2. UNIT OBJECTIVES
After 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
3. 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
6. 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
7. 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 draw
of 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 to start motors directly.
9. 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 rated
at a lower amperage than the primary contacts.
• Contacts and coils can be replaced, the material most used on the
contacts is silver.
• Use the exact replacement whenever possible
11. WHEN THE COIL
IS ENERGIZED,
THE CONTACTS
ARE PULLED
CLOSED
The most common coil voltage is 24 volts.
12. L1 T1 L2 T2 L3 T3
STATIONARY
CONTACTS
MOVABLE
CONTACTS
SPRINGS
COIL
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 capacity
and they help obtain equal contacts
against the stationary contacts.
13. MOTOR STARTERS
• Contactor equipped with overload protection, which
actually are designed to protect the motor where fuses
or 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
zero volts
14. CLEAN CONTACT
DIRTY, PITTED CONTACT
Dirty contacts don’t necessarily mean
they are pitted, the sure sign of pitted
contacts 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.
15. MOTOR PROTECTION
• Motors are expensive and must be protected
• Fuses and circuit breakers protect the entire circuit, not
the individual circuit components
• Motors can operate under an overcurrent condition for a
short period of time
• Most small motors have no overload protection
• The larger the motor, the more elaborate the method of
motor protection should be
• Motor protection can be inherent (internal) or external
• Motor service factor is determined by the motor’s
reserve capacity
17. 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 bimetal
control and the motor
– Snap action opens contacts if the motor temperature rises
above the desired level
18. 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 are
determined by the manufacturer
• The overload device takes the service factor of the
motor into consideration
19. NATIONAL ELECTRIC CODE (NEC)
STANDARDS
• Sets standards for electrical installations
– Conductor 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
20. 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
21. MAGNETIC OVERLOAD DEVICES
• The most accurate means of electrical overload
protection
• Device is not attached to the starter
• Device is not affected by increased ambient
temperatures
• The contacts within the device will open to de-
energize the motor at the desired amperage level
22. RESTARTING THE MOTOR
• Motors should not be restarted immediately
• Cause for the overload condition must first be
located 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 possible
problem which caused the starter to trip in the first
place.
• Some controls reset automatically after a
predetermined time delay
• Time delay feature prevents short cycling
23. 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
