Why limiting starting current in large motor is essential?
Restriction of the starting current in the large motors is important for various of reasons but all of them are associated with reduction of mechanical and electrical stresses, improvement of reliability and suitability of the equipments for the intended functions.
Mechanical Stress Reduction
Larger motors are in most cases energized when excited by inrush currents that are much higher than the rated full load current and could be from five to seven times or more.
This high starting current can result to creation of high mechanical losses specifically affecting the rotor and stator.
The jerky thrashing generates appropriate amount of turbulence in the form of vibrations and oscillations within the machines mechanical components such as bearings and these can lead to premature failure.
Some of these problems are avoided when the mechanical load is regulated through the limitation of the starting current.
Electrical System Stability
As mentioned earlier, large motors drag large currents from the electrical system and hence they result in a large voltage drop. This can influence not only the motor but also other connected equipment it may cause unstable operation or disconnection of parallel loads.
Ex: It can decrease to approximately 20% at motor start-up, which directly brings decreased torque for parallel motors, with consequent intermissions in operations.
Restriction of the momentary current of start-up is useful for the stabilization of the power supply and the prevention of possible outages rising from a chain reaction throughout the electrical power network.
Thermal Management
At start-up, motors produce much heat as a result of high currents passing through the windings.
This heat is not shed during the first stage of operation as intended, which results in temperature spikes that may approach or even cross emergent maximums (up to 500 o C in some cases), if not controlled.
Hot climate conditions can cause a deterioration of insulation materials and internal system problems.
In this manner, thermal stress is controlled by managing the inrush current so as to prolong the life of motors besides lowering the cost of maintenance.
High Durability and Dependability
High starting currents produce fatigue damage in the motor parts, broken bars in the rotor and insulation material damage.
Reducing starting current possesses the following advantages and therefore helps to increase the general dependability of the motor: In addition, it also permits multiple initiations without threatening the stability of the motor system.
Some of the approaches like soft starters, Variable Frequency Drives (VFDs) will help in controlling the amount of starting currents and at the same time achieve gradual starting.
Economic Considerations
High starting currents not only pose a threat to equipment and their life span but also create a problem of frequent power penalties by utilities which penalize for high kVAR and low power factor.
However, those start current surges can be reduced in detail; thus, firms will be able to avoid those costs as they keep their operations smooth.
Hence, it is important to limit the amount of starting current in large motors in-order to reduce mechanical stress, improve electrical stability, handle thermal conditions, increase reliability and minimize costs of operation. They are essential to ensure large motors are used optimally and their service time is long.
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