Soft Starter, Its Circuit Diagram, Operation, Advantages & Applications
Our industries use various kinds of machines. The induction machine is one of the most used three phase AC machine which is almost 70% of the motors used in industries. their robust construction and high efficiency makes it the best choice for every industrial sector. But they do require protection devices & equipment used for their safe operation so it can operate safely and prevents any potential damage to the motor and also increase their life span. The most essential equipment used for three phase induction motor is a motor starter.
Motor Starter
A motor starter is an electrical device that is used for safely starting and stopping an electrical motor. It also offers protection against overcurrent & low voltage protection.
Since the induction motor is extensively used for various kinds of applications in industries, they need motor starter to safely start & stop it. The induction motors draws a huge amount of current at starting. It is due to the low impedance of the windings of the motor at standstill.
It is very essential for safe operation of induction motor. It is due to the low rotor impedance of the motor in rest position. The rotor impedance depends upon the slip (Relative speed between the rotor & stator) of the induction motor. The slip of induction motor is not constant & varies throughout its operation thus the rotor impedance also varies. It is inversely proportional to the slip of motor.
At standstill (rest position), the slip of induction motor is maximum i.e. 1, thus the rotor impedance is minimum. Connecting the motor to power supply draws a huge amount of current in the stator winding due to this low impedance called inrush current. The alternating current in the stator generates a rotational magnetic field (RMF) that induces current in the rotor windings.
The rotor current generates its own magnetic field that try to cancel its cause & starts rotating in the direction of RMF. Thus the rotor experience torque & as its speed starts increasing the slip of the motor decreases (i.e. the rotor RMF speed approaches near stator RMF speed). Since the slip is reduced, the impedance of the rotor increases & the motor start drawing normal rated current.
The High inrush current is 5-8 time larger than the full load rated current of the motor. Induction motor cannot tolerate such amount of current as it can quickly damage or burn the windings, reducing the performance & lifespan of the motor. Such high currents can also cause huge dip in the line voltage which is dangerous for other appliances connected to the same line.
In order to prevent such high inrush current, we use motor starters that reduce the initial current for a short duration. Once the motor gains certain speed, the normal power supply is resumed. It also offers protections against low voltage & overcurrent.
These motor starters are usually used for heavy power rated motors. Small motors below 1Hp does not require motor starter due to their high impedance. However, they do need overcurrent protection that is available in DOL starter.
Soft Starter, Its Circuit Diagram, Operation, Advantages & Applications
Our industries use various kinds of machines. The induction machine is one of the most used three phase AC machine which is almost 70% of the motors used in industries. their robust construction and high efficiency makes it the best choice for every industrial sector. But they do require protection devices & equipment used for their safe operation so it can operate safely and prevents any potential damage to the motor and also increase their life span. The most essential equipment used for three phase induction motor is a motor starter.
Motor Starter
A motor starter is an electrical device that is used for safely starting and stopping an electrical motor. It also offers protection against overcurrent & low voltage protection.
Since the induction motor is extensively used for various kinds of applications in industries, they need motor starter to safely start & stop it. The induction motors draws a huge amount of current at starting. It is due to the low impedance of the windings of the motor at standstill.
It is very essential for safe operation of induction motor. It is due to the low rotor impedance of the motor in rest position. The rotor impedance depends upon the slip (Relative speed between the rotor & stator) of the induction motor. The slip of induction motor is not constant & varies throughout its operation thus the rotor impedance also varies. It is inversely proportional to the slip of motor.
At standstill (rest position), the slip of induction motor is maximum i.e. 1, thus the rotor impedance is minimum. Connecting the motor to power supply draws a huge amount of current in the stator winding due to this low impedance called inrush current. The alternating current in the stator generates a rotational magnetic field (RMF) that induces current in the rotor windings.
The rotor current generates its own magnetic field that try to cancel its cause & starts rotating in the direction of RMF. Thus the rotor experience torque & as its speed starts increasing the slip of the motor decreases (i.e. the rotor RMF speed approaches near stator RMF speed). Since the slip is reduced, the impedance of the rotor increases & the motor start drawing normal rated current.
The High inrush current is 5-8 time larger than the full load rated current of the motor. Induction motor cannot tolerate such amount of current as it can quickly damage or burn the windings, reducing the performance & lifespan of the motor. Such high currents can also cause huge dip in the line voltage which is dangerous for other appliances connected to the same line.
In order to prevent such high inrush current, we use motor starters that reduce the initial current for a short duration. Once the motor gains certain speed, the normal power supply is resumed. It also offers protections against low voltage & overcurrent.
These motor starters are usually used for heavy power rated motors. Small motors below 1Hp does not require motor starter due to their high impedance. However, they do need overcurrent protection that is available in DOL starter.
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