DESIGN AND ANALYSIS OF DC POWER SUPPLY (28 V) FOR

AEROSPACE APPLICATION

V. Jayashree Shivkumar, R. Periasundaram

#, M.Vadivel

#

Scientist ‘E’,

#T O ‘B’, Aircraft Projects Division, LCA, CVRDE, Avadi, Chennai – 600054.

#Project Associate, Electrical Department, IIT Madras, Chennai- 600036

Abstract: The present day trend in efficient power

generation is to use Permanent Magnet Generator for

Aircraft Applications. The technology used in the

military / aircraft applications for power generation

comprises three-stages / two stages generator

configuration. The advantages of Permanent Magnet

Generator over conventional generators are discussed

in this paper. Since the rotor excitation winding has

been replaced by Permanent Magnets, the efficiency of

the generator has improved as the copper losses in the

rotor has been eliminated.

A 1 kW Permanent Magnet Generator using rare earth

permanent magnet has been designed, developed,

extensively analyzed for its performance parameters.

This promising technology can overcome the problems

in a constant speed machine in terms of power

requirement, reliability, ease of maintenance and

higher operating speed and temperatures. Keywords: Brushless Permanent Magnet Generator,

and VSCV concept.

I. INTRODUCTION

The Electrical generators can be classified as brush

type and brushless type. The advantages and

disadvantages of both the types of generation is

detailed below.

Advantages of brushed scheme

•Simple and conventional

•Cost effective

Disadvantages

•Maintenance

•Ageing

•Reliability

•Power loss

Advantages of Brushless generator

•Good power density

•Reduced size

•No EMI/EMC interferences

Disadvantages:

•No Excitation control

•De-magnetizing effects

In-view of the lower weight and size and due to low

EMI/EMC interference, it is decided to select

permanent magnet brushless generators for the design

of Aerospace Applications.

A comparison table on the different types Generators

is shown in Table 1.

Table 1. Comparison of Different Types of

Generators

Induction

Generator

Surface

Mounted

Permanent

Magnet

(SPM)

Generator

Interior

Permanent

Magnet

(IPM)

Generator

Cubic Volume 1.0 0.7 0.7

Efficiency 1.0 1.05 1.05

Power factor 1.0 1.0 1.2

Constant Power

range 1.0 0.2 0.8

Max. Rotor

surface speed 1.0 0.3 0.6

* The values are based on relative index

with respect to Induction generator.

Permanent magnet synchronous machines

have found wide applications in various fields.

Compared to other electrical machines, PM machines

combine the advantages of induction machines and

common synchronous machines in high efficiency,

power factor and power density, low size and weight.

Various types of permanent magnets like

Alnico, ferrites, Samarium cobalt and NdFeB are

available. After exploring the B-H characteristics of

all these permanent magnets it is decided to choose

NdFeB PM magnets for the design of generator. The

choice of permanent magnets calls for high residual

flux density with high MMF to demagnetize the

magnets.

2

Typical range of NdFeB grades is shown in

Fig.1,

Fig.1. Typical Range of NdFeB Grades

For our application other essential factors

like very high overload-factor and the maximal

permitted short-circuit current without permanent

demagnetization of the magnets, had to be

considered.

II. PROTOTYPE DESIGN

A 1 kW PERMANENT MAGNET

SYNCHRONOUS MACHINE is designed and

developed using NdFeB as permanent magnets. The

SURFACE MOUNTED PERMANENT MAGNET

on rotor is designed and built to prove the new

technology concepts in the optimized configurations.

A shielding cylinder surrounding the magnets on

rotor protects the magnets against centrifugal forces.

The ratings of the 1 kW generator are as follows

Power output : 1 kW

Input Speed : 200 RPM to 900 RPM

The generator laminations are cobalt iron

Vanadium featuring very good magnetic and

mechanical strength. It is a 12-pole machine with 36

slots .The machine is designed to generate 28V.dc. It

caters maximum torque near the rated speed. The

geometry and the number of poles are fixed. The

design of the generator is obtained from the

conventional fundamental equations for the given

output ratings. The variable voltage derived as a

function of speed is rectified and converted to a

constant 28VDC voltage (VSCV) for variable load

and speed conditions using MOSFET power switches

with PWM concepts.

The generator design parameters are listed below

Table 2. Input data of 1 kW Generator

Parameters Value

Power Factor, pf 0.8

Speed, N (rpm) 775

Stacking Factor, stf 0.96

Slot Filling Factor, Ks 0.5

Short Pitching (Slot) 1

Magnet MQ3G32SH

No of Poles, P 12

No of Slots/pole/phase, q 1

Stator Outer Dia, ods (mm) 145

Rotor Outer Dia, odr (mm) 83.5

Machine Length, Lc (mm) 114

Magnet Length, lm (mm) 8

Slot Depth, Sh (mm) 18.5

Tooth Width, Wts (mm) 4

Core Depth, dcs (mm) 13.5

No. of Turns per Coil 7

No. of Coils per Phase 12

Current Density, J (A/Sq.mm) 5

III. SIMULATION ANALYSIS

A powerful Electro-Magnetic Simulation

package was used for design analysis and

performance validation at no load and full load at

transient conditions. To make the model suitable to

optimize the machine design, all the parameters are

calculated from the dimensions and material

properties of the machine. The simulation data

ensures that there is less armature reaction in the

machine. For an optimum design, Ampere-turns

spent by magnet should be approximately equal to

the Ampere-turns spent by stator core teeth and

airgap. Using the simulation packages the flux

density at various critical parts of the generator is

derived. The no load flux density of the magnet is

shown in Fig. 2.

