Troubleshooting & Servicing

HVACR

Electrical Systemsfor Non-Electricians

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Section 1Section 1WHAT IS ELECTRICITY?WHAT IS ELECTRICITY?

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ELECTRONSAtoms 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 DIFFERENCEAn 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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There must be a potential difference for the meter to register a voltage reading.

POTENTIAL DIFFERENCE

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POTENTIAL DIFFERENCEThe 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 DIFFERENCENEVER TOUCH AN ELECTRICAL WIRE BECAUSE NEVER TOUCH AN ELECTRICAL WIRE BECAUSE A ZERO VOLTAGE READING WAS OBTAINED!!A ZERO VOLTAGE READING WAS OBTAINED!!

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.

WARNING!WARNING!

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AMPERAGEAmpere, 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

Resistance refers to anything offering opposition to current flow.

Electron flow is energy in motion and must be controlled.

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 LAWThe 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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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”.

Ohm’s Law Pie Chart

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TYPES OF RESISTANCEPure Resistance

Inductive Reactance

Capacitive Reactance

Pure resistance remains constant, such as in a heating element or a light bulb.

Inductive reactance is caused by the magnetic field that develops around a conductor, especially in coils or motors.

Capacitors store and discharge electrons that create an opposition to current flow.

The total of pure resistance, inductive reactance, and capacitive reactance is called Impedance.

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MEASURING RESISTANCEAn 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 = I x E746 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 2Section 2SAFETY and HAZARD SAFETY and HAZARD

PREVENTIONPREVENTION

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ELECTRICAL SHOCKCurrent 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 3Section 3SCHEMATIC SCHEMATIC

DIAGRAMS & PICTORIALSDIAGRAMS & 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 SCHEMATICReading 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. NEXT

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 4Section 4

CIRCUITS & THEIR CIRCUITS & THEIR COMPONENTSCOMPONENTS

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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:

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Check your math! The total resistance in a parallel circuit will always be less than the smallest resistance in the circuit!

THREE PHASE CIRCUITSThe 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

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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.

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 WIREThe 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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INSULATORSInsulators 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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SEMICONDUCTORSThe 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 MATERIALIf 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 MATERIALIf 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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DIODEBy 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 SWITCHESManufacturers 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

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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.

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.)NEXT

RELAYSA 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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CONTACTORSA 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.

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Any number of switches may be located in the contactor coil control circuit.

LINE STARTERSA 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. NEXT

DEFROST TIMERA 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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THERMOSTATSOne 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 single-pole double-throw, allowing them to be used for heating and cooling. NEXT

HEAT ANTICIPATORThe 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 Section 5

MOTORSMOTORS

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INDUCTION MOTORSThere 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 POLESTwo (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 ROTORA 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 MOTORSA 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 ROTATIONRotation (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.)

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GE

OTHERS

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. NEXT

MOTOR SPEEDThe 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 AMPSAt 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 AMPSFull 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 PROTECTORSMany 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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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

Capacitor Details

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Capacitor Ratings

Volts of AC Current (VAC)

Capacitance in Microfarads

(f or MFD) Start Capacitor Ratings:

21 to 1600f Run Capacitor Ratings:

1.5 to 70f

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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 f = 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 Start-Induction 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 Hi- 2 poles activated Medium- 4 poles activated Low- 6 poles activated

Table of Common Wire CodeTAP COLOR

COMMON WHITE HIGH BLACK MEDIUM YELLOW LOW REDCAPACITOR 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 NEXT

More on 3 Phase MotorsOnly one end of each winding is brought out to the power sourceAll other connections are made within the motor during assembly The three poles alternate

polarity from North to South, to ZeroThe zero position allows the other two poles to produce the rotating push-pull effect on the rotorThe rotation of the poles can be reversed by switching any two supply wires

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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 MotorsMany 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 PlateInstructions 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. NEXT

Section 6

ELECTRIC HEAT

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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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Power Supply

Heating ElementsMade 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

OpenClosedNEXT

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 MethodsLineal 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 Search1. 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

Measures Circuit for continuity & level of resistance

Used only when no power is applied to the circuit

1/8 4.5 16

1/6 4.0 16

1/5 2.5 13

1/4 2.0 17

HP

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Checking Motor Windings

Identifying Hermetic motor windingsThree readings are required to locate the common terminal as shown to the right

Scratch good connection

Grounded to frame

Zero resistance (Dead Short)

Open Winding

Infinite resistance (No Circuit)

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

“Ringing Out” motor windingsUse 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

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.

I = E/RAmps = Wattage/VoltageAmps= 3000/240 =12.5 Amps per element

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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. NEXT

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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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

A simple diagnosis may turn difficult when: Introducing loads into a switch series

Introducing Switches in parallel

This circuit is closed and the motor is running

Troubleshooting Switches

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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-over-heals 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. Open run capacitor

4. Open start capacitor

5. 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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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VOLTAGE ON LOAD SIDE (BOTTOM) OF

MOTOR CONTACTOR

VOLTAGE ON LOAD SIDE (TOP) OF

MOTOR CONTACTOR

NO

NO

YES

YES

MOTOR START COMPONENTS

CAPACITORS

INTERNAL OVERLOADS

MOTOR WINDINGS

ON/OFF SWITCH

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

MOTOR TROUBLESHOOTING FLOW CHART

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THIS IS THE END OF THE ELECTRICITY

PRESENTATION ACCOMPANIMENT