Alternators and Generators

Question 1

What is a Dynamo?

Answer 1

A dynamo is a rotating electrical machine that converts mechanical energy into electrical energy or electrical energy into mechanical energy. An electrical dynamo could function as a dc generator or a dc motor.

Question 2

What is a generator?

Answer 2

A generator is a rotating electrical machine that converts mechanical energy into electrical energy. Generators are rated by the kilowatts or megawatts of power that they can deliver at rated voltage and speed without overheating.

Question 3

What is an Alternator?

Answer 3

An alternator, also called a synchronous generator, creates alternating current when being rotated by a mechanical means. Alternators are rated by the volt amps (VA) or kilovolt amps (kVA) or megavolt amps (MVA) that they can deliver at rated voltage at a specific speed without overheating. The speed of these machines is normally synchronous.

Question 4

What is Synchronous speed?

Answer 4

Synchronous speed is a definite rotor speed required to obtain a standard frequency.

Question 5

What can an alternator also be called?

Answer 5

An alternator can also be called a synchronous generator.

Question 6

What is a Prime Mover?

Answer 6

A dc generator or an ac 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. A prime mover can be anything that causes the rotor to rotate.

Question 7

What are some common prime movers?

Answer 7

Turbines powered by falling water, steam, or wind.

diesel engines

gasoline engines

natural gas engines

Question 8

What is a motor?

Answer 8

A motor is 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 rated voltage and speed without overheating.

Question 9

What is a stator?

Answer 9

A stator is the stationary part of an electrical machine.

Question 10

What is a Rotor?

Answer 10

A Rotor is the rotating part of an electrical machine.

Question 11

What is a Field Winding?

Answer 11

On a dc machine, the field windings are typically located on the stator. On an ac machine, the field windings are typically located on the rotor. Regardless of their location, their role is to create magnetic flux.

Question 12

What is an Armature Winding?

Answer 12

In the traditional dc dynamo, the armature windings are located on the rotor. The armature 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 13

What is a Slip Ring?

Answer 13

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 and insulated rings called slip rings. These rings are mounted on the shaft of the ac machine and rotate with the coil and the shaft.

Question 14

What is a Commutator?

Answer 14

The commutator of a dc machine consists of a number of copper segments that are assembled into a cylinder which is secured to, but insulated from, the shaft. Commutators are also called split rings. Each commutator segment is soldered to the ends of the armature coils.

Question 15

What is the purpose of a commutator?

Answer 15

The purpose is either to:

1. Convert induced ac voltage on the armature coils into dc voltage, as in a generator or
2. To convert applied dc voltage to ac voltage on the armature coils, as in a dc motor.

Question 16

What is Commutation?

Answer 16

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 17

What is the Neutral Plane?

Answer 17

The neutral plane is the point at which the armature coil cuts the fewest lines of flux.

Question 18

What are brushes normally made of?

Answer 18

Carbon of varying degrees of hardness.

Question 19

What does a brush do in a dc machine?

Answer 19

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

Question 20

What does a brush do in an ac alternator or synchronous motor?

Answer 20

The brushes slide on slip rings and are the electrical connection between the field windings and the external circuit.

Question 21

On an ac machine, the armature windings are typically located on the ____________, and the field windings are typically located on the _________.

Answer 21

stator, rotor.

Question 22

What is the role of the field windings in an ac or dc machine?

Answer 22

To create magnetic flux.

Question 23

The field windings of an ac alternator may be connected to an external circuit by using what?

Answer 23

Slip rings

Question 24

What causes the induced voltage in the armature conductors of a generator?

Answer 24

The mechanical force rotates the armature winding conductors through a magnetic field which causes electromagnetic induction.

Question 25

Describe Flemings Left-Hand Rule.

Answer 25

Thumb: Pointed in the direction of motion of the conductor through the magnetic field.
Pointer Finger: Pointed in the direction of the main field flux from N to S
Middle Finger: Pointed in the direction of polarity of the emf induced into the conductor. It points at the negative.

Question 26

What effects the magnitude of voltage induced into a conductor?

Answer 26

The flux density
The effective length of the conductor within the field
The velocity of the conductor

Question 27

What is the formula for voltage induced into a conductor moved through a magnetic field?

Answer 27

e = Blv

Where:
e is the instantaneous voltage
B is the flux density in teslas (T)
l is the effective length of the conductor exposed to the field in meters (m) and
v is the velocity of the conductor in meters per second (m/s)

Question 28

What happens to the emf when a conductor moves past opposite magnetic poles?

Answer 28

The polarity changes

Question 29

How much emf or voltage is induced into an armature conductor that is in the neutral plane position? How much current will be at the load

Answer 29

Zero V, therefore we will have zero current.

Question 30

The voltage induced into the armature conductor of a dc generator is (ac or dc) voltage.

Answer 30

ac

Question 31

A variable resistor connected in series with a field winding is called a:

Answer 31

Rheostat

Question 32

Name 4 things that affect the amount of voltage produced by electromagnetic induction in a generator.

Answer 32

• Flux density (or magnetic field density)
• The relative speed of the conductor in the field.
• Length of conductor exposed to the field (which is effected by the number of turns)
• The angle at which the conductor cuts the lines of force.

Question 33

When the armature loop of an ac generator is at 180 degrees, what is the value of the induced voltage?

Answer 33

ZERO

Question 34

Can motors or generators be overloaded for a brief time?

Answer 34

Yes

Question 35

The temperature of the cooling medium associated with a motor or generator is called the ___________ temperature.

Answer 35

ambient

Question 36

What does FLC mean?

Answer 36

Full Load Current. The value of current that occurs at rated speed and at rated voltage.

Question 37

How many Watts is in 1 Horsepower?

Answer 37

1 hp = 746W