Module 3: DC Motor/Generator Theory

Module 3: Electrical Fundamentals - Topic 3.12: DC Motor/Generator Theory

• This topic covers DC motor and generator theory, components, construction, and operation.

• Students should be able to describe basic motor and generator theory.

• Students need to identify the components of a DC generator and describe their construction and purpose.

• Students need to describe the operation of DC generators, including factors affecting:

  • Output
  • Direction of current flow

• Students need to describe the operation of DC motors, including factors affecting:

  • Output power
  • Torque
  • Speed
  • Direction of rotation

• Students need to describe the operation and features of the following DC motors:

  • Series wound
  • Shunt wound
  • Compound

• Students need to describe the construction and operation of starter generators.

• The direction of a magnetic field around a conductor is dependent on the direction of current flow.

• The left-hand rule is used to determine the direction of the magnetic field around a conductor.

• To use this rule, grasp the conductor in your left hand with your thumb pointing in the direction of current flow. Your fingers will curl in the direction of the magnetic field. This rule applies ONLY to electron flow.

• Magnetic field patterns around current-carrying conductors are perpendicular to the conductor.

• Parallel conductors carrying current in the same direction will have magnetic lines of force that combine, increasing the strength of the field around the conductors.

• Parallel conductors carrying current in opposite directions will have magnetic lines of force that oppose, deforming the field around the conductors and causing them to repel each other.

• One method of producing an electromotive force(emf) is through magnetism and movement.

• A changing magnetic field induces an emf in a conductor.

• An emf is induced in a conductor when placed in a magnetic field if the field is changing around the conductor or the conductor is moving through the field

• This is called electromagnetic induction

• Three conditions must exist before emf is produced by magnetism

  • A conductor must be present in which the voltage is produced.
  • A magnetic field must be present near the conductor.
  • There must be a relative motion between the conductor and the magnetic field.

• Left Hand Rule for DC Generators

• Position your left hand with your fingers pointing in the direction of the magnetic field.

• Point your thumb in the direction of rotation.

• Your index finger will point in the direction of the magnetic flux (from North to South).

• Your middle finger will point in the direction of the electron current flow.

• Basic Alternating-Current Generator

• Conductor rotated clockwise through a magnetic field between the poles of a permanent magnet.

• The orange half is parallel to the lines of force, cutting no lines of force.

• The purple half is doing the same, cutting no lines of force.

• No emf is induced in this position.

• As the loop rotates toward a new position, it cuts more lines of force and induces an increasing voltage.

• At 90 degrees, the conductor is cutting directly across the field and the induced voltage is maximum.

• As the loop rotates, the voltage decreases and then drops to zero in positions C and 360 degrees.

• The value of induced voltage at various points during rotation is a sine wave.

• In DC generators, AC is always induced in the output windings, and the generated current is rectified to DC mechanically by a commutator and brushes.

• A single-loop generator has each conductor end connected to a segment of a 2-segment metal ring, which is the commutator, that replaces the slip rings of an AC generator.

• Commutator mechanically reverses in order to change the polarity of the voltage in the armature loop and convert the AC voltage to a pulsating DC voltage. This is known as commutation.

• Voltage developed across the brushes fluctuates and is unidirectional.; this is known as ripple.

• More coils and segments of the commutator will reduce the ripple.

• Typical DC generator construction includes:

  • Field frame or yoke
  • Armature
  • Commutators
  • Brushes

• Note that laminated field poles are used to reduce eddy currents.

• Generator field frame (or yoke): completes the magnetic circuit and provides mechanical support for other generator parts.

• In small generators, the yoke is made from one piece of iron; larger generators use multiple pieces bolted together.

• The yoke is highly permeable and forms the majority of the magnetic circuit along with pole pieces.

• The magnetizing force is produced by an electromagnet, a coil called a field coil, that is bolted to the inside of the frame. The core is usually laminated to decrease eddy currents and concentrates the lines of force produced by the coils.

• Effects of adding coils:

• With 4 commutator segments, a new segment will pass each brush every 90 degrees, which allows for the brushes to switch between the white and red coils. This evens out the voltage fluctuations in the coil.

• Increased numbers of coils and commutator segments result in a less-rippled AC voltage.

• Increased numbers of pole pieces produce a more powerful magnetic field, in turn increasing the voltage output based on the number of flux lines cut per revolution.

• Nearly all generators employ electromagnetic poles instead of permanent magnets. These poles consist of coils of insulated copper wire wound on soft iron cores.

