Transcript Electrical Safety - HCC Learning Web

Air-Conditioning Control Systems
Electricity for Refrigeration, Heating and Air
Conditioning 7th Edition
Chapter 15 Air-Conditioning Control Systems
Air-Conditioning Control Systems
Upon completion of this chapter the student will be able to:
•
•
•
•
•
•
•
•
Understand the electrical circuitry of a residential condensing unit
Make all electrical connections to install a condensing unit in a residential
application
Troubleshoot a residential condensing unit
Understand the basic control systems used in residential air-conditioning
control systems
Understand the control systems used in light commercial air-cooled and
water-cooled packaged units
Make all electrical connections for a complete residential installation
Understand the control systems used In gas heat electric air-conditioning
packaged units
Troubleshoot residential air conditioning system
Key Terms
•
•
•
•
•
•
•
•
•
•
Air-Cooled Package Unit
Condensing Unit
Factory-Installed Wiring
Field Wiring
Gas Pack
Indoor Fan Relay Package
Internal Pressure Relief Valve
Packaged Air-Conditioning Unit
Short-cycling
Water-Cooled Packaged Unit
Basic Condensing Units
•
•
•
•
A condensing unit is the portion of a split air-conditioning system that is
mounted outside and contains the compressor, the condenser, the
condenser fan motor, and the necessary devices to control these
components.
A split system is one that is divided into two parts, usually a condensing unit
(outside) with a fan coil unit (inside).
In most cases, the condensing unit is used with some type of equipment
that will produce the air flow, and a coil must be mounted in the air flow.
Most condensing units used in the industry today are air-cooled condensing
units.
Condensing Unit Components for a Simple
Control System
•
•
•
•
•
Most modem condensing units pick up their 24-volt power supply to operate
the control system from the furnace transformer or from an indoor fan relay
package.
The evaporator fan motor usually is controlled by a relay when the system
contains a furnace or fan coil unit and the condensing unit.
The compressor and condenser fan motor in a condensing unit are usually
controlled by a contactor.
The simplest control system used on condensing units today is a contactor
that controls the operation of the entire condensing unit with the exception
of the internal overloads in the compressor and condenser fan motor.
Most condensing units without a high-pressure switch will have an internal
pressure relief valve in the compressor that opens if the discharge pressure
exceeds an unsafe level.
Condensing Unit Components for Complex
Control Systems
•
•
•
•
•
The more complex systems incorporate a high-pressure and a
low-pressure switch.
If any of the safety controls open, the compressor and condenser fan
motor stop.
Several manufacturers also use a device that protects the system from
short-cycling.
This device maintains a certain period of time between the cycles of the
equipment so the system does not cut on and off in rapid succession
(short-cycling).
This control system adds a relay and a time clock mechanism for this
purpose.
Wiring
•
•
All condensing units come from the manufacturers with a wiring diagram
and, in some cases, with an installation wiring diagram.
Usually it is not difficult to follow the installation instructions and wire the
condensing unit correctly.
Troubleshooting
•
•
Any troubleshooting of the condensing unit can be done from the schematic because of the simplicity of most
control systems.
By using the schematic diagram, and having an understanding of the components, you should have no trouble
with diagnosing a residential control system.
Packaged Units
•
•
•
•
•
•
•
•
•
A packaged air-conditioning unit is built with all the components housed in one unit.
In most cases, packaged units are complete except for the power connections and the control connections.
Air-cooled packaged units are the most widely used types of packaged units.
The units are used on applications ranging from small residential cooling units to large commercial and industrial
air-cooled units.
The smaller residential systems may be installed when the structure is built or at a later date.
The air-cooled packaged unit contains the compressor, evaporator fan motor, condenser fan motor if not of
remote design, and all the necessary controls.
Water-cooled packaged units are usually used on commercial and industrial systems.
The water-cooled unit requires a cooling tower or some means of supplying water to the condenser.
If a water-cooled unit is used, there must be interlocks to ensure that the cooling tower pump is operating when
the compressor is operating.
Control System for Packaged Air-Conditioning
Systems
•
•
•
•
•
Water-cooled packaged units utilize a line voltage control system with some
type of interlock to start the cooling tower pump or other accessories that
are required.
Air-cooled packaged units can incorporate some form of heating as well as
cooling.
The smaller packaged units usually have some easy means of installing
electric resistance heat into the equipment.
