7/27/2019 Generation Power Flux

1/3

GENERATION

energize - August 2006 - Page 46

Power flux test on a stator coreby Kobus Stols, Eskom

The purpose of a power flux test is to test the integrity of the insulation between the lamination plates in the core of a stator.

The EL-CID (electromagnetic core

imperfection detection) is the preferred

test, but in some cases there is a need for

a power flux test. The resistance between

laminations is not always linear under

different voltage levels and a power flux

test with a higher axial potential difference

between laminations may therefore reveal

core faults that are not detectable by an

EL-CID test.

The axial potential differences between

adjacent lamination plates are explained

with the aid of Figs.1 and 2.

The relevant polarity of the voltages that drive

the Eddy current is shown in the Fig. 2. Note

the opposing polarities on two adjacent sides

of the insulation.

Flux of between 80% and 105% of rated

flux is normally used to perform a power flux

test. The percentage flux level refers to the

flux in the back of the core and not to the

flux per pole.

Test equipment setup

The ideal setup to perform the test is illustrated

in Fig. 4.

The fol lowing provides the essential

calculations for a power flux test

Definition of symbols

C = Number of conductors in series per

phase

Ntp

= The number of turns per phase

Np

= The number of parallel paths per

phase

= Useful flux per pole

p = Number of pole pairs

n = Speed in r.p.s. (revo lution per

second)

f = Frequency

Kd

= The winding distribution or spread

factor

Kp

= The coil pitch or cording factor

Basis formula

The following formula forms the basis of theory

behind the flux test.

Fig. 1

Fig. 2

Fig. 3 Fig. 4

The following illustrates the derivation of this

formula:

The amount of magnetic flux that cuts a

conductor in 1 revolution:

= x(Number of poles)

There are 2 poles in 1 pole pair. The formula

therefore becomes:

= x 2 xp

7/27/2019 Generation Power Flux

2/3

GENERATION

energize - August 2006 - Page 47

The amount of magnetic flux that cuts a

conductor in 1 second is therefore:

= x 2p xn

Therefore the average emf generated per

conductor is:

Eaverage= 2 x xpn

The average emf generated per phase

therefore becomes:

Eaverage

= 2 x xpn xC

If a sinusoidal waveform is assumed, the

average emf can be converted to an RMS

value by multiplication with the following

factor:

The figure forKp

is 1,0 if the coil is fully pitched

(i.e. 180 electrical).

When the coi l pitch or cording factor is

accommodated, the formula changes as

follows:

Erms

= 4,44 xfxKd

xKp

xNtpx

The number of parallel paths per phase (Np)

must also be considered. The formula then

becomes:

Flux in the back of the core

It is important to know the rated flux in the

back of the core before the number of turns

for the test can be calculated.

The rated flux per pole when the formula of

the previous section is manipulated to extract

the flux element:

The flux from a pole divides into 2 as soon

as it enters the stator core. This is illustrated in

Fig. 5. The flux in the circumferential direction

of the stator core yoke is therefore half of the

flux per pole.

The flux voltage (VF) required for rated flux

in the back of the core is:

The figure x in the formula is the percentage

flux level chosen for the test. Good results can

be obtained when the test is performed at

flux levels that vary between 75% and 105%

of the rated flux level.

The sinusoidal voltage of a given magnitude

and frequency dictates the magnitude of

the steady state flux regardless of the core

dimensions and the property of the core

material.

Test voltage

The ideal is to have a variable supply, but

since this is seldom available a fixed voltage

is normally used.

Any of the readily available supplies can be

used for the test. The chosen supply is referred

to as the test voltage (VS). The effect of the

different voltage sources is to change the

number of turns and the current required for

the test. The availability of a specific cable

for the test normally dictates the voltage

source.

Number of turns

The number of turns for the rated flux test can

be calculated as follows:

NT

is the number of is turns, VS

is the test

voltage and VF

is the flux voltage.

Flux density calculation

The flux is distributed through the cross-

sectional area as shown by the light red

colour in Fig. 6.

The slots in the core cause a high reluctance

path in the inner path of the core. This high

reluctance path is shown in a light yellow

colour in Fig. 7.

