Assignment: 55aKB9
ELET B55a Knowledge Base Assignment #9
Relays
Text Chapter 7
Student‘s Name:



ANSWER KEY
For “Knowledge Base” assignments, each student needs to submit their own completed assignment.
A score of zero will be recorded for all students involved who submit duplicate work from another
student.
You should always retain a backup copy of this document in your college “home” drive or on your computer.
You can always e-mail yourself a copy of the document file as an attachment.
Knowledge Checklist
The knowledge and skills you should retain after completing this assignment are:
1. Compare electromagnetic, solid state, timing, and latching relays in terms of
construction, operation, symbols, applications, and ratings.
2. Describe the operation of on-delay and off-delay timer relays.
3. Discuss the use of relays as control elements in motor circuits.
Assignment
Answer the following questions, or perform the requested activities. When you have completed
this assignment, save it using the proper file name format, and then upload and submit it,
following the instructions your instructor has given you.
Textbook Chapter 7: Parts 1-5
Part 1: Electromechanical Control Relays
1. The two basic parts of an electromechanical relay are the
__Coil (input)_______ and the __Contacts (output)____ .
2.
The two circuits associated with a relay are the:
a. Primary and secondary
b. Control and load
c. Normally open and normally closed
d. High and low
ET55a Knowledge Base Assignment #9
Answer ___b___
Page 1
3.
Complete the relay wiring diagram for Figure 7-1 to operate so that
opening and closing the switch will control current to the load through the
relay contact.
4.
Relays are used to control large loads of 15A or more.
(True/Fa1se) ___False_____ (less than 15A)
5.
An electromechanical relay normally will have only one ___Coil_______ ,
but it may have a number of different ____Contacts_________ .
6.
Normally closed contacts are defined as those contacts that are closed
when the coil is ____not energized_____ .
7.
State the correct type (N.O. or N.C.) and label for each of
the contacts associated with the relay coil of Figure 7-2.
(a) _______N.O._________
(b) _______N.C._________
ET55a Knowledge Base Assignment #9
Figure 7-2
Page 2
8. Which of the following is not a common application for a relay?
a. To control a 120-V lighting circuit with a 12-V
control circuit.
b. To control several switching operations by a
single separate current.
c. To change alternating current to direct current.
d. To switch a small servo motor with a lowcurrent sensor signal.
9.
Answer ___c____
The coil and contacts of a relay are not normally electrically insulated from
each other. (True/Fa1se) ___False________ (they are insulated)
10. Complete the relay wiring diagram for Figure 7-3 to operate so that when
the push button is closed, the green light switches from on to off and the
red light switches from off to on.
11. Relay options that aid in troubleshooting may include:
a. ________On-Off Indicator______________________
b. ________Manual Override______________________
12. It is possible for a relay coil to be rated for 24 V DC and its contacts to be
rated for 240 VAC. (True/Fa1se) ___True_____
ET55a Knowledge Base Assignment #9
Page 3
13. Electromechanical relay contacts may be constructed as
a. N.O. or N.C. contacts.
b. Single-break or double-break contacts.
c. Fixed and movable contacts.
Answer ___d____
d. All of these
14. State the number of poles, throws, and breaks for each of the relay
contacts shown in Figure 7-4.
a. __1___ pole, ___2___ throw,
__1___ break
b. ___2__ pole, ___2____ throw,
___1__ break
c. ___1__ pole, ___2____ throw,
___2__ break
d. ___3__ pole, ___2____ throw,
___2__ break
15. The load-carrying capacity of contacts is normally given as a current value
for a:
a. Resistive load
b. Inductive load
c. Capacitive load
Answer ____a____
d. Shorted load
16. A relay contact rates 12A at 12VDC can switch _____3_____ A at 48VDC.
P = I x E = 12A x 144W
I = P/E = 144W / 12V = 3A
ET55a Knowledge Base Assignment #9
Page 4
Part 2: Solid-State Relays
1.
