This document provides details on the design of a turbo alternator. It includes: 1) Key parameters that must be considered in turbo alternator design such as diameter, speed, air gap length, and output equation modifications for low speed. 2) Construction details of the stator including smaller diameter, larger axial length, ventilating holes, and conductor design. 3) Methods for estimating the air gap length and length of the armature. 4) The rotor uses a smooth cylindrical non-salient pole design to prevent exceeding the safe peripheral speed limit due to high rotation.

1. Shroff S.R. Rotary Institute of Chemical Technology
Principal Supporter & Sponsor- UPL LTD./ Shroff family
Managed By Ankleshwar Rotary Education Society
Approved by AICTE, New Delhi, Govt. of Gujarat & GTU Affiliated
Guided By:
Mr. Jignesh Joshi
Assistant Professor, Department of Electrical Engineering,
Shroff S. R. Rotary Institute of Chemical Technology, Vataria, Ankleshwar
Name of Student Sachin V. Chauhan
Enrollment No. 170993109001
Semester 7th
Name of Topic Design Of Turbo Alternator
Name of Course Design Of AC Machine
Course Code 2170909
Academic Term June-October, 2019
Year of Admission 2016
Department of Electrical Engineering
Active Learning Assignment (ALA)

2. CONTENT
2
 Design of low speed and vertically operatedturbo alternator
 Length of air-gap
 Flow chart for overall design of alternator
 Flow chart for overall design of alternator

3. DESIGN OF TURBO ALTERNATOR
3
𝐷 =
𝑉
𝜋𝑛𝑠
The diameter
𝑉 = 𝜋𝐷𝑛 𝑠
Is limited by the maximumIn low speed turbo-alternator
peripheral speed,
The output equation of ac machine can be modified by using the above
relation.
The kVA rating or Output kVA, Q = C0 𝐷2 𝐿𝑛 𝑠
where,
C0 = 11 𝑎𝑐 𝐵𝑎𝑣 𝐾 𝑤 ∗ 10 − 3
Substitute
,
0
and the expression for C in the equation for Q
Q=11 𝑎𝑐𝐵𝑎𝑣𝐾 𝑤
𝑉
𝜋𝑛 𝑠
2
𝐿∗10−3
Q=1.11 𝑎𝑐𝐵 𝑎𝑣
𝑤
𝐾 𝐿
𝑉
𝑛 𝑠
2
∗10−3
𝐷 =
𝑉
𝜋𝑛𝑠

4. High speed alternators are called as Turbo alternators.
To provide high speed of rotation, the rotor diameter is reduced and the axial
length is increased.
Two or four poles are normally used in the turbo alternators.
Mostly steam turbines are used as prime movers.
Introduction

5. Turbo Alternator Stator Construction:
• The stators of turbo alternators are almost similar to the low-speed alternators except
few differences in the external appearance. The differences are:
• Smaller air gap diameter.
• Larger axial length.
• Greater depth of stampings behind the teeth.
• To increase the cooling effect, axial ventilating holes are provided. Air or hydrogen is
passed through these holes for the purpose of cooling.
• Stator stampings are made up of silicon alloy steel sheets.
• By increasing the percentage of silicon, the iron loss can be reduced.But this leads to
increasing brittleness.
• Stampings are prepared in segments and they are assembled for the required size.
• The stator conductors used in the turbo alternator may be in the form of bars or strips.
• These strips are placed in the slots and connected in parallel.
• They are twisted/transposed in the slot to nullify the effect of local differences in the
magnetic flux density. Because of this, the eddy current losses in the conductors are
greatly reduced.

