Wound Rotor Induction Motor
Hi friends, I hope you are fine and having fun. In my last article, we have discussed Squirrel Cage Induction Motor which is a type of 3 Phase Induction Motor. Today, I am going to tell you about a Wound Rotor Induction Motor , which is also a type of 3 phase induction motor. This motor is the one which uses a wound rotor. I will tell about the construction of a wound rotor later in this article. A Wound Rotor Induction Motor is also known as slip ring induction motor.
I will tell you about the construction of Wound Rotor Induction Motor in the first section. In the second section, I would be throwing light on the operation and working principle of Wound Rotor Induction Motor. After that, I will tell you some of the key features and uses of Wound Rotor Induction Motor.
Construction of Wound Rotor Induction Motor
As in other induction motors, the basic parts are stator and rotor. Stator of this motor is the same as the one used in squirrel cage induction motors. Rotor is the part which distinguishes it from other induction motors. Let me give you a brief idea of stator first and then I will tell you about the structure of rotor.
Stator of Wound Rotor Induction Motor
- As the name implies, stator is the stationary part of Wound Rotor Induction Motor.
- The stator and rotor are separated from each other by a small air gap.
- Stator of any type of induction motor is a cylindrical frame inside which rotor rotates.
- The cylindrical frame has grooves on its internal periphery to carry electrical circuitry.
- The stator winding is excited by AC supply.
- That was the basic structure of stator, now let’s talk about rotor.
Rotor of Wound Rotor Induction Motor
- In Wound Rotor Induction Motor, the rotor has a 3 phase winding similar to stator winding.
- Rotor is also cylindrical in shape and has slots to carry winding.
- The winding is placed evenly on slots of the rotor.
- are connected to 3 slip rings.
- These slip rings are mounted on the shaft.
- Each phase is connected to one of the three slip rings. These slip rings are associated with brushes.
- The three slip rings rotate with rotor, while brushes remain stationary.
- This is all about construction of rotor. Now let’s discuss the working principle.
Working of a Wound Rotor Induction Motor
- The working principle of Wound Rotor Induction Motor is as same as in any other induction motor.
- AC supply is given to stator winding, which produces a magnetic field that is rotating because of the time changing AC supply.
- The flux lines of the magnetic field will cut the rotor and induce an emf according to Faraday’s law.
- The induced emf will induce a current that will generate another magnetic field. This magnetic field is called rotor magnetic field.
- The magnetic field of stator and the magnetic field of rotor will interact and give rise to a torque which will turn the rotor.
This motor is different because the rotor windings are connected to external resistances through slip rings. The speed and torque can be adjusted in these motors by changing resistance. Now I am going to tell you about some of the key features of these motors.
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Key Features
Some of the features of wound rotor induction motors are as under:
Speed Control
- It is possible to control the speed of such motors. The torque can also be controlled.
High Starting Torque
- These motors have high starting torque.
Maintenance
- Wound Rotor Induction Motor require more maintenance because of slip rings and brushes.
Cost
- Such motors are expensive than other induction motors. The maintenance cost also counts.
Low Starting Current
- This motor draws less current at start as compared to squirrel cage induction motors.
Efficiency
- Wound Rotor Induction Motor are less efficient than squirrel cage induction motors.
Power Factor
- These motors have a low power factor.
Now I am moving towards the last section, in which I am going to tell you all about some of the application areas of Wound Rotor Induction Motor .
Applications
The uses and applications of this induction motor are:
- They are used in areas where high starting torque is required. And where squirrel cage induction motors cannot be used because of their high starting currents.
- These motors are used with high inertia loads.
- Wound Rotor Induction Motor is used in applications which require smooth start and adjustable speed.
- Some of the applications of this motor include cranes, mills, hoists and conveyors.
- Wound rotor induction motor is also used in fans, blowers and mixers.
- They are used in large pumps in water industry.
Squirrel Cage Induction Motor
Hello Friends, I hope you all are fine and doing great. I am here with another article on Induction Motors. In this article I am sharing some basic knowledge on Squirrel Cage Induction Motor. In my previous article named as 3 Phase Induction Motor, I have given a brief overview of this Squirrel Cage Induction Motor.
A 3 phase induction motor has two types based on the rotor construction, named as: Squirrel Cage Induction Motor and Wound Rotor Induction Motor. Former is cheaper and broadly used as it requires less maintenance than later one. Firstly, I am going to tell you about the structure of squirrel cage induction motor. Afterwards I will cover the working principle and features of Squirrel Cage Induction Motor. In the last sections, I will share some advantages and uses. In simple words, the type of 3 phase induction motor which uses a squirrel cage rotor is called squirrel cage induction motor.
Construction of Squirrel Cage Induction Motor
All induction motors have a rotor and a stator. Basically, it is the construction of rotor which makes Squirrel Cage Induction Motor different from Wound Type Induction Motor. Stator is same in both types of motors. Let’s first talk about stator of Squirrel Cage Induction Motor:
Stator of Squirrel Cage Induction Motor
- Stator is that component of motor which is stationary.
