Describe the construction of the starters used to start a three-phase slip ring induction motor.

The starting methods of the three-phase induction motor are usually direct online starting, low voltage starting and soft starting.

Direct online match

This type of starting mode is the most basic and simplest when starting the engine. The method is characterized by less investment, simple equipment and a small amount. Although the starting time is short, the torque is lower when starting and the current is large, which is suitable for starting small capacity motors.

Starting with reduced voltage

The reduced voltage starting method can be introduced in medium and large size induction motors to restrict the starting current. When the engine finishes starting, it will run again at full pressure. However, the result of the reduced voltage start will decrease the starting torque. Therefore, the reduced voltage starter is only suitable for starting the motor in unloaded or lightly loaded conditions. Some common methods of starting low voltage are as follows.

Stator circuit series resistance start

A three-phase electrical ballast is inserted into the motor stator winding circuit. The electric ballast can simply be considered as a coil, which can produce induced electromotive force to reduce the direct input power frequency voltage.

Star-delta match

In normal operation, the three-phase induction motor whose stator winding is designed to connect the delta connection can be started in star during the start, to reduce the voltage of each phase of the motor and then reduce the starting current. After finishing the game, he is connected to the delta.

Delta starter is widely used due to its advantages, including simple starter equipment, low cost, more reliable operation and easy maintenance.

Starting the autotransformer

The reduced voltage start of the autotransformer refers that the reduced voltage of the mains energy is connected to the motor stator windings until the speed approaches a constant value and then the motor is connected to the mains.

When starting, the switch is pulled to the "start" position, and the autotransformer is connected to the network, followed by connection to the motor stator windings to obtain a reduced voltage start. When the speed of rotation approaches the nominal value, the key will be pulled to the "operational" position and the motor will directly access the grid under full pressure operation, cutting the autotransformer.

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Starting the three-phase induction motor autotransformer

The reduced voltage start of the autotransformer is introduced in the star connection for the large capacity motor or normal operation with the start of a certain load. Depending on the load, the transformer tap is chosen according to the required starting voltage and starting torque. At this point, the starting torque is still weakened, but it is not reduced by a third (compared to the reduced voltage start of the star triangle). However, the autotransformer is large and light in size, with the high cost and inconvenient maintenance, which cannot be moved frequently.

Smooth start

The soft starter is a new type control device whose main advantages include soft start, light load and energy-saving and speed. One of the most important features is that the electronic circuit is conducted in the silicon-controlled rectifier of the motor under the tandem connection of the power supply. Using the soft starter to connect the power supply to the motor and different methods to control the driving angle on the silicon-controlled rectifier can cause the motor input voltage to gradually increase from zero and transfer all voltage to the motor from the beginning at the end, what is called a soft start. When starting in this way, the motor torque will gradually increase with the optimized speed. In fact, the soft starter is a voltage regulator that only changes the voltage without changing the frequency at startup.