Possibly the most in depth look at this instrument ever featured online!
By Dorian Stonehouse
First let’s take a look how the meter movement works:
The “wee” megger was an ingenious insulation tester of yesteryear.
It consisted of a hand-cranked generator, a sensitive meter and associated components.
It was sold as a “moisture meter” – presenting a high voltage low current source across the appliance under test.
Should the appliance be leaky, a tiny current flows, causing the meter to register a certain resistance:
There are many descriptions online on how the megger works, and the following explanation is just my take on things.
Refer to the drawing immediately above
The movement consists of two needle deflector coils:
As soon as the generator is cranked, L1 (in series with R1) are across the generator supply voltage.
When this voltage appears across L1 (VL1), it will push the meter needle towards the left – “infinity” position:
L2 is in series with R2 and the appliance under test (my finger). Continued below:
R1 and R2 are wirewound coils, serving as resistors (this was during the days of carbon resistors, which broke down under high voltage), so wirewound was the only alternative:
Evershed & Vignoles “Wee” Megger series 3 insulation (moisture) tester
When a tiny current flows through my finger and L2 (VL2), the meter needle swings to the right – short circuit direction.
(See “zero” on above scale).
L1 and L2, therefore, work in opposition.
First case scenario: appliance under test is okay – no leaks:
When the generator is cranked, VL1 is higher than VL2.
L2 is now in series with a very high resistance – the appliance under test .
The higher value VL1 (which is effectively across the meter movement), propels the meter needle to the left – “infinity.”
With no current flowing through the appliance under test (no leakage), VL2 will be very small.
VL2 cannot, therefore, push the needle to the right.
VL1 wins the match, pushing the meter needle towards “infinity” – left.
Second case scenario: appliance under test is shorted out:
When the generator is cranked, if the appliance under test is short circuited, coil L2 now senses a higher current through it.
VL2 is, therefore, greater than VL1.
VL2 swings the meter needle to the right – “zero.”
L1 is now partly shunted by the short circuit (partly bridged) by the appliance under test…
Thus, VL1 cannot oppose VL2.
The result is a hard swing to the right and a match win for VL2.
Third case scenario: what happens if your megger encounters a betwixt and between value resistor ?
If there is a partial short circuit in the appliance under test, VL1 and VL2 will again produce opposing forces on the meter needle.
Depending whether VL1 or VL2 emerges the greater, the position of the needle will now be between “zero” and “infinity,”
Reflecting the resistance of the defective appliance, the scale is marked in thousands, then millions of ohms, (see photo of megger scale).
Repairing the field coils and reassembling the Evershed & Vignoles “Wee” Megger generator:
These generators were the Rolls-Royce of the electrical trade.
They were so well made, that many are still running like clockwork, more than 70 years later!
While grabbing the output terminals with one hand, I gingerly cranked the generator with my other hand. However, I felt no tingling sensation whatsoever, suggesting the presence of a fault – somewhere!
Working back towards the generator, it soon became apparent that the 250 volts had dropped to only 2 volts.
In response to this situation, I decided to first check the brushes, but they appeared to be okay.
Onward and upward, I proceeded to measure commutator to commutator resistance. drat! – open circuit!
Off came the commutator housing – could it be the dreaded armature coil?
Suddenly, however, there was a glimmer of hope, as I could see what appeared to be a tiny break in one of the wires connecting the commutator to the coil!
It transpired, therefore, that this wire was, indeed, open circuit, and that the armature coil was most likely intact.
The break is shown in the photograph above.
Refitting spindle and worm wheel:
Evershed & Vignoles “Wee” Megger series 3
View from the top: Gear housing and armature worm wheel:
Preview of how the worm wheel fits into the gear housing (before being bolted to the armature)
The coil to commutator repair was fairly straightforward, as was bolting everything back on the armature.
The armature was soon back together again:
Appearing on the reverse side seems to be the resistance value and the number of turns of wire; suggesting that servicing was probably on the forefront of the designer’s mind
Large permanent magnet used in Evershed & Vignoles “Wee” Megger
Magnet is slipped back over armature assay :
Evershed & Vignoles “Wee” Megger series 3
Continued from above:
Now for those dreadful roller bearings
Before the armature worm wheel is pushed back to mesh with the gears, the roller bearings must be reinstated.
Shown below is a retaining barrel, a brush and pressure spring. The roller bearings also appear on the list:
When removing and reinserting the armature assay, great care must be taken to ensure the roller bearings do not scatter everywhere and get lost, thereby destroying the chances of a successful repair!
A bit of paper tape to block the exit of the roller bearings and light greasing with Vaseline to ensure bearings adhere to the gear housing, and all should be good for lift-off
Slowly, slowly catchy monkey is the only way forward when dealing with the “Wee” Megger armature repairs!
The armature assay is gradually pushed into the gear and rear housing
Long bolts are pushed through rear housing to bolt on to the gear housing
The generator is fitted back into the case:
Monochrome aspect: Evershed & Vignoles “Wee” Megger DC 250 volt generator
Evershed & Vignoles “Wee” Megger DC 250 volt generator:
Evershed & Vignoles “Wee” Megger
250 volt DC generator: reverse view
Having refitted the generator in the case, I carried out the tingling hand test once again: holding the output terminals with one hand, while cranking the generator handle with my free hand.
This time, however, I got more than I bargained for – a nasty shock, as the Megger burst into life…
More pictures to follow