Soft starter pdf

It generally consists of solid-state devices like thyristors to control the application of supply voltage to the motor. The starter works on the fact that the torque is proportional to the square of the starting current, which in turn is proportional to the applied voltage. Thus the torque and the current can be adjusted by reducing the voltage at the time of starting the motor.

Open Control : A start voltage is applied with time, irrespective of the current drawn or the speed of the motor. For each phase, two SCRs are connected back to back and the SCRs are conducted initially at a delay of degrees during the respective half-wave cycles for which each SCR conducts. This delay is reduced gradually with time until the applied voltage ramps up to the full supply voltage. This is also known as Time Voltage Ramp System.

Closed-Loop Control : Any of the motor output characteristics like the current drawn or the speed is monitored and the starting voltage is modified accordingly to get the required response.

The current in each phase is monitored and if it exceeds a certain set point, the time voltage ramp is halted. Thus the basic principle of the soft starter is by controlling the conduction angle of the SCRs the application of supply voltage can be controlled. The level voltage is generated using the comparator LM whose inverting terminal is fed using a fixed voltage source and the noninverting terminal is fed through a capacitor connected to the collector of an NPN transistor.

The charging and discharging of the capacitor cause the output of the comparator to change accordingly and the voltage level to change from high to low. This output level voltage is applied to the noninverting terminal of another comparator LM whose inverting terminal is fed using a ramp voltage. This ramp voltage is produced using another comparator LM which compares the pulsating DC voltage applied at its inverting terminal to the pure DC voltage at its noninverting terminal and generates a zero voltage reference signal which is converted to a ramp signal by the charging and discharging of an electrolyte capacitor.

The 3 rd comparator LM produces a High pulse width signal for every high-level voltage, which decreases gradually as the level voltage reduces. This signal is inverted and applied to the Optoisolator, which provides gate pulses to the SCRs. As voltage level falls, the pulse width of the Optoisolator increases and more the pulse width, lesser is the delay and gradually the SCR is triggered without any delay. The bypass contactors can be internal or external.

The internal bypass contactors are embedded inside the power switches. Each SCR have a bypass switch in parallel that supply the current under normal condition. While the external bypass contactors are connected externally in parallel with the soft starter. Such soft starter are bulky.

The bypass contactors are not meant to break or make the current supply to the circuit, thus it can be a low rated contactors. Smooth Startup: Unlike conventional motor starter, it provides very gradual increase of voltage thus speed that results in a very smooth startup.

There is no mechanical stress whatsoever or jerks that can damage the motor. This is used in application where startup acceleration needs to be adjusted. No Power Surges: Since the conventional motor starter allows full voltage across the motor, a huge inrush current start flowing into the motor that cause a power surge in the circuit.

However, soft starters drastically increases the number of startups for a motor in a specific duration. Reduction of Overheating: The motor overheating is a very serious problem. It occurs due to the high winding current during its startup. The soft starter allows a very small amount of starting current which prevents the overheating of motor. Increased Life Span: The soft starter as compared to a conventional starter improves the life time of the motor.

Less Maintenance: Due to its smooth operation, the induction motor is less likely to have any mechanical faults, which is why it require less maintenance as opposed to conventional motor starter. Efficiency: A conventional motor starter supply full voltage very high inrush current to the motor that consumes too much energy. Also the power switches are controlled using very low voltage level.

It improves the overall efficiency of the motor. Unlike other motor starters, it has very small size. Low Cost: compared to other starters such as VFD, this sure does cost cheaper.

No Speed regulation: The soft starter only allows the control of input voltage supply i. The speed of induction motor is regulated by varying the supply frequency below or above the line frequency according to the need.

Such feature is only available in VFD variable frequency drive. Heat dissipation : The semiconductor switches inside the soft starter dissipates some energy in the form of heat.

Therefore, it also requires heat sinks for cooling the power switches. Reduced starting Torque: Since it reduces the input voltage that corresponds to the input current which is directly proportional to the starting torque of the induction motor, it significantly reduces the starting torque.

This is why Soft starters are used for low or medium starting torque application. Fans: The huge fans used in industries runs at a constant speed. However, they do require the startup protection. A soft starter is a best option for such fans. It wears the belt that couples it to the load.

A soft starter offers a smooth starting for such motor applications. A conventional starter may generate enough pressure at startup to break the line. Benshaw Medium Voltage Products Catalog Benshaw Control Products Catalog Benshaw Pilot Devices Catalog Benshaw Product Line Overview Brochure.

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