M6-S4: Applications of The Motor Effect: DC Motor

 

 

Recall the following concepts:

  • Motor effect: a current-carrying conductor experiences a force in an external magnetic field. 

  • Using the right-hand palm rule to determine the direction of the Motor Effect force

 

Motor Effect in Motors

  • A motor is a device which converts electrical energy into mechanical (kinetic) energy
  • DC motor runs on direct current (DC), usually connected to a battery which only allows current (or electrons) to flow in one direction.
  • When the battery is switched on, current flows through the coil creating a current-carrying conductor. The coil moves as it is influenced by an external magnetic field as a result of the Motor Effect.

 

 

 

Components of a DC Motor

Component

Description

Function

Coil & Armature

·       Multiple turns (n) of wire are wound around the armature to form a coil.

·       The overall force resulted from the Motor Effect is proportional to the number of turns

·       Coil: carries current to produce mechanical energy as a result of the Motor Effect.

·       Armature refers to the entirety of the coil. It is effective the surface of the conductor which interacts with external magnetic flux to produce the Motor Effect and back emf.

Axle

·       Located in the middle of the armature

·       The pivot point of applied torque and where the armature rotates about

·       Provides a point of rotation for the armature

·       Transfers the rotational mechanical energy to another appliance e.g. fan

Magnets

·       Can either be permanent or electromagnetic. Electromagnetic is better as the strength of magnetic field can be changed by altering current

·       Usually radial magnets are used to produce a radial magnetic field. Radial magnetic fields ‘smooth’ out the rate of change in flux, such that the motor speed becomes more dependent on current size.

·       Produces an external magnetic field

Split-ring commutator

·       Semi-crescent shaped, conductive material that is connected to the coil/armature

·       Rotates with the coil when the motor is running

·       Connects the armature with the brush and power source.

·       Reverses the current direction periodically (every half revolution) to keep torque direction constant and rotation continuous

Brush

·       Conductive material e.g. carbon

·       Connected to external battery or other kinds of DC power source

·       Does not rotate with coil when the motor is running

·       Requires maintenance and replacement as frequent contact with split-ring commutators produce friction which wears out the material

·       Not physically fixed to split-ring commutators. Acts as a conductive connection between split-ring commutators & armature and power source (battery)

Battery

·       Most common source of electromotive force (emf) which forces electrons to travel in one direction, hence producing direct current (DC)

·       Produces current à produces motor effect as the coil becomes a ‘current-carrying conductor’ in an external magnetic field

  • Diagram below:
    • Radial magnets produce a radial external magnetic field
    • Single turn coil that is connected to split-ring commutators
    • Applying the right-hand rule for electrons, the left side of the coil is acted by an upward force and the right side of the coil is acted by a downward force. Consequently, the armature rotates clockwise as shown by the red arrow.
    • Split-ring commutators: notice the ‘split’ or gap is always perpendicular to the plane of the coil. Therefore, when the coil is in a vertical position, it loses contact with the DC supply as the ‘split’ becomes aligned with the brush. At this instance, the momentum of the coil carries it past the vertical position. Afterwards, the electron movement in each side of the coil is reversed to allow the rotation to continue in the same direction.

  • The Motor Effect force acting on the armature remains constant throughout its rotation within the magnetic field. This is true for both parallel and radial magnets as the angle between the coil and magnetic field lines is 90º in both scenarios.
    • The magnitude of force remains constant
    • The direction of force changes periodically (every 180º)

 

Torque Allows for the Rotational Motion Observed in a Motor

  • The total force acting on the armature is equal to the force acting on a single turn of coils multiplied by the number of total turns

 

Recall torque from Module 5 – Advanced Mechanics:

  • Since most armatures are quadrilateral in shape:

 

 

  • In the old syllabus, an alternative expression of this equation was taught. Students are recommended to know both variations.

