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    Q1. In a 3-phase slip-ring induction motor, the rotor windingterminals are brought out through slip rings to

    A.  connect extra resistance across them during starting.

    B.  connect them either in star or in delta as per need.

    C.  connect to 3-phase ac supply.

    D.  close the rotor circuit externally.

    View Answer   4 -2 Explanation:- Answer : A

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    स्रोत : studysite.org

    [Solved] A three

    Concept:  When the slip rings are shorted, the induced current in rotor oppose the relative motion between rotor conductor & stator field according to

    Home Electrical Machines Three Phase Induction Motor


    Download Solution PDF

    A three-phase slip-ring induction motor, provided with a commutator winding, is shown in the figure. The motor rotates in clockwise direction when the rotor windings are closed.

    If the rotor winding is open circuited and the system is made to run at rotational speed fr with the help of prime-mover in anti-clockwise direction, then the frequency of voltage across slip ring is f1 and frequency of voltage across commutator brushes is f2. The values of f1 and f2 respectively are

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    f + fr and f f - fr and f f - fr and f + fr f + fr and f - fr

    Answer (Detailed Solution Below)

    Option 1 : f + fr and f

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    When the slip rings are shorted, the induced current in rotor oppose the relative motion between rotor conductor & stator field according to the Lenz's law. So, the rotor also starts rotating in the same direction as that of stator field. As the rotor rotates in clockwise direction, the direction of stator field is also in clockwise direction.

    When the slip rings are opened, then rotor is rotate in the Anticlockwise direction.


    Rotor is made to rotate with speed


    (in ACW direction) when slip ring are opened.

    Relative motion between rotor & stator field

    = Ns + Nr

    Therefore frequency of induced EMF in rotor winding

    f1=p120×(Relative motion)

    f1=p120×(Ns+Nr) f1=NsP120+NrP120 f1 = f + fr

    At the same time, the frequency of voltage across commutator brushes is same as stator frequency.

    ⇒ f2 = f

    Download Solution PDF

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    More Three Phase Induction Motor Questions

    Q1. Which of the following is NOT an effect of single phasing in a three-phase induction motor?Q2. For which of the following type of motor, least maintenance is required?Q3. Frequency of rotor current in an induction motor is given by:Q4. In a three phase induction motor, speed of the rotating magnetic field is called :Q5. For a 3-Phase induction motor, motor copper losses at a slip 4% is 625 W. Find the motor power output.Q6. Which of the following quantity will be reduced due to skewing of motor in a three-phase squirrel cage induction motor? 1. Parasitic torque 2. Starting torque 3. Pullout torqueQ7. What will happen if any two phases of an induction motor are interchanged?Q8. A 12-pole, 3-phase alternator driven at a speed of 500 rpm supplies power to an 8 -pole, 3 -phase induction motor. If the slip of motor at full load is 3%, calculate the full load speed of the motor.Q9. An 8 pole, 3-phase, 50 Hz induction motor is operating at 720 rpm. The frequency of rotor current isQ10. Which of the following starter is NOT used for starting of induction motors?

    स्रोत : testbook.com

    Slip Ring Induction Motor

    This Article Encompasses the Working Principle of Slip Ring Induction Motor, Construction, Speed Control, Resistance Calculation and Its Applications

    An induction motor is an electrical device that converts electrical energy into mechanical energy. It is most widely used for industrial applications due to its self-starting attribute. Slip ring induction motor is one of the types of 3-phase induction motor and is a wound rotor motor type. Because of various advantages like low initial current, high starting torque, and improved power factor, it is used in applications that require high torque, cranes, and elevators. The rotor windings consist of more number of windings, higher induced voltage, and less current compared to the squirrel-cage rotor. The windings are connected to external resistance through slip rings, which helps to control the torque/speed of a motor.

    What is a Slip Ring Induction Motor?

    Definition: A slip ring induction motor is referred to as an asynchronous motor as the speed at which it operates is not equal to the synchronous speed of a rotor. The rotor of this type of motor is wound type. It comprises of a cylindrical laminated steel core and a semi-closed groove at the outer boundary to accommodate a 3-phase insulated winding circuit.

    Slip Ring in Induction Motor

    As seen in the figure above, the rotor is wound to match the number of poles on the stator. The three terminals of a rotor and three start terminals connecting through slip rings are connected to a shaft. The aim of the shaft is to transmit mechanical power.


    Before we discuss the working principle of slip ring Induction motor, knowing the slip ring induction motor construction is important. So let’s begin with the construction which includes two parts: Stator and Rotor.

    Stator Rotor


    The stator of this motor comprises of various slots that are arranged to support the construction of a 3-phase winding circuit connecting to a 3-phase AC source.


    The rotor of this motor consists of a cylindrical core with steel laminations. Besides this, the rotor has parallel slots to accommodate 3-phase windings. The windings in these slots are arranged at 120 degrees to each other. This arrangement can reduce noise and avoid irregular pausing of a motor.

    Working of Slip Ring Induction Motor

    This motor runs on the principle of Faraday’s law of electromagnetic induction. When a stator winding is excited with an AC supply, the stator winding produces magnetic flux. Based on faraday’s law of electromagnetic induction, the rotor winding gets induced and generates a current of magnetic flux. This induced EMF develops torque that enables the rotor to rotate.

    However, the phase difference between the voltage and current do not meet the requirements to generate high starting torque as torque developed is not unidirectional. The external resistance of high value is connected with the circuit to improve the phase difference of a motor. As a result, inductive reactance and phase difference between I and V is reduced. Consequently, this reduction helps the motor to generate high stating torque. The slip ring induction motor diagram is shown below.

    Slip Ring Induction Motor Connection Diagram

    Why Slip Rings are used in an Induction Motor?

    Slip is defined as the difference between the flux speed and the rotor speed. For an induction motor to produce torque, at least some difference should be there between stator field speed and rotor speed. This difference is called ‘slip’. The Slip Ring” is an electromechanical device that aids in transmitting power and electrical signals from stationary to a rotating component.

    Slip rings are also known as rotary electrical interfaces, electric rotary joints, swivels, or collector rings. Sometimes, based on the application, the slip ring requires higher bandwidth to transmit data. Slip rings improve the efficiency and performance of a motor by improving system operation and eliminating wires that are dangling from motor joints.

    Slip Ring Induction Motor Resistance Calculation

    The peak torque occurs if

    r = Smax. X —— (I)

    Where, Smax = Slip at pull-out torque

    X = Inductance of a rotor

    r = resistance of rotor winding

    Adding external resistance R to equation (I),

    r+R= (Smax)’. X —— (ii)

    From equation (i) and (ii),

    R = r(S’ max/Smax – 1) —— (iii)

    By definition of Smax, we get Smax = 1 – (Nmax/Ns) —— (iv)

    Putting S’max =1 in equation (iii), we get

    R = r. (1/Smax-1) —— (v)

    Let’s say, Ns = synchronous speed of 1000rpm and pull-out torque happens at 900 rpm, equation (iv) reduces to Smax = 0.1 (i.e., 10% slip)

    Substitute in equation (v),

    R = r. (1/0.1 – 1)R = 9. r

    ‘r’ is measured using a multimeter. The resistance value of 9 times higher than of a slip ring rotor resistance is connected externally to experience maximum starting torque.

    Slip Ring Induction Motor Speed Control

    स्रोत : www.elprocus.com

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