The larger air-cooled packaged units make electric and gas heat available
to the customer.
Water-cooled packaged units and air-cooled packaged units usually use a
hot-water coil, steam coil, or resistance heat for heating purposes.
Small Air-Cooled Packaged Units
•
•
•
•
The small air-cooled packaged units usually have a simple and easy-tofollow control system.
The control system uses a contactor to operate the compressor and the
condenser fan motor and an evaporator fan relay to control the evaporator
fan motor.
All necessary safety components are also included in the control system.
The installation of the air-cooled packaged unit is relatively simple because
the only necessary connections are from the power source and the control
source or thermostat.
Gas-Electric Air Conditioners
•
•
•
The basic control system for the air conditioning is the same as the one for
most small residential units.
However, several components are added to take care of the heating
control system.
The installation instructions should be followed when installing this type
of system.
Rooftop Units
•
•
•
•
The air-cooled packaged units used in rooftop applications are complex
because they usually have some type of heating to go along with the air
cooling and, in some cases, some type of zone control.
These units are usually installed on the roof of a structure.
But they could just as easily be installed on ground level when the
necessary changes are made in the ductwork connections.
Often the electric connections of these systems are hard to install because
of the number of wires required for the control circuit and the power wiring.
Field Wiring
•
•
•
•
Wiring that must be installed by the installation technician and is called
field wiring.
The factory-installed wiring is the wiring installed at the factory.
The factory wiring has been sized, color-coded, and installed in the control
system to operate the equipment properly.
The remainder of the wiring, whether it be the power wiring or control wiring,
must be connected in the field.
Power Wiring
•
•
•
There are two power connections that must be made on a split airconditioning and heating system: the connections to the condensing unit
and the connections to the evaporator fan motor or furnace.
The electric supply wiring of a commercial and industrial system is somewhat more complex than the simple residential systems.
The use of three-phase current in commercial and industrial structures does
not add to the complexity of the system, only to the number of power wires
that must be supplied to the system.
Sizing Wires and Fuses
•
•
•
•
The distance that the power wiring must be run is important because of the
voltage drop that can occur on long circuit runs.
The installation instructions usually give a wiring chart with the length that is
allowable for each size of wire.
It is also important to follow the manufacturer’s recommendations for the
fuse or breaker size.
Before any heating, cooling, or refrigeration system can be expected to
operate properly, it must first be supplied with the correct size wire and fuse
to deliver the proper voltage to the system.
Control Wiring
•
•
•
•
The control wiring of heating and air-conditioning systems is just as
important as the supply power wiring.
Low-voltage control systems are used on residential and small commercial
applications.
On large commercial and industrial applications, line voltage control
systems are often used.
Many large commercial and industrial systems are controlled by special
control systems: electric, pneumatic, or electronic.
Residential
•
•
•
•
•
All residential packaged air conditioners use a low-voltage control system
that connects the thermostat to the unit.
Residential split systems are usually easy to install.
They all incorporate a 24-volt control system, with the low-voltage power
being supplied from an indoor fan relay package, the furnace, or the
condensing units.
The control system of a residential split system can be connected several
different ways, and the manufacturer’s installation instructions should be
followed.
The more modern condensing units are usually designed without the
low-voltage transformer but with two low-voltage connections instead.
Industrial
•
•
•
Control systems for relatively small commercial units are similar in design to
the residential low-voltage system.
The only exception is that the system uses a control relay that supplies the
condensing unit with 240 volts while using a normal 24-volt control system.
Two-stage heating or two-stage cooling can also be used with a low-voltage
control system.
Sizing Wire
•
•
•
•
•
Control wiring in most cases is a small wire (No. 18 to No. 20) that is usually
covered by a rubber jacket when using a low-voltage control system.
Thermostat wire can be purchased single-stranded or multi-stranded, with
or without a rubber jacket protecting the small thermostat wires.
The wiring of air-conditioning and heating systems is one of the most
important factors in the installation of equipment.
The life of the equipment can be drastically cut by undersized wire or by
having loose connections.
The installation instructions are the best place to look for the proper method
of wiring and the proper connection points.
Schematic Diagram of a Simple Air-Cooled
Condensing Unit
Schematic Diagram of a Condensing Unit with
Safety Controls
Schematic Diagram of a Small Air-Cooled
Packaged Unit Used in a Residence
Schematic Diagram for Electric Air-Conditioning
and Gas-Heating Unit
Control System of a Furnace