testing lucas alternators aug 2013

Testing Lucas Alternators

1

Some background Originally I wanted to know if the alternator on my T100 was working. The engine was in bits so I

mounted the stator and rotor onto my bench grinder. This ran at about 3000 RPM but my grinder

struggled to maintain speed when the

stator was connected to a load. I later

bought a more powerful three-phase

motor and inverter and built a small

test rig (on the right.)

This rig can drive an alternator from 0

to 4000 RPM under load. The top limit

is arbitrary as I don’t know the

maximum speed of the motor!

To make measurements I used a

digital

multimeter

(DM) and a Picoscope (software oscilloscope.)

Lucas specifies a 1 Ohm load resistor to test

their alternators and initially I made my own

from resistance wire. There is a fair amount of

heat to dissipate by the load (at least 120W) and

after burning my hand a couple of times I fitted

a cooler running 200W device below left. I also

added a selection of resistors to simulate ignition and lighting loads.

Although I started out to test my own alternator, over time this has changed and I have taken the

opportunity to test any alternator I can get my hands on. I have logged the test results at the end of

this document.


2

Stators Stators are pretty simple devices, just a long length of copper wire wound around coil formers.

Generally they will either work or they won’t. There were two types latterly produced by Lucas,

single-phase and three-phase. You can identify each type by its number of output wires and coils.

A single-phase stator has two wires and six coils (below left), while a three-phase stator has three

wires and 9 coils (below right).

Confusingly the early single-phase stators also had three wires. This was because they were initially

used on 6 volt electrical systems which had no voltage regulation (no zener). To increase alternator

output whenever lights were used, extra stator coils (the third wire) were brought into use. These

stators can also be used on 12v systems by connecting the green/yellow and green/black wires

together to make a two wire single-phase stator.

Some even earlier stator types had 5 wires but these were used on AC only bikes.

Lucas stators have an ID number and date of manufacture stamped on them. To find your numbers

you may have to remove your stator. Aftermarket stators tend to have only a name at best.


3

Rotors As far as I can tell there is no difference between single and three-phase rotors. Rotors have

identifying numbers and sometimes a date.

The rotor keyway is only required for timing purpose, it is not required to drive the rotor. I never use

a key on my rig and only ever just nip up the rotor nut, it never moves.

You must use a key if you run a battery-less ignition system (including with a capacitor) or if you use

the marks on the rotor for ignition timing.

There may be rub marks on your rotor but these are not necessarily a bad thing. However there

must be a gap between the rotor and stator, check yours with a feeler gauge. The gap size is not

critical but make sure you can get a feeler all the way around.

If the rub marks have any depth or formed grooves, check the nearest main bearing for wear.

Magnetism There is a lot of forum talk about how much magnetism a rotor loses over time and what impact this

might have on alternator output. I have a collection of old rotors and some are good, some are bad.

Shown below two of my old Lucas rotors are very close to a new Wassell on output (C & E), two are

close and rotor B wants chucking in the bin.

Meter Reading Scope reading Power 1

Ohm No load 1

Ohm No

load Output %

AC AC Peak Peak Compared to

ID Make Year ID Watts RPM Volts Volts Volts Volts Wassell

A Lucas 54215824 149 3000 12.2 22.1 23.2 58.2 70

B Lucas 11 68 54212006 108 3000 10.4 18.8 20.1 51.2 51

C Lucas 204 3000 14.3 25.3 27.2 63.7 96

D Wassell ww10105p 213 3000 14.6 25.9 27.2 63.7 100

E Lucas 44 78 54212006 202 3000 14.2 24.8 26.7 61.4 95

F Lucas 5 69 54212006 166 3000 12.9 23.2 24.1 60.4 78


4

I have tested a Sparx high-output rotor and this gave a useful extra 50W output on a single phase

stator and 25W extra on a three phase. Take note that this increase would stress a standard zener

through overheating. A modern rec/reg should be OK with the extra output though.

I used a two wire stator 47205A with 1 Ohm load

at 3000 RPM in this test.

Tight Fit Rotor While mentioning Sparx, their rotors are now an interference fit on the crankshaft. Until we know

different I’d suggest using an expanding reamer or hone in the Sparx rotor bore to give yourself a bit

of clearance.

