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<H1>Proper
Motor Protection</H1>
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With electric motors driving the majority of today's
manufacturing processes, proper motor protection is critical. Not
surprisingly, a significant amount of effort and resources have recently
been invested in motor protection technology, resulting in cost-effective
solutions to many of today's common motor problems.
<UL type=square>
<LI>
Motors
fail for a number of reasons 
<LI>
Moisture
and contamination, 
<LI>
short
circuits, 
<LI>
Mechanical problems and 
<LI>
Old
age </LI></UL>
But the primary reason is excessive heat, caused
by
<UL>
<LI>
Excess
current (greater than normal motor full load current), 
<LI>
High
ambient temperatures, and 
<LI>
Poor
ventilation of the motor. </LI></UL>
If a motor is continuously overheated by only 10
degrees, its life can be reduced by as much as 50%.
Levels of protection Various levels of
protection can be achieved depending on the type of motor protective
device applied. In the area of overload protection, traditional
electromechanical overload relays are used in a motor circuit to protect
motors and circuit conductors from damage caused by continuous
over-current conditions. These electromechanical relays (eutectic alloy or
bimetallic) do not measure current directly, but rather operate by passing
current through a heater element, which simulates the actual heating
effect that is taking place in the motor.
During overload conditions, the heat generated within
the heater element reaches a level that causes a mechanism to operate and
an auxiliary contact to open. In typical installations, the contactor coil
is wired in series with this contact. When the auxiliary contact opens,
the contactor is de-energized, removing current from the motor. New
solid-state overload relays, on the other hand, measure current directly,
resulting in better accuracy and multiple protective functions. Current
can be measured in a variety of ways, but the most typical method is using
current transformers. Once an overload condition is reached, the
electronic circuit of the motor protective device operates due to the
increased level of current, causing a contact to open, de-energizing the
contactor and removing current from the motor.
Besides basic overload protection, new solid-state
overload relays offer a broad range of protective features that previously
required several additional protection devices in the motor circuit. In
addition, by measuring parameters such as current, temperature, and phase
imbalance, damage to the motor's stator and rotor can be prevented. this
can also provide an early warning of possible trouble with another part of
the mechanical system, such as conveyors, belts, gears, and
bearings.
Advancements in solid-state technology now incorporate
motor control and communications functions. The ability of new solid-state
protective devices to communicate with device-level networks enables users
to manage multiple motors and processes for greater productivity and lower
life-cycle costs.
Through network communications, vital motor
information can be collected, processed, and displayed, including
real-time application parameters. This information can be helpful for
diagnosing problems before they occur, allowing operators to monitor the
system to prevent a device from tripping at a critical stage in a process.
For example, many solid-state devices have prewarning levels for various
causes of trip conditions (like ground fault, starting time, underload, or
jam) and are also capable of displaying data in the form of LEDs on the
devices themselves. In many cases, this communication can take place
simultaneously with a programmable controller.
By properly analyzing application requirements and
selecting appropriate protective components, motor management can be
applied to maximize motor life and the performance of the application.
Protecting motors means protecting your business.</TD></TR>