Direct Current Machines

Question 1

What are the three basic parts of all DC generators?

Answer 1

1. A set of permanent magnets or electromagnets
2. An armature winding that is rotated within the field by a machanical prime mover
3. A commutator and brush set

Question 2

What is Reluctance?

Answer 2

• Reluctance is a common magnetic property
• It is the opposition to flux in a magnetic circuit

Question 3

What is Flux Density?

Answer 3

• 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)

Question 4

What is Electromagnetism?

Answer 4

• 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

Question 5

Describe the Left-Hand Rule for Conductors?

Answer 5

• 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)

Question 6

How is the current flowing through a conductor drawn in a cross-sectional view of the conductor?

Answer 6

• 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

Question 7

Describe the Left-Hand Rule for determining the Polarity of an electromagnet?

Answer 7

• 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

Question 8

What factors determine the strength of the magnetic field or flux density at any given point on a coil?

Answer 8

1. The value of current in the conductors of the coil
   
   1. Flux density increases as the coil current increases
2. The number of turns of conductor in the coil’s construction
   
   1. A coil with more turns produces a stronger field
3. The material composition of the coil’s core
   
   1. For a set amount of coil current, an iron core will produce a stronger flux density than an air core

Question 9

What is Magnetomotive Force (mmf)?

Answer 9

• 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

Question 10

Describe Magnetic Saturation, what does it mean for a magnetic core to become saturated?

Answer 10

• 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

Question 11

Can coils with cores made of air or other non-magnetic materials saturate?

Answer 11

• 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

Question 12

What makes of the magnetic circuit of a DC Machine?

Answer 12

1. The pole pieces (pole core and pole shoe)
2. The yoke (frame)
3. The armature core (rotor)
4. The air gap

Question 13

What is a Commutator?

Answer 13

• 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

Question 14

What is Lorentz Force?

Answer 14

• Lorentz Force is the force that pushes a current carrying conductor at rangle angles to a magnetic field

Question 15

What is Armature Reaction?

Answer 15

• Armature reaction is the distortion of the flux pattern in a machine caused by the cross-magnetizing effect of the armature-current flux

Question 16

What kind of problems and in what areas does Armature reaction lead to in a DC generator?

Answer 16

1. The interpolar zone ( the area between the poles of the machine
   
   1. 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
2. The Polar Zone (the area directly under the pole faces of the machine)
   
   1. 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

Question 17

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) ?

Answer 17

• 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

Question 18

What is the most effective way of reducing the effects of armature reaction in the polar zone of a DC generator?

Answer 18

• 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

Question 19

Describe the Right-Hand Rule for determining Lorentz Force?

Answer 19

• 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

Question 20

What equation would you use to calculate the generated voltage of a generator?

Answer 20

E_G = K x ø x n

• E_G = Induced EMF
• K = a constant
• ø = flux per pole
• n = rotational frequency (r/min)

Question 21

What equation would you use to calculate the magnitude of CEMF produced by a motor?

Answer 21

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

Question 22

How would you calculate current in the armature of a motor at start and once it is at speed?

Answer 22

• Current through the armature at start: I_A = V_A / R_A
  
  • I_A = Current through the armature
  • V_A = The applied voltage
  • R_A = The resistance of the armature
• Current through the armature once it’s at speed:
  
  • I_A = (V_A - CEMF) / R_A

Question 23

What is the Fundamental Motor Equation?

Answer 23

V_A = CEMF + (I_ARM x R_ARM)

Question 24

What are two ways of reducing the high starting currents of a DC Motor?

Answer 24

1. Place a resistor in series with the armature at start, and once the motor increases, gradually remove the resistance from the circuit
2. Apply less than rated voltage to the motor at start, then slowly increase the voltage as the motor comes up to speed

Question 25

What three factors affect the speed of a DC motor?

Answer 25

1. The amount of load on the motor
2. The intensity of the magnetic field
3. the amount of armature voltage

Question 26

What happens when the developed torque of a motor is greater than the counter-torque?

Answer 26

• The motor will accelerate until the counter-torque equals the developed torque

Question 27

What happens when the counter-torque of a motor is greater than the developed torque of a motor?

Answer 27

• The motor speed will decrease until the counter-torque is equal to the developed torque

Question 28

Describe the process of a load being added to a DC motor

Answer 28

1. The motor’s speed will slow because the developed torue is not sufficient to maintain the current speed
2. The CEMF will decrease because the motor speed has decreased
3. The armature current increases because CEMF limits the armature current and it just decreased
4. The torque of the motor will increase until it equals the counter-torque of the load

Question 29

What is a common method of controlling the speed of a DC motor?

Answer 29

Varying the field current

Question 30

How are DC Motors and Generators categorized?

Answer 30

By the way the field and armature windings are connected

Question 31

What are the three different ways the field and armature windings of a DC Motor or Generator could be connected?

Answer 31

1. Shunt Wound (windings connected in parallel)
2. Series Wound
3. Compound Wound (series and parallel)

Question 32

How is a DC Shunt Motor connected?

Answer 32

• The armature windings and the main field windings are connected in parallel
• There is equal voltage across both windings
• The branch with the least resistance will have the most current, and in a shunt motor the largest current is through the armature because of its large conductors and low resistance
• The shunt field winding is made up of many turns of fine wire, so it has a higher resistance and lower current than the armature conductors

Question 33

How is a series motor connected?

Answer 33

• The motor field winding is connected in series with the armature winding
• The field winding carries the same current as the armature winding
• Even though the field winding may only be a few turns of larger gauge wire, the high currents that flow through it and armature can result in strong magnetic fields

Question 34

How does a runaway occur in a shunt motor?

Answer 34

• Speed control of a shunt motor is often achieved by varying the shunt field resistance
• If the field resistance is really high when the motor starts, the result is very low field current and a very weak or non-existent magnetic shunt field, and this leads to very little or no CEMF being generated.
• With no CEMF being generated, this allows the the armature current increases to a high value and results in a runaway and can cause damage to the motor
  
  • This hazardous condition can be prevented by adding resistors in series with the armature during startup

Question 35

How much starting torque can a series motor develop in comparison to its full-load torque?

Answer 35

• A series motor can develop as much as 500% starting torque as compared to its full-load torque

Question 36

How is a Compound DC Motor connected?

Answer 36

• A compound DC motor has a series field winding in series with the armature and a shunt field winding in parallel with the armature
  
  • The series winding is made up of a few turns of heavy wire
  • The shunt is made up of many turns of fine wire

Question 37

What are the two ways that the field windings of a Compound DC Motor may be connected?

Answer 37

1. Cumulative-compounded: designed so that the current through both the series and shunt field are in the same direction so that the magnetic fields aid each other
   
   1. In a DC Dynamo, the shunt field winding can be connected in two ways: long shunt and short shunt
   2. Long Shunt: The shunt winding is connected in parallel to the combination of both the armature winding and the series field winding
   3. Short Shunt: The shunt winding is connected directly in parallel to the armature winding, the series field winding is connected in series with both of them
2. Differentially Compounded: The current in the series winding flows in the opposite direction of the current in the shunt winding

Question 38

What would happen if a load was placed on a differentially compounded motor, and that load was too much for the motor to handle?

Answer 38

• If too great of a load is placed on a differentially compounded motor, the series field flux opposes the shunt flux, which weakens the total field flux. When the load becomes too great, the series field intensity becomes greater than the shunt field intesity and the motor reverses direction rapidly with destructive results