The crankshaft diameter is 0.750in.

All dimensions in inches.

Rotor Offset A rider said his rotor has a 0.25” (6.35mm) offset from his stator and wondered what effect it would

have on output. I did a quick check using a standard Lucas rotor with single-phase 47205a stator

running at 3000rpm without and with a load.

That offset caused a 9% drop in output so worth

getting it right. It would be interesting to know if

the offset is standard or has been caused by a

mismatch of parts ie wrong size spacer or stator

studs.

Shorted Stator Output There has been some forum discussion about the power consumption of Lucas alternators, i.e. how

much effort does it take to rotate a rotor. Most thought it would be at maximum effort when the

output leads are shorted together. However one lone voice declared that the drive power would be

at minimum with leads shorted.

To test the theory I set the frequency of my inverter to 17Hz (1000 RPM) and noted the output

current of the inverter with no load, with a 1 Ohm load and with the stator leads shorted together.

To just drive the motor alone (no rotor or stator) the inverter current was 1.9 amps.

Rotor Voltage Watts

Power Output %

Compared to Wassell

Lucas 10.8 117 96

Wassell 11.0 121 100

Sparx 13.1 171 141

Rotor Bore size

Clearance

Lucas 0.753 0.003

Wassell 0.7525 0.0025

Sparx 0.7505 0.0005

Voltage No load

Voltage 1 Ohm

Watts

Offset 33.4 10.2 104.0

Aligned 35.5 10.7 114.5


5

The tests showed that minimum power is required with shorted leads. Not surprising then that most

modern rec/regs work using this system as the following example

shows:

http://mastercircuits.blogspot.co.uk/2010/05/motorcycle-

voltage-regulator.html

I was surprised at how little difference there is between the no

load and 1 ohm load values. If anyone is wondering why the no load values are so high don't forget

that you have a set of rotor magnets trying to hang onto the stator poles.

Since doing this test I have found Lucas circuit diagrams

that show output leads shorted together in some

lighting switch positions (Lucas Workshop Instructions,

Section L-2, Part E, Page 3 Nov 1960).

The following text was included, “In the lights ‘off’

position the alternator output is regulated to a

minimum value by the interaction of a magnetic flux set

up by the current flowing in the short-circuited coils

with the magnetic flux of the rotor – the latter flux being distorted and therefore less effective.”

Will a shorted stator overheat? Another forum member suggested that although it takes less power to rotate a shorted stator, over

time a shorted stator would still fatally overheat. The accompanying chart shows stator J with its

output leads shorted together

running with a Lucas rotor at

3000 RPM. After 40 minutes

the temperature is stabilising

around 80 deg C.

The output leads of the stator

were only warm to the touch

suggesting that the

temperature rise is mostly

caused by eddy currents.

I am not sure what the internal

temperature of a Triumph primary chaincase would be after 40 minutes running but I would imagine

that it is likely to be 80 deg C or more.

Motor current (A)

Stator D Stator J

No load 3.3 2.9

1 ohm 3.6 3.1

shorted 2.8 2.5


6

0

10

20

30

40

50

60

70

80

90

100

0 2000 4000 6000

Peak Volts DC

RPM

1 Ohm load

No load

Testing Finally to give an idea what an alternator is doing have a look at the plot below, the output of stator

M. As you can see, with no load the stator output is a straight line. If you double the speed of the

rotor you double the output voltage of the stator. With a load, the curve is flattened.

Testing your alternator RM stators are robust things; it is difficult to destroy one. If you are having charging problems it is

likely to be something other than a bad alternator. Also if you replace your alternator without first

testing it then the chances are you are wasting your money.

If your bike suddenly develops a flat battery then to test your alternator:

First charge your battery (so you can start your bike and keep it running)

Disconnect your stator

Measure the stator resistance and compare your readings to mine in the tables below.

When measuring very low resistances the meter lead resistance can be higher than the

stator resistance so measure the lead resistance first (connect the meter leads together and

note the meter reading), then measure your stator resistance and subtract the lead

resistance to give actual stator resistance. Remember that with stator resistance results,

near enough is good enough. If my stator measures 0.5 Ohm and yours is 0.62 Ohm then

that is OK. If you have an infinite or very high stator resistance then you have a broken stator

coil or stator lead.

