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

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
3/3/2013
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