Electric generators & counter EMF (Back EMF)

Everything You Need in One Place

Homework problems? Exam preparation? Trying to grasp a concept or just brushing up the basics? Our extensive help & practice library have got you covered.

Learn and Practice With Ease

Our proven video lessons ease you through problems quickly, and you get tonnes of friendly practice on questions that trip students up on tests and finals.

Instant and Unlimited Help

Our personalized learning platform enables you to instantly find the exact walkthrough to your specific type of question. Activate unlimited help now!

?
Intros
Lessons
  1. Introduction to motors and electric generators
  2. ac generator
  3. dc generator
  4. voltage Vs. Time graph ( ac generator Vs. dc generator)
  5. Counter emf (Back emf )
?
Examples
Lessons
  1. A motor is connected to a 12V dc supply and draws 5.0A when it first starts up. What will be the back emf when the motor is operating at full speed and drawing 1.2 A?
    1. 7.0 A
    2. 7.8 A
    3. 9.1 A
    4. 10.8 V
    1. A motor has an armature resistance of 3.5Ξ©\Omega and is connected to a 12.0V source. At full speed the current through the armature is 0.18A. what is the back emf at full speed?
      1. 0 V
      2. 0.63 V
      3. 11.4 V
      4. 12.0 V
      1. A dc motor is connected to a 12.0V power supply. When the armature is rotating, the current through it is 0.78A and the back emf is 10.6 V. What is the resistance of the armature?
        1. 1.4 Ξ©\Omega
        2. 1.8 Ξ©\Omega
        3. 14 Ξ©\Omega
        4. 15 Ξ©\Omega
        1. The load on an electric motor is gradually increased. Which of the following quantities decreases? (Input voltage remains constant.)
          1. current
          2. back emf
          3. armature resistance
          4. heat produced by armature
          1. The graph below shows the emf produced by an ac generator Vs. time. At which point in time is the rate of flux change in the generator the greatest?

            Electric Generator and Counter EMF

            1. a
            2. b
            3. c
            4. d
            Free to Join!
            StudyPug is a learning help platform covering math and science from grade 4 all the way to second year university. Our video tutorials, unlimited practice problems, and step-by-step explanations provide you or your child with all the help you need to master concepts. On top of that, it's fun - with achievements, customizable avatars, and awards to keep you motivated.
            • Easily See Your Progress

              We track the progress you've made on a topic so you know what you've done. From the course view you can easily see what topics have what and the progress you've made on them. Fill the rings to completely master that section or mouse over the icon to see more details.
            • Make Use of Our Learning Aids

              Last Viewed
              Practice Accuracy
              Suggested Tasks

              Get quick access to the topic you're currently learning.

              See how well your practice sessions are going over time.

              Stay on track with our daily recommendations.

            • Earn Achievements as You Learn

              Make the most of your time as you use StudyPug to help you achieve your goals. Earn fun little badges the more you watch, practice, and use our service.
            • Create and Customize Your Avatar

              Play with our fun little avatar builder to create and customize your own avatar on StudyPug. Choose your face, eye colour, hair colour and style, and background. Unlock more options the more you use StudyPug.
            Topic Notes
            ?

            In this lesson, we will learn:

            • Alternating Current (ac) VS. Direct Current (dc)
            • Motor VS. Electric Generator
            • ac Generator Principles
            • dc Generator Principles
            • Counter emf (Back emf)

            Notes:



            Alternating Current (ac) VS. Direct Current (dc)


            ac: A current that changes its sense with time is called the alternating current. (Example; ac generator)

            Electric Generator and Counter EMF


            dc: A current that flows only in one direction is called the direct current. (Example; dc Motor)

            Electric Generator and Counter EMF


            Motor VS. Electric Generator

            Motor: transfers electrical energy into mechanical energy.


            Electric Generator and Counter EMF


            Electric generator or Dynamo:transforms mechanical energy into electrical energy.


            Electric Generator and Counter EMF


            AC generator: A generator that consists of many loops of wire wound around an armature that rotates in a magnetic field. The axle connected to the armature rotates by some mechanical means such as; falling water, steam turbine, etc. As the coil rotates, an emf is induced, therefore, an electric current is the output of the generator.

            The current in external circuit changes direction.


            Ac Generator Principles


            Assumption: the coil rotates in clockwise direction

            0-90
            As the coil rotates and moves to its vertical position side AB goes up and side CD goes down. The change in flux is maximum, therefore, the induced current is maximum.

            90-180
            The coil would be in its horizontal position, change in flux is zero, no induced current, so current does not change direction when the loop is parallel to the filed.

            180-270
            The loop is again in its vertical position, where the change in flux is maximum, the current is induced in the opposite direction.

            270- 360
            The loop is back to its horizontal position where there is no change in flux and no induced current.

            Electric Generator and Counter EMF


            Dc generator: A generator that consists of many loops of wire wound around an armature that rotates in a magnetic field. The coil is connected to a source of electricity. Since the coil is placed in a magnetic field, as the current flows through the coil, an electromotive force will be produced, thus mechanical energy would be the output of dc generator.

            dc Generator Principles


            0-90
            As the coil rotates and moves to its vertical position side AB goes up and side CD goes down. The change in flux is maximum, therefore, the induced current is maximum.

            90-180
            The coil would be in its horizontal position, change in flux is zero, no induced current, so current does not change direction when the loop is parallel to the filed.

            180-270
            The loop is gain in the vertical position, where the change in flux is maximum, current is induced.

            270- 360
            The loop is back to its horizontal position where there is no change in flux and no induced current.

            • The current in the external circuit always flows in one direction.
            • When the loop is in vertical position the brushes are in contact with the rings, maximum current flows through the external circuit.
            • When the loop is in horizontal position, the brushes are not in contact with the rings, no current flows through the external circuit.

            Electric Generator and Counter EMF



            Voltage Vs. Time Graph

            dc generator

            Electric Generator and Counter EMF


            ac Generator

            Electric Generator and Counter EMF



            Counter EMF (Back EMF)

            • Due to the rotation of the coil, the magnetic flux through the coil changes, thus, an induced EMF is produced.
            • According to the Lenz’s law, the induced EMF acts to oppose the motion which is called the Counter EMF or Back EMF.
            • The greater the speed of rotation, the greater the counter EMF.
            • If there is no load, the speed of the motor keeps on increasing till the back EMF will be equal to the input voltage.
            • As the mechanical load increases, the motor rotates at lower speed thus smaller back emf is produced.

            Example
            The armature of a dc motor has a resistance of 8.0 Ξ© \Omega . The motor is connected to a 110-V power supply, when the motor reaches its maximum speed the back emf is 90-V.

            1. Calculate the current in the motor as it starts up.
            2. Calculate the current when the motor reaches its highest speed.


            1. At the beginning the motor turns very slowly, so there is no back emf.

              I=VR=110V8.0 Ω= I = \frac{V} {R} = \frac{110V}{8.0 \, \Omega} = 13.75 A≃A \simeq 14AA

            2. The motor is rotating at its full speed, the back emf is induced and should be included in the circuit,

            Electric Generator and Counter EMF


            VAcrossβ€…β€Štheβ€…β€Šresistor=V_{Across\;the\;resistor} = 110 -90 = 20VV


            I=VR=20V8.0 Ω= I = \frac{V} {R} = \frac{20V}{8.0 \, \Omega} = 2.5 AA