Direct current (DC) machines Flashcards
What is the definition of a direct-current (DC) Machine?
A DC Machine is a rotary electrical device that is used to convert mechanical energy to DC electrical energy (generator)
OR
converts DC electrical energy into mechanical energy (motor)
What is a yoke and what is it’s purpose?
it is the cast-iron or cast-steel casing of a DC machine.
It’s purpose is to:
•Provide mechanical protection to the motor
•Provide mechanical support for the field system
•carry magnetic flux produced by the poles
What is the terminal box of a DC machine?
a box within all the electrical connections of a DC machine are made
What are Field coils?
Also called Field windings.
They consist of insulated copper wire wound around the field poles.
They produce the main magnetic flux (ϕ) when they are energized or excited by a field current (I_f)
What are the pole shoes?
They evenly distrust the main magnetic flux over the air gap between the stator and rotor.
What are the methods for improving armature reaction?
•Increase the brush contact resistance
•Shift the brushes
•Use interpols
•Use compensating windings
What is the commutator?
part of the armature assembly that makes electrical connection with armature conductors
it ensures that the current flows as direct current in the case of a generator
What else is a coil called? and what are the two most common types?
Windings
• Lap-Windings
• Wave-Windings
What are the other names for lap windings? And what are their applications?
Overlap or Parallel windings
They are ideal for high-current, low-voltage.
What is another name for wave windings? And what are their applications?
Series Windings
They are ideal for high-voltage and low to medium current
What are the two important definitions associated with armature windings?
•Pole Pitch - the peripheral distance between the centers of the two field poles that are next to each other
•Coil Pitch - represents the distance measured in terms of armature slots between two sides of a coil
What are the functions of a commutator?
•Allow for an electrical connection with the rotating armature conductor by means of brushes
•Collect all the current from the armature conductors (when generator)
•Convert alternating current into direct current
What are the types of brushes?
•Carbon brushes
•Graphite brushes
•Electro-Graphite brushes
•Copper-Graphite brushes
What is the use of carbon brushes?
Mainly used in DC machines.
Can withstand very high operating temperatures.
Polishes the commutator and improves the electrical connection.
What is the use of graphite brushes?
And what can Electro-Graphite do?
•Mainly used in small machines because they are weak.
Their operation is almost silent and noiseless
•Electro-Graphite brushes can carry larger currents and have a low coefficient of friction
What is the advantage of Copper-Graphite brushes and what is the disadvantage?
•Advantage - improves the current-carrying capacity of the brush
•Disadvantage - the copper compromises the hardness of the brush
Define armature reaction?
The distortion of the main magnetic field entering and leaving the armature caused by the magnetic field of the armature.
What are the types of DC motors?
•Series
•Shunt
•Compound
○Differential (long or short shunt)
○Cumulative (long or short shunt)
The formula for a DC series motor?
Iₗ = Iₛₑ = Iₐ
Pᵢ = VIₗ
Pₒ = mechanical power output
The formula for DC shunt motor?
Iₗ = Iₛₕ + Iₐ
Iₛₕ = V/Rₛₕ
Iₐ ≠ V/Rₐ
Pᵢ = VIₗ
What is the equation for the general EMF of a DC machine?
E = 2ϕNpZ/60c
= 2ϕnpZ/c
The Formula for the back EMF in the armature of a series motor? And what is it if it includes brush contact voltage drop?
•E = V - IₐRₐ - IₛₑRₛₑ
•Brush contact voltage drop;
E = V - IₐRₐ - IₛₑRₛₑ - IₐRᵦᵣᵤₛₕ
The Formulathe back EMF in the armature of a shunt motor?
E = V - IₐRₐ -IₐRᵦᵣᵤₛₕ
Formula for armature conductors?
Z = 60Ec/2ϕNp
The formula to calculate Torque? and what is the alternative formula?
T = Fr
T = 0.318IₐϕpZ/c
The formula for a DC motors constant?
k = 2pZ/60c
Formula for deducing the characteristics of a DC motor?
N = E/kϕ = V-IₐRₐ/kϕ
When are series motors used?
•Cranes
•Trains
•Trams
•motor vehicle starter motors
•gate motors
When are shunt motors used?
•Lathes
•woodworking machines
•printing machines
Name the types of compound Moters.
•Cumulative compound motors
•Differential compound motors
What are the effects of armature reaction on a DC machine?
It causes demagnetizing and cross-magnetizeing
Explain the Demagnetizeing effect?
•It reduces the strength of the main fluxes
•The motor attains a speed above the rated speed
•The torque delivery capacity of the motor is reduced
Explain the Cross-magnetizeing effect?
It distorts the main flux