• Advantages of electromagnetic poles are increased field strength; and allowing for more control of the field's strength which affects the output voltage.

• Brushes ride on the commutator surface. They serve as electrical contacts between the armature coils and the external circuit. They are made of high-grade carbon materials sometimes containing molybdenum for added lubrication.

• Brush holders, which are insulated from the frame, allow the brushes to freely slide.

• Flexible braided copper conductors (pigtails) connect brushes to the external circuit.

• Commutator located at one end of the armature comprises wedge-shaped segments of hard drawn copper. Each segment is insulated (mica). These segments are held in place with steel V-rings.

• Leads from armature coils are attached to the risers or soldered to slits in segment ends.

• Commutation is defined as the successful reversal of current in an armature coil while it's under short circuit, converting AC to DC. Voltage is generated in the coil when it moves under a brush in the commutator, causing a reversal of current in the coil.

• When a current flows through coils in the armature, it sets up lines of flux around its conductors, interacting with the main field. This interaction may cause a shift in the neutral plane, and to maintain proper commutation, the brushes must be moved to the new neutral plane.

• Interpoles are placed between main poles; consisting of coils of heavy insulated copper wire wound and placed to have the same polarity. They cancel the effects of armature reaction; and cause a shift in the neutral plane contrary to the shift caused by armature reaction; therefore, maintaining an even output regardless of the change in load.

• Motor reaction in generator occurs when the armature is delivering current, causing a magnetic force that acts contrary to the rotation of the armature.

• When current flows through conductors, it creates a flux (magnetic field) around the conductor.

• Interaction between the conductor field and the main field weakens the field above the conductor where there is a reduction in field strength.

• The strengthening of the field below the conductor creates an upward-directed reaction force.

• The increase in current in the conductor results (with increased load) in increased motor reaction force.

• With no armature current, there is no magnetic (motor) reaction. As armature current increases, motor reaction increases.

• Armature losses are losses in any electrical device attempting to decrease the efficiency; affecting the armature are:

  • Copper loss in the winding
  • Eddy current loss in the core
  • Hysteresis loss (a sort of magnetic friction).

• Copper loss is the power lost in the form of heat in a generator's armature winding.

• Heat is generated every time a current flows through a conductor known as I2R loss.

• Eddy current loss occurs when a conductor experiences induced currents, when in the presence of a magnetic field. In generator armatures, eddy currents are undesirable power losses in the form of heat.

• Soft iron is an electrically conducting material with good magnetic characteristics.

• The resistance of a material in which current flows is inversely proportional to the cross-sectional area.

• Eddy current losses are less with laminated generators compared to solid core generators.

• Laminated cores with insulated laminations from each other reduce the amount of eddy current compared to solid cores.

• Hysteresis loss is heat loss attributed to magnetic properties of the armature core.

• Continuous movement of magnetic particles produces molecular friction that transmits to armature windings.

• Compensating for hysteresis losses, use heat-treated silicon steel laminations.

• Armature windings are coated to prevent oxidation.

• DC generator output voltage is a product of three factors.

  • Number of conductor loops in series in the armature
  • Armature speed
  • Magnetic field strength

• Field windings are supplied with DC voltage in order to set up a steady magnetic field. This is called field excitation.

• Separately excited generators receive current from an independent current source.

• Self-excited generators receive current from their own output.

• Series motors: Field winding is wired in series with the armature. These motors have very high starting torque, frequently used in small electric equipment, portable electric hand tools, cranes, winches and other devices. Speed differs greatly between no-load and full load.

• Shunt motors: Field winding is wired in parallel with the armature. These motors maintain a relatively consistent speed regardless of load; and they are useful in constant speed applications like blowers.

• Compound wound motors: combination of both series and shunt motors. This is often better for starting load characteristics.

• Separately excited motors: have their field current supplied by separate DC power source.

• Methods of reversing DC motor direction:

  -The direction of the armature rotation can be reversed by connecting the power to the armature wires in a reversed manner. -Two field windings on the same pole with opposite directions in their coils can be used to reverse the motor direction.

• Armature reaction is the same for DC motors as for DC generators..

• The effect of armature reaction shifts the neutral plane, opposite from the direction of rotation.

• To correct the reaction, the brushes must be moved; this is corrected through interpoles in the large generators, due to the expense of compensating windings, as they are equivalent in generators and motors.

• Starter generators are combined starter and generator functions to provide an electric starter and power generation.

• Starter generators supply both starters and generators and are frequently used in small turbine engines for their appreciable weight saving potential.