EducationWound Rotor Induction Motors: Construction, Operation, and Applications

Wound Rotor Induction Motors: Construction, Operation, and Applications

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Wound Rotor Induction Motors (WRIMs) are a type of induction motor characterized by their rotor construction. Unlike the more common squirrel-cage rotor found in standard induction motors, the rotor in WRIMs is composed of a winding wire that is similar to the stator winding. The key features of WRIMs include:

  1. Rotor Windings: The windings are connected to slip rings on the motor shaft, allowing for external resistors or controllers to be connected to the rotor circuit.
  2. Control Flexibility: This configuration enables improved control over the motor’s speed-torque characteristics.
  3. High Starting Torque: WRIMs provide high starting torque with low starting current.
  4. Speed Control: The motor speed can be adjusted by varying the resistance in the rotor circuit, making WRIMs suitable for applications requiring speed variation under load.
  5. Applications: They are commonly used in heavy-load applications such as elevators, conveyors, and hoists, where variable speed control and high starting torque are necessary.

WRIMs, while offering distinct advantages in certain applications, are more complex and costly than standard squirrel-cage induction motors due to their rotor design and the need for additional external equipment for speed control.

Induction machines are part of the category that comprises AC electronic machines which is where the transformation of electrical energy into mechanical energy, or the reverse. The energy conversions occur within induction machines by means of the magnetic field. They are due to the dynamic force that the field exerts the conductors (windings) that are placed inside the machine. In these machines, currents flow which are driven to be pushed by electromotive forces (sem) caused by the magnetic field. An induction machine is comprised of 2 windings (electric circuits) generally located in grooves that are components of the ferromagnetic core that are separated with the air gap. The windings aren’t galvanically connected to one another. One winding are connected with the network for industrial use and the other is directly shorted. The transformation of electrical energy into mechanical energy or vice versa happens due to being able to induce semi’s inside windings that are shortcircuited (unpowered) windings that force the flow of current through the winding.

Apart from that electric motors can also be divided by supply current. There is DC electric motors and AC electric motors that are split into the synchronous and asynchronous. AC motors are also divided according to the principles operating into synchronous induction, and commutator models. Additionally, you can find a distinction between motors based on the shape of the rotor. These are ring-induction motors as well as the squirrel-cage induction motors. Alongside the previously mentioned classifications, there is a second category of electric motors. which are known as universal motors. typically used in appliances for home use. They can be powered by direct current or alternating current.

wound rotor induction motor

Induction motor

Induction motors, also called asynchronous motors, are popular in various industries. These motors are known for their simple design and range in power from small milliwatts to large megawatts. Synchronous AC motors are also common in household appliances.

Typically, induction motors are used in applications that don’t require speed control. These motors often have squirrel-cage rotors and power ranges from a few milliwatts to several hundred kilowatts. They operate at nominal voltages between 0.4 and 6 kV.

Wound rotor motors, or ring motors, range in power from 2 kW to several megawatts, operating at the same voltages. A ring motor differs from a squirrel cage motor in its construction. It has three-phase windings in the rotor grooves, with short-circuited ends and connections to slip rings on the rotor shaft. Brushes on the rings connect the rotor winding to a speed or starter controller.

Ring-rotor motors designed without speed control have circuits to short-circuit the rings and lift the brushes. These motors have lower starting currents and allow for speed control but are more complex and expensive compared to squirrel-cage motors.

Wound rotor induction motor construction

Core Components of Wound Rotor Induction Motors

The Rotor: The Heart of Motion

Central to the wound rotor induction motor is the rotor itself. Unlike standard induction motors, the rotor in these machines is ‘wound’ — meaning it consists of a series of coils laid into slots along the rotor’s length. This design is crucial for its ability to offer controlled motor performance. Each coil is meticulously connected to the rotor, ensuring precision in the magnetic field generation.

Stator: The Stationary Powerhouse

The stator, a stationary cylinder, encircles the rotor. It consists of a series of electrical windings or coils on an iron core. When an alternating current passes through these windings, a rotating magnetic field is generated. This field is what ultimately induces the rotor to turn. The design and construction of the stator play a pivotal role in determining the efficiency and performance of the motor.

