AC Machines: Three-Phase Alternators

BASICS:

Alternators produce most of the electric power in the world. Some applications for small single-phase alternators are used as portable generators for home emergency or to provide the power for portable power tools on a work site, but most alternators are three phase. This article…

• explains the principles of operation for almost all alternators regardless of size.
• discusses what determines output frequency and how output voltage is controlled.
• explains how alternators are connected in parallel to provide more power when needed.
• discusses the different types of alternators and the operation of each.
• explains how to interpret the NEC when determining how to connect and determine protective devices.

TOPICS:

• Intro to Three-Phase Alternators
• The Rotor
• The Brushless Exciter
• Alternator Cooling
• Frequency
• Output Voltage
• Paralleling Alternators
• Sharing the Load
• Field-Discharge Protection
• Summary / QUIZ

TERMINOLOGY:

• Alternators
• Brushless exciter
• Field-discharge resistor
• Hydrogen
• Parallel alternators
• Phase rotation
• Revolving-armature-type alternator
• Revolving-field-type alternator
• Rotor
• Sliprings
• Stator
• Synchroscope

LEARNING GOALS:

• discuss the operation of a three-phase alternator.
• explain the effect of speed of rotation on frequency.
• explain the effect of field excitation on output voltage.
• connect a three-phase alternator and make measurements using test instruments.

INTRO

Most of the electric power in the world today is produced by AC generators or alternators. Electric power companies use alternators rated in gigawatts (1 gigawatt =1,000,000,000 W) to produce the power used throughout the United States and Canada. The entire North American continent is powered by AC generators connected together in parallel. These alternators are powered by steam turbines. The turbines, called prime movers, are powered by oil, coal, natural gas, or nuclear energy.

Three-Phase Alternators

Alternators operate on the same principle of electromagnetic induction as DC generators, but they have no commutator to change the AC produced in the armature into DC. There are two basic types of alternators: the revolving-armature type and the revolving-field type. Although there are some single-phase alternators that are used as portable power units for emergency home use or to operate power tools in a remote location, most alternators are three phase.

Revolving-Armature-Type Alternators:

The revolving-armature-type alternator is the least used of the two basic types. This alternator uses an armature similar to that of a DC machine with the exception that the loops of wire are connected to sliprings instead of to a commutator. Three separate windings are connected in either delta or wye. The armature windings are rotated inside a magnetic field. Power is carried to the outside circuit via brushes riding against the sliprings. This alternator is the least used because it’s very limited in the amount of output voltage and kilovolt-ampere (kVA) capacity it can develop.

++++1 Basic design of a three-phase armature. Shaft; Windings; Sliprings

++++2 The armature conductors rotate inside a magnetic field.

++++3 Basic design of a three-phase alternator. Phase 2 Phase 1 Phase 3 Center connection; N -- S

++++4 The alternator produces three sine wave voltages 120 degree out of phase with each other.

++++5 Wound stator.

Revolving-Field-Type Alternators:

The revolving-field-type alternator uses a stationary armature called the stator and a rotating magnetic field. This design permits higher voltage and kilovolt-ampere ratings because the outside circuit is connected directly to the stator and is not routed through sliprings and brushes. This type of alternator is constructed by placing three sets of windings 120 degree apart. The winding of Phase 1 winds around the top center pole piece.

It then proceeds 180 dgr around the stator and winds around the opposite pole piece in the opposite direction. The second phase winding winds around the top pole piece directly to the left of the top center pole piece. The second phase winding is wound in an opposite direction to the first. It then proceeds 180 degree around the stator housing and winds around the opposite pole piece in the opposite direction. The finish end of Phase 2 connects to the finish end of Phase 1. The start end of Phase 3 winds around the top pole piece to the right of the top center pole piece. This winding is wound in a direction opposite to Phase 1 also. The winding then proceeds 180 degree around the stator frame to its opposite pole piece and winds around it in an opposite direction.

The finish end of Phase 3 is then connected to the finish ends of Phases 1 and 2. This forms a wye connection for the stator winding. When the magnet is rotated, voltage is induced in the three windings. Because these windings are spaced 120 degree apart, the induced voltages are 120 degree out of phase with each other.

The stator shown is drawn in a manner to aid in understanding how the three phase windings are arranged and connected. In actual practice, the stator windings are placed in a smooth cylindrical core without projecting pole pieces. This design provides a better path for magnetic lines of flux and increases the efficiency of the alternator.

++++6 The rotor contains pole pieces that become electromagnets. Pole pieces Sliprings

++++7 Rotor of the salient pole type.

++++8 The brushless exciter uses stationary electromagnets. Rotor of small alternator Stationary electromagnet DC supply