Troubleshooting & Servicing
HVACR
Electrical Systems
for Non-Electricians
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Section 1
WHAT IS ELECTRICITY?
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ELECTRONS
Atoms are made of particles called protons, neutrons, and electrons.
Protons have a positive charge. +
Electrons have a negative charge. -
(Neutrons have no charge and have no electrical effect.)
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POTENTIAL DIFFERENCE
An imbalance of electrons is called a potential difference, or
an electromotive force (emf).
A potential difference can be created by:
Friction (static electricity)
Chemical action (batteries)
Magnetic activity (generators)
Thermoelectric (heat)
Photoelectric (light)
The unit of measurement of emf is the VOLT.
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MEASURING VOLTS
Voltmeters are used to measure potential
difference between two specific points.
Voltmeters are available in analog or
digital types.
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POTENTIAL DIFFERENCE
There must be a potential difference for the meter to register a
voltage reading.
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POTENTIAL DIFFERENCE
The voltage tester reads zero when no potential difference exists
between the two probes.
Likewise, if the voltage is 120 at both probes, the meter reads zero.
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POTENTIAL DIFFERENCE
NEVER TOUCH AN ELECTRICAL WIRE BECAUSE
A ZERO VOLTAGE READING WAS OBTAINED!!
WARNING!
You may be reading the same
potential (no difference) between
the probes.
Additional tests are required to
determine if voltage is or is not
present.
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AMPERAGE
Ampere, amperage, amps, and current are terms
commonly used to describe the quantity and intensity
of electrons moving through a conductor.
When current flows through a conductor, a magnetic
field is created.
The clamp-on ammeter is most commonly used on
AC circuits.
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RESISTANCE
Electron flow is energy in motion and must be controlled.
Resistance refers to anything offering opposition to current flow.
There are several types of resistance that will be discussed,
but a basic understanding of Ohm’s Law is is necessary before that
discussion.
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OHM’S LAW
The relationship between Volts ( E ), Amperes ( I ),
and Resistance ( R ) can be expressed
mathematically in the formula E = I x R.
Therefore, if two of the values are known, you can
solve the equation to find the other.
The following pie chart example may help
you remember the formula.
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Ohm’s Law Pie Chart
Using the pie chart, cover the value that you want to find.
By covering the “I”, you
see that the formula is
“E” divided by “R”.
By covering the “R”, you
see that the formula is
“E” divided by “I”.
By covering the “E”, you
find that the formula is
“I” times “R”.
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TYPES OF RESISTANCE
Pure
Pure resistance remains constant, such as in a
Resistance heating element or a light bulb.
Inductive
Reactance
Inductive reactance is caused by the magnetic field
that develops around a conductor, especially in coils
or motors.
Capacitive Capacitors store and discharge electrons that create an
Reactance opposition to current flow.
The total of pure resistance, inductive reactance, and capacitive
reactance is called Impedance.
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MEASURING RESISTANCE
An ohmmeter is used to measure pure resistance. Batteries
inside the meter provide a power supply to measure electron
movement. NEVER connect an ohmmeter to a circuit with the
power on or damage to the meter may occur.
Also, be sure that the component you are measuring is
electrically disconnected to prevent a feedback circuit and false
readings.
Resistance can be calculated on live circuits by measuring
voltage and amperage, then using Ohm’s Law, voltage divided
by amperage equals resistance.
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WATTAGE
Electrical power is the rate at which electricity is
used to perform useful work.
The work performed is measured in units called
watts.
Watts are calculated by multiplying amperage x voltage.
W=IxE
746 Watts is equal to 1 horsepower.
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WHEEL OF ELECTRICITY
Volts ( E ), Amps ( I ), Ohms ( R ) or Watts ( W )
can be calculated if you know two of the values.
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Section 2
SAFETY and HAZARD
PREVENTION
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ELECTRICAL SHOCK
Current is the killing factor in electrical shock.
Currents between 100 and 200 mA generally
cause the heart to fibrillate.
A 110 volt power circuit will generally cause
between 100 and 200 mA current flow through the
bodies of most people.
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LOCKOUT – TAGOUT
PROCEDURES
Whenever a piece of equipment is being
worked on, it should be disconnected from
the power source and locked.
The person working on the equipment
should carry the only key to prevent
accidental activation.
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DO NOT WORK ALONE
If you must test a live circuit, have someone with you
ready to turn off the power, call for help, or give
cardiopulmonary resuscitation (CPR).