Slip Rings: The Connection Conduits

Unique to the wound rotor motor are the slip rings. These rings, attached to the rotor’s shaft, provide a means of connecting external resistances or control circuits to the rotor windings. This feature is what sets wound rotor motors apart, allowing for greater control over the motor’s speed and torque characteristics. The construction and material choice for the slip rings are critical, as they must endure constant electrical and mechanical stress.

Material Selection in Wound Rotor Induction Motor Construction

Copper Windings: Conducting Efficiency

In wound rotor induction motors, the windings are usually made of copper. Copper is preferred for its excellent electrical conductivity, which allows for efficient current flow with minimal energy loss. The rotor’s windings, intricately coiled, benefit greatly from copper’s pliability and resistance to corrosion, ensuring long-term reliability.

Steel Core: The Strength Within

The core of both the rotor and the stator is typically made from laminated steel. This lamination minimizes eddy current losses, a form of energy dissipation that occurs in alternating magnetic fields. The steel’s high permeability makes it an ideal material for the magnetic core, as it enhances the motor’s magnetic properties, thus improving its efficiency and power output.

Insulation Materials: Safeguarding Performance

Insulation is a crucial aspect of motor construction. Materials like mica, polyester, and polyamide are used to insulate the windings and other electrical components. These materials are selected for their thermal resistance and electrical insulating properties, which are vital in preventing short circuits and motor burnout under high operating temperatures.

Slip Rings and Brushes: Ensuring Connectivity

The slip rings in a wound rotor motor, usually made from copper or brass, provide a durable, conductive path for the rotor windings. Similarly, the brushes, often made of carbon or graphite, ensure a consistent electrical connection to the rotating slip rings while offering low friction and wear resistance.

Now, let’s delve into the final point of our article, “Design Innovations and Technological Advancements in Wound Rotor Induction Motor Construction.”

Design Innovations and Technological Advancements

Embracing Efficiency with Enhanced Cooling Systems

One of the most significant advancements in wound rotor induction motor design is the integration of advanced cooling systems. These systems are crucial for managing the heat generated during operation, thereby improving efficiency and prolonging the motor’s lifespan. Innovations in cooling technology, such as liquid cooling and improved ventilation designs, have allowed these motors to operate at higher loads without the risk of overheating.

Smart Sensors: The Gateway to Intelligent Performance Monitoring

The incorporation of smart sensors in wound rotor induction motors marks a leap towards intelligent machinery. These sensors monitor various parameters like temperature, vibration, and load, providing real-time data for predictive maintenance and performance optimization. This technological leap not only enhances the motor’s efficiency but also significantly reduces downtime and maintenance costs.

Advanced Materials for Unparalleled Efficiency

The evolution of material science has brought forth new materials that offer better performance and efficiency. The use of high-grade electrical steel in cores, improved insulation materials, and more conductive copper alloys for windings, has elevated the overall efficiency and reliability of these motors. These materials not only enhance electrical properties but also offer greater resistance to wear and environmental factors.

Integrating with Digital Control Systems

The synergy of wound rotor induction motors with digital control systems has revolutionized their application. These systems allow for precise control over speed and torque, adaptable to varying load conditions. This integration paves the way for more energy-efficient and responsive motor control, essential in today’s increasingly automated and energy-conscious industrial landscape.

Basic parameters of engines

If your motor connects to a 3-phase network the stator winding creates the field, whose speed of rotation depends on the number pole pairs. When the rotor located within the stator, is able to rotate freely, as well as its windings have been shortcircuited (either directly or via resistance) there is a torque created by the effect of the field’s rotation on the currents generated by these windings, which causes the rotor’s position to change in directions of field.

The speed of the rotor should be at least a percentage lower thanthe field’s synchronous speed.

The rotor speed should be several percent lower than the synchronous speed of the field. The power factor cosφN and efficiency ηN depend on the degree of its load on the shaft.

