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Switching Synchronous Motors Smoothly Through Electronic Circuits

written by: Swagatam • edited by: Lamar Stonecypher • updated: 5/6/2011

A soft starter is a electronic starter for a synchronous motor that supplies gradually increasing voltage to the motor during start up to avoid high starting current taken by the motor.Let's find out how the soft starter works.

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    What is Soft Starting?

    Soft starting, as the name indicates, is a smooth starting of a motor without any moving contactors. It employs a electronic circuit for starting having thyristors on a common circuit board. Soft starting is considered an efficient electronic switching technique.

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    Why We Need Soft Starting for Synchronous Motors?

    Let us take a case where the synchronous motor is started initially as an induction motor and brought to synchronous condition later. In this, when a synchronous motor is connected directly to a three phase AC supply without any starter, a very large stator current flows through the stator winding. The stator current drawn by the motor during startup will be up to five to eight times the full load current. This is due to the fact that the rotor cuts a maximum rate of flux, creating a large induced current in the rotor of the motor due to large induced EMF. Because of this increased stator and rotor current, the supply power factor will be very low typically about 0.2 lagging. This rises to 0.5 lagging on no load then to about 0.85 lagging on full load. This starting surge current reduces as the motor accelerates up to its running speed.

    The motor operating at low power factor (0.5 lagging) at the light load condition reduces the efficiency as the supply current is relatively high causing significant I2R resistive losses (copper losses). In order to attain good efficiency from the motor, the power factor has to be improved.

    The only way to improve the power factor of the motor at light load is to reduce the supply voltage given to the motor. This can be achieved with the applicable voltage controller. A type of voltage controller working on electronic circuits employed to control the supply voltage of the motor is called a soft starter.

    The soft starter, which is an electronic voltage controller, is also known as an energy manager. It can match the supply voltage to the startup and load conditions. Such a controller aims to maintain the operating power factor as high as possible to minimize supply current and power losses. An important thing to note is that this type of voltage controller does not control the speed of the motor; that can be controlled by frequency of the supply. However this speed control is only possible in induction motor whereas the synchronous motor is a constant speed motor that runs at its synchronous speed.

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    Soft Starting of Synchronous Motor

    The soft starters employed for starting synchronous motor have back to back thyristors or triacs in each three phase AC supply line to the motor. These thyristors are gated to delay turn-on with each AC half cycle, and the delayed switching applies a reduced average AC voltage to the motor, which means it supplies gradually increasing AC voltage during start up. The applied motor voltage is gradually ramped up by the starter software program until the full voltage level is reached. Once the motor achieves the rated speed, the thyristor allows the rated current to flow in the lines to the motor and hence the full voltage is applied there by achieving maximum efficiency. Strictly speaking, the electronic switching circuit is bypassed for normal running. It also employs a gate control circuit which controls the thyristor function. Start and stop commands can be given from this control circuit. The thyristor operates according to the command given to the gate control circuit.Synchronous Motor Soft Start, Block Diagram, Image  

    A soft starter may be further adapted to become a voltage controller or a voltage regulator over the load range of the motor operation, and hence it is called an energy manager. It supplies voltage according to the load of a motor. In this type of efficient energy manager application, the controller monitors the motor power factor which is a means of motor loading and according to the power factor of the motor, the soft starter supplies voltage to the motor.

    On light load and the full voltage, the power factor is low so the controller reduces the motor voltage, which reduces current while improving the power factor and efficiency.

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    Soft Starter Handbook -

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