6. The length of armature , L can be estimated from above equation.
6
The value of specific loading for conventionally cooled alternators
are,
𝑎𝑐 = 50000 𝑡𝑜 75000 𝑎𝑚𝑝.
𝐵 𝑎𝑣 = 0.54 𝑡𝑜 0.65 𝑊𝑏/𝑚2
𝑐𝑜𝑛𝑑
𝑚
The specific loading for conventionally cooled alternator are,
𝑃 𝑎𝑣 = 0.54 𝑡𝑜 0.65 𝑊𝑏/𝑚2
𝑐𝑜𝑛𝑑
𝑎𝑐 = 180000 𝑡𝑜 200000 𝑎𝑚𝑝.
𝑚

7. LENGTH OF AIR-GAP
7
The length of air gap can be estimated from the ratio lg/τ = 0.02 to 0.025. It
can be also estimated from value of SCR.
mmf for airgap = 800000 𝐾 𝑔 𝐵 𝑔 𝑙 𝑔
mmf for airgap = 80% of no load field mmf 𝐴𝑇 𝑓0
Here, ATf0=SCR x 𝐴𝑇
𝑎;
where 𝐴𝑇 𝑎 =𝑎𝑐(𝜏/2);
Note:- 𝑆𝐶𝑅 = 0.5 𝑡𝑜 0.7 for turbo alternators and 𝐵 𝑔= 𝐵 𝑎𝑣/𝐾 𝑓

8. On equating the two equation of mmf and mmf for airgap,
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800000 𝐾 𝑔 𝐵 𝑔 𝑙 𝑔 = 0.8 𝐴𝑇 𝑓0
800000 𝐾 𝑔 𝐵 𝑔 𝑙 𝑔 = 0.8 x 𝑆𝐶𝑅 x 𝐴𝑇 𝑎
800000 𝐾 𝑔 𝐵 𝑔 𝑙 𝑔 = 0.8 x 𝑆𝐶𝑅 x 𝑎𝑐 x (𝜏/2)
𝑙𝑔=
0.8 x 𝑆𝐶𝑅 x 𝑎𝑐(𝜏/2)
800000𝐾
𝐵
𝑔 𝑔
=
0.5 x 𝑆𝐶𝑅 x𝑎𝑐x𝜏
𝑔 𝑔𝐾 𝐵 x 106

9. =
0.5 x𝑆𝐶𝑅 x𝑎𝑐 x 𝜏 x 10 − 6
9
𝐾 𝑔 𝐵 𝑎𝑣/𝐾 𝑓
The armature slot, winding, turns per phase and conductor design of
turbo-alternator are same as that of salient pole alternator.

10. • FLOW CHART
FOR
• MAIN DIMENSION
OF
ALTERNATOR
start
P=(120*f)/n
𝐶 𝑂 =11* 𝐾 𝑤 * 𝐵 𝑎𝑣*ac*10−3
𝐷2L= Q/(𝐶 𝑂 *(n/60))
D=𝑉 𝑎𝑚/(П ∗𝑛)
L=(𝐷2L)/𝐷2
τ= П*D/P
Selectthe suitable value ofD
and L
end
𝐵𝑎𝑣,KVA, n, f ,m, pf, ac,ƞ,𝐾 𝑤

11. Rotor of Turbo alternator
• Smooth cylindrical(non-salient pole) type rotor is used in the turbo alternator.
• Owing to the high-speed, they have either two poles or four poles. In order to
prevent the peripheral speed from exceeding the safe limit, the diameter of the
rotor is kept down.
• The core of cylindrical rotor and shaft generally forged in one piece. Radial
slots are cut around the outer periphery to receive the field windings.
• Remember that rotor poles are the non-salient type.
• It means they do not project out from the surface of the rotor. Over certain
regions, the slots are omitted to form the surface of the rotor.
• At this moment it is good to refresh the pros of non-salient type rotor over
salient type rotor.

12.  The rotor winding ( field winding) is very easily provided firmly in its place.
 The air gap length is uniform throughout the periphery of the rotor.
 A better mechanical balancing of the rotor is obtained.
 Noise less running is possible as the surface of the rotor is smooth cylindrical
shape.
 The winding provided in the rotor is a distributed one like DC armature.
Hence a better cooling surface is available.