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    Principle and Structure of Slip-Ring Induction Motor

    Slip-ring induction motors, also known as three-phase wound-rotor AC asynchronous motors, must be connected in series with a resistor or resistors in the motor rotor in order to reduce the starting current and increase the starting torque and power factor, effectively improving the motor starting performance. This article describes the principle and structure of slip-ring motors.

    Working principle of slip-ring induction motor

    Slip-ring induction motors are asynchronous motors because the rotor never runs synchronously with the stator pole. In the case of squirrel-cage induction motors, the rotor resistance is very low, so the current in the rotor is high, which makes its starting torque poor. However, adding external resistance to a slip-ring motor makes the rotor resistance high at start-up, so the rotor current is low and the starting torque is maximum. In addition, the slip required to produce the maximum torque is proportional to the rotor resistance, and in a slip-ring motor, the rotor resistance is increased by adding an external resistor, so the slip increases. Because of the high rotor resistance and higher slip, "pull-out" torque can be achieved even at low speeds, and when the motor reaches its base speed (full rated speed), it operates in the same manner as a squirrel cage induction motor when the external resistance is removed and under normal operating conditions. Therefore, these motors are best suited for very high inertia loads, which require almost zero speed pull-out torque and acceleration to full speed with a minimal current draw in a very short period of time.

    Slip-ring induction motor construction

    • Slip-ring induction motor stator

    Squirrel cage induction motors and slip-ring induction motors have the same stator construction. The main difference between them is the rotor construction and operation. Some variations in the stator may be encountered when using slip-ring motors in a cascade system because the power supply from the motor comes from the power control of the rotor of another slip-ring motor where an external resistor is mounted on its rotor.

    • Slip-ring induction motor rotor

    Slip-ring motors usually have phase wound rotors, this type of rotor has a three-phase double distribution winding which consists of the coils used in an alternator, the rotor core consists of a steel laminate with slots to accommodate the 3 phase single phase windings formed, these windings are electrically separated by 120 degrees.

    Even though the stator is wound in two phases, the rotor is wound with as many stators as there are in the stator, and always in three phases. These three windings are led off at the other end internally and connected to three insulated slip-rings mounted on the rotor shaft itself. The three terminals contact these three slip-rings with the help of carbon brushes, which are fixed to the rings with the help of a spring assembly.

    The three-carbon brushes are further connected externally to the varistor of the three-phase starting connection. The slip-rings and the external varistor increase the external resistance of the rotor circuit, giving it a higher resistance at start-up and thereby increasing the starting torque.

    During operation under normal conditions, the slip-rings are automatically looped through a metal sleeve ring which is pushed along the shaft, thus bringing the three rings into contact with each other. In addition, the brushes are automatically lifted off the slip-ring to avoid frictional losses and wear. Under normal operating conditions, the wound rotor acts in the same way as the squirrel cage rotor.

    Below is a related ATO video about how the slip-ring connector is for rotating electrical connections.

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