High power induction motors need starters for the internal protection of the motors. Starters are also used to control the starting speed of the motor.
These are the basic functions of the starters:
Protection against overload: Relay devices in the starter circuit trips when there is overload on the input side. An induction motor may need high input current for starting, above the nominal current needed for the motor. After a time heating due to this excess current damage the winding of the motor.
Protect from the hazards of shortcircuit: Fuses and the electromagnetic circuit breakers protect the motor high current flow due to short circuit. Overcurrent is the result of a short circuit.
Isolate the motor from the main supply: Their role is to insulate circuits safely from their energy source (mains power supply) to ensure the protection of goods and people.
Initial speed controller: Used in soft starter control, the speed of the motor is controlled in ramp function manner
There are mainly two types of starter systems
Direct-on-line starters or DOL
Simplest type among starters. A DOL includes both the overload and overcurrent protection.
The figure above shows the DOL starter with both the overload and overcurrent protection, if any of them goes wrong one of the protection circuit trips.
In overcurrent protection circuit fuses and circuit breakers are commonly used. Fuses and circuit breakers are set to break the connection if the current in the line exceeds the nominal current of the motor.
Overload protection is commonly implemented with overload relay. These relays protect the motor from overheating due to high current. Relays are selected based on their tripping class.
A Soft starter is used for the input voltage control apart from the overload and overcurrent protection.
A soft starter device uses solid-state electronics like thyristor to control start-up voltage. The soft starter device connects series with the motor, an external start/stop signal triggers the motor,
A high running voltage is not directly applied the motor, the soft starter applies the voltage in increasing order.
A controller circuit sends firing signal to the thyristor gates to vary the starter output voltage as desired. The control circuit use thyristor firing control technique called phase angle modulation to produce an appropriate voltage output.
The controller circuit fires each of the thyristor to limit the voltage, such as the thyristor starts to conduct when the voltage phase-angle is greater than the firing angle.
Soft start ramp does not start with zero value because the motor needs a minimal starting voltage. The ramp increase until the motor reaches full speed/maximum running voltage. The motor runs in this voltage till it commands to stops.
There is a soft end ramp when the motor is commanded to stop the soft end ramp start decreasing, the voltage starts decreasing till it reaches zero.