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LEARN FIRST AID
Anyone working on electrical
equipment should take the time
to learn CPR and first aid.
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ELECTRICAL BURNS
Do not wear rings or jewelry when working on electrical circuits.
Never use screwdrivers or other conductive tools in an electrical
panel when the power is on.
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PORTABLE ELECTRIC
TOOLS
Electric tools constructed
with a metal frame should
have a safety ground wire
in the power cord.
When using an adapter for
a two prong receptacle to a
three prong cord, be sure
the adapter is properly
grounded.
More modern hand held
tools are constructed in a
plastic case for double
insulation.
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NON-CONDUCTING
LADDERS
Aluminum ladders can be
hazardous if they come in
contact with power lines.
Fiberglass or wood
ladders should be used.
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Section 3
SCHEMATIC
DIAGRAMS & PICTORIALS
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PICTORIAL & SCHEMATIC
DIAGRAMS
Pictorial diagrams show how
components are actually wired.
However, pictorial diagrams
become cumbersome when many
components are involved.
Schematic diagrams present the
logic of the circuit in an organized
fashion. Schematic diagrams are
less cluttered because they use
symbols to represent components.
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LADDER DIAGRAMS
A ladder diagram is arranged with the power supply lines
drawn vertical as the legs of a ladder. Each horizontal line
contains one load and its control switches. Each load line
may be numbered for ease of identification.
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READING A WIRING SCHEMATIC
Reading a wiring schematic is easier if you follow a few simple rules.
 Schematics are read like a book, top to bottom, left to right.
There must be a complete circuit for current to flow through a
component.
Electrical contacts and switches are always shown in their normal
position (power off).
When a relay is energized, all of its contacts will change position.
Normally open contacts will close. Normally closed contacts will open.
Switches or components that are used to provide the function of stop
are normally closed and generally wired in series.
Switches or components used to provide the function of start are
normally open and wired in parallel.
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START – STOP PUSH BUTTON CIRCUIT
Notice there is no complete
circuit to motor starter coil
“M” because the start switch
and auxiliary contacts ( M )
are open.
When the start button is
pressed, both “M” contacts
will close and the motor will
run.
The auxiliary contacts will
serve as a hold-in circuit to
keep the circuit complete
when the start switch is
released.
The circuit will remain energized until
the stop button is pressed, interrupting
current flow to the “M” coil.
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Section 4
CIRCUITS & THEIR
COMPONENTS
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SERIES CIRCUITS
A series circuit has
one single path for
current flow.
If the connection is
broken or if one of
the components fail,
current flow stops in
the entire circuit.
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TOTAL RESISTANCE IN A
SERIES CIRCUIT
A series circuit has only one path for current flow.
Therefore, the total resistance is the sum of all of the
resistances in the circuit.
Rtotal = R1 + R2 + R3 ….
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PARALLEL CIRCUITS
A parallel circuit has more than
one path for current flow.
Current flows through each
load independent of the others.
The current flow through each
load is not necessarily equal,
but the voltage supplied across
the load is always equal.
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TOTAL RESISTANCE IN A
PARALLEL CIRCUIT
Since a parallel circuit has more than one path for current flow, adding
additional paths (loads) will decrease the total resistance in the circuit.
The formula to calculate the total resistance in a parallel circuit is:
Check your math! The total resistance in a parallel circuit
will always be less than the smallest resistance in the circuit!
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THREE PHASE CIRCUITS
The power plant generator rotates
three conducting loops, each
spaced 120 degrees apart, through
a magnetic field. The induced
power pulses take turns changing
polarity from positive to negative
to zero at a rate of 7200 times per
minute (60 times per second).
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THREE PHASE CIRCUITS
Each wire has the same voltage but different polarity ( + vs. - ).
The potential between any two wires is additive.
120 volts positive plus 120 volts negative equals 240 volts.
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SINGLE PHASE CIRCUITS
Some loads are designed to
operate with just two hot
wires from a three phase
system. These two wires
will alternate from positive
to negative polarity. This
“push-pull” effect can be
obtained with any two
phases.
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THE NEUTRAL WIRE
The earth is always at zero potential (no voltage) and can be used to
complete an electrical circuit. Many electrical loads operate with
just one hot wire from a three phase source and another wire called
the neutral. A potential difference exists because the hot wire has
voltage and polarity but the neutral wire is connected to the Earth
(grounded) which is zero volts. The neutral wire is a current
carrying conductor, but has no voltage.
