A soft starter is a device designed to detect the presence of rotating shafts. Soft starters are typically used on rotating electrical machines (such as power generators) and in electricity distribution networks.
When the rotor, usually a squirrel-cage induction motor or servomotor, rotates a sufficient number of times or reaches a critical speed, it produces an electrical impulse via its coils. This impulse flows through the distributor network and induces an electromotive force in any connected load. The induced current may be as large as 15kA during normal operation when there are no problems with the soft starter.
What is a Soft Starter?
A soft starter is a device designed to detect the presence of rotating shafts. Soft starters are typically used on rotating electrical machines (such as power generators) and in electricity distribution networks.
The soft starter consists of a magnetic winding mounted on a rotating machine’s characteristic rotor (rotor shaft). It also includes an autotransformer used to isolate open circuit faults from the normal operating range of the machine. The soft starter is connected to the primary winding(s) (typically fed from line frequency or synchronous motors or generators) via a resistance network, diodes, relay coils, or capacitive couplings.
The soft starter causes the machine to produce an electrical current over that required to operate the device (when powered down) by inducing an electromagnetic field near the rotor. With a synchronous motor, this means that the engine is shut down. Without a soft starter, further reduction in motor speed reduces output power as the current supply falls below the minimum level necessary to keep rotor speed up to speed.
Soft starters are common on generators and electric motors at or near line voltage. Alternatively, they can be connected between line power (e.g., three-phase) and converter or rectifier station power (typically 208V electricity).
Working Principle of Soft Starters
The soft starter is a device designed to detect the presence of rotating shafts. Soft starters are typically used on rotating electrical machines (such as power generators) and in electricity distribution networks.
Soft Starters work essentially like this :
The soft starter may be connected in series with the primary winding of the machine via a resistance network, relay coils, or capacitive couplings.
With a synchronous motor, the motor is shut down, and no current is drawn from the supply at line voltage to operate the machine. The output rotational speed of a synchronous machine will be reduced when cross-power-frequency excitation fails.
Without a soft starter, further reduction in speed simply reduces output power as the current supply falls. Soft starters are especially important when there is not sufficient inertia in the load to allow it to continue turning (i.e., generating electricity) if its’ driving motor comes to rest.
Soft starters are common on generators and electric motors at or near line voltage. Alternatively, they can be connected between line power (e.g., three-phase) and converter or rectifier station power (typically 208V electricity).
Applications of Soft Starters
This is a very discrete approach that identifies the application of soft starters.
The list of applications of soft starters is given below :
- Critical speed: in this case, the soft starter reduces the power frequency induced to a safe level. If this happens, the motor cannot start at full speed (100% duty cycle). Soft Starters are an important part of any rotating machine as they can detect when starting/stopping and control it accordingly. The main reason for continuous control is to achieve synchronous operation between different devices and speeds while reducing energy losses.
- Inrush load: the soft starter reduces the inrush current during starting of motors. Soft starters help a lot in reducing electricity consumption.
- Inrush current: when an electrical machine comes into operation, the Voltage across the motor is high, creating a high current flow. In this case, if the soft starter is not there, then excess electrical energy will be wasted (due to over-current flow), and it will consume more energy, whereas with a soft starter, only a small amount of electricity is consumed by a motor, and therefore, the overall power consumption will be less.
- Over-current: a soft starter limits the maximum inrush current and the maximum operating current of a machine.
- Overload: a machine is designed to work at a certain load and with a certain voltage, but when the machine reduces its rotation speed (overload), then it cannot handle that amount of power produced by itself, due to which the machine is broken. With the help of a Soft starter, machines can be used extensively without breaking because they produce pulsating DC voltages. Otherwise, these machines would break easily if they do not have soft starters.
- Over speed : in this application, a soft starter is used to control the speed of the machine. A soft starter helps in controlling the rotation speed of a machine, and at the same time, it helps in maintaining constant torque on the machine. If a soft starter is not present, then the max speed for this application would be 100%.
- Over Voltage: when Voltage goes beyond safe limits, then it will damage sensitive components like motors, transformers, etc. Soft starters are also known as ride-through fuses, and they let electric machines run at safe Voltage during failure as well as after failures; that occur due to overload or over Voltage.
- Under Voltage: in case of under Voltage, Soft start prevents the machine from rotating with low Voltage. This helps to protect the machine from damage as well as saves energy.
Conclusion
Soft starters are a type of transient power protection that helps to protect electrical machines and transformers during starting or overload conditions. Soft starter is used in several loads (e.g., rotary-phase AC motors and generators) that cannot support dynamic inertia during overload conditions.
Soft starters have mostly been used for electrical machines but can be used for other types of loads as well, including heating, air conditioning, fans, pumps, and gas burners. A soft starter will help protect an electrical machine from excessive load current during starting if it is able to provide enough Voltage to meet the permanent magnet speed requirements before the machine’s rotational speed starts to decrease.
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