Alternators and Generators

Question 1

What is a Dynamo?

Answer 1

• a rotating electrical machine that converts electrical energy into mechanical energy, or mechanical energy into electrical energy
• An electrical dynamo could function as either a DC generator or a DC motor

Question 2

Could an electrical dynamo function a DC generator and a DC motor?

Answer 2

Yes

Question 3

What is a Generator?

Answer 3

• a rotating electrical machine that converts mechanical energy into electrical energy
• generators are rated by the kilowatts of power that they can deliver at a rated voltage and speed without overheating

Question 4

How are Generators rated?

Answer 4

Rated by the Kilowatts (or Megawatts) of power that they can deliver at a rated voltage and speed without overheating

Question 5

What is an Alternator?

Answer 5

• Also called a Synchronous Generator
• a machine that creates alternating current when being rotated by a mechanical means
• Alternators are rated by the Volt Amps (VA) they can deliver at a rated voltage at a specific speed without overheating
• Speed of these machines is normally Synchronous
• Synchronous Speed is a definite rotor speed required to obtain a standard frequency

Question 6

How are Alternators rated?

Answer 6

• Rated by the Volt Amps (VA) they can deliver at a rated voltage at a specific speed
• The speed of these machines is normally synchronous. Synchronous speed is a definite rotor speed required to obtain a standard frequency

Question 7

What is a Prime Mover?

Answer 7

• A DC generator or DC Alternator is driven by a mechanical machine, usually called a Prime Mover
• The Prime Mover supplies the rotational mechanical force to turn the rotor of the alternator or generator
• Common Prime Movers include:
  
  • Turbines power by falling water, steam or wind
  • Diesel engines
  • Natural gas engines

Question 8

What does a Prime Mover do?

Answer 8

• Supplies the rotational mechanical force to turn the rotor of the alternator or generator, the prime mover can be anything that causes the rotor to rotate

Question 9

What are some examples of Prime Movers?

Answer 9

1. Turbines powered by falling water, steam or wind
2. Diesel engines
3. Natural gas engines

Question 10

What is a motor?

Answer 10

• A rotating electrical machine that converts electrical energy into mechanical energy
• Motors are rated by the power in kilowatts or horsepower that they can deliver at a rated voltage and speed without overheating

Question 11

How are motors rated?

Answer 11

• Motors are rated by the power in kilowatts or horsepower that they can deliver at a rated voltage and speed without overheating
• 746W = 1 Horsepower

Question 12

How many Watts are in one unit of Horsepower?

Answer 12

746W = 1 HP

Question 13

What is a Stator?

Answer 13

The stationary part of an electrical machine

Question 14

What is a Rotor?

Answer 14

The rotating part of an electrical machine

Question 15

What is the purpose of the Field Windings of a motor?

Answer 15

• Their role is to create the magnetic flux
• On a DC machine, the field windings are typically located on the stator
• On an AC machine, the field windings are typically lcoated on the rotor

Question 16

What is the purpose of the Armature Windings in a motor?

Answer 16

• In a traditional DC Dynamo, the artmature windings are located on the rotor
• The Artmature winding is where voltage develops in a DC generator, and where torque develops in a DC motor
• On an AC machine, the armature windings are located on the stator

Question 17

What are Slip Rings?

Answer 17

• The Field Windings of an AC alternator or synchronous motor may be connected to an external circuit by connecting the coil terminals to two continuous uninsulated rings called slip rings
• The rings are mounted on the shaft and of the AC machine and rotate with the coil and the shaft

Question 18

What is the commutator of a DC machine?

Answer 18

• The purpose of the commutator is either:
  
  • In a generator, to convert the induced AC voltage on the armature coils in DC voltage
  • In a DC motor. to convert the applied DC voltage into AC voltage on the armature coils
• consists of a number of copper segments that are assembled into a cylinder that is secured to the shaft
• Commutator is insulated from the shaft
• Also called Split Rings
• Each commutator end is soldered to the ends of the armature coils

Question 19

What is Commutation?

Answer 19

Commutation is the process of reversing the direction of current in an armature coil as the commutator segments to which the coil is connected pass under the brush

Question 20

What is the Neutral Plane?

Answer 20

The point in the rotation of the armature that it cuts the fewest lines of flux

Question 21

How do the brushes operate in a DC machine?

Answer 21

They ride on the face of the commutator and form the sliding electrical connection between the armature windings and an external circuit

Question 22

How do the Brushes operate in an AC machine?

Answer 22

In AC alternators and synchronous motors, the brushes slide on slip rings and are the elctrical connection between the field windings and an external circuit

Question 23

How does a generator produce an electromotive force (EMF)?

Answer 23

• By rotating the armature winding conductors within the magnetic field produced by the windings
• The input to the generator is the mechanical energy that is required to rotate the rotor that contains the armature windings
• The voltage induced into the armature conductors is a result of electromagnetic induction

Question 24

What is Electromagnetic Induction?

Answer 24

• Electromagnetic Induction is the production of a voltage in a conductor because of the relative motion between the conductor and a magnetic field
• Whenever a conductor cuts across lines of flux, an EMF is induced in the conductor
• The polarity of the induced EMF and the resulting current depends upon the direction of the relative motion between the conductor and the field

Question 25

How do you calculate the magnitude of the instantaneous voltage induced into a straight conductor moving in a straight line through a magnetic field?

Answer 25

• e = ßIv
• e = Instantaneous induced voltage in volts (V)
• ß = flux density in teslas (T)
• I = effective length of conductor exposed to the field in metres (m)
• v = velocity of the conductor in metres per second (m/s)

Question 26

How do you calculate the amount of voltage generated in the armature conductors of an alternator?

Answer 26

• e = ßIv
• e = the magnitude of the instantaneous voltage
• ß = the flux or magnetic field density
• I = the amount of armature winding effectively exposed to the field
• v = rotational frequency or speed of rotation of the armature

Question 27

If the ends of the armature conductor are connected to a split ring, what is formed and what kind of voltage will it generate?

Answer 27

• If the ends of the armature conductor are connected to a split ring, it forms a commutator
• By using this commutator instead of slip rings, a switching or rectifying action takes place and this machine will produce a pulsating DC voltage

Question 28

What kind of voltage do machines that utilize split rings produce?

Answer 28

Pulsating DC Voltage

Question 29

How could you increase the ouput voltage of a single-loop armature coil?

Answer 29

• Increase the amount of loops in the armature windings
• A three-loop armature coil will produce three times the output voltage of a single-loop armature coil

Question 30

How do you improve or smooth out a pulsating DC waveform?

Answer 30

• Increase the number of armature coils and commutator bars
• The output voltage still fluctuates and has ripples but not to the extreme of the one-coil winding
• Increasing the number of armature coils has two functions
  
  • It decreases the magnitude of the ripple
  • it increases the average value of voltage at the brushes

Question 31

What effect does increasing the number of armature coils and commutator bars have on the output voltage of the machine?

Answer 31

1. It decreases the magnitude of the ripple
2. It increases the average value of voltage at the brushes