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    DC Motor

    Speed Control Of DC Series Motors

    • Categories DC Motor
    DC-Series-Motor

    Although a far greater percentage of electric motors in service are ac motors, the dc motor is of considerable industrial importance.

    The principal advantage of a dc motor is that its speed can be changed over a wide range by a variety of simple methods. Such a fine speed control is generally not possible with ac motors.

    In fact, fine speed control is one of the reasons for the strong competitive position of dc motors in modern industrial applications.

    The speed control of d.c. series motors can be obtained by two methods

    1. Flux control method
    2. Armature-Resistance control method.
    Series Motor Speed Control Methods
    Series Motor Speed Control Methods

    The armature-resistance control method is mostly used.

    Related: Speed Control of DC Series Motor

    Table of Contents

    • Flux control method
      • 1. Field diverters
      • 2. Armature diverter
      • 3. Tapped field control
    • Armature-resistance Control
      • Related Articles

    Flux control method

    In this method, the flux produced by the series dc motor is varied. The variation of flux can be achieved in the following ways:

    1. Field Diverters
    2. Armature Diverter
    3. Tapped Field Control

    1. Field diverters

    In this method, a variable resistance (called field diverter) is connected in parallel with series field winding.

    A part of the line current passes through this diverter and thus weakens the field. Since N ∝ 1/ϕ , speed also varies with field flux.

    Field diverters
    Field diverters

    The lowest speed obtained by this method is the normal speed of motor when the current through diverter is zero, ie, diverter open circuited.

    2. Armature diverter

    In order to obtain speeds below the normal speed, a variable resistance (called armature diverter) is connected in parallel with the armature of dc series motor.
    The diverter reduces the armature current. As a result flux get increased. So the speed decreases since N ∝ 1/ϕ.

    3. Tapped field control

    In this method, the flux is reduced (and hence speed is increased) by decreasing the number of turns of the series field winding.

    Tapped field control
    Tapped field control

    The switch S can short circuit any part of the field winding, thus decreasing the flux and raising the speed.

    With full turns of the field winding, the motor runs at normal speed and as the field turns are cut out, speeds higher than normal speed are achieved.

    Armature-resistance Control

    In this method, a variable resistance is directly connected in series with the supply. This reduces the voltage available across the armature and hence the speed falls. 

    Armature-resistance Control
    Armature-resistance Control

    By changing the value of variable resistance, any speed below the normal speed can be obtained. This is the most common method employed to control the speed of d.c. series motors.

    Related Articles

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      Speed Control of DC Motor (Shunt, Series and Compound)

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    • dc motor characteristics
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      In this post, we will look into the characteristics of a DC series motor. A…

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      Armature reaction also occurs in a DC motor just like armature reaction in DC Generator.…

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    Previous post

    Speed Control of DC Shunt Motors
    November 22, 2013

    Next post

    Speed Control of DC Motor (Shunt, Series and Compound)
    November 28, 2013

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      1 Comment

    1. EllToopay
      October 8, 2019
      Reply

      Nice article. Well written

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