AC generators

Question 1

Describe the differences and similarities between the 3 voltages in a 3 phase supply

Answer 1

The voltages have the same amplitude and frequency but are a different phase (120∘)

Question 2

Describe briefly 3 methods of improving the output of a single loop generator

Answer 2

E = B L V
increase field strength
Increase number of windings
increase speed of rotation

Question 3

State the equation for speed that you would apply to an AC generator.

Answer 3

N = 60f/p

Question 4

State the advantages of placing the armature of an AC generator in the stator and using a rotating magnetic field

Answer 4

• Higher speeds possible
• Limits centrifugal forces
• Heavy duty brush gear not required

Question 5

What is measured by a synchroscope?

Answer 5

• The difference in frequency between the busbar and generator
• The difference between the phases of the 2 generators

Question 6

List and explain the 4 conditions that must be met before 2 AC generators are connected in parallel.

Answer 6

• The phase sequence of both running and oncoming generator must be the same so that all phases can synchronise with each other.
• Both generators must have the same terminal voltage (If not there will be a difference between the oncoming generator and busbar)
• The oncoming generators frequency must be slightly higher than the running generator
• Both generators must be in phase when paralleling 2 AC generators.

Question 7

Explain how a synchroscope is used when paralleling 2 AC generators.

Answer 7

The synchroscope should be rotating slowly in a clockwise direction, and the breakers should be closed when the needle is between 5 minutes to and 5 minutes past position.

Question 8

Briefly describe the 3 advantages of using a 3 phase supply.

Answer 8

• Power can be transmitted with greater economy.
• Makes self starting induction possible
• More power of an equivalent 1 phase

Question 9

What type of 3 phase generator can be connected to a neutral wire and when would this be done?

Answer 9

A star connected generator would use a neutral wire, but this would only be done for an unbalanced load.

Question 10

Name and briefly describe the 2 types of rotor found in an AC generator

Answer 10

Salient pole rotor, short length and large diameter
Cylindrical rotor, long length and small diameter

Question 11

Explain why 2 AC voltages at different frequencies cannot be connected in parallel.

Answer 11

Because although the peak and RMS values would be the same the instantaneous voltages would be different.

Question 12

Identify 3 ways in which the output of a simple AC generator can be increased.

Answer 12

E = B L V
Increase magnetic field strength
Increase length/number of loops
Increase velocity

Question 13

State the equation you would use to calculate frequency in regards to an AC generator.

Answer 13

f = NP/60

Question 14

Explain why the line voltage is greater then the phase voltage in a star connected 3 phase. What is the factor that connects line and phase quantities.

Answer 14

VL = √3 Vph

In a star connected system, the line is connected across 2 of the phases so VL is always different to Vph. The linking factor is √3