Thermal modeling protects motors without overtripping
Today’s modern microprocessor-based protection relays offer much more than the standard motor protection provided by electromechanical relays. Advanced features include motor-heating, time-to-trip and countdown-to-restart calculations %%MDASSML%% plus real-time diagnostics.
Even with the calculating power of microprocessor-based protection, most relays today still attempt to provide motor protection just by measuring current. The various manufacturers’ models calculate motor heating by thermal capacity or thermal register, where 0% is completely cooled and 100% is the trip threshold.
This thermal capacity is accumulated based on the measured current such that during motor starting, protection is essentially a function of the current squared multiplied by time (an I2t element), with maximum starting time dictated by the hot motor safe-stall time.
Limits of accuracy
While this type of protection dates back to electromechanical relays, the I2t model does not result in the most accurate motor protection, and limits true available horsepower. Thermal time constants and changing motor resistance are critical to proper protection.
Without proper compensation for these motor attributes, the overcurrent model overestimates motor heat, which causes it to trip prematurely. This is especially evident in cyclic overload conditions and in slow-starting, high-inertia load applications.
Cyclic overloads cause a motor to run for some time over the rated load and then for another period at less than the rated load. If the total heat buildup during the overload condition is less than the heat dissipated during the underloaded condition, then the motor thermal capacity is not exceeded. A true thermal model will calculate the heating and cooling time constants and properly apply them to the cyclic overloading condition.
An overcurrent-based protection scheme will overtrip the motor because the model overestimates heat buildup. Adjusting the cooling rate does not solve the problem because it is only correct for one known overcurrent and cyclic period. True thermal-based protection accurately models motor heat and provides accurate, proper protection.
Problems with the I2t model also arise when starting motors with high-inertia loads, as the time required to start the motor may approach %%MDASSML%% or even exceed %%MDASSML%% the hot safe-stall time. Highly accurate thermal-based protection includes a model that calculates motor slip (speed) during the start.
The relay calculates slip based on measured current, voltage and known full-load slip and locked-rotor torque (rated torque) from the nameplate. The relay uses the calculated slip to compute the positive- and negative-sequence rotor resistance throughout the motor start.
Motor resistance changes during starting by a factor of three or more. Without compensating for the resistance reduction during starting, an I2t scheme will trip before true thermal limits are reached, thereby restricting available horsepower during the start. Relays using a thermal model with a slip-based algorithm can automatically calculate maximum safe start times for each unique start sequence. Calculation of rotor resistance allows the relay to accurately reflect motor heating during a start and results in longer allowable acceleration times before tripping.
Motor protection can be greatly enhanced today with microprocessor-based relays with true thermal models that account for dynamic motor attributes. Relays that calculate thermal time constants and apply a thermal-based protection scheme can properly protect a motor during cyclic overloads without overtripping.
The slip-dependent thermal model protects the motor and allows for long acceleration times better than traditional I2t elements and electromechanical relays. Accurate calculation and tracking of motor heat by modern relays are valuable tools for improving motor protection and providing wider operating margins.
Mark Zeller is director of corporate marketing at
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