Now measure the resistance between the stator lead and the crankcase, it should always be

infinite (very high).

Start your bike and measure the AC voltage between leads at 3000 RPM (you don’t need the

1 Ohm resistor.) Compare your results with mine. Again, being close is good enough. Don’t

just blip the throttle to make measurements, hold the revs until you get a steady meter

reading.

If your resistances are OK but your voltage is much lower than mine then you have weak

magnets in your rotor.


7

If your bike passes these tests then you need to look at your rectifier and zener (or your rec/reg for

combined units) for your charging problems. There is further guidance here:

http://www.scribd.com/doc/152680652/Fault-Finding-Lucas-Motorcycle-Charging-Systems

The log, what it all means ID - is what I scribed onto a stator as I pulled it from the box so I could refer back if necessary.

Type – single (1) or three (3) phase

Number - is what was stamped onto the stator. I am not sure what the numbers mean and I suspect

that you could get the same alternator type but with different numbers for each bike manufacturer.

Where it says ‘unknown’ this refers to a rotor purchased new from eBay and I think of them as a

Royal Enfield (India) types of stator rather than of Lucas origin.

Year - what was stamped on the stator.

Wires - the number of, coming from the stator.

Number of laminations - counted.

Lead colours - if the majority of lead colour is brown with a thin blue stripe then this is brown-blue in

my log.

Resistance between leads - in Ohms.

As I said previously measuring low value resistance is tricky because part of the resistance you are

trying to measure is caused by the meter itself; often the meter lead resistance is greater than the

stator resistance. To overcome this in my tests I used the ‘four-wire’ resistance measuring method,

done using two meters (hence four-wire) and a separate power supply.

The power supply is used to provide a current flow through the stator being

tested and one meter is connected to measure this current flow. If we now

measure the voltage across the stator leads we can calculate its resistance

(R=V/I). The important point is that with this method the meter lead resistance

is of no concern.

Using the circuit on the left I adjusted the power supply until 1 amp was flowing

through the ammeter; from Ohm’s law the voltage measured across the stator

is the same as the stator resistance. So if I measure 0.5v across the stator coil

then its resistance is 0.5 Ohm.

Watts - an idea of what the stator output power is likely to be. I have multiplied the meter reading

with 1 Ohm load by itself. It is not accurate but a reasonable indication.

RPM – of the rotor used for the measurement.

Meter reading 1 Ohm load volts AC - what my meter measured with a 1 Ohm load. These readings

are relative rather than absolute. You need what’s known as a “true RMS” meter for accurate

results, my meter isn’t but it gives you an idea what to expect.


8

Meter reading no load - as above with 1 Ohm load disconnected.

Peak 1 Ohm volts - peak voltage measured on my Picoscope, the dotted lines shown below.

This is important as it gives an indication of whether at maximum load your battery will be charged.

Now a charged 12v battery measures 12.6V across its terminals, so to charge it we need more than

12.6 volts. Looking at the image above you can see the peak voltage is just over 10 volts which isn’t

enough.

Peak no load volts - as can be seen bellow you have plenty of voltage to charge your battery.

Stator C 4000 RPM 1 Ohm load

Meter reading 6.0 volts AC

Stator C 4000 RPM no load

Meter reading 52.1 volts AC


9

In practice you can never achieve “no load” as your electrical system will always have a load of some

sort (the battery itself, ignition coils, zener diode.)

Notes - some remarks. An ‘X’ in this column indicates the leads I used for measurement on three-

phase stators.

DerryUK

August 2013


10

T Y Meter reading Scope reading Y e Resistance 1 Ohm No load 1 Ohm No load P Stator a Between AC AC Peak Peak

ID E number r Wires Laminations Lead colours Leads Watts RPM Volts Volts Volts Volts Notes