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THE SAFETY GROUND WIRE
The safety ground is connected to the frame of a motor or appliance and
provides an alternate pathway for electrons to travel to ground should a
fault occur. The safety ground connects to the same terminal as the
neutral wire at the service panel. The neutral wire normally carries
current. The safety ground only carries current in the event of a short
circuit.
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CONDUCTORS
In general, any material that has three or less electrons in its outer
orbit is considered a conductor. Copper is the most commonly
used conductor. Wire size and type determine the current
carrying ability.
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INSULATORS
Insulators offer high resistance to current flow. Materials that
have five or more electrons in the outer orbit are considered
insulators. The type of insulation determines where a
conductor can be used safely.
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SEMICONDUCTORS
The outer ring of a pure silicon atom has 4 electrons , but
there is room for 8. The atoms share electrons.
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N-TYPE MATERIAL
If an impurity with only 3 electrons were added to the silicon, the
structure would have a “hole” and will allow an electron “in”.
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P-TYPE MATERIAL
If an impurity with 5 electrons is added, the structure would
already have an extra electron and will not allow more in.
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DIODE
By sandwiching a piece of N-type
and P-type material together, an
electrical “check valve” can be
produced.
Electrons would be allowed to flow
into the N-type material and out of
the P-type material.
However, electrons attempting to
enter the P-type material would be
blocked and no current would flow.
This simple solid state device is
called a diode.
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CIRCUIT PROTECTION
Fuses and circuit breakers are used
to protect a circuit against over
current.
The amperage rating of a fuse must
not be greater than the ampacity of
the wires being protected.
Fuses and breakers are used to
protect wires, not people.
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LOADS AND SWITCHES
Manufacturers design devices with the correct amount of resistance
for the device to perform the desired amount of energy conversion.
Electrical energy flows through the device and is converted to
another form of energy ( light, heat, motion, etc.).
A load cannot operate unless the circuit provides a complete path for
electrons to flow. Switches are used to control and / or provide
safety protection. Switches are wired in series with the load.
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LOADS AND SWITCHES
When more than one load is connected to a power source,
switches are connected in series with each load and each
load is connected in parallel with the power source.
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SINGLE PHASE
TRANSFORMERS
Transformers have two windings, a primary (incoming voltage),
and a secondary (outgoing voltage).
Voltage at the secondary (step-up or step-down) is determined by the
number of coils in the secondary versus the number of coils in the
primary. Single phase transformers are rated by VA (volts x amps) at
the secondary.
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THREE PHASE
TRANSFORMERS
Three phase transformers are wound in “wye” or “delta”configurations.
Combinations of wye and / or delta primary and secondary coils
provide a variety of voltage and current outputs.
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HIGH LEG SYSTEM
In a high leg system,
voltage from two of the hot
legs to neutral will read 115
volts.
However, one of the hot
legs to neutral will register
208 volts.
This is sometimes called
the high leg, stinger leg, or
crazy leg, and cannot be
used for 115 volt circuits.
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SOLENOID VALVE
When current flows through the coil of a solenoid valve, the
electromagnetism lifts the plunger, opening the valve.
(Some valves are designed to close when energized.)
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RELAYS
A relay uses electromagnetism to operate a switch (or contacts).
The electrical circuit to the relay coil is entirely separate from the
circuit through the contacts. A relay allows high current loads to
be controlled using low current control switches and safeties.
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CONTACTORS
A contactor is basically a large
rely. The contacts are much larger
and capable of carrying more
current. Contactor components
(contacts, coil, etc.) are
replaceable, whereas a relay is
generally replaced as a complete
unit.
Any number of switches may
be located in the contactor coil
control circuit.
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LINE STARTERS
A line starter, or motor starter, is basically a
a contactor with overload protection.
The overload contacts are connected in series in the circuit controlling the
contactor coil. There is one overload in each leg of the three phase power
supply to the motor. If an over-current should occur, the overload contacts
interrupt the coil control circuit and the motor stops.
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DEFROST TIMER
A cam that is gear driven by a synchronous motor opens a set of
contacts at a set time. The contacts change position, stopping the
cooling process and energize the defrost heater.
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THERMOSTATS
One common type of temperature
sensing device is a bimetal switch,
in which two different types of
metal are laid together. Because
the metals expand at different
rates, a change in temperature will
cause the bimetal strip to bend,
opening (or closing) a set of
contacts.
A mercury switch may be
attached to the bimetal strip. A
small drop of mercury is sealed
in a glass tube that also contains
a set of contacts. Most mercury
switches are designed as singlepole double-throw, allowing
them to be used for heating and
cooling.
