
f. 230 V
g. 0 V
h. 0 V
i. 0 V
If voltage is measured across a device such as a lamp or starter coil, but the device is not
functioning, what is the problem?
The device is open.
If the motor starter shown in Figure 2 is energized and functioning properly, what is the
voltage across the:
a. n.o. contact Ma
b. n.c. contact Mb
a. 0 V
b. 120 V
If the starter in Figure 2 is de-energized by pressing STOP, what is the voltage measured
across the:
a. n.o. contact Ma
b. n.c. contact Mb
a. 120 V
b. 0 V
If, when the START button is pressed, the coil of the starter energizes but drops out when the
button is released, what is the most likely problem?
The seal-in circuit is not working. This may be due to a:
• Loose wire
• Faulty contact
• Open connection
If the magnetic starter is properly energized initially but there is a loud chattering noise
during operation, what could be wrong?
Possible faults are:
• Low voltage
• Broken shading coil
• Poor contact in the seal-in circuit
Why is it sometimes necessary to use control relays in conjunction with magnetic motor starters?
Whenever a magnetic starter does not have enough auxiliary contacts for the control for the control circuit operations, then one (or more) control relays are used.
Describe what is represented by each of the following symbols.
Name three types of mechanisms used for sensing pressure.
Bourdon tube, bellows, diaphragm
With respect to pressure switches, what does the term differential mean
In a pressure switch, the differential is the differences between cut-in pressure (switch closes) and cut-out pressure (switch opens)
When a normally-open contact on a control relay is rated at 10 amperes AC, does this mean it can make and brake all values of current up to and including 10 A? Explain your answer
No, the value of 10 A is the continuous-current-carrying rating. For making or breaking inductive type circuits, the continuous current rating must be greatly reduced or a contacter must be used.
For sump pumping, the switch _____ (opens or closes) the circuit at a high liquid level, and _____ (opens or closes) the circuit at a low liquid level.
closes, opens
What is a latching relay?
A latching relay is a control relay that is activated electrically but held mechanically. The coil needs only to be energized momentarily for picking-up (latching) or dropping-out the relay.
A paddle arm is sometimes associated with which type of two-wire pilot device?
Flow switch
What type of switch is sometimes installed in ducts of commercial air heating systems to prevent the duct heaters from energizing when there is no air movement in the duct?
Air-flow switch
What device is used to activate the mercury switch contacts in a common thermostat?
Bimetallic element
What type of switch is commonly used to respond to temperature changes in liquids?
Remote bulb, temperature switch
What type of mechanical switch is used to prevent over-travel?
Limit switch
With respect to timing relays, what is meant by on-delay?
On-delay means that, after the relay is energized, the timed contacts do not activate until a preset delay. After this, they remain in the energized state until the relay is de-energized. On-delay is also referred to as time delay on energization (TDE) or time delay on pick-up (TDPU)
Which type of switches are the terms transmitter and receiver commonly associated?
Photoelectric switch
With respect to timing relays, what is meant by off-delay?
Off-delay means that the timing action takes place after the relay is de-energized. When the relay is energized, the timed contacts are activated immediately. The contacts remain in the energized state until the relay is de-energized. at which point they return to normal after the preset time delay. Off-delay is also referred to as time delay on de-energization (TDDE) or time delay on drop-out (TDDO).
A normally open, time-opening contact is associated with which type of time-delay relay?
Off-delay relay, the n.o. contact closes immediately when the relay is energized. It remains in the closed position as long as the relay is energized. When the relay is de-energized, it takes time (it is delayed) before it returns to its n.o. position.
What type of contact is illustrated by the symbol in Figure 1? *(H-2 LT2 Question 6)
Normally-open, time-closing (on-delay contact)
An oil-filled pot and piston assembly is associated with a _____ timer.
dashpot
The movement of air using a bellows or diaphragm assembly is associated with a _____ timer.
pneumatic
What type of timer is best suited for switching on and switching off a fan motor at one-hour intervals throughout the day?
Motor-driven timer
What fundamental principal do electronic or solid-state timers use in their operation?
The R-C time-constant principle. The time to charge a capacitor to a predetermined voltage value is directly proportional to the amount of resistance in series with the capacitor.
Do time-delay relays have instantaneous contacts?
Time delay relays are usually available with both instantaneous and timing contacts.
For the schematic in Figure 2, briefly explain the action of the relay on energization and on de-energization. *(H-2 LT2 Question 12)
The diagram shows an off-delay relay with one timed and one instantaneous contact. When the relay is energized, both sets of contacts activate immediately. The n.c. timing contact opens immediately, and the n.o. instantaneous contact closes. When the relay is de-energized, the instantaneous contact opens immediately, and the timing contact remains open for 5 seconds, after which it closes.
What other name is a plugging switch commonly known by?
Zero-speed switch
What would happen if the plugging-switch contacts in Figure 3 of Learning Task 3 are adjusted to open too late?
*(H-2 LT3 Question 2)
The motor would momentarily start up in the reverse rotation, and could possibly cause damage to the driven equipment, or create a hazard.
What is the purpose of the lockout relay used with a plugging switch?
This relay prevents activation of the plugging switch contacts until power is present at the motor. It is sometimes also called a safety-latch relay.
What is anti-plugging protection?
Anti-plugging uses a zero speed switch to prevent reversing a motor until it has come to a complete stop.
What is the purpose of a control relay?
Allows start/stop action for timing operations, and to provide under-voltage protection for the control circuit.
What type of delay is provided by timing relays? *(H-2 LT4 Question 4 b)
Both timing relays are on-delay relays.
What is another name for a schematic diagram?
ladder diagrams or elementary diagrams
What is another name for a wiring diagram?
connection diagrams
What does it mean to inch a motor?
Same meaning as to jog a motor
What does it mean to reverse the phase sequence?
To interchange any two of the three line leads feeding the motor in order to reverse it.