- It is the outer most frame in which rotor is placed.
- It has grooves on internal circumference to carry electric circuitry. This circuitry is excited by 3 phase supply.
- The 3 phase winding circuit is placed on the slots. These winding are 120 degrees apart connected as star or delta configuration.
- Now let’s move towards the construction of rotor.
Rotor of Squirrel Cage Induction Motor
- Rotor is the rotating part of a motor. It contains a cylindrical core.
- The rotor core is made in lamination to reduce eddy currents.
- Squirrel cage type rotor consists of bars of copper that we call conductors.
- The copper bars or conductors are longer than the rotor and are fixed in grooves of rotor core.
- These extended conductors are short circuited with each other by means of copper rings on each sides.
- Also rotor is sometimes provides with fans on each side for cooling purposes.
- This type of construction of bars and end rings is similar to a squirrel cage on which it is named.
- That was all about rotor construction. Other than rotor and stator, a motor has also other parts to support and protect the assembly.
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Working of a Squirrel Cage Induction Motor
In this section, I will share the working of a squirrel cage induction motor with you guys.
- When the winding of stator are given a 3 phase AC supply, a current will start flowing through it.
- This 3 phase AC will give rise to a rotating magnetic field in the rotor.
- The speed of rotation of this magnetic field can be found from the frequency of AC supply and number of poles.
- This speed is the synchronous speed of motor.
- The rotating magnetic field of stator will induce voltage in rotor because its flux lines cut through the rotor. This induced voltage will induce current in rotor winding and another magnetic field will be generated which is rotor’s magnetic field.
As you all know, that a current carrying conductor experiences a force on it in presence of a magnetic field. The rotor will also experience a force which would start turning it. This force will produce a torque and rotor will rotate.
Key Features
Now I am mentioning some important features of a squirrel cage induction motor. I will talk about speed, starting current, direction of rotation, slip and power factor. First on the list is speed.
Speed
A squirrel cage induction motor generally operates at a constant speed. This speed is synchronous speed.
Starting Current
Such motors require high starting currents. Which may result in fluctuations in voltage.
Direction of Rotation
The direction of rotation of these motors can be reversed if you interchange two power lines out of three.
Slip
As for other induction motors, the slip is defined as the difference in speed of rotating magnetic field of stator and rotating speed of rotor. The speed of rotation of magnetic field is called synchronous speed. Slip is expressed as a ratio with synchronous speed or in percentage.
Power Factor
Power factor is the ratio of actual power to apparent power. It is expressed in percentage. The power factor is low when motor is running at no load and it is high when motor is operating at full load.
Advantages
In this part, I will tell you guys some of the advantages of squirrel cage induction motor.
- Squirrel cage induction motors are:
- Cheap
- Robust
- Rugged
- Require less maintenance
- Due to cage structure of rotor, they require less material. So copper losses decrease.
- Due to absence of brushes, the chances of spark are reduced.
- These motors are provided with fans, so less heat is generated.
Now I am moving towards the last segment of my article, where I will tell you some of the uses of squirrel cage induction motors.
Uses
This type of motor finds its uses in industries because of their cost. They are extensively used in industrial applications instead of wound type induction motor. They are used in applications where low starting torque is needed. Such motors may also be used as generators.
3 Phase Induction Motor
Hi fellows! Hoping everyone is fine and doing great. Today, I am going to write about 3 Phase Induction Motor. As I mentioned earlier in my article on Introduction to Induction Motors, there are two types of Induction Motors, Single Phase Induction Motor and 3 phase Induction Motor.
3 phase induction motor is the one which operates on three phase AC supply. It is the most commonly used motor for high load and industrial applications. I would discuss advantages of a 3 phase induction motor later in this article. Three phase induction motors are further divided into two types; Squirrel Cage Induction Motor and Wound Rotor Induction Motor based on the construction of rotor. First of all, I am going to discuss about the construction of 3 Phase Induction Motor and then I will throw some light on its working.
Construction of 3 Phase Induction Motor
Talking about its construction, like any other motor a 3 phase induction motor consists of a rotor and a stator. Let’s revise the basic knowledge about rotor and stator first.
- Rotor is the rotating part while stator is the stationary part of motor.
- Rotor is separated from the stator by a small air gap.
- Stator is actually a cylindrical frame inside which the cylindrical core of rotor rotates.
- The stator of motor has slots on its internal side to carry the winding circuitry. This circuit is supplied with AC power. This winding is called stator winding.
- As I told earlier, rotor is a cylindrical core which is laminated and it acts as the output shaft of motor.
- Rotor of motor has also slots to carry conductors. These conductors make up rotor winding.
- Now, I am going to discuss types of rotor.
Types of a 3 Phase Induction Motor
Based on the rotor construction, a 3 phase induction motor is further classified into two types:
- Squirrel Cage Induction Motor
- Wound Rotor Induction Motor
Squirrel cage induction motor is cheaper than wound type, and because of the absence of brush assembly it requires less maintenance due to less wear problems.