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HEAT ANTICIPATOR
The heat anticipator is an adjustable resistance heater located
near the bimetal coil in a thermostat. It functions to slightly heat
the bimetal coil to prevent system overshoot.
The heat anticipator is set according to the amperage draw of the
heating control circuit.
If the anticipator is set at higher amperage, system overshoot will
occur. If it set to lower amperage, system lag will occur.
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Section 5
MOTORS
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INDUCTION MOTORS
There are two main parts of a motor, the rotor (the part that rotates)
and the stator (stationary electromagnetic coils arranged in a circular
pattern). The rotor is placed inside the the stator. End bells with
bearings are used on each end of the motor and the assembly is
bolted together.
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STATOR POLES
Two (or more) electromagnets (called poles) are positioned at
opposite sides of the stator. The poles have opposite polarity
because the coils are wound in opposite directions. The poles
will change polarity when the alternating current changes
direction, 120 times per second.
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THE ROTOR
A common type of rotor is the “squirrel cage”. Copper bars are mounted
in slots formed around the core of the rotor. The ends of the bars are
joined together forming a series of loops or a “cage”. The magnetic fields
of the stator induce current into the loops, creating a magnetic field that is
opposite that of the stator. Since opposite fields attract, the rotor is in a
locked position. However, if the rotor were given a spin, it would continue
spinning as the fields attract and repel each other.
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SPLIT PHASE MOTORS
A start winding is required to provide automatic starting. The start
winding establishes another magnetic field that is “out of step” with
the run winding. The start winding is made of smaller wire and has
more turns on the pole. The higher resistance produces the magnetic
field slightly behind the run winding.
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DIRECTION OF ROTATION
Rotation (clockwise or counter clockwise) is determined by the
direction of current flowing through the start winding. To reverse
rotation, reverse the two power supply connections at the start winding.
On open type motors, the electrical connections are located at one end
and the shaft exits the opposite end. Direction of rotation is normally
determined by viewing the shaft end. (General Electric motors call for
viewing the lead end.)
GE
OTHERS
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DISCONNECTING THE
START WINDING
The purpose of the start winding is to get the motor started.
The start winding will burn out if left energized. Single phase
open type motors use a centrifugal switch, located inside the
motor, to disconnect the start winding after the motor starts.
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MOTOR SPEED
The speed of a motor is determined by the number of stator poles.
Synchronous speed is determined by dividing the number of poles
into 7200 (the number of alternations per minute in a 60 Hz
circuit). A motor running at full load actually rotates at a speed
about 4% to 5% below synchronous. This difference in motor
speed is called slip.
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CALCULATING MOTOR
HORSEPOWER
Horsepower is rated by the power consumed by the motor. The power
consumed is rated in watts, and 746 watts equals 1 horsepower. A 5 hp
motor will consume 3730 watts (5 x 746 = 3730). This formula
assumes 100% efficiency. When the wattage and voltage are known,
Ohm’s Law makes it possible to determine the amount of amperage
draw at full load conditions.
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LOCKED ROTOR AMPS
At start-up and before the motor begins to turn, current flow is
determined by the resistance of the windings. Starting current is
about 6 times higher than normal running amperage. This high
current flow is called locked rotor amps (LRA).
FULL LOAD AMPS
Full load amps (FLA) refers to the amperage the motor draws
when it is at normal speed and fully loaded. Most induction
motors operate at less than FLA because the motor is rarely
working at fully loaded conditions.
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OVERLOAD PROTECTORS
Many motors have an overload protection device in addition to
any device that may be found in the power circuit. These are
usually a bimetal disc that will deflect and open the circuit if an
overload occurs.
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CAPACITORS
Single phase motors use capacitors to regulate the flow and
phase of electricity to the motor by storing and discharging
electrical energy.
There are two types of capacitors, Start and Run.
Many capacitors have a Bleed Resistor. This resistor allows the
capacitor to completely discharge while the circuit is open to
prevent the capacitor from becoming overcharged. The resistor
also reduces the possibility of arcing, which reduces the risk of
electric shock.
