AC machines

What two devices are used to eliminate the induced voltage produced in the rotor when the field excitation current is stopped?

1) field-discharge resistor (connected to the field via a DPST switch) 2) diode (connected in parallel with the field)

What are the advantages of an alternator operating in a hydrogen atmosphere?

1) hydrogen less dense than air at the same pressure therefore less windage loss of spinning rotor is reduced. 2) hydrogen has the ability to absorb and remove heat much faster than air

What three (3) factors determine the output voltage of an alternator?

1) length (# turns) of armature's or stator windings (conductors) 2) strength of the rotor's magnetic field 3) the rotor's speed

what are the two basic types of alternators?

1) revolving-armature type 2) revolving-field type

What are the two main parts of an alternator?

1) rotor 2) armature coils

What three conditions are required before two alternators can be connected in parallel?

1) the amount of output voltage of each machines should be the same 2) the phase rotation of each alternators must be the same 3) the phase connection of each alternator must be the same 4) the frequency of each machines should be the same

what two factors determine the output frequency of an alternator?

1) the number of stator poles 2) rotor's rotation speed

At what speed must the following alternators turn to produce 60 Hz: 2-pole alternator 4-pole alternator 6-pole alternator 8-pole alternator

3600 rpm 1800 rpm 1200 rpm 900 rpm

Is the rotor excitation current AC or DC?

DC

What must an alternator have before it can produce an output voltage?

Direct Current (DC)

what is the formula to calculate the amount of voltage induced in the stator winding?

E = BLV / 10^8 E (induced voltage) = B (flux density in gauss) x L (length of conductor in cm) x V (velocity in cm/s) divided by 18^8

What are two advantages of using hydrogen to cool alternator?

Less wind age loss due to spinning fan Absorbs heat better than air

When producing voltage in an alternator, does the armature turn inside the magnetic stator field OR does the magnetic stator field rotate around the armature?

Magnetic stator field rotates around the armature

What are two ways of cooling an alternator?

Small alternators use general airflow (fan) large alternators are enclosed and operate in hydrogen atmosphere

T/F: The magnets of the rotor in an AC machine are electromagnets and require some source of external DC to excite the alternator.

True

T/F: most alternators are 3-phase.

True

On the revolving-armature type alternator, the _______ are rotated inside the magnetic field.

armature windings

Why is the revolving-armature type alternator not used as much as the revolving-field type alternator?

because it has very limited output voltage and power rating

Why can the revolving-field type alternator generate higher power than the revolving-armature type alternator?

because the outside circuit is connected directly to its stator instead of sliprings and brushes

On the revolving-armature alternator, power is carried to the outside via the ______.

brushes riding against the sliprings

What do many large alternators use to supply DC to the rotor winding?

brushless exciter

How is tyhe output frequency in an AC machine controlled?

by adjusting the the speed of the rotor

How can you vary the amount of voltage induced in the rotor

by changing the amount of excitation current supplied to the electromagnets (thereby increasing or decreasing the strength of the magnetic field of the rotor)

what are two ways to test for phase rotation?

by connecting three lightbulbs between two 3-phase alternators (most common) with a synchroscope

How do you control the magnetic field strength of the rotor?

by controlling the D C excitation current to the rotor

How is electricity produced in an alternator?

by electromagnetic induction

How is the output frequency of an alternator determined?

by the number of stator poles and by the speed of the rotation

What is the most common way of checking phase rotation?

connect three lightbulbs between the alternators connected in parallel through a synchronizing switch

What does a bridge rectifier do?

converts the three-phase AC voltage produced in the armature into DC voltage before it is applied to the main rotor windings

On the revolving-armature type alternator, its three windings have what type connection(s)?

either delta or wye

Alternators operate based on what principle?

electromagnetic induction

What DC source is used to excite the rotor of an AC machine called?

excitation current

what is the DC supplied to field of an alternator called?

excitation current

What is the formula to determine frequency when the poles and rpm are known?

f = PS (# poles per phase) times RPM divided by 120 (phase difference)

When more than one alternator is used, how are they connected?

in parallel

Where are the field windings in an alternator located?

in the slots cut into the core material of the rotor

What does salient mean?

projecting (such as a salient pole type alternator)

What are sliprings used for on a revolving-field type alternator?

provide connection to the external circuit by way of brushes

What does the rotor do?

provides the magnetism needed to induce voltage into the stator windings.

terms used for a closed diode connection (no current flow through diode when excitation current flowing) and open diode connection (current flows through diode)

reversed-biased forward-biased

Which type of alternator has its windings connected to sliprings and are less frequently used than the other type due to it being limited to due to the amount of output voltage and kilovolt-amp (kVA) capacity it can develop.

revolving-armature type alternator

Which alternator type uses a stator and a rotating magnetic field to generate voltage?

revolving-field type alternator

Which of the two types of alternators contains electromagnets?

revolving-field type alternator

The _____ is the rotating member of an AC machine.

rotor

What device is be used to determine phase rotation and difference of frequency between two alternators?

synchroscope

What controls the alternator's output voltage?

the amount of DC excitation current applied to the electromagnets

when testing phase rotation using lightbulbs, the voltage drop across the bulbs is proportional to what?

the difference in voltage between the two alternators

In an alternator, how does the rotor and the armature coils work?

the rotor produces a rotating magnetic flux which induces electricity in the armature coils the armature coils are stationary

what does the excitation system do?

tries to keep the alternator's voltage at a preset level.

T/F: Unlike generators, alternators don't have a commutator that converts the internal AC voltage to DC voltage.

true