Motors’ fundamental parameters comprise:

Rated power:

P= √3 UN IηcosφN ∙ 10-3

Stator phase winding voltage:

When connected in delta: Us= UN

At the star connection:

Stator phase winding current:

At the triangle connection:

At the star connection: Ist = IN

Operating states of wound rotor induction motors

Idling of the induction motor

Induction motors are idling. is known as a condition of operation that includes:

  • The stator winding is supplied by voltage from the mains,
  • the rotor, which is not loaded with any torque spins at a very high speed , close to the synchronous speed
  • there are extremely small amounts of slip (about equal to 0.001) as well as frequency of the fs of the rotor, and losses in the steel rotor,
  • the electromotive force having a low magnitude is produced within the rotor. A very tiny amount of I2 current flows through it, so losses in the winding of the rotor are minimal,
  • the power generated to the motor’s shaft is negligible (the motor shaft isn’t fully loaded),
  • the power generated from the motor is used entirely to cover the losses in the stator winding ∆Pcu0, and in the stator steel, ∆PFe0 as well as mechanical losses ∆Pm,hence:

Po = ∆Pcu0 + ∆PFe0 +∆Pm

Idle losses ∆Po (do not depend on the load) and are:

P = Po – ∆Pcu0

     The idle current in induction machines at rated voltage is:

Io = (025 – 0,5)IN

      The power factor at idle is:

cosφo = 0,1 – 0,2

     A lower power factor means that the motor running at idle draws almost entirely inductive reactive energy from the grid, which causes power supply losses to increase and reduces power consumption from the mains power source.

In this case it is essential to employ compensating equipment (e.g. capacitor banks and compensaters synchronous) and ensure that loaded induction motors are removed from the grid.

Short circuit condition of induction motor

The condition of a short-circuit in the induction motor can be referred to as an induction motor condition. It is characterized by:

  • The stator’s winding gets power from the mains
  • the circuit of the rotor is short-circuited and the rotor has not been started, i.e. when n = 0 and si = 1 The current in the rotor is high, and it may be as high as (4 + 10)IN,
  • the electrical energy used by the motor in the short-circuited condition is completely converted to energy (mechanical electricity is not used since the rotor doesn’t move),
  • due to the relatively small loss in the central core area, it is possible to be roughly believed that the energy consumed by Po is designed to cover loss of load.

∆Pcu1 – in the stator winding;

∆Pcu2 – in the rotor winding,


Po = ∆Pcu1 + ∆Pcu2

The short-circuit voltage (uz) of an induction engine (uz) can be described as the current that has to apply to the windings of one side such as the stator. This is so that, when the opposite side is short-circuited, and the rotating rotor is deactivated the current rated flows to the supply side.

The short-circuit voltage of an motor is calculated in relation to the voltage that it is rated at:

The short-circuit voltage for induction motors varies from 10 to 25 percent UN.

Current in the short-circuit of an Induction Motor

An induction motor operating in the short-circuit state, supplied with the rated voltage is drawn a short-circuit current

The equation is: Is = (4 (or 10)IN (and reversed when it is supplied with an in-circuit short-circuit voltage, it draws the current that is rated). The relationship between short-circuit voltage (Iz) (Iz), the rated (IN) (IN) as well as short circuit voltage (uz percent) is as the following:

The short-circuit problem in squirrel-cage motors occurs each when that the circuit is connected even if voltage reduction devices aren’t utilized. The short-circuit current of the ring motor is decreased through increasing resistance in the winding circuit.

Load condition of induction motor. Stable and unstable operation

The load-condition that an induction engine experiences happens in the event that it is connected to the machine being driven and the stator winding is supplied by the line voltage.

Under load conditions these parameters for the motor’s induction are as the following:

  • the power conditions for the supply of the motor are generally unchanged,
  • the speed at which the equilibrium between the torque produced to the motor by M, and that of the braking force Mh are established.
  • at every load change the torque released by the motor is adjusted to the braking torque.
  • the slip flow of currents through the windings power ratio that the motor has, the actual energy consumed, as well as the power balance changes,
  • the force produced by the motor is changed until the balance of moments is established M = Mh.

The characteristics of an induction motor are determined by its operating characteristics, which is the relation of these parameters to the amount of torque (or power P). The most often used characteristic of motion of induction motors can be seen in the figure M = f(n) and the equation U1N = const. and f1N = const. This is also known as mechanical characteristics.

The characteristics of the motor’s operation that operate in the load state are influenced by their stability and instability, as well as the the motor-driven unit as well as to transient or steady states that include:

In steady state, the speed of rotation (n) that is generated by the machine is constant and the torque produced through the motor identical in magnitude, but in a different direction from the torque for static loads of the machine.

