Direct Current Machines Flashcards
What are the three basic parts of all DC generators?
- A set of permanent magnets or electromagnets
- An armature winding that is rotated within the field by a machanical prime mover
- A commutator and brush set
What is Reluctance?
- Reluctance is a common magnetic property
- It is the opposition to flux in a magnetic circuit
What is Flux Density?
- Flux density is the intensity of a magnetic field at a given point
- Points closer to the poles of a magnetic have a higher flux density than the points that are farther away
- Usually described as flux per unit area
- Quantity Symbol for flux = beta (ß)
- Unit of measurement = tesla (T)
What is Electromagnetism?
- Electromagnetism is the relationship between magnetism and electricity
- When an electric current is ran through a conductor, a magnetic field is set up around that conductor, the density of this field is proportional to the amount of current flowing through it
Describe the Left-Hand Rule for Conductors?
- Using your left hand, if you grasp a conductor with your thumb pointing in the same direction as the current flowing through the conductor, your fingers indicate which direction the magnetic lines of force are traveling around that conductor (clockwise)
How is the current flowing through a conductor drawn in a cross-sectional view of the conductor?
- Think of the current flow as an arrow with a cross at the tail end of it
- If the arrow is coming towards you (out of the page), all you will see is the dot of the head of the arrow, which is how it is drawn in a cross-sectional vew of a conductor: current coming out of the page is represented by a dot in the center of the conductor
- If the arrow is going away from you, all you will see is the crossed tail feather on the back of arrow, so if current is flowing into the page, it will be drawn as a cross in the center of the conductor
Describe the Left-Hand Rule for determining the Polarity of an electromagnet?
- Using your left hand, grasp the electromagnetic with your fingers curling around the coil in the direction of current flow, your thumb will then point towards the north pole of that magnet
What factors determine the strength of the magnetic field or flux density at any given point on a coil?
- The value of current in the conductors of the coil
- Flux density increases as the coil current increases
- The number of turns of conductor in the coil’s construction
- A coil with more turns produces a stronger field
- The material composition of the coil’s core
- For a set amount of coil current, an iron core will produce a stronger flux density than an air core
What is Magnetomotive Force (mmf)?
- Magnetomotive force is the measure of a coil’s ability to produce flux
- The amount of magnetic flux in a coil depends on:
- The current in the coil
- The number of turns of conductor in the coil
- The unit of measurement for Magnetomotive force is called Ampere-Turns (The product of the two values)
- ex. A 10-turn coil carrying 50A produces 500 apere-turns of mmf
Describe Magnetic Saturation, what does it mean for a magnetic core to become saturated?
- As an un-magnetized iron core is subjected to an increasing magnetic field these processes occur:
- The magnetomotive force is applied to the core and increases
- A greater percentage of the magnetic domains within the core material becomes aligned
- The flux density (ß) of the core increases
- Once most of the magnetic domains near the surface of the material are aligned, a much greater value of magnetomotive force must be applied in order to produce even a small increase in flux density
- Only coils that have a core made of magnetic material can saturate
- Coils with cores made of air or other non-magnetic material show a direct relationship between field instensity and flux density, this results in a straight line on a graph
Can coils with cores made of air or other non-magnetic materials saturate?
- No they cannot
- Coils that have a core made of air or other non-magnetic material show a direct relationship between field intensity and flux density, this results in a straight line on a graph
What makes of the magnetic circuit of a DC Machine?
- The pole pieces (pole core and pole shoe)
- The yoke (frame)
- The armature core (rotor)
- The air gap

What is a Commutator?
- One of the most important parts of a DC Machine
- It keeps the coils in the correct polarity to interact with the main field
- The commutator is the connector from the armature windings to the electrical load or supply
- This connection to the electrical load or supply is achieved via brushes

What is Lorentz Force?
- Lorentz Force is the force that pushes a current carrying conductor at rangle angles to a magnetic field
What is Armature Reaction?
- Armature reaction is the distortion of the flux pattern in a machine caused by the cross-magnetizing effect of the armature-current flux
What kind of problems and in what areas does Armature reaction lead to in a DC generator?
- The interpolar zone ( the area between the poles of the machine
- Armature reaction causes the neutral plane to shift or rotate in the same direction as the rotation of the rotor. With the neutral plane shifted, the brushes are now shorting out commutator segments that have voltage across them, which causes arcing and effectively reduces the life of the brushes and the commutator
- The Polar Zone (the area directly under the pole faces of the machine)
- The disortion of the flux pattern caused by the armature reaction results in a distortion of the waveform of the generated voltage and a reduction of generated voltage. As the flux is forced to one edge of the pole faces, it causes the saturation of the iron in that area, which reduces the total flux. And since the generated EMF is proportional to the amount of fluc, the generated EMF is therefore reduced
What is the most effective way of reducing the effects of armature reaction in the interpolar zone (The area between the poles of the machine) ?
- Adding Interpoles with commutating windings to the generator
- Interpoles are narrow poles placed halfway between the main poles and directly in line with the no-load magnetic neutral plane
- The interpoles are wound with windings called commutating windings
- The polarity of the interpole must be such that it produces an interpole flux in direct opposition to the armature flux
What is the most effective way of reducing the effects of armature reaction in the polar zone of a DC generator?
- Use Compensating Windings
- Compensating windings are placed in slots or holes in the main pole faces of the pole shoe
- Compensating winding is connected in series with the armature and commutating windings and carries the full load current
- They run parallel to the armature conductors and carry current in the opposite direction of the armature windings right next to them
- The magnetomotive force of the compensating winding is then opposite to the magnetomotive force of the adjacent armature winding. If these two forces are equal, they cancel each other out, therefore eliminating the effects of armature reaction
Describe the Right-Hand Rule for determining Lorentz Force?
- The right-hand rule is used to find the direction of the force pushing a conductor through a magnetic field at right angles (Lorentz Force)
- Using your right hand, your first finger points in the direction of the main field flux (north to south)
- The middle finger indicates the direction or polarity of the EMF induced into the conductor
- The thumb points in the direction of the Lorentz Force exerted on the conductor

What equation would you use to calculate the generated voltage of a generator?
EG = K x ø x n
- EG = Induced EMF
- K = a constant
- ø = flux per pole
- n = rotational frequency (r/min)
What equation would you use to calculate the magnitude of CEMF produced by a motor?
CEMF = K x ø x n
- CEMF = the magnitude of the CEMF produced
- K = a constant
- ø = flux per pole
- n = rotational frequency (r/min)
- note: the value of the CEMF generated when the motor is running is lower than the voltage applied to the motor
How would you calculate current in the armature of a motor at start and once it is at speed?
- Current through the armature at start: IA = VA / RA
- IA = Current through the armature
- VA = The applied voltage
- RA = The resistance of the armature
- Current through the armature once it’s at speed:
- IA = (VA - CEMF) / RA
What is the Fundamental Motor Equation?
VA = CEMF + (IARM x RARM)
What are two ways of reducing the high starting currents of a DC Motor?
- Place a resistor in series with the armature at start, and once the motor increases, gradually remove the resistance from the circuit
- Apply less than rated voltage to the motor at start, then slowly increase the voltage as the motor comes up to speed