 

 

Split-Ring Commutators Overcome the Problems of a DC Motor

  • Problem #1: When the armature is perpendicular to the magnetic field, zero torque acts on the armature. As the net force acting on the pivot point is now zero, this prevents the armature from further rotation
  • Problem #2 Direct current flowing through the coil leads to a unidirectional force vector acting on the coil. This imposes a problem as the armature cannot complete a revolution of rotation due to it being ‘stuck’ when it becomes perpendicular to the external magnetic field
  • The usage of a split-ring commutator overcomes these two problems
  • The split-ring commutator is set-up such that it is in contact with the brush throughout the armature’s rotation except when it becomes perpendicular to the magnetic field. When the commutators lose contact with the brush, current becomes zero in the coil as the electrons are no longer affected by the electromotive force (emf) provided by the battery. This temporary stoppage of current allows the armature to continue spinning from its previously possessed momentum
  • After the armature overcomes its perpendicular position with the magnetic field, the pair of split-ring commutators swaps contact with the respective brushes. This reverses the current direction flowing through each side of the armature. The overall current direction of the circuit remains the same. This allows the armature to continue rotating to complete its revolution
  • Hence, it is said that split-ring commutators help the armature in a DC motor to achieve sustained rotational motion


Diagram Explanation:

(a) The armature is in a horizontal orientation with side LK experiencing an upward force and MN a downward force. In this position, the split-ring commutators are in contact with the brush, allowing the circuit to be complete.

(b) The armature is about to adopt a vertical orientation. Sides LK and MN experience the same force as in (a)

(c) The moment retained from before has allowed the armature to bypass the vertical position to adopt the new orientation in (c). Here, the split-ring commutator has reversed the current direction flowing through side LK and MN.

Notice the change of current direction in the second row of diagrams. This reversal of current keeps the torque constant which allows for continual motion. Additionally, notice the pair of split-ring commutators have switched their contact points with the brush.

(d) The armature has returned to its horizontal plane, but this time LK is experiencing a downward force, MN an upward force due to the reversal of current that has taken place before.

(e)The armature is about to reach a vertical orientation.

 

Movement of the Armature Within the External Magnetic Field Produces Back Emf

  • Faraday’s Law: as the armature rotates within the external magnetic field, its surface of area A cuts through magnetic flux lines. The magnetic field density remains constant but the angle between the surface’s normal and the flux lines changes throughout rotation. As a result, the coil in the armature experiences changes in magnetic flux. Finally, this causes an emf to be induced (Faraday’s Law).
  • Lenz’s Law: this induced emf opposes the magnetic field which first produced it. Thus, the induced emf’s direction is always opposite to the emf generated by the battery. The emf which travels in the opposite direction is called back emf
  • Back emf reduces the overall net emf à current is reduced as resistance of the coil remains the same.

 

  • Back emf reduces the speed of a DC motor. From the above equation, reduced current leads to a smaller torque acting on the armature. Consequently, the rotational motion of the motor becomes slower. Its efficacy is reduced.
  • Back emf is only induced when the rotor experiences change in flux. Thus, when it is not rotating, no back emf is induced.
  • Back emf limits the efficacy of DC motors. The energy conversion from electrical into mechanical energy becomes more and more inefficient as torque (rotational speed) of the armature increases. This is because the magnitude of back emf is proportional to change in magnetic flux (Faraday’s Law).
    • Motors operate at a speed which produces maximum efficiency – a compromise is made: maximising speed whilst minimising back emf.

 

 


 Practice Question 1

(a) Describe the components that make up a DC motor (3 marks)

(b) Explain how the components you described in (a) achieve the function of a DC motor (2 marks)

 

Practice Question 2

A rectangular wire loop is connected to a DC power supply. Side X of the loop is placed next to a magnet. The loop is free to rotate about a pivot.

When the power is switched on, a current of 20 A is supplied to the loop. To prevent rotation, a mass of 40 g can be attached to either side X or side Y of the loop.

(a) On which side of the loop should the mass be attached to prevent rotation? (1 mark)

(b) Calculate the torque provided by the 40 g mass. (2 marks)

(c) Calculate the magnetic field strength around side X. (3 marks)

 

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