Squirrel Cage Induction Motor
- First I will talk about squirrel cage type of induction motor.
- The rotor of a squirrel cage type motor consists of bars of copper.
- These copper bars or conductors extend out from the rotor length and are fixed in slots of rotor core.
- The bars which are extended out are short circuited together by means of copper rings on each sides as shown in figure.
- This type of construction of bars and end rings is similar to a squirrel cage, from where the name comes. Next is the construction of a wound type induction motor.
Wound Rotor Induction Motor
- In wound rotor induction motor, there is a 3 phase winding which is similar to stator’s winding.
- This winding is placed uniformly on slots of the rotor.
- The endings of these windings are connected to 3 slip rings on the shaft.
- Each phase out of three phase is connected to one slip ring.
- The slip rings of each phase rotate with rotor and are further associated with brushes which are stationary. The construction is shown in figure.
- That’s all about construction and classification of 3 phase induction motor. Now let’s discuss about working principle.
Working Principle of 3 Phase Induction Motor
In this section of the article, I am going to talk over the working principle and operation of a 3 phase induction motor. Before diving into the details, let’s revise some basic concepts and laws that govern its operation.
Faraday’s Law:
- Faraday’s law is the basic law of electromagnetic induction which plays the most important part in the working of a 3 phase induction motor.
- According to which an emf is induced in a conductor when it is placed in a varying magnetic field or if the conductor is rotated in a magnetic field. In other words, emf is induced whenever there is a relative motion of the conductor and magnetic field.
- Also, if the conductor is a closed circuit, a current is produced termed as induced current.
Lenz’s Law:
- Following Faraday’s Law as explained earlier in the previous point, when an emf is generated in a conductor, its polarity would be such that it will produce current whose magnetic field will oppose the change that is producing current. This is called Lenz’s law.
Lorentz Force:
- Lorentz force is the force which is responsible to move the conductor in the magnetic field.
- Whenever, a current is flowing in a conductor in the presence of a magnetic field, the conductor will experience this force.
That was all about the basics, now I am going to throw light on its working and development of torque.
First of all, a 3 phase AC voltage is supplied to the stator windings due to which a 3 phase current starts flowing in it. These currents flowing in windings will produce a magnetic field that rotates due to AC oscillations. Now, the flux of this rotating magnetic field will cross the rotor conductors and a voltage is induced in them which depends on three factors; the relative speed of conductor and magnetic field, magnetic flux density and length of the conductor. The result of this induced voltage is the generation of induced current in rotor, which lags behind the rotor’s induced voltage. This leads to the production of rotor’s magnetic field. Finally, the torque induced is the scaled product of stator magnetic field and rotor magnetic field and the rotor of 3 phase induction motor starts rotating.
The rotor of induction motor never achieves the synchronous speed to keep itself rotating as I discussed in my article on Induction Motors. Next, I am going to give some equations and formulas for calculating synchronous speed, slip and rotor frequency of 3 phase induction motor.
- Let’s first talk about Synchronous speed. Synchronous speed is the speed of rotating magnetic field of stator. It is represented as n_s. If “f” is the frequency of AC supply and “p” is the number of poles then n_s = 2*f/p.
- Second term on the list is slip of induction motor. Slip is denoted by “p” and it is the difference of synchronous speed and rotors mechanical speed. If synchronous speed is n_s and rotor speed is n_r then s = (n_s - n_r). Slip is also expressed as a ratio per unit or as percentage of synchronous speed. This expression is used to find rotor speed, when slip of motor is known.
- Rotor frequency is also discussed for 3 phase induction motors. This frequency is directly proportional to the slip. Represented by f_r, it is expressed by the following formula: f_r = s * f
That was all about the working of a 3 phase induction motor, so before telling the advantages I am going to give a brief idea on its equivalent electric circuit.
Equivalent Circuit of a 3 Phase Induction Motor
A 3 phase induction motor is also called a rotating transformer as working principle of both are quite similar. So, the equivalent circuit is also similar to that of a transformer as shown in image. Right side of the image shows rotor circuit and left side is stator side.
- On the stator side, there is resistance and self-inductance in stator windings. R1 is the resistance of stator and X1 is stator reactance.
- E1 is the stator voltage. Er is the induced voltage in rotor windings while Vp is the applied voltage to the machine.
- On the rotor side, Rr is the rotor resistance and Xr is the rotor reactance.
- A_eff denotes the turn ratio of a transformer which associates E1 and Er.
- Xm is the magnetizing reactance.
- I1 is the stator current while Ir is the current in rotor.
In the last section of my article, I am gonna give some advantages of a 3 phase induction motor.
Advantages
- Let’s now take a look on the advantages of a 3 phase induction motor.
- 3 phase induction motors are simple and require easy maintenance.
- They are preferred because of low price.
- They can be used in rugged environments.
- A 3 phase induction motor is brushless so the problem of maintenance and wear is avoided.
- Speed of this motor can also be controlled.