A 1 47197A 67 5 18

Brown Brown Red Black-white

brown-blue red brown-blue black Yellow

0.916 0.451 0.484

4.790

27 45

9

3000

5.2 6.7

3.0

13.2 26.4

75.9

9.1

11.2

4.5

35.5 92.5

203.0

No connection Stop Lamp Lights Ignition coils

B 3 47252 85 3 19 Green-yellow Green-yellow White-green

Green-black White-green Green-black

0.847 0.851 0.851

34

3000 5.8 38.0 9.6 67 X

C 3 47252 89 3 21

Green-yellow Green-yellow White-green


0.849 0.858 0.860

34 36

1000 2000 3000 4000

4.5 5.5 5.8 6.0

13.2 26.3 39.4 52.1

7.8 9.1 9.5 10

22.3 48.0 72.0 96.3

D 1 47161A 64 2 16 Green-yellow Green-pink 0.234 149 3000 12.2 22.1 23.2 58.2

E 3 47244A 88 3 20 Green-yellow Green-yellow White-green


0.529 0.530 0.517

53 3000 7.3 30.0 12.1 56.8

F 3 472442 83 3 20



0.519 0.519 0.519

53 59

1000 2000 3000 4000

4.9 6.6 7.3 7.7

9.8 19.6 29.2 38.6

8.5 11.4 12.2 12.8

16.4 33.5 50.4 67.0

X

G 3 47252A 79 3 20 Green-yellow Green-yellow White-green


0.829 0.831 0.844

36 3000 6.0 37.3 9.8 60.3 X

H 1 Unknown 5 17

Brown Brown Brown Brown

Clear Red Purple Mauve

0.704 1.102 2.350 1.092

18 1 1 1

3000

4.2 1.0 1.1 1.2

20.4 7.5

16.5 10.1

6.9 1.7 1.9 1.9

55.7 30.1 54.9 37.8


11

T Y Meter reading Scope reading y e Resistance 1 Ohm No load 1 Ohm No load p Stator a Between AC AC Peak Peak

ID e number r Wires Laminations Lead colours Leads Watts RPM Volts Volts Volts Volts Notes

I 1 7188B 5 18

Red Brown Brown Black-white

Brown-blue Black-white Black-yellow Black-yellow

0.499 3.950 0.400 4.340

42 6

35 8

3000

6.5 2.5 5.9 2.8

26.6 73.8 16.0 89.5

10.2 3.7 9.8 4.1

78.2 225.0 37.9

265.0

Marked C15

J 1 47205A 69 2 18 White-green Green-yellow 0.424 42 90

117

1000 2000 3000

6.5 9.5

10.8

11.1 22.2 33.1

13.6 17.6 17.9

31.3 62.3 92.4

K 1 Unknown 3 19 Light green Light green Dark green

Mid green Dark green Mid green

0.597 1.120 1.700

Not tested Stator did not fit my rig

L 1 Unknown 3 23 Green Green White

White Purple Purple

0.735 2.040 1.330

71 1

29 3000

8.4 0.3 5.4

27.8 1.9

26.0

14.0 1.0 9.0

75.1 12.3 67.1

M 1 Unknown 4 32

Orange Purple Orange

Yellow Purple Yellow

0.531 0.960

92 41

149

3000

9.6 6.4

12.2

23.0 25.8

17.6 11.16

23.9

66.2 79.7

Windings In-phase

N 1 47209A 3 26

White-green White-green Green-black White-green

Green-black Green-yellow Green-yellow Green-black+ green-yellow

1.199 0.584 1.788

31 88

129

3000

5.6 9.4 0

11.4

40.1 38.5 1.5

38.8

8.55 14.6 0.38

17.9

104.7 101.0

4.6

101.6

Windings In-phase

P 1 472308 73 3 Red Red Green

Green Yellow Yellow

Broken leads


12

T Y Meter reading Scope reading

y e Resistance 1 Ohm No load 1 Ohm No load

p Stator a Between AC AC Peak Peak

ID e number r Wires Laminations Lead colours Leads Watts RPM Volts Volts Volts Volts Notes

Q 3 Sparx

Hi-output 3 20


White-green Green-black Green-black

0.495 0.497 0.497

62 3000 7.9 28.0 12.78 40.9 Wassell rotor

Q 3 Sparx

Hi-output 3 20


White-green Green-black Green-black

0.495 0.497 0.497

87 3000 9.32 31.8 15.13 46.2 Sparx

hi-output rotor

testing lucas alternators aug 2013

Documents

stators stators

lucas stators

wire singlephase stator

extra stator

testing lucas alternators

earlier stator types

rotor keyway

rotor nut