ELECTRICAL
CONTROL AND
INSTRUMENTATION
14.1 INSTRUMENTATION AND
CONTROL
This chapter concentrates on the electrical equipment associated with the
refrigeration system. It focuses on controlling the electrical supply to the system
and on electrical and/or electronic sensors and actuators. Because the full range
of electrical topics is so vast, a selection has been made, as illustrated in Fig.
14.1, such that the subjects treated in this chapter are those most intimately
connected with the refrigeration system. Those topics not covered or only
mentioned in passing include, for example, electrical circuits (single- and threephase,
wye and delta), conductor sizing, circuit protection, power factors and
their corrections, and transformers. Another important body of knowledge that
is not covered is that of motors—the types of industrial motors and their
characteristics.
The electrical principles that will be explained are those related to controlling
the power to electric motors and other equipment and the electrical types of
instrumentation and actuators. The first major topic presented will be ladder
diagrams, which are the widely accepted means of showing control logic and
are the plans followed by electricians. Means of executing ladder diagrams
include electromechanical relays, programmable controllers, and computer
controllers. Instruments providing visual indications (pressure gauges,
thermometers, etc.) are standard and should continue to be installed. The shift
in recent years to electronic and/or computer control, however, stimulates a
greater need for electric and electronic sensors, transducers, and actuators.
These devices will be explained in the latter sections of this chapter.
14.2 LADDER DIAGRAM SYMBOLS
Ladder diagrams serve two purposes—they represent the plan for hardwiring
a panel of electromechanical devices and they also represent the logic of the
control plan. Logic means the conditions that must be met before a certain
action is taken. The ultimate action of most ladder diagrams is to provide electric
power or to interrupt power to motors and other electrical devices. The symbols1
that are probably the most used in ladder diagrams are shown in Fig. 14.2 for
manual switches, Fig. 14.3 for switches controlled by physical variables and
other conditions, Fig. 14.4 for timing switches, Fig. 14.5 for symbols referring
to the controls for the power portion of the electric system, and Fig. 14.6 for
miscellaneous symbols.
A toggle switch, as shown in Fig. 14.2, retains its position (open or closed)
until manually changed. Push buttons are designed for momentary contact or
interruption. A dashed line indicates a mechanical linkage between two push
buttons, which in Fig. 14.2 shows one push button opening and the other closing
a contact when the button is pressed.
In Fig. 14.3, the switch changes from its normal position when the sensed
variable increases above its setting. For example, the normally closed (NC)
temperature switch set for 40°C (104°F) is closed when the sensed temperature
is below 40°C (104°F) and open when the temperature is above the setting. If a
low-temperature cutout is to open a switch when the temperature drops below
0°C (32°F), for example, a normally open (NO) switch would be chosen and set
for the temperature. During satisfactory operation above 0°C (32°F), the switch
is in its non-normal state (closed).
The symbols for another class of components used in ladder diagrams are
shown in Fig. 14.4 and apply to timing switches. Most timing switches are
single-throw, but double-throw switches are also available, as shown in Figs.
14.4e and 14.4f. One class of timing switches is indicated by the upward-pointing
arrow ↑ and another by a downward-pointing arrow ↓, representing a delay on
and a delay off, respectively. The energizing of the coil initiates the time delay
of a delay-on switch, while the denergizing of the coil initiates the time delay of
a delay-off timing switch.
Figures 14.4a and 14.4c show NC switches, and Figs. 14.4b and 14.4d show
NO switches. The NC, timed-open, delay-on switch of Fig. 14.4a begins the
timing upon energizing of the coil and opens the contacts following the specified
delay. When the coil is deenergized, the contacts immediately return to their
NC status. Should the coil be deenergized during the delay period, the switch
remains closed and the timer is reset to zero. The NO, timed-closed, delay-on
timing switch of Fig. 14.4b begins the timing operation upon energizing of the
coil and closes the switch following the delay. When the coil is deenergized, the
contacts immediately return to their NO status. Should the coil be deenergized
during the delay period, the contacts remain open and the timer is reset to zero.
The down arrow ↓ timer switches in Fig. 14.4c and 14.4d are in the status
shown (normally closed or normally open, respectively) when the coil has been
deenergized for some time. When the coil is energized, the switch changes
instantly to its nonnormal status, at which condition it remains so long as the
coil is energized. When the coil is deenergized, timing begins, and following the
specified delay, the contacts revert to their normal position. If the coil should
be energized during the delay period, the contacts return to their nonnormal
status and the timer resets to zero.
The double-throw timing switch of Fig. 14.4e is a combination of the switches
in Figs. 14.4a and 14.4b. The status of the blade shown occurs when the coil has
been deenergized for a period of time. When the coil is energized, the timer
begins, and following the specified delay, the blade changes from the NC contact
to the NO contact, where it remains so long as the coil is energized.
