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SECTION 4
ELECTRIC MOTORS
UNIT 19
MOTOR CONTROLS
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UNIT OBJECTIVESAfter studying this unit, the reader should be able to
Describe the differences between relays, contactors and starters
Explain why the locked rotor amperage (LRA) affects the choice of a
motor starter
List the component parts of contactors and starters
Name two types of external motor overload protection
Explain the differences between external overload protection devices Explain the conditions under which motor safeties can be reset
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INTRODUCTION TO MOTOR
CONTROL DEVICES
Relays, contactors and starters pass power to the
motor by closing sets of contacts
Contacts controlled by coils in the control circuit
Starting relays are only in the active circuit for a
short period of time
The type of motor control used is determined by
the size and application of the motor used
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L1 L2
CONTROLCIRCUIT
RELAY OR
CONTACTOR
MOTORRUN
START
START
RELAY
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L1 L2
CONTROLCIRCUIT
RELAY OR
CONTACTOR
MOTORRUN
START
START
RELAY
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MOTOR AMPERAGES Running load amperage (RLA)
Similar to full load amperage (FLA)
Amperage drawn by the motor while operating
Locked rotor amperage (LRA)
Amperage drawn by motor on startup
Five to seven times greater than RLA or FLA
Both LRA and RLA must be considered when
choosing a control device
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THE RELAY
Uses a magnetic coil to open or close one or more sets of
electric contacts The most common control voltage for both relays and
contactors is 24 volts.
Relays are not repaired. Replace on failure.
Used for light duty applications
Can be used as a pilot-duty relay
The relay contacts must be able to handle the amperage drawof the load being controlled
Pilot relays are designed to switch on and off larger contactors
or starters. They are very light duty and are not designed tostart motors directly.
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COIL
NORMALLY OPEN CONTACTS
NORMALLY CLOSEDCONTACTS
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THE CONTACTOR Larger version of the relay
Has movable and stationary contacts Often times only one set of contacts opens which opens only one
side of the power to condensing units. This provides power for off-cycle heat to the compressor.
Holding coils are rated at different voltages Can have one or more sets of contacts
Some are equipped with auxiliary contacts, which are usually ratedat a lower amperage than the primary contacts.
Contacts and coils can be replaced, the material most used on thecontacts is silver.
Use the exact replacement whenever possible
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HOLDING COIL
STATIONARY
CONTACTS
STATIONARY
CONTACTS
MOVABLE
CONTACTS AND
ARMATURE
COIL
CONNECTIONS
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WHEN THE COIL
IS ENERGIZED,
THE CONTACTS
ARE PULLEDCLOSED
The most common coil voltage is 24 volts.
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L1 T1 L2 T2 L3 T3
STATIONARY
CONTACTS
MOVABLE
CONTACTS
SPRINGSCOIL
STATIONARY
ELECTROMAGNET
ARMATURE
When contactor is energized what resistance would you expect to read
between L1 to T1, L2 to T2 and L3 to T3? -
If placing one lead of your voltage meter on T1 and the other on L1, what
voltage would you expect to read if the coil is energized?
Springs maintain pressure on the
contacts for current carrying capacityand they help obtain equal contacts
against the stationary contacts.
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MOTOR STARTERS
Contactor equipped with overload protection, whichactually are designed to protect the motor where fusesor circuit breakers protect the entire circuit.
High amp draw will cause the starter to open.
Coils, contacts and heaters can be replaced
Contacts become pitted over time Pitting increases the resistance across the contacts, if the
pitting is bad enough it can cause motor burn out.
The voltage across the contacts will increase
The voltage across a good set of contacts should be about zerovolts
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CLEAN CONTACT
DIRTY, PITTED CONTACT
Dirty contacts dont necessarily mean
they are pitted, the sure sign of pittedcontacts is increased voltage across the
contacts.
Sanding contacts will only speed
up the deterioration of the
contact surfaces.
The most common material used for the
contacts is silver.
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MOTOR PROTECTION Motors are expensive and must be protected
Fuses and circuit breakers protect the entire circuit, notthe individual circuit components
Motors can operate under an overcurrent condition for ashort period of time
Most small motors have no overload protection
The larger the motor, the more elaborate the method ofmotor protection should be
Motor protection can be inherent (internal) or external
Motor service factor is determined by the motors
reserve capacity
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L1 L2
3A25A
FUSES PROTECT THE
ENTIRE CIRCUIT, NOT
THE INDIVIDUAL
COMPONENTS
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INHERENT (INTERNAL) MOTOR
PROTECTION
Thermally activated devices in the motor winding. Internal thermal overloads
Usually embedded in the motor windings
Open on a rise in temperature
Thermally activated bimetal snap discs
Positioned so that contact is made between the bimetalcontrol and the motor
Snap action opens contacts if the motor temperature risesabove the desired level
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EXTERNAL MOTOR PROTECTION
Devices that pass power to the holding coil of the
starter or contactor
Devices open when an overcurrent condition exists
The trip point and type of overload protector aredetermined by the manufacturer
The overload device takes the service factor of the
motor into consideration
NATIONAL ELECTRIC CODE (NEC)
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NATIONAL ELECTRIC CODE (NEC)
STANDARDS
Sets standards for electrical installationsConductor sizes and ampacities
Cable materials and applications
Electrical devices
Sets standards for motor overload protection
The published code book should be consulted if
questions or concerns are encountered on the job
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TEMPERATURE-SENSING DEVICES
Bimetal elements
Devices called heaters wired in series with the load The heater is exposed to the current draw of the load
The bimetal warps and open when it gets too warm
The open bimetal de-energizes the starter holding coil
Solder pot
Uses solder with a low melting point
The solder melts when excessive temperature is sensed
Excessive heat results from the overcurrent condition Look @ Figure 19-21 on page 366
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MAGNETIC OVERLOAD DEVICES
The most accurate means of electrical overloadprotection
Device is not attached to the starter
Device is not affected by increased ambienttemperatures
The contacts within the device will open to de-
energize the motor at the desired amperage level
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RESTARTING THE MOTOR Motors should not be restarted immediately
Cause for the overload condition must first belocated and repaired
Motor must be given ample time to cool
Many control devices are manually reset, this is to
ensure that technicians are aware of a possibleproblem which caused the starter to trip in the firstplace.
Some controls reset automatically after a
predetermined time delay Time delay feature prevents short cycling
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UNIT SUMMARY Relays, contactors and starters pass power to motors
Contacts controlled by control circuit holding coils Relays are used for light duty, contactors for heavier
duty applications, starters have built-in overloads
Contacts on contactors and starters can be replaced
Fuses and circuit breakers protect the entire circuit,
not the individual circuit components
Motor protect can be inherent or external