21-05-2014, 11:56 AM
Motor Protection Principles
Motor Protection.pdf (Size: 679.51 KB / Downloads: 242)
Various Industry Motor Applications
Fans, Blowers
Pumps, Compressors
Grinders, Chippers
Conveyors, Shredders
Crushers, Mixers
Cranes, Extruders
Refiners, Chillers
Thermal Stress Causes Motor Failure
• Most of the motor failure contributors and failed motor components are
related to motor overheating.
• Thermal stress potentially can cause the failure of all the major motor parts:
Stator, Rotor, Bearings, Shaft and Frame.
Overload Protection - Thermal Model
A motor can run overloaded without a fault in motor or supply
A primary motor protective element of the motor protection relay is the
thermal overload element and this is accomplished through motor
thermal image modeling. This model must account for thermal process
in the motor while motor is starting, running at normal load, running
overloaded and stopped. Algorithm of the thermal model integrates
both stator and rotor heating into a single model.
Motor Thermal Limit Curves
• Thermal Limit of the model is dictated by overload curve
constructed in the motor protection device in reference to thermal
damage curves normally supplied by motor manufacturer.
• Motor protection device is equipped with set of standard curves
and capable to construct customized curves for any motor
application.
Thermal Overload Pickup
• Set to the maximum allowed by the
service factor of the motor.
• Set slightly above the motor service
factor by 8-10% to account for
measuring errors
• If RTD Biasing of Thermal Model is
used, thermal overload setting can
be set higher
• Note: motor feeder cables are
normally sized at 1.25 times motor’s
full load current rating, which would
limit the motor overload pickup
setting to a maximum of 125%.
Thermal Model Behavior - Long Starts
Issue ÎDuration of a high inertia load start is longer than the allowed
motor safe stall time.
• For these starts, thermal model must account for the current change during
acceleration and also use the acceleration thermal limits for TCU calculations.
• Motor thermal limit is growing along with motor rotation speed during
acceleration.
• Starting current is proportional to system voltage during motor acceleration,
thus voltage could be a good indication of the current level corresponding to the
locked rotor conditions.
• Voltage dependant dynamic thermal limit curve is employed to enhance the
thermal model algorithm.
• Motor relay will shift acceleration
thermal limit curve linearly
and constantly based on
measured line voltage during
a motor start.