The flux during the test will tend to follow thepath with the least reluctance i.e. the path

shown in red. The high reluctance path (the

yellow area) should therefore be ignored

= 0,707/0,635= 1,11

The RMS voltage generated per phase is

therefore:

Erms

= 1,11 x 2 x xpn xC

= 2,22 xCxnp x

The numbers of conductors in series (C), can

be replaced with the number of turns per

phase (Ntp) in the formula. Since there are

2 conductors in series per turn, a factor of

2 should be used when using (Ntp) instead

of (C).

Erms

= 2,22 x (C) xnp x

= 2,22 x (2Ntp) xnp x

= 2,22 x 2 xNtp

xnp x

= 4,44 xNtp

xnp x

Convert the speed and the number of

poles to frequency. Note that n is already

expressed in revolutions per second, and

not per minute:

f = np

When replacing np with f , the formula

changes as follows:

Erms= 4,44 xnp xNtp x= 4,44 xfxN

tpx

The winding distribution or spread factor (Kd)

has the following ratio:

Fig. 5

Fig. 6

Wh e n th e w i n d i n g d i s t r i b u t i o n i s

accommodated, the formula changes as

follows:

Erms

= 4,44 xfxKd

xNtpx

The coil pitch or cording factor (Kp) is the

following ratio:

7/27/2019 Generation Power Flux

3/3

GENERATION

energize - August 2006 - Page 48

when the cross-sectional area is calculated.

The length of the core is determined by thelamination thickness, number of laminations,

vent il at ion space di stance, number of

vent ilat ion spaces and the th ickness of

interlamination insulation

The total core length is not made of magnetic

material. The stacking factor is basically used

to obtain the effective length of the core from

a magnetic material perspective.

The area mentioned in Fig. 6 is calculated

as follows:

Area

= Core length x Back of Core depth x Stacking factor

The flux density (B) in Tesla at the back of the

core is given by:

Test current calculation

The magnetising current depends on the size

of the core and the type of material used. It

is important to know how many amperes will

be drawn by the winding in order to size the

cable correctly.

Take the flux density (B) calculated previously

and read the corresponding magnetic field

intensity (H) from the B-H curve that is relevant

to the specific core material (Fig.10).

The magnetic field intensity (H) is expressed

in ampere-turns/meter. It is therefore required

to calculate the length of the magnetic path

before the magneto magnetic force (MMF)

can be calculated.

The length of the flux path is:

Length of flux path

= Average back of core diameter x

Back of core diameter

= Outside diameter {Inside diameter +

(2 x slot depth)}

The total ampere-turns (MMF) required to

induce the required flux density (B) in the backof the core area is:

MMF = Hxlength of fluxpath

The steady state supply current in ampere

during the test is:

The test current will decrease with an increase

in the number of turns as can be seen in the

example shown in Fig. 11.

The initial current, immediately after the

supply is switched on, will be higher than the

steady state current (IS) due to the transient

inrush currents. The level of the inrush current is

not predictable in practical terms. The reason

for this is that the cores remnant flux and its

polarity are not known and the point of the

sine wave, where the supply voltage will be

switched on, is not predictable. In the worst

condition, the newly applied voltage may

attempt to set up a flux in the same direction

of the remnant flux, thereby driving the core

into saturation with a significant increase in

magnetising current being a result.

Test duration

The duration of the test depends on the size

of the core and whether the stator bars are

still fitted, but usually varies between 30 and

70 minutes.

Core evaluation

The temperatures in the core depend on the

flux density. The temperature in the teeth will

therefore be lower than the temperature in

the area at the back of the core as can be

seen in the infrared picture shown in Fig. 12.

The influence of the emissivity of the core

material and the reflection of light from

external sources should be considered when

using an infrared temperature measuring

device. A calibrated temperature meter that

utilises a different technology can be used

to confirm the temperature of a suspected

hotspot.

It is important to compare areas with similar

flux density levels when evaluating the core. A

difference in hot spot versus average core

temperature of less than 10C is normally

acceptable when the test is performed atflux levels between 85% and 100% of the

rated level.

Disclaimer

It should be noted that a power flux test has

the potential to destroy a stator core if not

performed correctly or if any test values are

incorrectly calculated. The author of this article

therefore does not take any responsibility if the

information provided in the article is used,

perused, disseminated, copied or stored as

a reference by anybody.

References[1] ISBN 0-471-6144 7-5, Operation and

Maintenance of Large Turbo-Generators (by

Geoff Klempner & Isidor Kerszenbaum)

[2] ISBN 0-582-41144-0, Electrical Technology

(by Edward Hughes)

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Fig. 11

Fig. 12