3

Fig. 2. No Load Flux Density in Magnet

The simulation test results are given in table 3. The

results confirm that the performance requirements of

the Generator will be met.

Table 3. Output data of 1Kw generator

Quantities Simulation

program

Voltage, V (Volts) 72.155 (VL)

Current, I (Amps) 15 (IL)

Output Power, Pout (Watts) 1248.3

Efficiency, Eff 0.9117

Fig. 3. Single stage PM Generator

IV. PROTOTYPE PERFORMANCE VALIDATION

Various load tests have been carried out on the

generator, which is shown in fig3. The Test Data is

matching with simulation data analysis and it is

tabulated in table 4. Various Performance tests like

open circuit, load tests and efficiency analysis have

been conducted and the test validation is matching

with the simulation data and the performance graphs

are shown in Figs.4, 5, 6, 7 & 8.

Table 4. Test data matching with simulation data

Quantities Test data Simulation

program

Voltage, V (Volts) 82.9 (VL) 72.155

Current, I (Amps) 13.6933 (IPh) 15 (IL)

Output Power,

(Watts)

1121 1248.3

Efficiency, Eff 80% to 90% 0.9117

Fig. 4. Open Circuit Characteristics

4

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200

Output Power (in Watts)

Eff

icie

ncy

(in

%)

Speed ~ 800 RPM

Fig. 5. Load Graph at 800 RPM

Fig. 6. Speed Vs Power

Fig. 7. Volt Ampere Characteristics at Different

Speeds

Fig. 8. PM Generator efficiency

V. THERMAL ENVELOPE

(a) The temperature study was carried out to

determine the temperature rise on various parts

of the generator and to evaluate the magnet

characteristics with increased temperatures.

The resistances (cold) of the winding at ambient

temperature are:

Rr = 0.387 ohms

Rb = 0.384 ohms

Ry = 0.381 ohms

Ambient Temperature, T1 = 30 oC

After noting the initial values, the machine is

loaded fully and is run for nearly Two hours. As

the machine reaches a steady temperature of

60oC and it is switched off.

The readings are shown below.

Table 5. Heat run Test data Time Resistance Time

(1 Hour +)

Resistance Time

(2 Hour +)

Resistance

(Minutes) (Ohms) (Minutes) (Ohms) (Minutes) (Ohms)

10 0.421 10 0.411 10 0.406

20 0.422 20 0.410 20 0.405

30 0.417 30 0.410 30 0.404

40 0.413 40 0.409 40 0.403

50 0.412 50 0.407 50 0.403

60 0.411 60 0.406 60 0.401

5

The resistance of the hot wire is raised upto a

value of 0.422ohms and after 2 hours it started to

decay exponentially.

(b) There could be about 60 % increase in

the value of current density by increasing the

Power output to nearly 1.6 kW from the design

value of 1.0 kW. As the temperature rise is well

with in limits of the insulation there is no

requirement to increase the insulation level.

INFERENCES

[A] A feasibility study has been carried in the field,

variable speed constant voltage operation in

single stage configuration.

[B] The generator shall be loaded upto 1.5 kW. For 5

minutes.

[C] The design of insulation system is compact.

[D] Max. temperature rise is 27°C.

FUTURE SCOPE OF WORK

Based on the simulation and test results, the research

work can be further extended to 5 kW power

requirements at high-speed operation in single stage.

This in turn results in a more compact configuration

for airborne requirement.

ACKNOWLEDGEMENTS

We wish to express our gratitude to Dr. D.

Hanumanna, Outstanding Scientist and Director

CVRDE, Avadi, Shri. S. Sundaresh, Sct. ‘G’,

Additional Director (Technology), Prof. G. Sridhara

Rao, EEE, IITM, Shri. C. Chandrasekaran, PD-AP

and Prof. P. Sasidhara Rao, EEE, IITM for the

constant guidance and immense support for doing

this project in CVRDE.

REFERENCES

[1] Electrical Machine Design – A. K. Sawhney.

[2] Brushless Permanent Magnet and Reluctance

Motors Drives – T. J. E. Miller.

[3] Performance and Design of AC Machines –

M. G. Say.

[4] Electric Motor Drives – R. Krishnan,

Virginia Tech, Prentice Hall of India, 2002.

[5] Design and Analysis of 42V Permanent

Magnet Generator for Automotive

Applications’, Ali Keyhani, IEEE Trans on

Energy Conversion, March 2003.