Deenergization of the coil returns the blade to the NC position instantly. Some
commercial timing switches of this type are supplied with power continuously,
and what is referred to as energizing and deenergizing of the coil is achieved by
closing and opening, respectively, external contacts.
The double-throw, delay-off timing switch of Fig. 14.4f combines the functions
of the switches in Figs. 14.4c and 14.4d in the following manner. The position of
the switch shown is what occurs when the coil has been deenergized for a period
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of time. Energization of the coil changes the blade instantly from the NC post
to the NO post. Deenergization of the coil initiates the timer. Following the
specified delay, the blade changes to contact the NC post.
Several other symbols sometimes associated with ladder diagrams that refer
to the power portion of the electrical network-are shown in Fig. 14.5. To show
that two crossing electrical lines are not physically connected, it is only necessary
to show the cross without a heavy dot. This practice is standard in the electronic
industry since the diagrams sometimes become massive. For drawings applicable
to refrigeration systems often used in industrial workplaces, the semicircular
bypass avoids the risk of misreading a soiled drawing.
The symbols for coils of relays, solenoids, contacts, and several other
components used in ladder diagrams are shown in Fig. 14.6
14.3 LADDER DIAGRAMS
The ladder diagram gets its name from the appearance of the network, which
consists of the horizontal rungs extending from one vertical rail to the other.
The left rail is designated L1 and the right rail L2. Usually, L1 is hot and is at
a voltage of 115 V ac, for example, and L2 is usually neutral. In some cases,
however, L2 is 115 V ac of opposite polarity to L1, so that 230 V prevails across
the ladder. When a voltage is applied across a control relay (CR), any contact
associated with this relay is switched to the opposite of its normal position. A
simple ladder diagram depicting the operation of the compressor and condenser
fan motors and the heater for the compressor crankcase is shown in Fig. 14.7.
The compressor is started by pressing a push button, designated 2PB, and
stopped by pressing a different button, 1PB. When the start button is pressed,
the crankcase heater comes on if needed until the temperature of the oil rises
to a temperature of 30° C (86°F). When the desired oil temperature has been
reached, the condenser fan starts and runs for 10 s before the compressor motor
starts. Once the compressor is operating, there is no further need to warm the
oil, and the heater shuts off.
The plan expressed by the ladder diagram in Fig. 14.7 is as follows. To start
the sequence, press push button 2PB, which feeds power to the left side of 1CR
Control relay 1CR changes the status of all contacts associated with it. One of
these contacts is 1CR-1, which is wired in parallel to 2PB. Closing this contact
permits 1CR to remain energized even after 2PB is released. Another contact
associated with 1CR is 1CR-2, which closes. Because 3CR-1 is normally closed,
power is fed to the crankcase heater, 1HTR. The final contact associated with
1CR is 1CR-3, which closes. If the oil temperature is not yet up to the desired
value of 30°C (86°F), temperature switch T1 is open and no power reaches 2CR
and TR-1. When the oil temperature rises to the point that T1 closes, 2CR and
TR-1 are energized. The energizing of 2CR closes the contacts in the 3-phase
lines to the condenser fan motor and also closes the contacts in line 6. Energizing
TR-1 starts the timing switch, which after a 10 s delay energizes 3CR. Energizing
3CR closes the contacts in the 3-phase lines to the compressor motor, thus
starting that motor. Another contact served by 3CR, namely 3CR-1, turns off
the crankcase heater.
To stop the compressor, press 1PB, which interrupts power to 1CR. Contact
1CR-1 reverts to its NO position and deenergizes 1CR even after 1PB is released.
Contact 1CR-2 returns to its normally-open position, but the heater had been
off anyway, so the status of the heater remains unchanged. Contact 1CR-3 opens
so that 2CR is deenergized, which opens the contacts in the 3-phase lines to the
condenser motor and also opens 2CR-1, which deenergizes 3CR to stop the
compressor motor.
Also illustrated in Fig. 14.7 is a recommended numbering pattern. Each of
the rungs is numbered in sequence down the left rail, and each of the control
relays, contacts, and other switches are given unique numbers. The designation
2CR-1(4), for example, indicates a contact whose status is changed by control
relay 2CR. The number 1 indicates that this contact is the first one associated
with 2CR, and the number 4 in parentheses designates the rung number in
which 2CR is located. The numbers down the right rail indicate the rungs in
which contacts are located that are associated with any CR of that rung. In
rung 1, for example, 1CR activates contacts in rungs 2, 3, and 4.
Example 14.1. At the moment that 3CR is energized, starting the compressor
motor and shutting off the crankcase heater, some remaining liquid
refrigerant in the crankcase oil evaporates to cool the oil and open the
temperature switch T1. What is the response of the control logic thereafter?
