If you do not want a motor to re- start when power is lost, then is

Recall : What are the 2 sections
of a Manual Motor Starter?
You are going to use a Thermostat to control a
large blower-motor in line with a manual motor
starter. This motor should turn on when the
temperature is at or above 85,°sketch the
switch that is closed because the temperature is
currently 90°.
What rating should the thermostat have?
Begin with the End in Mind
• Review Chapter 4 questions
• Chapter 3 and Chapter 4 will be combined
for one homework assignment.
If you do not want a motor to restart
when power is lost, then is
restored, you need one with what
option?
CHAPTER 4
Overload Relays
Objectives
• Discuss difference between fuses and
overloads.
• List different types of overload relays.
• Describe operation of thermal overload
relay.
• Describe operation of magnetic overload
relays.
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Fuses/ Circuit Breakers (1 of 2)
• Difference between fuse/circuit breaker
protection and overload protection:
– Fuses (with the exception of dual element time
delay types) and circuit breakers protect circuit
from grounds and short circuits only.
– Protect Motor Circuit and Power system from a
short in the motor circuit.
– C.B.’s are more expensive but can be reset, Fuses
are less expensive but can be real pain to replace.
Overload Relays (2 of 2)
• A fuse or circuit breaker does not protect
the motor from an overload.
Dual Element Fuses
• They provide both short circuit and overcurrent
protection.
• Fuse link provides short circuit protection.
• Solder link provides over-current
protection with time delay.
• Work well, are cheaper than Resetable
types, but cannot be reset
Overloads (1 of 2)
• Overload protection provides protection to the
motor from overload conditions (excessive
current through the motor windings).
– “Protect against a little too much current for a little
too long”
• “A little too much Current” is typically 125% Name-plate
• “A little too long” is typically 2 minutes
Overload Properties
• Must have means of sensing motor current
• Must have some type of time delay
• Two operating sections
– Current sensing and contact section
Current
Sensor in
Motor
Circuit
OL Contacts
in Control
Circuit
2 Categories of OL Relays
1) Thermal
Responds to the Heat due to Current
Flow
2) Magnetic
Responds to the Magnetic field due to
Current Flow
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Thermal Overload Relays
• Thermal overload relays
• Both may use the same symbol on a
drawing
• Two types
– Solder pot
– Bimetal strip
Thermal Overload Relays (2 of 5)
• The spring pushes the contact open when heat melts
alloy causing the ratchet wheel to turn.
Thermal Overload Relays (1 of 5)
• Solder pot
Thermal Overload Relays (3 of 6)
Thermal Overload Relays (4 of 6) Thermal Overload Relays (6 of 6)
• Bimetal strip
– Same principle as melting alloy-type, but
uses metal strip to sense heat rather than
melting alloy
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•
Bimetal Strip
Overload Relays
Note that Bi-Metalic OLs can
be adjusted.
A Solder-Pot Ol only works for
one current level.
Three-phase Overloads (2 of 2)
Bimetal Strip Relay
Three-phase Overloads (1 of 2)
• Three-phase overload relays are used to
sense the current in each of the three
phase lines.
Single-phase Overload
• Control circuit
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Three-phase Overloads (1 of 2)
• Control circuit (This OL has 3 separate contacts)
Magnetic Overload Relays
• Two types
– Elelctronic
– Dashpot
Electronic OLR
This does a Whole
lot more than
simply open the
circuit if the
current a bit too
high for a bit too
long! Click-it for
more information.
Three-phase Overloads (2 of 2)
• Control circuit (This OL has one contact)
Magnetic Overload
Relays Electronic (1 of 2)
• Use current transformers to sense motor
current.
– (like and Amp-Clamp)
Electronic Overload Relays (2 of 2)
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Dashpot Overload Relay (1 of 8)
• Uses dashpot timer to provide time delay
• Consists of a container, piston, shaft, and
oil
-Like a Shock Absorber
Dashpot Overload Relay (3 of 8)
Dashpot Overload Relay (2 of 8)
Dashpot Overload Relay (4 of 8)
• Contains current coils in series with the motor
Note that the
Coil symbols
in the text are
wrong
Dashpot Overload Relay (5 of 8) Dashpot Overload Relay (6 of 8)
• Changing shaft length adjusts for current
values.
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Dashpot Overload Relay (7 of 8)
• Line at top of shaft shows desired amount
of current when flush with top of container.
Overload Contacts
• All overload relays contain normally closed
contacts.
• Some manufacturers install both NC and
NO contacts.
• NC Contacts always installed in series with
MS Coil.
N.O. Contacts Providing
Signal to PLC Input
Dashpot Overload Relay (8 of 8)
• Nameplate
• Starter schematic
showing an SPDT
set of contacts
What does
the Amber
light tell you?
Starter Schematic
Protecting Large
Horsepower Motors (1 of 3)
• Large motors drawing hundreds of amps
require Current Transformers that reduce
the amount of current to the overload
heaters.
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Protecting Large
Horsepower Motors (2 of 3)
• Current
transformers
used to reduce
overload heater
current
Protecting Large
Horsepower Motors (3 of 3)
• The Current
transformers
provide the
current for the
OL Heaters.
Remote Reset (Larger Motors) C.T. Example
Short-Term Memory Test:
What does an Overload do?
1) In this case, If the FLC of the motor is
100A, what would the OLs see at FLC?
2) What would the Typical OL Setting be?
What are the 2 sections of an
Overload?
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2 classes of Overload Sensors:
2 Overload types that use
Electro-Magnetism
Summary
• In this chapter, we discussed:
– The difference between fuses and
overloads
– Different types of overload relays
– The operation of thermal overload relay
– The operation of magnetic overload relays
2 Overload types that use heat
What do you do if the current to
the motor is “too hot to handle?”
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