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Capacitor Details
The Start Capacitor
 Connected in series with the
motor start winding
 Fragile construction
 Electricity flows through,
simultaneously charging one
side as it discharges the other
 The energy is discharged to the
start winding to improve startup
torque
 The Capacitor is only used for
startup of the motor, and is
typically activated for less than
1 second
The Run Capacitor
 Also connected in series with
the start winding
 Hefty construction
 Low Capacitance, Constant
Operation
 Improves running torque by
regulating low amounts of
energy to the start winding
 Hot wire is connected to
marked terminal
 Will open circuit breaker if
shorted to prevent damage to
motor
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Capacitor Ratings
 Volts of AC Current (VAC)
 Capacitance in Microfarads
(mf or MFD)
 Start Capacitor Ratings:
21 to 1600mf
 Run Capacitor Ratings:
1.5 to 70mf
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Testing Capacitors with an
Ohmmeter
1. Discharge the capacitor using a 15,000 ohm resistor across the
terminals.
2. DO NOT directly short terminals as this may cause damage to the
capacitor. Any Bleed Resistors must be removed prior to testing.
3. Once discharged, connect the ohmmeter across the terminals. If the
capacitor is holding a charge properly, the needle should deflect from
zero towards an infinite reading.
4. If this fails, check both terminals by bridging the ohmmeter from the
terminal to the outside surface of the capacitor.
The ohmmeter should show an infinite reading. Capacitors that fail the
test are called “open” or “shorted”. Although a Capacitor may pass the
ohmmeter test, it my not carry its full capacitance rating.
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Testing Capacitance for
Start Capacitors
 Use special capacitor tester or ammeter
 Test capacitance only after ohmmeter test
 Close momentary switch for no more than
3 seconds and record current flowing into
the capacitor
 Capacitance mf = 2650 X Amps
Applied voltage
If the tested number is within 20%
of its rating, the capacitor is good.
 Run capacitors my be tested without the
momentary switch. Make sure there isn’t
continuity between the terminals and the
capacitor housing.
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Capacitors & Motors
 CSIR Motor: (Capacitor StartInduction Run) used for high-load
start applications. Uses start
capacitor and winding only during
startup for additional torque.
 PSC Motor: (Permanent Split
Capacitor) used for fans and
devices where load is speed
dependent. Uses run capacitor to
throttle energy to the start winding.
This aids the run winding under full
load. The run capacitor is
connected to the start circuit
permanently.
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Multi Speed Motors
 Speed is determined by the number
of stator poles under power




Several taps engage/disengage poles
Hi2 poles activated
Medium4 poles activated
Low6 poles activated
Table of Common Wire Code
TAP
COMMON
HIGH
MEDIUM
LOW
CAPACITOR
COLOR
WHITE
BLACK
YELLOW
RED
PURPLE (2 WIRES)
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Three Phase Motors
 More compact in design than
single phase motors.
 More Efficient
 No start winding or
capacitors
 High start & run torque
 Carries safety ground (green
wire) to allow escape of
electrons from the metal
frame in case of short
 3 pairs of stator poles, 3
north, 3 south spaced 60
degrees apart
 Windings of equal resistance
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More on 3 Phase Motors
The three poles alternate
polarity from North to South, to
Zero
The zero position allows the
other two poles to produce the
rotating push-pull effect on the
rotor
The rotation of the poles can be
reversed by switching any two
supply wires
Only one end of each winding
is brought out to the power
source
All other connections are
made within the motor during
assembly
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Checking Resistance of
Windings
Resistance is tested, using an ohmmeter,
from one motor lead to another. If the
ohmmeter reads zero resistance, the
winding is shorted. If a reading is
obtained from a lead to the ground, the
winding is grounded. An infinite reading
indicates the winding is open. In any of
the 3 cases, the motor must be re-wound
or replaced.
In three phase motors, resistance drops
with size, ranging anywhere from less
than 1 to 50 Ohms.
Dual voltage motors have half the
resistance of the main winding in the 2nd
winding.
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Dual Voltage 3 Phase Motors
Many three phase motors are designed for connection to either of two voltages
(240/480). Instead of just three external connection wires, they have nine. Tagged
1-9, these wires are easy to identify. These motors have an extra set of three
windings, with 2 wires for each for a total of 6 additional connection wires. At the
lower voltage, the windings run in parallel, and at the higher voltage, they connect
in series. Three power supply wires are ALWAYS connected to motor numbers T1,
T2, & T3.
Low & High Voltage
connections for a WYE
(Star) Connected motor
Low & High voltage
connections for a Delta
connected motor
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The Motor Name Plate
Instructions for making electrical connections to a motor are normally included
on the motor nameplate (a.k.a. data plate). The nameplate should be carefully
reviewed before selecting, replacing, or wiring a motor. Here’s an Example:
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Motor Name Plate Definitions
 Frames & Type: Motors of a certain horsepower rating are built in a certain
size of frame or housing. NEMA has standardized the frame size and shaft
heights to be used for each integral horsepower motor. This permits easy
replacement or interchanging of motors.