In the state of transient – the speed of rotation changes in the transient state; there is no equilibrium of the torque generated through the motor (M) and the brake the torque (Mh).

There is a difference in these instances:

M ‒ Mh = Md

     is called the dynamic moment (Md).

If Md > 0 (that is, M > Mh ), the system accelerates;

If Md < 0 (i.e., M < Mh ), the system decelerates.

Transients in the operation of rotating machinery most often occur during:

  • start-up,
  • speed control,
  • load changes,
  • changes in power supply conditions,
  • stopping of the system.

Depending on the characteristics of the induction motor – driven machine system, there can be two ranges of operation:

  • stable operation, if the system returns to equilibrium after a brief disturbance,
  • unstable operation, if after a short disturbance the system stops or tends to coast.

Ranges of stable and unstable operation of induction motor

Stable operation range of induction motor at 0 < s < sk

If the induction motor is filled with a braking force Mh = const lower than the initial torque Mr (at the point of beginning n = 0 and the s value is 1) It will be supplied by line voltage, and the motor starts to spin in the direction of dynamic torque Md. The moment Md gives acceleration to the rotor, and consequently the speed of the rotor rises and slip s decreases.

The length of an electric motor in the form M = f(s) can be described in the following manner:

  • The motor’s operating point is moved in the direction of that M = f(s) beginning at to the point of s=1 towards the smaller slips, and then across the line (Mk, sk);
  • The steady-state at A. The lines (b) the steady state at point A, where M = f(s) and intersects the Mh line. Then, the torque generated by the motor M = Mh while the torque dynamic Md is M + Mh is 0, rotor’s speed is fixed at the point A, at a speed of n1;
  • The steady state is at B. the torque Mh grows until it reaches the value M’h. Then M’h > M, i.e., it will be higher than the torque generated by the motor. The motor’s speed decreases, and slip grows. The operating point is shifted towards more slip, and the torque produced by the motor will be equal to the torque of braking in the braking system. A new steady state will be created at point B. It will be characterized by the sII as well as nII.

Range of motor unstable operation at sk < s < 1

In the event that an instantaneous equilibrium state M = M’k is observed at the point C (the braking torque increases, Mh) The motor’s speed is reduced and the motor operating point shifts towards the slip s=1.

The torque produced by the motor as well as the rotor’s speed will decrease until, when 1 s, the motor is stopped.

In a different scenario, if the braking torque is reduced when it is at equilibrium at the point C, the force generated by the motor will be higher than the braking torque M exceeds Mh which causes the rotor’s speed to increase.

The motor’s operating point will move through it’s point (Mk, the sk ) towards smaller slips. This will result in the simultaneous increasing the torque and speed. The steady state change will take place at the point B.

Overload capacity of an induction motor

The torque of motors is proportional magnitude of the voltage Any decrease in the line voltage will drastically reduce the amount of torque generated by the motor. This is why the motor’s operating point must be in the steady operating range that is far away from its critical torque in order that the decreased voltage doesn’t make the engine stop.

The majority of the time, the nominal torque (MN) that an induction engine produces is two times smaller than the crucial torque (Mk). The motor’s overload power is calculated using the formula:

The overload capacity in a motor is assumed to be about 2.5. In motors performing responsible tasks, such as in crane motors, it can even exceed 4.5.

Wound rotor induction motor starting methods

Wound rotor induction motors, a cornerstone in industrial machinery, leverage their unique design to offer remarkable control and efficiency in starting and operating heavy loads. Unlike their squirrel cage counterparts, these motors feature windings on the rotor connected through slip rings to external resistances. This design permits various starting methods, each tailored to reduce the initial high starting current and minimize stress on the motor and power grid. Among these methods, the most prominent include the use of external resistors, auto transformers, and star-delta starters. Each method has its nuances and applications, making them a subject of keen interest for engineers and technicians in the field. The external resistor method, for instance, involves gradually decreasing the resistance in the rotor circuit, enabling a smooth acceleration. Auto transformers, on the other hand, temporarily reduce voltage during startup, effectively managing the current. Lastly, the star-delta method alters the motor’s winding connection from star to delta, reducing the voltage and current during the initial phase. Understanding these methods is not only critical for the efficient operation of these motors but also plays a pivotal role in prolonging their lifespan and ensuring safety in industrial settings.

