Constructional Features of Alternators

i Learning Objectives

By the end of this chapter you should be able to:

Name the parts of the stator and state the material and function of each.
Describe the salient-pole rotor, including its spider, poles, field winding and damper winding.
Describe the cylindrical (non-salient) rotor and explain why it suits high-speed turbines.
Explain how the length of the air gap is chosen.

Demonstrative Video

Section 72-1

Main Parts

A synchronous machine has three groups of parts: the stator (stationary, carrying the armature), the rotor (rotating, carrying the DC field), and miscellaneous parts such as brushes, slip rings, bearings and the shaft.

Parts of an alternator — Main parts of an alternator.

Section 72-2

The Stator

Part	Material	Function
Stator frame	Cast iron or fabricated steel	Outer body. It supports and protects the core and winding. It carries no working flux, so it is chosen for mechanical strength.
Stator core	Laminated silicon-steel stampings, insulated from each other	Provides a low-reluctance path for the flux and holds the armature winding. Lamination limits eddy-current loss.
Stator winding	Enamelled or insulated copper	The three-phase (or single-phase) armature winding, placed in slots on the inner periphery of the core.

Section 72-3

Salient-Pole Rotor

In a salient-pole rotor the poles project outward from a central hub. It is cheaper to build but cannot withstand the centrifugal stresses of high speed. It is therefore used for low and medium speeds, typically 100–375 rpm with hydraulic turbines and somewhat higher with diesel engines.

Spider: a cast-iron hub keyed to the shaft, which provides a path for the flux. The pole cores are attached to its outer surface.
Pole core and pole shoe: the pole core is a low-reluctance path for the field flux. The pole shoe spreads the flux over the periphery and is shaped to give a nearly sinusoidal EMF. The pole shoe usually covers about 2/3 of the pole pitch, and the poles are laminated to reduce eddy-current loss.
Field (exciting) winding: wound on a former and placed around each pole core. It is fed with DC through slip rings.
Damper winding: copper bars in holes in the pole faces, short-circuited at both ends by rings. The bars on adjacent poles are usually joined into a complete squirrel cage.

Why a damper winding? It damps rotor oscillations (hunting) during transients, helps the machine operate smoothly under unbalanced loads, and in synchronous motors provides starting torque.

Vertical-shaft hydro alternator — Hydro alternator, usually mounted with a vertical shaft.

Salient-pole rotors have a large diameter and short axial length, because a low speed needs many poles for 50 Hz. In hydro-electric stations the shaft is usually vertical.

Section 72-4

Cylindrical (Non-Salient) Rotor

Steam and gas turbines run at high speed, 3000 rpm for a 2-pole machine at 50 Hz, so only 2 or 4 poles are needed. The rotor is a smooth cylinder of small diameter and long axial length, which limits the centrifugal stress.

Rotor core: a solid forging of high-strength steel (smaller machines may use laminations), keyed to the shaft. Slots are milled along part of the periphery.
Field winding: placed in the rotor slots and connected so that the required number of poles is formed. The unslotted parts form the pole centres.

Non-salient pole rotor — Cylindrical (non-salient) rotor.

Section 72-5

Comparing the Two Rotors

Table 72.1 Salient-pole versus cylindrical rotor.
Feature	Salient pole	Cylindrical
Speed	Low/medium (≈100–1000 rpm)	High (1500–3000 rpm at 50 Hz)
Poles	Many	2 or 4
Shape	Large diameter, short length	Small diameter, long length
Air gap	Non-uniform	Uniform
Reactance	Different on the d and q axes (\(X_d \ne X_q\))	Single synchronous reactance \(X_s\)
Flux waveform	Shaped by the pole shoe	Nearly sinusoidal, giving a better EMF waveform
Windage and noise	Higher	Lower, quieter running, better dynamic balance
Damper winding	Usually fitted	Not essential, because the solid rotor acts as a damper
Prime mover	Hydraulic turbine, diesel engine	Steam or gas turbine

Why the air gap matters. With a uniform gap, the self-inductance of a stator coil does not depend on rotor position. With a salient rotor it does, so the stator inductances vary with time. That is why salient-pole machines need separate d-axis and q-axis reactances.

Section 72-6

Miscellaneous Parts and the Air Gap

Brushes and slip rings: carbon brushes slide on the slip rings to carry the DC field current to the rotor winding.
Bearings: support the shaft and reduce friction.
Shaft: mild or alloy steel. It transmits mechanical power to or from the machine.

Gap too small

The synchronous reactance \(X_d\) is high, which hurts regulation and stability.
Stray-load loss increases.

Gap too large

A larger field MMF and excitation current are needed.
The field system becomes bigger and costlier.

As a compromise, the ratio of air-gap length to pole pitch is typically between 0.008 and 0.02.

1 Worked Example — Choosing the rotor type

Problem. Two 50 Hz generators are to be coupled to (a) a steam turbine at 3000 rpm and (b) a Kaplan turbine at 150 rpm. Find the number of poles and the likely rotor type for each.

\[P_a = \frac{120\times50}{3000} = 2, \qquad P_b = \frac{120\times50}{150} = 40\]

(a) needs a 2-pole cylindrical rotor (turbo-alternator). (b) needs a 40-pole salient-pole rotor of large diameter, usually with a vertical shaft.

Section 72-7

Summary

The stator is made up of the frame (strength), the laminated core (flux path) and the armature winding (in slots).
The salient-pole rotor (spider, poles, field coils, damper bars) suits slow hydro and diesel drives.
The cylindrical rotor (solid forging, slotted field winding) suits fast steam and gas turbines.
Air-gap length trades excitation requirement against synchronous reactance.

Section 72-8

Common Mistakes

Thinking the frame carries the flux. The flux returns through the laminated core, not the frame.
Assuming the damper winding carries current all the time. At synchronous speed it carries no current. It acts only when the rotor departs from synchronous speed.
Using a single \(X_s\) for salient-pole machines. The non-uniform gap requires \(X_d\) and \(X_q\).

Section 72-9

Chapter Review

Multiple-Choice Questions

MCQ 1. The stator core is laminated to reduce
(a) copper loss (b) eddy-current loss (c) windage loss (d) friction loss

Show answer
(b) eddy-current loss.
MCQ 2. Turbo-alternators have
(a) large diameter and short length (b) small diameter and long length (c) salient poles (d) many poles

Show answer
(b).
MCQ 3. Damper windings are placed in
(a) the stator slots (b) the pole faces (c) the spider (d) the frame

Show answer
(b) the pole faces.
MCQ 4. A 50 Hz generator driven at 250 rpm has
(a) 12 poles (b) 24 poles (c) 48 poles (d) 6 poles

Show answer
(b) 24 poles.

Conceptual Questions

Why is the stator frame made of cast iron even though it carries no flux?
Why are salient-pole rotors unsuitable at 3000 rpm?
Give two reasons why cylindrical rotors produce a better EMF waveform.
Explain the trade-off involved in choosing the air-gap length.