In the symbol of the solid-state relay (SSR) shown
in Figure 7-5, the #1 set of leads is comparable to
the ______Coil_______________ leads of an
electromechanical relay, and the #2 set to the
_______Contact______________ leads.
2.
List three common main switching semiconductors used in solid-state
relays:
a. ______Bipolar Transistors___________________________
b. ______MOSFETs_____________________________________
c. ______SCR’s or Triacs_______________________________
3.
A common method used to provide isolation is to have the input section
illuminate which of the following items below, which will activate a
photodetective device?
a. LED
b. SCR
c. Transistor
d. Triac
4.
Answer ___a____
Identify the components of the solid-state relay shown in figure 7-6:
a. ____LED________________________
b. ____Phototransistor____________
c. _____Trigger Circuit_____________
d. _____Triac_______________________
ET55a Knowledge Base Assignment #9
Figure 7-6
Page 5
5.
The input section of the solid-state relay (SSR) acts like the contacts of an
electromagnetic relay (EMR). (True/Fa1se) ____True______
6.
The use of solid-state relays is not recommended for loads that must be
switched continually and quickly. (True/Fa1se) __False__
(greatly recommended)
7.
Most SSRs have a variable input voltage range. (True/Fa1se) ___True____
8.
A majority of SSRs are multiple-pole devices. (True/Fa1se) ___False__
(Most are unipolar – l pole)
9.
Complete the solid-state relay wiring diagram in Figure 7-7 to operate so
that when the control contact is closed, three-phase power is delivered to
the load.
ET55a Knowledge Base Assignment #9
Page 6
10. Complete the three-wire motor control diagram for Figure 7-8 utilizing the
SCR and solid-state relay. The circuit is to operate so that the starter coil
is energized when the start button is momentarily pressed and deenergized
whenever either of the two stop buttons is momentarily pressed.
11. What type of SSR switching is primarily designed to work with resistive
loads?
a. Instant-on switching
b. Peak switching
c. Phase-shift switching
d. Zero switching
Answer __d____
12. ___Instant On_______________________ SSR switching initiates the output
at the exact same time the input signal is received.
13. Zero switching of resistive loads increases the inrush current.
(True/Fa1se) ____False_____ (no effect on inrush current)
ET55a Knowledge Base Assignment #9
Page 7
14. Compared to electromechanical relays, solid-state relays:
a. Do not exhibit contact bounce
b. Have a much faster response time
c. Do not generate as much
electromagnetic interference
d. All of these
Answer __d_____
15. Compared to EMR contacts, the SSR switching semiconductor has a much
lower on-state resistance. (True/Fa1se) __False____ (the opposite is true)
Part 3: Timing Relays
1.
Timing relays are used to ___delay________________ the opening or closing
of contacts for circuit control.
2.
Contacts of a synchronous clock timer are tripped by
a. Hand
b. An electromagnet
c. Tabs set along a timing wheel
d. A control signal
3.
Answer ___c___
Synchronous clock timers are not accurate enough for critical timing
processes. (True/Fa1se) ___True_____
4.
Timing is managed in a dashpot pneumatic timer by controlling
____Fluid or Air___________ flow through a small orifice.
ET55a Knowledge Base Assignment #9
Page 8
5. Pneumatic timers have a relatively narrow timing range.
(True/Fa1se) ___True______
6.
Compared with dashpot timers, solid-state timers:
a. Provide much more accurate timing
b. Have a narrower timing range
c. Are initiated by an electromagnetic coil
d. All of these
Answer ___a___
7.
Complete the solid-state timer wiring
diagram for Figure 7-9. The circuit is to
operate so that load is energized at 120
V AC following the closure of the float
switch and preset time delay period.
8.
Draw the symbol for the timer contacts listed below:
NOTC On-Delay
Normally open, delay on energize
NOTO
Off-Delay
Normally open, delay on deenergize
NCTO
On-Delay
Normally closed, delay on energize
NCTC
Off Delay
Normally closed, delay on deenergize
ET55a Knowledge Base Assignment #9
Page 9
9.
Examine the timing diagrams of Figure 7-10 and identify the timer type as
DOE, DODE, one-shot, or recycle.