 Max Amb: The maximum ambient temperature at which the motor can be
operated
 Temperature Rise: The Amount of temperature rise permitted above ambient
air at rated load
 Duty: Continuous or Limited. Delivering rated horsepower continuously or
for a specified period of time without overheating
 Thermal Protection: Indicates Oil, Air, or other types of thermal protection
used, if any
 FLA: Rated amps at full load
 LRA: Rated amps when motor is unable to turn
 KVA Code: Starting amperage required, relative to LRA
 Insulation Class (INSL): type of insulation used
 Service Factor: The amount of overload the motor can tolerate on a
continuous basis at rated voltage and frequency.
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Section 6
ELECTRIC HEAT
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Power Supply
Most electric heat systems require 240 Volt, 3
wire service run direct from the load center.
A fused disconnect must be installed and be
capable of locking open.
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Heating Elements
Made of nickel-chromium wire with a resistance.
3 types include open-wire, ribbon, and enclosed.
Open wire and ribbon wires must be shielded to prevent
burns and electric shock.
Most are direct-wired with a fuse to prevent over current.
Air-Flow is pertinent to safe operation.
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Limit Switch
A bi-metal disc opens the
circuit if an element reaches
extreme temperatures.
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Sequencers
Contains a bi-metal strip heated by
low voltage that activates elements in
a sequence to minimize load, and is
dependent on element temperature.
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Baseboard Heating
Contain a heating element encased in a way
to disperse heat by natural convection.
Advantages: Individual thermostat,
compactness, quietness & no moving parts
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Duct Heaters
Used as boosters during temperature
extremes to aid larger systems in
efficient heat distribution.
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Electric Radiant Heat
Use of the infrared light wavelength of 4.0
microns or less at (900-2500MHz) to heat
objects upon which the light strikes. The
light is absorbed as heat.
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Radiant Panel Heat
Installed in walls and ceilings and
concrete floors, these cables vary
between 500-5000 watts. 60% of
the heat is produced by radiation.
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Section 7
Troubleshooting
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The Voltmeter
 Measures the electrical
potential between 2 points
 Load switch can be open
or closed, as long as the
voltmeter is bridged
across the load
Closed
Open
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Power Passing Devices & the
Voltmeter
 Since switches and fuses do
not consume power under
normal conditions, they are
called power passing devices.
Potential measured across
these devices is zero.
 Any reading of .02 volts or
higher on a 120 volt circuit
may indicate device failure.
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Troubleshooting methods with the
voltmeter for HVAC Circuits
 Most HVAC circuits have
more than one switch that
must close to activate the
circuit
 Shown on the left is a
typical HVAC circuit used
to control the operation of
a motor
 2 test methods may be
used to troubleshoot the
circuit
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Search Methods
Lineal Search
1.Test overall circuit first,
between L1 & N
2. L1 to Thermostat
3. L1 to High pressure switch
4. L1 to Low pressure switch
5. L1 to Overload
6. L1 to motor terminals
 A fail-proof test
S p l i t Search
1. Test a midpoint in the
circuit to determine
which half of the circuit
is at fault
 In larger circuits, the
power passing device at
fault can be located
faster by avoiding a test
of each component.
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The Ohmmeter
HP
1/8
4.5 
16 
1/6
4.0 
16 
1/5
2.5 
13 
1/4
2.0 
17 
 Measures Circuit for
continuity & level of
resistance
 Used only when no
power is applied to the
circuit
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Checking Motor Windings
Identifying Hermetic motor windings
Three readings are required to locate the
common terminal as shown to the right
Start and Run is
highest resistance
Common and
Run is lowest
resistance
Common and
Start is middle
resistance
The highest reading is Start
and Run. Therefore the other
terminal is common
Scratch good
connection
Zero resistance
(Dead Short)
Grounded
to frame
Open
Winding
Infinite resistance
(No Circuit)
“Ringing Out” motor windings
Use the ohmmeter to measure resistance
across the winding; when open and closed
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The Ammeter
 The ammeter is used to
measure current flowing
through a circuit
I = E/R
Amps = Wattage/Voltage
Amps= 3000/240 =12.5 Amps per element
 Monitors load activation &
operation on circuits
 Using Ohm’s Law, I = E/R,
once can use the wattage
rating and voltage of the load
to calculate the expected
amperage draw of the load.
