Slip ring motors, also known as wound rotor motors, are the core of many high-power transmission applications. They are widely used in raw material industries such as mining, mineral raw material industries or adhesive industries like cement, limestone, and gypsum in various crushing, roller pressing, and grinding processes. They are also applied in large fans, pumps, and conveyors.

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The stator:

The stator structure of squirrel-cage motors and slip-ring motors is the same. The main difference of slip-ring induction motors lies in the rotor structure and operation mode. When slip-ring motors are used in cascade systems, some changes in the stator may be encountered because the power supply of the motor comes from the power control of the rotor of another slip-ring motor, where external resistors are installed on its rotor.

Rotor:

What is the slip ring? The slip ring motor usually has a rotor with phase-wound stator. This type of rotor has a three-phase double-layer distributed winding, which is composed of the coils used in the AC generator. The rotor core is made of steel laminations and has slots to accommodate the formed three-phase single-phase windings. These windings are electrically separated by 120 degrees. Even if the stator is wound in two phases, the number of stator windings wound on the rotor is the same as the number in the stator and is always three-phase. These three windings are led out from the other end inside and connected to three insulated slip rings installed on the rotor shaft itself. The three terminals contact these three slip rings with the help of carbon brushes, which are fixed on the rings by spring components. These three carbon brushes are further connected externally to the three-phase starting connection rheostat. The slip ring and the external rheostat can increase the external resistance of the rotor circuit, making it have a higher resistance during startup, thereby increasing the starting torque. Under normal operating conditions, the slip ring automatically completes the circuit through the metal sleeve ring. The metal sleeve ring is pushed along the shaft, causing the three rings to contact each other. In addition, the brushes automatically lift from the slip ring to avoid friction losses and wear. Under normal operating conditions, the function of the slip ring rotor is the same as that of the squirrel-cage rotor.

What will happen if an external resistor is added? In the case of squirrel-cage induction motors, the rotor resistance is very low, so the current in the rotor is very high, which makes the starting torque worse. However, if an external resistor is added in the case of slip-ring motors, the rotor resistance is increased at startup, so the rotor current is low and the starting torque is maximum. Moreover, the slip required to generate the maximum torque is proportional to the rotor resistance. In slip-ring motors, by increasing the external resistor to increase the rotor resistance, the slip increases. Due to the high rotor resistance, the slip is greater, so even at low speeds, “pull-out” torque can be achieved. When the motor reaches its basic speed (full rated speed), when the external resistor is removed and under normal operating conditions, its operation mode is the same as that of squirrel-cage induction motors. Therefore, these motors are most suitable for very high inertia loads, which require almost zero pull-out torque at low speeds and acceleration to full speed, and absorb the minimum current in a very short time.

Advantages of slip-ring induction motors:

The main advantage of slip-ring induction motors is that they can be easily controlled in speed. Even at zero speed, “pull-out torque” can be achieved. Compared with squirrel-cage induction motors, it has a higher starting torque. The full-load torque is approximately 200-250% of the full-load torque. Squirrel-cage induction motors account for 600-700% of the full-load current, but the starting current of slip-ring induction motors is very low, approximately 250-350% of the full-load current.

Three Phase Slip Ring Motor Wound Rotor Starter – Control & Power Diagrams

Power and Control Diagrams of Self Start Slip Ring 3-Phase Induction Motor Wound Rotor Starter

Three-phase slip-ring (aka wound rotor) induction motors are widely used in heavy load and torque applications. While, three-phase induction motors can be challenging to start, especially when they are driving heavy loads. During startup, the rotor experiences a large inrush current, which can lead to overheating and mechanical stress. To mitigate these issues, various starting methods are employed, and one such method is the slip-ring rotor starter. In the following wiring guide, we will show how to control the starting and speed control of the three-phase wound rotor (slip-ring) induction motor with the help of schematic, power and control wiring diagrams.

Table of Contents

What is a Slip Ring Motor?

A slip ring motor, also known as a wound rotor motor, is a type of three-phase induction motor with a unique rotor design. Unlike the standard squirrel-cage rotor, a slip ring motor has two windings, stator and rotor where the rotor windings connected to external slip rings, which are in turn linked to resistors. This configuration allows for greater control over the motor’s starting characteristics and speed regulation.

