Direct current (DC) machines

Question 1

What is the definition of a direct-current (DC) Machine?

Answer 1

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)

Question 2

What is a yoke and what is it’s purpose?

Answer 2

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

Question 3

What is the terminal box of a DC machine?

Answer 3

a box within all the electrical connections of a DC machine are made

Question 4

What are Field coils?

Answer 4

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)

Question 5

What are the pole shoes?

Answer 5

They evenly distrust the main magnetic flux over the air gap between the stator and rotor.

Question 6

What are the methods for improving armature reaction?

Answer 6

•Increase the brush contact resistance
•Shift the brushes
•Use interpols
•Use compensating windings

Question 7

What is the commutator?

Answer 7

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

Question 8

What else is a coil called? and what are the two most common types?

Answer 8

Windings
• Lap-Windings
• Wave-Windings

Question 9

What are the other names for lap windings? And what are their applications?

Answer 9

Overlap or Parallel windings
They are ideal for high-current, low-voltage.

Question 10

What is another name for wave windings? And what are their applications?

Answer 10

Series Windings
They are ideal for high-voltage and low to medium current

Question 11

What are the two important definitions associated with armature windings?

Answer 11

•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

Question 12

What are the functions of a commutator?

Answer 12

•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

Question 13

What are the types of brushes?

Answer 13

•Carbon brushes
•Graphite brushes
•Electro-Graphite brushes
•Copper-Graphite brushes

Question 14

What is the use of carbon brushes?

Answer 14

Mainly used in DC machines.
Can withstand very high operating temperatures.
Polishes the commutator and improves the electrical connection.

Question 15

What is the use of graphite brushes?
And what can Electro-Graphite do?

Answer 15

•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

Question 16

What is the advantage of Copper-Graphite brushes and what is the disadvantage?

Answer 16

•Advantage - improves the current-carrying capacity of the brush
•Disadvantage - the copper compromises the hardness of the brush

Question 17

Define armature reaction?

Answer 17

The distortion of the main magnetic field entering and leaving the armature caused by the magnetic field of the armature.

Question 18

What are the types of DC motors?

Answer 18

•Series
•Shunt
•Compound
○Differential (long or short shunt)
○Cumulative (long or short shunt)

Question 19

The formula for a DC series motor?

Answer 19

Iₗ = Iₛₑ = Iₐ
Pᵢ = VIₗ
Pₒ = mechanical power output

Question 20

The formula for DC shunt motor?

Answer 20

Iₗ = Iₛₕ + Iₐ
Iₛₕ = V/Rₛₕ
Iₐ ≠ V/Rₐ
Pᵢ = VIₗ

Question 21

What is the equation for the general EMF of a DC machine?

Answer 21

E = 2ϕNpZ/60c
= 2ϕnpZ/c

Question 22

The Formula for the back EMF in the armature of a series motor? And what is it if it includes brush contact voltage drop?

Answer 22

•E = V - IₐRₐ - IₛₑRₛₑ
•Brush contact voltage drop;
E = V - IₐRₐ - IₛₑRₛₑ - IₐRᵦᵣᵤₛₕ

Question 23

The Formulathe back EMF in the armature of a shunt motor?

Answer 23

E = V - IₐRₐ -IₐRᵦᵣᵤₛₕ

Question 24

Formula for armature conductors?

Answer 24

Z = 60Ec/2ϕNp

Question 25

The formula to calculate Torque? and what is the alternative formula?

Answer 25

T = Fr
T = 0.318IₐϕpZ/c

Question 26

The formula for a DC motors constant?

Answer 26

k = 2pZ/60c

Question 27

Formula for deducing the characteristics of a DC motor?

Answer 27

N = E/kϕ = V-IₐRₐ/kϕ

Question 28

When are series motors used?

Answer 28

•Cranes
•Trains
•Trams
•motor vehicle starter motors
•gate motors

Question 29

When are shunt motors used?

Answer 29

•Lathes
•woodworking machines
•printing machines

Question 30

Name the types of compound Moters.

Answer 30

•Cumulative compound motors
•Differential compound motors

Question 31

What are the effects of armature reaction on a DC machine?

Answer 31

It causes demagnetizing and cross-magnetizeing

Question 32

Explain the Demagnetizeing effect?

Answer 32

•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

Question 33

Explain the Cross-magnetizeing effect?

Answer 33

It distorts the main flux