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Using the Ammeter with Small
Currents
 By introducing a coil into the circuit, you can greatly increase
the accuracy of the ohmmeter in low voltage applications.
 Connected in series with the circuit, the coil will multiply the
current measured by the ammeter by the number of coils wound
through the ammeters clamp.
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Troubleshooting Open Motor Circuits
In this figure, the circuit is open and the voltage can be measured
between L1 & L2. While L1 & L2 both terminate at the pressure
switch, L2 is able to pass through the motor and closed overload,
and thus can be used to test the motor and overload switch with a
voltmeter.
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Troubleshooting Closed Motor Circuits
Here, the circuit is closed and the motor is running. This causes
L1 to extend to terminal C of the motor, while L2 is terminated
at terminal R of the motor, rather than passing through.
Voltages can be measured Across the circuit as shown.
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Troubleshooting Switches
A simple diagnosis may turn difficult when:
 Introducing loads into a switch series
This circuit is closed
and the motor is running
 Introducing Switches in parallel
 In this circuit, two switches are
wired in parallel with each other,
yet still in series with the motor.
 Making the distinction between L1
& L2 is vital to diagnosis. Voltage
across the same line is 0. Voltage
across L1 & L2 is that of the power
source
 Only one switch needs to be closed
in order to close the circuit
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Using a Voltmeter
when Troubleshooting
This schematic is of a 230V system in the
refrigeration mode.
Voltages can be measured across the
areas shown.
Voltages measured across L1 & L2 will
equal supply voltage (230V)
Voltages measured across the same line
(L1 to L1 or L2 to L2) will be zero.
After Line 1 and Line 2 are identified,
the remainder of the diagnosis simplifies.
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Systematic Troubleshooting
Reduces diagnosis time
Increases profitability
Saves the customer money
A Win Win Situation
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Case Study in Systematic
Troubleshooting
In the slides to follow, an example of the systematic
troubleshooting method is evaluated incorporating a
symptoms/cause method.
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This diagram shows a time clock
controlling a defrost circuit and a
refrigeration circuit. Notice in
the refrigeration circuit that the
compressor’s run winding is
open due to a motor overheating
problem. The service call is a
“no cooling” call for a low
temperature walk-in cooler. Once
the technician looks the system
over and listens for clues that
may determine the problem, the
electrical schematic, if available,
should be studied. Understanding
the logic or sequencing of the
circuits before diving head-overheals into the problem is of
utmost importance in systematic
troubleshooting.
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In this scenario, an open run winding will give certain symptoms
that will not exist for other possible system problems. For
example, the technician listened to and examined the
refrigeration system and then studied the electrical schematic
drawing. The service technician then lists the symptoms.
Symptoms
1. Compressor motor hums and will not turn.
2. Compressor motor draws Locked Rotor Amps (LRA).
3. Compressor motor’s overload trips soon after drawing LRA.
Resets after two minutes.
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The service technician then turns power off to the refrigeration
system to let the motor cool down. After studying the electrical
diagram again, the technician lists some of the possible causes that
will correlate to “every” symptom listed. If a possible cause does not
correlate to every symptom listed, it cannot be a possible cause.
Possible Causes
1.
Open start winding
2.
Open run winding
3.
4.
5.
Open run capacitor
Open start capacitor
Compressor mechanically stuck
6.
Potential relay contacts between 1 and 2 stuck open
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Notice that every possible cause listed correlated to all the
symptoms. Now, all the service technician has to do is to check
only the six possible causes to find out which one is causing the
symptoms, instead of blindly checking out the entire system.
With the power off, disconnect a wire from the start winding and
ohm the winding. The ohmmeter reads 4 ohms, the start winding is
not open. Now remove a wire from the run winding and ohm the
winding. The ohmmeter read infinite ohms indicating an open run
winding.
The compressor has to be replaced. With either of the windings
open, the compressor has no phase shift for starting and will lock
its rotor, drawing LRA until the overload trips.
NEXT
If either capacitor was bad, the motor may not have had enough
phase shift to start. In certain cases, the motor may turn slowly.
If the compressor was mechanically stuck, such as something
wedged between the piston and cylinder, the motor would lock its
rotor and draw LRA.
If the contacts between terminals 1 and 2 of the potential relay
were stuck open for some reason, the start capacitor would be out
of the circuit. This again will probably not cause enough phase
shift to start the motor turning. The motor would again draw LRA.
Notice that in every case, all the symptoms were met.