By adjusting the resistance in the rotor circuit, slip ring motors can provide smoother starting, higher torque at low speeds, and improved control over the motor’s performance, making them well-suited for applications where precise speed and torque control are essential, such as in heavy industrial machinery and certain specialized applications.

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• The rotor windings are connected to the external rotor resistances, that’s why it is known as slip ring motor. These external resistances automatically adjusts and runs the motor at full load and speed.
• The slip ring induction motor has a high starting torque because the resistance of the motor winding increases with the addition of external resistance. The starting torque is enhanced as the power factor of the rotor circuit improves during the startup process.
• The slip ring induction motor has its rotor winding connected to an external variable resistor using slip rings. This configuration reduces the starting current during the initial stage of motor startup.

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Components Needed

1. Three-Phase Slip-Ring Motor
2. 4 Nos. of Contactors
3. Thermal overload Relay
4. 3P-MCCB
5. ON-OFF Pushbuttons
6. Three Phase Power Supply
7. Wires & Cables

Power & Control Wiring Diagrams

Power Diagram

Schematic Diagram

Control Circuit Diagram

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What is a Slip Ring Rotor Starter?

A slip ring rotor starter, also known as a wound rotor starter, is a specialized device used to control the starting and speed of a three-phase induction motor. It consists of several key components:

1. Rotor with Slip Rings: In a slip ring rotor starter, the rotor of the induction motor has three slip rings mounted on its shaft. These slip rings are electrically insulated from the rotor and are connected to external resistors.
2. External Resistors: The slip rings are connected to external resistors via brushes. These resistors are often made of wire-wound ceramic or other high-resistance materials. The resistors are placed in series with the rotor windings.
3. Control Panel: The control panel houses the necessary electrical components for operating the starter. This includes contactors, switches, and sometimes a rheostat for controlling the resistance in the rotor circuit.

How Does a Slip Ring Rotor Starter Work?

The operation of a slip ring rotor starter can be broken down into several steps:

1. Starting: When the motor is initially started, the slip rings are short-circuited. This means that the rotor windings are connected directly to the stator windings, creating a high resistance to the rotor current. As a result, the rotor experiences a reduced inrush current during startup, preventing excessive heating and mechanical stress.
2. Speed Control: As the motor accelerates, the resistance in the rotor circuit is gradually decreased. This is typically done by adjusting the position of the brushes or using a rheostat. Reducing the resistance in the rotor circuit allows the rotor to approach synchronous speed more quickly.
3. Running: Once the motor reaches near synchronous speed, the slip rings are completely disconnected from the rotor circuit. At this point, the motor operates like a standard squirrel-cage induction motor, running efficiently at its rated speed.

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Advantages of Slip Ring Rotor Starters

Slip ring rotor starters offer several advantages, making them suitable for specific applications:

1. Reduced Starting Current: By introducing resistance in the rotor circuit during startup, slip ring rotor starters significantly reduce the inrush current, preventing voltage drops and minimizing mechanical stress.
2. Smooth Starting and Speed Control: These starters provide smooth acceleration and allow precise control of the motor’s speed during startup, which is crucial for applications with varying load demands.
3. High Torque at Low Speeds: Slip ring rotor starters enable the motor to produce high torque even at low speeds, making them ideal for applications requiring heavy initial loads, such as conveyor belts and crushers.
4. Improved Efficiency: Once the motor reaches its rated speed and the slip rings are disconnected, the motor operates with minimal rotor resistance, maximizing its efficiency.

Applications of Slip Ring Rotor Starters

Slip ring rotor starters find application in various industries and settings, including:

1. Mining: Crushers, mills, and conveyor belts often use slip ring rotor starters to handle heavy loads and provide smooth starting and speed control.
2. Cement Industry: Cement kilns and crushers benefit from slip ring rotor starters to manage the starting torque and speed control requirements.
3. Metallurgy: Rolling mills and large pumps in metallurgical plants require slip ring rotor starters for precise control and efficient operation.
4. Marine and Offshore: Slip ring rotor starters are used in ship propulsion systems where precise control of motor speed and torque is critical.

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If you are looking for more details, kindly visit Rotor of Motor.

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