Importance of Starting Methods in Wound Rotor Induction Motors

The starting phase of a Wound Rotor Induction Motor (WRIM) is a critical juncture that demands careful consideration. Unlike synchronous motors or standard induction motors, WRIMs present unique challenges during their starting process, primarily due to their construction and operational principles. These challenges can significantly impact both performance and efficiency.

  1. High Starting Currents: One of the most significant issues during the startup of a WRIM is the high inrush of current. This surge can be as high as six to eight times the motor’s full-load current, leading to a substantial thermal and mechanical stress on the motor. Such stress, if not managed correctly, can lead to a reduction in the motor’s lifespan and increased maintenance costs. It can also cause voltage drops in the power supply network, affecting other equipment connected to the same network.
  2. Mechanical Stress: The torque produced during the start-up can be abrupt and intense. This sudden force can be detrimental to both the motor and the driven equipment. Ensuring a smooth acceleration is vital to reducing mechanical stress and enhancing the longevity of the motor and its connected machinery.
  3. Impact on Performance: The method chosen for starting directly influences the motor’s performance. An inappropriate starting method can lead to inefficient operation, increased wear and tear, and higher energy consumption. It can also cause operational issues such as vibration and noise, which are undesirable in precision-demanding industrial environments.
  4. Efficiency Considerations: The efficiency of a WRIM during its start-up phase can significantly affect its overall energy consumption. Efficient starting methods help in reducing the initial power requirement and contribute to a lower carbon footprint, which is increasingly important in today’s energy-conscious world.
  5. Safety Aspects: Safety is another critical factor. The starting method must ensure that the motor starts reliably and safely every time. This reliability is crucial in applications where motor failure can lead to hazardous situations or significant production losses.

Overview of Common Starting Methods

The starting methods for Wound Rotor Induction Motors (WRIMs) are vital for ensuring efficient and safe operation. Three common methods are widely used in the industry: Direct-on-line starting, Star-delta starting, and Auto-transformer starting. Each method has its own set of advantages and is suited for different operational requirements.

Direct-OnLine Starting

This is the simplest and most straightforward method of starting a WRIM. It involves directly connecting the motor to the power supply, which causes a high inrush current and a strong starting torque. Direct-on-line starting is typically used for small motors where the high starting current will not cause excessive voltage drop in the power supply network.

Star-Delta Starting

Star-delta starting is a method designed to reduce the starting current of the motor. Initially, the motor is connected in a ‘star’ configuration, which reduces the voltage across each winding and, consequently, lowers the starting current. After the motor reaches a certain speed, it is switched to a ‘delta’ configuration to allow full voltage across the windings for normal operation. This method is suitable for larger motors where controlling the starting current is crucial.

Auto-Transformer Starting

This method utilizes an auto-transformer to reduce the voltage applied to the motor during the start-up phase. By doing so, the starting current is also reduced. Once the motor reaches the desired speed, the transformer is bypassed, and the motor runs at the full line voltage. This method is effective for large motors that require a smooth start with minimal impact on the power supply network.

Each of these starting methods plays a significant role in the performance and longevity of WRIMs. The choice of starting method depends on various factors, including the size of the motor, the mechanical load, and the capacity of the power supply network.

Advanced Starting Techniques for Enhanced Performance

Advancements in technology have led to the development of more sophisticated starting methods for Wound Rotor Induction Motors (WRIMs). These methods not only address the issues of high starting currents and mechanical stress but also offer enhanced control over the motor’s starting process. Two notable advanced techniques are Soft Starters and Variable Frequency Drives (VFDs).

  1. Soft Starters: Soft starters are devices that control the voltage supplied to the motor, allowing for a gradual increase in power. This method significantly reduces the inrush current and mechanical torque during start-up, leading to a smooth and controlled acceleration of the motor. Soft starters are particularly beneficial in applications where the mechanical systems connected to the motor are sensitive to sudden jolts of torque.
  2. Variable Frequency Drives (VFDs): VFDs offer the most advanced control over the starting process by adjusting both the frequency and voltage supplied to the motor. This precise control allows for a smooth ramp-up to full speed and can be tailored to the specific requirements of the application. VFDs not only improve the starting process but also enhance the overall efficiency of the motor during its operation. They are ideal for applications requiring variable speed control and high precision.