Solution. The opening of T1 deenergizes 2CR and TR-1. The condenser
fan stops, and the contacts of 2CR-1 change to their NO status, which
deenergizes 3CR, stopping the compressor motor and allowing the contacts
of 3CR-1 to return to their NC status. Coil 1CR has remained energized, so
the heater resumes operation and another startup is attempted
14.4 LADDER DIAGRAM FOR A
SCREW COMPRESSOR
To illustrate a more complex control logic, Fig. 14.8 shows a ladder diagram for
the implementation of many of the functions needed to start, stop, and reset a
screw compressor package. Most of the standard features are incorporated, but
to provide a gradual progression to full realism, the control of the slide valve
and the capability of automatic remote control have been omitted. A complete
example is presented in Reference 2, which in turn is taken from a
manufacturer’s manual.3 Those functions that are incorporated are the following:
1. Push-button start.
2. Cutouts to stop the compressor and oil pump in the event of:
• oil pressure failure lasting longer than 6 s.
• low oil temperature.
• high temperature of discharge refrigerant.
• low suction pressure.
• high discharge pressure.
• overloads of motors serving compressor and oil pump.
3. Push-button stop and reset. The function of the reset is to prevent the
compressor from automatically starting up again after a cutout has stopped
operation.
4. Operation of the oil pump for 30 s prior to starting the compressor in order
to allow the slide valve to move to its unloaded position (the slide valve
operation is not shown in Fig. 14.8).
5. Anti-recycle timing, which prevents a restart of the compressor motor within
20 min of the previous start.
6. Keeping the oil warm during compressor shutdown and turning the heater
off during compressor operation.
The following is a brief, line by line description of each switch, contact, and
relay in Fig. 14.8:
1. 1TR-1 is normally open (NO) but closed if 1TR is energized, and it opens 6
s after 1TR is deenergized. 1TS is NO and closes on satisfactory high oil
temperature. 2TS is normally closed (NC) but opens on high refrigerant
discharge temperature. 1PS is NO but closes on adequately high suction
pressure. The motor overload cutouts are NC but open on high motor
current. 2PS is NC but opens on high discharge pressure. 1PB is a NC
push button for stopping. 2PB is a NO push button for starting. 1CR is a
relay coil.
2. The pilot lights are in parallel with each cutout and are off when the cutout
is closed, because most current passes through the switch. If the switch is
open, the light is illuminated. 1CR-1 continues to energize 1CR after the
start button is released
3. 3PB is a NO push button reset mechanically connected to the STOP button.
4. 2TR-1 is NC but opens 30 s after energizing 2TR, providing power to 1TR
on startup. 1TR is a relay coil that remains energized if the oil pressure
differential is above 140 kPa (20 psi).
5. If the oil pressure differential is greater than 140 kPa (20 psi), then the
contact of 3PS is with A; but if the differential pressure is very low, the
blade is in contact with B.
6. 4TR-1 is a special timing switch, combining both an electrical and pneumatic
capability. It is NC and opens when 4TR is energized. The switch remains
open for 20-min before reclosing. Because of the pneumatic capability, the
20 min timing does not require electric power. 1M is the oil pump motor
relay coil.
7. A pilot light, which is in parallel with 4TR-1, lights when 4TR-1 is open.
2TR is a relay coil initiating the two 30 s time delays of 2TR-1 and 2TR-3,
as well as closing contact 2TR-2.
8. 2TR-2 contact is closed by energizing 2TR and continues to provide power
to 2TR as long as 1CR-2 is closed.
9. 2TR-3 is NO but closes 30 s following the energization of 2TR to provide
power to the compressor relay coil, 2CR. 2CR-3 is NC and opens by relay
coil 2CR. 4TR is an anti-recycle relay coil that is energized only momentarily
on startup of the compressor.
10. 4TR-2 is NO and closes at the momentary energizing of 4TR and remains
closed for 20 min.
11. When 4TR-2 is closed, 2CR relay coil is energized, which closes 2CR-2 to
maintain power to 2CR even after 4TR-2 reopens.
12. 2M is the compressor motor relay.
13. 3CR-1 is a NO contact serving the oil heater.
14. 3TS is a thermostatic switch that closes when the oil temperature drops
lower then 43° C (110°F) and opens when the oil temperature rises above
49°C (120°F).
Example 14.2. Is the heater on or off when the oil temperature is (a) 40°C (104°F),
(b) 50°C (122°F), and and (c) 45°C (113°F)?
Solution. If the compressor is running, 2CR is energized and the NC contact in
line 14 is open, deenergizing 3CR. Therefore, the heater will be off, regardless of
the oil temperature. If the compressor is not operating (a) 3 TS is closed, 3CR is
energized, and the heater is on; (b) 3TS is open, 3CR is deenergized, and the heater
is off; (c) could be either on or off depending upon whether the oil is cooling off following
the heater switching off or warming up following the heater switching on.
Example 14.3. What are the states of relay coils, switches, and contacts when the
compressor and oil pump have been operating for more than 20 min
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