NEXT
What about an open overload or an open potential relay coil
between terminals 2 and 5 of the potential relay?
If the overload were opened at the beginning, from too high of a
compressor amp draw, the compressor would not hum or draw
LRA. This would not correlate with all the symptoms listed and
could not be a possible cause.
If the coil of the potential relay were open, the contacts between 1
and 2 of the potential relay would stay in their normally closed
position and not open. This would cause the start capacitor to be in
the circuit all the time, and the motor would turn, draw higher than
normal amperage, and eventually open the overload. These
symptoms do not correlate with the original symptoms listed, thus
cannot be a possible cause.
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Once the service technician has replaced the compressor and the system is up and
running, it is important to run a system check to see what caused the compressor
overheating that opened the winding.
Evaporator superheat, total superheat, and condenser subcooling, along with suction
pressure and head pressure, must be taken for the system check. In this case, the
technician took a system check and found the evaporator superheat to be very high at 40°
and the total superheat to be very high at 90°. Condenser subcooling was fine at 12°.
Both suction and head pressure were low (see Table below).
The problem that caused the overheating was a faulty thermostatic expansion valve. The
valve would not open enough, and the entire low side of the system was being starved.
The compressor was a refrigerant-cooled compressor. This caused the compressor to
overheat and cycle on its overload until the run winding finally opened. Without this final
system check, the new compressor would fail within a short time.
NEXT
Voltmeter or Ohmmeter?
Troubleshooting the power consuming device or load.
Service technicians often encounter switches in series or
parallel with electrical loads. Keeping the electrical power
on and using a voltmeter to voltage troubleshoot is the
fastest and most reliable method. However, there will be
times when a technician must switch to an ohmmeter and
shut the electrical power off in order to get to the root of
the problem.
NEXT
Shown is an electric PSC motor in
series with two switches that are in
parallel with one another. The voltage
between points A and B (the open
switch), in this case, would be zero
volts because the voltmeter would be
measuring between Line #1 and Line
#1. The voltage between points C and
D (the closed switch) would also be
zero volts because of the voltmeter
measuring between Line #1 and Line
#1 again. Remember, the motor is
running and dropping all of the 230
volts while it is consuming power. A
voltmeter across the R and S terminals
of the PSC motor would read 230
volts because the meter is measuring
the voltage between Line #1 and Line
#2, which is 230 volts.
NEXT
Notice that a voltmeter placed
across the R and C terminals of
the motor (the opened winding)
will again read 230 volts. In
fact, all the voltages in the
previous diagram and the one
shown here are the same. This
drawing illustrates that whether
the motor is running properly or
if it has an opened winding, the
voltage will still read 230 volts
across R and C. So, how does
the service technician determine
if the run winding is opened or
not? The answer is with an
ohmmeter.
NEXT
The service technician must shut
the power and disconnect one
wire, either from the R or C
terminal of the motor (Shown).
Disconnecting the wire will
prevent electrical feedback from
the ohmmeter’s internal voltage
source through another parallel
electrical circuit.
The technician must then place
an ohmmeter across the R and C
terminals of the motor. The
measurement will read “infinite
ohms” if the winding is open.
This is the only way the service
technician can tell if the winding
is open or not.
NEXT
This diagram shows a
feedback circuit from the
ohmmeter’s internal
voltage source caused by a
failure to disconnect a wire
from motor’s terminals.
In this case, the ohmmeter
reading would be 2 ohms.
This could fool technicians
into thinking the winding
was still good.
NEXT
MOTOR TROUBLESHOOTING FLOW CHART
VOLTAGE ON LOAD SIDE
(BOTTOM) OF
MOTOR CONTACTOR
MOTOR START
COMPONENTS
YES
CAPACITORS
NO
INTERNAL
OVERLOADS
VOLTAGE ON LOAD SIDE
(TOP) OF
MOTOR CONTACTOR
YES
ON/OFF
SWITCH
MOTOR
WINDINGS
NO
MOTOR
CONTROL CIRCUIT
CHECK FUSES OR
CIRCUIT BREAKERS
HIGH
PRESSURE
SWITCH
LOW
PRESSURE
SWITCH
LIMIT
SWITCH
SOLID
STATE
MODULE
MOTOR TIME
DELAY OR
INTERLOCK
EXTERNAL
OVERLOADS
INTERNAL
OVERLOADS
NEXT
THIS IS THE END OF THE
ELECTRICITY
PRESENTATION
ACCOMPANIMENT