What is Soft Starter? Its Working, Diagram and Applications
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 machines 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 prevent 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 a motor starter to safely start & stop it. The induction motors draw 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 the 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 tries 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 a 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 a certain speed, the normal power supply is resumed. It also offers protections against low voltage and overcurrent.
These motor starters are usually used for heavy power rated motors. Small motors below 1 HP do not require a motor starter due to their high impedance. However, they do need overcurrent protection that is available in DOL starter.
The motor starter uses various kinds of techniques to start a motor such as
Full voltage or across the line starting technique; It connects the motor to the full voltage of the supply. It is used for small motor
Reduced Voltage Starter; It reduces the supply voltage during motor startup to reduce the inrush current. The soft starter uses this technique to start an induction motor.
Multispeed Starter; the motor is designed to have multiple preselected speeds that is achieved through pole (winding) configuration. Gradual increases speed reduce the inrush current.
What is Soft Starter?
The soft starter is a type of motor starter that uses the voltage reduction technique to reduce the voltage during the starting of the motor.
The soft starter offers a gradual increase in voltage during motor startup. This will allow the motor to slowly accelerate & gain speed in a smooth fashion. It prevents any mechanical tear & jerking due to sudden supplying of full voltage.
The torque of an induction motor is directly proportional to the square of current. & the current depends on the supply voltage. So the supply voltage can be used to control the starting torque. In a normal motor starter, applying full voltage to the motor generates maximum starting torque which possess mechanical hazard to the motor.
Therefore we can say that a soft starter is a device that reduces starting torque & gradually increases it in a safe manner until it reaches its rated speed. One the motor attains its rated speed, the soft starter resumes the full voltage supply through it.
During motor stopping, the supply voltage is gradually reduced to smoothly decelerate the motor. Once the speed reaches zero, it breaks the input voltage supply to the motor.
The main component used for the regulation of voltage in a soft starter is a semiconductor switch such as a Thyristor (SCR). Adjusting the firing angel of the thyristor regulates the voltage supplying through it. Other components such as OLR (overload relay) used for overcurrent protection is also used.
Diagram of Soft Starter
In a three phase induction motor, two SCRs are connected in an anti-parallel configuration along each phase of the motor making it a total of 6 SCRs. These SCRs are controlled using a separate logic circuitry that can be a PID controller or a microcontroller. The logic circuitry is powered from the mains using a rectifier circuit as shown in the figure.
What is Soft Starter? Its Working, Diagram and Applications
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 machines 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 prevent 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 a motor starter to safely start & stop it. The induction motors draw 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 the 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 tries 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 a 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 a certain speed, the normal power supply is resumed. It also offers protections against low voltage and overcurrent.
These motor starters are usually used for heavy power rated motors. Small motors below 1 HP do not require a motor starter due to their high impedance. However, they do need overcurrent protection that is available in DOL starter.
The motor starter uses various kinds of techniques to start a motor such as
Full voltage or across the line starting technique; It connects the motor to the full voltage of the supply. It is used for small motor
Reduced Voltage Starter; It reduces the supply voltage during motor startup to reduce the inrush current. The soft starter uses this technique to start an induction motor.
Multispeed Starter; the motor is designed to have multiple preselected speeds that is achieved through pole (winding) configuration. Gradual increases speed reduce the inrush current.
What is Soft Starter?
The soft starter is a type of motor starter that uses the voltage reduction technique to reduce the voltage during the starting of the motor.
The soft starter offers a gradual increase in voltage during motor startup. This will allow the motor to slowly accelerate & gain speed in a smooth fashion. It prevents any mechanical tear & jerking due to sudden supplying of full voltage.
The torque of an induction motor is directly proportional to the square of current. & the current depends on the supply voltage. So the supply voltage can be used to control the starting torque. In a normal motor starter, applying full voltage to the motor generates maximum starting torque which possess mechanical hazard to the motor.
Therefore we can say that a soft starter is a device that reduces starting torque & gradually increases it in a safe manner until it reaches its rated speed. One the motor attains its rated speed, the soft starter resumes the full voltage supply through it.
During motor stopping, the supply voltage is gradually reduced to smoothly decelerate the motor. Once the speed reaches zero, it breaks the input voltage supply to the motor.
The main component used for the regulation of voltage in a soft starter is a semiconductor switch such as a Thyristor (SCR). Adjusting the firing angel of the thyristor regulates the voltage supplying through it. Other components such as OLR (overload relay) used for overcurrent protection is also used.
Diagram of Soft Starter
In a three phase induction motor, two SCRs are connected in an anti-parallel configuration along each phase of the motor making it a total of 6 SCRs. These SCRs are controlled using a separate logic circuitry that can be a PID controller or a microcontroller. The logic circuitry is powered from the mains using a rectifier circuit as shown in the figure.
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