Both soft starters and VFDs extend the lifespan of the motor and connected equipment by reducing wear and tear. They also contribute to energy savings and operational efficiency, making them a preferred choice for modern industrial applications.

Protection of induction motors

Electric motors can have a substantial effect on electricity grids, for example, current overcurrents when motors start as well as voltage dips and fluctuations and substantial reactive power consumption from motors that are idle.

Motor protection basics include:

  • short-circuit protection,
  • overprotection,
  • (c) protection from the negative effects of the effects of voltage reduction.

To protect motors from short-circuits that have voltages less than 1 kV, three-phase fuse or three-phase solenoid devices are utilized.

For motors that have an output of more than 1 kV The function of short-circuit protection is fulfilled with fuses in 3 phases, paired with a disconnector, or time delay relays. For motors that have an output of greater than 2 MW , and when six terminals are exposed with longitudinal differential protection, two phases is utilized. Motors that exceed 1MW have ground fault protection. It is that is supplied by an electric Ferranti transformer.

The setting current of the motor short circuit protection is to remain as minimal as it is but in a way that it doesn’t operate at high levels of currents during normal operation , and also when the motor is starting up.

The current that is rated for fuse-links to protect AC motors is determined by the connection:

Ibn > Ins

in which:

Irs – motor inrush current, in A;

Ins – rated motor current, in A;

kr – inrush current multiplication factor;

α – starting coefficient (under average conditions equal to 2 for fast-acting inserts

and 2.5 for inserts with delayed action).

     Triggers or electromagnetic relays that are short-circuit protection for the motor are set to current:

Iwe ≤ 1,2 ·Irsmax

Where: Irsmax – the highest inrush current of the motor.

Triggers, thermal relays and temperature sensors are employed to provide overload protection for motors with a rating of up at 1kV. If motors are rated above one kV, delayed protection is utilized.

Motor short-circuit protection must be employed in three phases of three-phase systems. However, in DC systems, it should be applied to two poles or in one when the other pole is grounded. Short-circuit protection shouldn’t be utilized in circuits that excite.

Each motor must have its own short-circuit protection. Short-circuit protection that is common to groups of motors is permissible, which means that should there be the short circuit occurring within one of the motors the group protection will be activated. Overload protection must be present for each motor , with the exception of:

  • motors that have ratings of lower than A,
  • motors that do not exceed 10 kW in continuous operation and are unlikely to overload (pumps or fans),
  • motors that form one unit, each with its own safety,
  • motors that operate intermittently which are protected with temperature sensors isn’t economically viable.

In three-phase systems, with the neutral point being grounded, overload protection is employed in 3 phases, but not with a natural point that is grounded in two phases. Overload protections are configured to have a limit of not more than 1.1 Ins.

Protection against overvoltage is provided as over-voltage delay single- or two-phase. This protection should be utilized in the following situations:

  • The reduction in voltage prevents the motor from working,
  • self-starting of a short circuit motor is not recommended,
  • self-starting of an ring motor is unacceptable,
  • it is recommended to detach a variety of less critical motors to ensure the self-starting of the other motors.

The arrangement of two relays undervoltage, which are which are switched by the line-to-line voltages, is utilized to guard a group of motors or a single motor important from the standpoint of the technology. A system with only one relay undervoltage should be utilized to safeguard specific motors that are less important.

Wound rotor induction motor characteristics

The red color indicates features of the motor at the positive spin rate. If the slip is that is greater then zero (motor speed lower than synchronous speed: oos) the machine acts as a driver, whereas for slips that are negative (motor speed higher than the synchronous speed: o>os) the machine goes from brake operation. The blue color indicates characteristics of the slower rate of field rotation that can be achieved by switching between 2 phases in the voltage of supply to the motor.

Mk – critical torque,

Mr – starting torque,

MN – rated torque,

ωN – rated speed,

ωs – synchronous speed.

