Connect the lead from the battery to one brush.
Connect the other lead from the battery to the load.
From the other side of the load connect to a brush adjacent to the first brush.
Now measure the voltage drop between each pair of commutator bars in turn, between the powered brushes.
The voltage drop between any two adjacent commutator bars will be equal in a good rotor.
This limits you to 1/4 of the total number of the bars to test.
You may complete a number of tests and then turn the rotor to test more bars.
You may also measure the other 3/4s of the bars on the back side but the readings will be lower.
You are looking for one reading of a set that is noticeably different fro the others in the same set.
After turning the rotor the readings may be slightly different due to the brush contact.
Not to worry, after turning you are starting a new series of tests and all readings in the new test should be equal.
No growler? Not to worry. You probably have what you need and don't realize it.
I hope that you like this one, Pete.
Are you familiar with the poem?
"The Cremation of Sam McGee."
.."The Northern Lights have seen queer sights,
But the queerest they ever did see
Was that night on the marge of Lake Lebarge
I cremated Sam McGee."
Link
I was sitting with a very sick generator a mile or so from the "marge of Lake Lebarge".
The exciter would fail when it came up to operating temperature.
We had it growler tested in a shop in Whitehorse, Yukon Territory, and it tested good.
The shop hesitated to open up a rotor that tested good.
They heated it in their oven but the differential expansion was not the same as when it was heated in service and it tested good.
Took it back to the site and it failed again.
How to test it when it was hot and failed when the nearest growler was almost an hours drive away.
Back to basics.
How does a growler work?
It induces a flux in the rotor and any shorts divert the flux into leakage flux which can be detected by a hacksaw blade.
First, how can I induce a flux and second how can I detect any irregularities?
Then I started to think about the brush neutral test.
The field is excited with AC and the induced AC voltage at the brushes is measured.
The brushes are shifted until the voltage is the lowest, ideally zero volts.
Let's see what happens;
I ran the set until the exciter failed.
I energized the field with AC and connected a voltmeter to the brushes as if doing a brush neutral position adjustment.
Then I turned the rotor. (Not part of the brush neutral test, but not a bad idea,).
As I turned the field the voltage varied as the shorted coils changed position in relation to the flux.
Back to the shop where I explained my test.
They knew the basics and ageed that there was a shorted coil.
"Now that we are sure of the problem we can fix it."
A few days later we were up and running.
Now Adam,
What do you have on hand to replicate this test with a series motor.
Do you have an extension cord that you are willing to cut?
Do you have some appliance that draws about 10 or 12 Amps.
Most shunt motors or generators will easily withstand 120 VAC across the field windings.
Not so a series motor.
You need to limit the current.
If this is a motor from any kind of an electric vehicle it should withstand 10 or 12 Amps through the field.
If you are not familiar with safety precautions working with live 120 Volts AC find someone who is to help you.
You need an extension cord and an electric appliance. An electric kettle (with water in it. grin), a toaster or some appliance that draws about 10 or 12 Amps.
Disconnect the series field.
Wire the appliance in series with the field and connect the extension cord so that the current flows trough the appliance and the field.
Now you have a flux established through the rotor. Connect your meter across two adjacent brush holders and check for AC voltage.
Turn the rotor while watching the voltmeter. It should stay at or near zero.
Any variation of voltage indicates a shorted winding.
There may be minor variations as the brushes cross from one bar to the next.
One shorted turn will show a voltage rising to a peak and dropping back to zero over 1/4 turn of the rotor.
Verification.
When zero is a good test result, how are we sure that the test was performed correctly?
Easy.
With the current flowing through the test setup, put one voltmeter lead on one of the brushes.
Touch the other voltmeter probe to an armature bar next to the brush.
Drag the probe across the commutator bars to the other brush holder.
You should see the voltage rise to a peak at the halfway point and then drop back don as you get near the other brush.
Good luck.
Bill
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"Why not the best?"
Jimmy Carter