(a) ____DODE__________
(b) ____DOE____________
(c) ____Recycle__________
(d) ____One-Shot________
10. Answer the following with reference to the timer circuit of Figure 7-11:
a. What is the state (open or closed) of contact
TR1-1 prior to the switch being closed?
Answer _Open__
b. What is the state of contact TR1-2
immediately after the switch is closed?
Answer _Closed_
c.
What is the state of contact TR1-1 after 5
seconds of the switch being closed?
Figure 7-11
Answer _Open__
d. What is the state of contact TRl-2 after 10
seconds of the switch being closed?
Answer _Open__
e.
What is the state of contact TR1-1 as soon as
the switch is opened?
Answer _Open__
ET55a Knowledge Base Assignment #9
Page 10
11. Answer the following with reference to the timer circuit of Figure 7-12:
a. What is the state (open or closed) of contact
TR1-2 prior to the switch being closed?
Answer _Closed_
b. What is the state of contact TR1-1 immediately
after the switch is closed?
Answer _Closed_
c. What is the state of contact TR1-2 immediately
after the switch is opened?
Figure 7-12
Answer _Open__
d. What is the state of contact TR1-1 after 10
seconds of the switch being opened?
Answer _Open__
e. What is the state of contact TR1-2 after 10
seconds of the switch being opened?
Answer _Closed_
12. Answer the following with reference to the plug-in timer base wiring
diagram of Figure 7-13:
a. What are the pin numbers for the normally
open contact?
Answer _3 and 1_
b. What are the pin numbers for the normally
closed contact?
Answer _4 and 1_
c. Identify the pin numbers where the constant
voltage source is to be connected:
Answer _2 and 7_
d. Identify the pin numbers where an external
trigger switch is to be connected:
Answer _5 and 6_
ET55a Knowledge Base Assignment #9
Page 11
13. The one-shot timer shown in Figure 7-14 is set for a time delay period of 5
seconds. For how long would the load be energized for each of the
following scenarios (conditions)?
a. The initiate button is held closed for 1 second:
Answer _5 seconds___
b. The initiate button is held closed for 5 seconds:
Answer _10 seconds___
c. The initiate button is held closed for 10
seconds:
Answer _15 seconds___
d. The initiate button is held closed for 2 seconds,
but power is lost after 1 second:
Answer _1 second___
14. Answer the following
with reference to the
recycle timer shown in
Figure 7-15:
a. What type of recycle timer (symmetrical or asymmetrical) is this?
Answer ___Asymmetrical_________________________
b. How is the on period timing adjustment accomplished?
Answer __Separate Adjustment Knob _____________
c. How is the off period timing adjustment accomplished?
Answer __Separate Adjustment Knob _____________
d. How is the on/off recycling of the contacts stopped?
Answer __Remove Input Voltage_____________________
ET55a Knowledge Base Assignment #9
Page 12
15. The term __Multifunction________ timer is reserved for a group of solidstate timers that perform more than one timing function.
16. A programmable logic controller can be programmed to implement
conventional timer functions. (True/Fa1se) __True______
Part 4: Latching Relays
1. Latching relays use a _____Mechanical_________ latch or permanent
______Magnet_______________ to hold the contacts in their last energized
position.
2. In a latching relay that uses two coils, the coils are identified as the
____Latch______ coil and the __Unlatch or Release____________ coil.
3. Latching relays can provide circuit continuity during power failures.
(True/Fa1se) __True______
4.
In a two-coil latching relay, the latch coil requires what type of a flow of
current to set the latch and hold the relay in the latched position?
a. Direct
b. Alternating
c.
Continuous
d. Pulsed
5.
Answer ___d____
On electrical diagrams, the contacts of a latching relay are normally
shown:
a. in their latched position
b. in their unlatched position
c.
energized
d. deenergized
ET55a Knowledge Base Assignment #9
Answer ___b____
Page 13
6.
Answer the following with reference to the latching relay circuit of
Figure 7-17:
a. How is the pilot light turned on?