Advantages and disadvantages of wound rotor induction motor

Advantages of the induction motor

  1. One of the main advantages of an induction motor is the fact that its construction is very simple. Stator designs are identical to that of the synchronous and induction motors. However the synchronous generator needs the use of a slip ring to supply DC energy to be delivered to the motor’s rotor. Slip rings are not necessary in a squirrel-cage induction motor as the windings are always shorted. As compared to the DC motor Induction motors have no brushes, and maintenance requirements are minimal. This results in a simpler design.
  2. The motor’s operation is completely independent of the conditions in the environment. This is due to the fact that the motor is robust and durable mechanically.
  3. Squirrel cage induction motors do not have slip rings, Brushes and commutators. Because of this, the price for the motor can be affordable. But slip rings are utilized to create Wound Induction Motors to provide an external source of resistance for the windings of the rotor.
  4. Due to the absence brush, there aren’t sparks that could ignite the motor. It is also able to operate in hazardous environments.
  5. Unlike motors that are synchronous 3 phase induction motors has a large starting torque, great control of speed and decent overloading capabilities.
  6. Induction motors are highly efficient with an efficiency of 85-97 percent.

Disadvantages of the induction motor

  1. A single-phase induction motor unlike a three-phase motor, is not equipped with a start-up torque. It requires auxiliary equipment to begin a single-phase motor.
  2. Under low load conditions the power efficiency of motor decreases to a low level. This is due to the fact that during the initial phase the motor, it draws a significant magnetic current to overcome the reluctance created due to the air space that is created between the rotor and stator. Additionally that an induction motor is able to draw smaller current from power source. The combination of the magnetizing and load present is deferred by approximately 75-80 degrees. This means it’s power efficiency is very low. Due to the large magnetizing current, the losses of copper in the motor will increase. This results to a reduction in motor efficiency.
  3. Controlling the speed for an induction motor can be very difficult to achieve. This is due to the fact that the three-phase motor is one that is constant speed that is, for the entire range of load, the rate of change in motor speed is minimal.
  4. Induction motors possess large surge currents at the input and are also referred to by the name magnetization surge currents. This leads to a decrease in voltage once the motor is first started.
  5. Due to the lower beginning torque of the motor, it is unable to be utilized for tasks that require large beginning torque.

Wound rotor induction motor applications

In the realm of industrial machinery, the wound rotor induction motor (WRIM) stands out as a pivotal component driving innovation and efficiency. These motors, known for their robust construction and customizable performance, are integral to numerous industrial operations. Despite their technical complexity, WRIMs have a straightforward purpose: to offer reliable, adjustable speed control and high starting torque, which are essential in heavy-duty applications.

From the bustling floors of manufacturing plants to the demanding environments of mining operations, WRIMs are the unsung heroes ensuring smooth and efficient processes. We will explore how these motors have become indispensable in various industries, shedding light on their versatility and the reasons behind their widespread adoption.

Manufacturing Sector: Wound rotor induction motors are extensively used in the manufacturing industry, particularly in applications that require high torque and variable speed. Their ability to withstand heavy loads and maintain a constant speed under varying loads makes them ideal for driving conveyor belts, cranes, and large milling machines. In the automotive manufacturing sector, WRIMs are instrumental in the assembly line, where precise control and high starting torque are crucial.

Mining and Mineral Processing: The mining industry benefits significantly from the ruggedness and reliability of wound rotor induction motors. They are employed in conveyors, crushers, and drills where they must operate under harsh conditions. Their high torque capabilities are essential for starting large loads typical in mining operations, such as heavy conveyors carrying tons of raw material.

Pulp and Paper Industry: In the pulp and paper industry, consistency and control are key. WRIMs are used in various stages of production, from pulping to rolling. Their ability to provide controlled speed and torque ensures the smooth operation of paper machines, which is vital for maintaining the quality of the paper produced.

Water and Wastewater Treatment: Wound rotor induction motors are a backbone in the water industry, especially in large pumping stations. They provide the necessary torque for starting large centrifugal pumps and can be easily adjusted to accommodate changes in flow rates, making them ideal for applications where varying demand is common.

Renewable Energy Sector: With the growing focus on renewable energy, WRIMs have found a place in applications such as wind turbines and hydroelectric power plants. In wind turbines, they are used for their ability to handle fluctuating loads and in hydroelectric plants, for their high starting torque and controlled speed, which is essential for maintaining grid stability.

Michal Pukala
Electronics and Telecommunications engineer with Electro-energetics Master degree graduation. Lightning designer experienced engineer. Currently working in IT industry.