Electric Generator Questions and Answers

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AC Generator Working

State the purpose of each of the components used in an AC generator.

  • AC generators consist of field windings, permanent magnets, an armature, slip rings, and brushes.
  • Field windings are electromagnets used to produce the stationary magnetic field in a generator.
  • An armature is the movable coil of wire in a generator that rotates through the magnetic field.
  • Slip rings are metallic rings connected to the ends of the armature and are used to connect the induced voltage to the brushes.
  • A brush is the sliding contact that rides against the commutator segments or slip rings and is used to connect the armature to the external circuit.

Explain how AC voltage generated in the armature is delivered to an external circuit.

  • As the armature is rotated, each half cuts across the magnetic lines of force at the same speed. Thus, the strength of the voltage induced in one side of the armature is always the same as the strength of the voltage induced in the other side of the armature.

Each half of the armature cuts the magnetic lines of force in a different direction. Therefore, the voltage induced in one side of the coil is opposite to the voltage induced in the other side of the coil. The voltage in the lower half of the coil enables current flow in one direction, and the voltage in the upper half enables current flow in the opposite direction.

  • Since the two halves of the coil are connected in a closed loop, the voltages add to each other. The result is that the total voltage of a full rotation of the armature is twice the voltage of each coil half. This total voltage is obtained at the brushes connected to the slip rings and may be applied to an external circuit.

AC Generator Types

Explain how a single-phase generator produces the positive and negative parts of a sine wave as the armature rotates through the magnetic field of the field windings.

  • Each complete rotation of the armature in a single-phase (one wire) AC generator produces one complete alternating current cycle.
  • Before the armature begins to rotate in a clockwise direction, there is no voltage and no current in the external (load) circuit because the armature is not cutting across any magnetic lines of force (0° of rotation).
  • As the armature rotates, each half of the armature cuts across the magnetic lines of force, producing current in the external circuit. The current increases from zero to its maximum value in one direction. This changing value of current is represented by the first quarter (90° of rotation) of the sine wave.
  • As the armature continues to rotate, current continues in the same direction. The current decreases from its maximum value to zero. This changing value of current is represented by the second quarter (91° to 180° of rotation) of the sine wave.
  • As the armature continues to rotate, each half of the coil cuts across the magnetic lines of force in the opposite direction. This changes the direction of the current. During this time, the current increases from zero to its maximum negative value. This changing value of current is represented by the third quarter (181° to 270° of rotation) of the sine wave.
  • As the armature continues to rotate, the current decreases to zero. This completes one 360° cycle of the sine wave.

Explain how a three-phase AC generator produces three simultaneous voltages that are 120° out of phase from each other.

  • The principles of a three-phase AC generator are the same as a single phase AC generator except that there are three equally spaced armature windings 120° out of phase with each other.
  • The output of a three phase AC generator results in three output voltages 120° out of phase with each other.
  • A three phase AC generator has six leads coming from the armature coils. When these leads are brought out from the generator, they are connected so that only three leads appear for connection to the load. Armature coils can be connected in a wye (Y) connection or a delta (A) connection. The manner in which the leads are connected determines the electrical characteristics of the generator output.

Describe a three phase AC generator with neutral wye-connected system.

  • A wye (Y) connection is a connection that has one end of each coil connected together and the other end of each coil left open for external connections.
  • The three ends can be safely connected at the neutral point because no voltage difference exists between them. As phase A is at its maximum, phases B and C are opposite to A. If the equal opposing values of B and C are added vectorially, the opposing force of B and C combined is exactly equal to A.
  • The net effect is a large voltage (pressure) difference between the A1, B1, and C1 coil ends, but no voltage difference between the A2, B2, and C2 coil ends.
  • In a three phase wye-connected lighting circuit, the three phase circuit is balanced because the loads are all equal in power consumption. In a balanced circuit, there is no current flow in the neutral wire because the sum of all currents is zero.
  • All large power distribution systems are designed as three phase systems with the loads balanced across the phases as closely as possible.
  • A wye connection can be used to obtain phase-to-neutral voltage (single phase low voltage), phase- to-phase voltage (single phase high voltage), and phase-to-phase-to-phase voltage (three phase voltage).
  • In a three phase wye-connected system, the phase-to-neutral voltage is equal to the voltage generated in each coil.

Describe a three phase AC generator with neutral delta-connected system.

  • A delta (A) connection is a connection that has each coil end connected end-to-end to form a closed loop. As in a wye-connected system, the coil windings are spaced 120° apart.
  • In a delta-connected system, the voltage measured across any two lines is equal to the voltage generated in the coil winding. This is because the voltage is measured directly across the coil winding.
  • Following any line in a delta-connected system back to the connection point shows that the current supplied to that line is supplied by two coils.
  • In a balanced three phase delta-connected system, the line current is equal to 1.73 times the current in one of the coils.
  • The delta connection is similar to a parallel connection because there is always more than one path for current flow. Since these currents are 120° out of phase with each other, vector addition must be used when finding the sum of the currents.
  • A delta-connected system permits different voltage possibilities.

Describe the differences between momentary, temporary, and sustained power interruptions.

• Voltage changes in a system may be categorized as momentary, temporary, or sustained.

  • A momentary power interruption is a decrease to 0 V on one or more power lines lasting from 0.5 cycles up to 3 sec. All power distribution systems have momentary power interruptions during normal operation.
  • A temporary power interruption is a decrease to 0 V on one or more power lines lasting for more than 3 sec up to 1 min.
  • A sustained power interruption is a decrease to 0 V on all power lines for a period of more than 1 min.

 

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