Answer _Press on button-this latches contact
on
b. How is the pilot light turned off?
Answer _Press off button-this unlatches
contact
Figure 7-17
c. Assume the unlatch coil is faulted open. How would this affect the
operation of the circuit?
Answer The contact will stay closed–will not be able to open it up.
______________________________________________________________________
______________________________________________________________________
7.
Single-coil magnetic latching relays are designed to be polarity sensitive.
(True/Fa1se) __True______
8.
Alternating relays are used to equalize the ____run______________
_________time_________________ of two loads.
9.
_______crossed____________ - _______wired____________ alternating relays
have the ability to alternate the loads during normal workloads and then
operate both when the load is high.
ET55a Knowledge Base Assignment #9
Page 14
10. Answer the following with reference to the alternating relay circuit of
Figure 7-18:
a. What configuration of relay contacts is used?
Answer ______DPDT________________________
b. What initiates the switch in output between Load
A (LA) and Load B (LB)?
Answer ___Opening Control Switch S1_____
c. Remote pilot lights are to be connected to the
unused contacts of the relay to indicate which
load is active. To which two pins would the pilot
lights for load A (LA) and load B (LB) be
connected?
Figure 7-18
Answer __Load A (pin 9) and Load B (pin 10)__
Part 5: Relay Control Logic
1. Identify the two states of the digital
signal shown in Figure 7-19:
a. ___Low (Off)____________
b. ___High (On)___________
2.
Relay control logic and ladder diagrams are the primary programming
language for ___PLC’s (Programmable Logic Controllers)_____ .
3.
The two separate and distinct parts of a motor control circuit are the
primary and secondary sections. (True/Fa1se) _False__ (input and output)
4.
The input section of a control circuit provides the:
a. Signals
b. Source
c. Actions
d. Load
ET55a Knowledge Base Assignment #9
Answer ___a____
Page 15
5. Answer the following with reference to the control circuit of Figure 7-20.
a. List all the component parts that are
considered to be inputs:
Answer ___On/Off switches, limit switch
Float switch, temperature switch, relay
contacts____________________________
b. List all the component parts that are
considered to be outputs:
Answer __Relay coil, horn, solenoid, ____
Contactor coil, pilot light, heater unit___
___________________________________________
c. Which output would be energized with power
applied and the circuit in the state shown?
Answer ____Solenoid_____________________
___________________________________________
d. Assume that the contactor coil becomes
faulted open. In what way would the normal
operation of the circuit be affected when the
temperature switch closes?
Answer ____Contactor coil would not close,
and heater unit would not energize. _____
____________________________________________
____________________________________________
6.
All input devices can be classified as being loads.
(True/Fa1se) ___False____ (output devices)
ET55a Knowledge Base Assignment #9
Page 16
7.
Logic functions describe how inputs interact with each other to control the
outputs. (True/Fa1se) __True ____
8.
Logic is the ability to make decisions when one or more different factors
must be taken into consideration. (True/Fa1se) __True ____
9.
Identify the control logic function associated with each of the contact
configurations shown in Figure 7-21.
a. ____OR______________________
b. _____NOT____________________
c. _____NAND__________________
d. ____AND____________________
e. _____NOR____________________
f. _____AND/OR________________
Figure 7-21
10. State the type of logic function described in each of the following control
circuit scenarios:
a. Two or more inputs are connected in series and they all must be closed
to energize the load. Answer ______AND____________
b. Two or more inputs are connected in parallel and anyone of the inputs
can close to energize the load. Answer _____OR_____________
ET55a Knowledge Base Assignment #9
Page 17
11. Draw an OR logic circuit in which a normally open push button and
temperature switch control a resistive heating element:
12. Draw a NOT logic circuit in which a solenoid and pilot light are controlled
by a single push button. Both loads are to be energized when the button is
not pressed, and deenergized when the button is pressed:
13. Draw a combination NOR/OR logic circuit that uses a three-wire control to
energize a magnetic motor starter. The motor can be started and stopped
from three locations. Include overload protection for the motor:
ET55a Knowledge Base Assignment #9
Page 18