"Growler" armature tester

If it's an old-fashioned 'growler' like what I'm thinking, the wiring is very simple. The winding at the base of the unit is simply connected across the line to create a nice alternating magnetic field on the pole pieces.

When the armature is set on in the pole pieces, you slowly shift it around. If at some point the noise of the thing increases (that is, it 'growls' at you), you've found a shorted winding (the shorted winding acts like a shorted secondary winding in a transformer and the primary winding is the growler's. Shorted turns make the current a lot stronger in the primary and the stronger magnetic field causes the rotor iron to vibrate on the pole pieces)

Laying a light piece of steel along the slot on top of the rotor as you turn it can help you find which slots have the shorted turn.

You can also use a meter or low-voltage lightbulb with a couple of leads to test each circuit on the commutator to look for opens.

Hope this helps

daestrom

In a generator rebuild shop we used a hacksaw blade both to check for shorted windings and to check for opens by shorting the edge of the commutator bars and observing the spark intensity.

Note that the growler will rapidly overheat if energized without an armature in place.

Don Young

Yes, I know essentially how they are used. The existing wiring has two probes wired directly to the 120V line (not even to the cold side of the switch!!!) and a 40 watt bulb is simply wired across the switched line (in parallel with the coil). I suspect the probes are meant to be wired only to the bulb with no connection to the 120V line. That's the way I'm rewiring it -- before I get electrocuted ! Jim H.

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A 120 volt test light was often provided for testing the armature for grounded windings. It was wired in series with the probes so that continuity between the probes would light the bulb. It was definitely a hazardous arrangement but those who used them knew to keep away from any contact with the probes. No matter which side of the line the probes were in, one of them was hot to ground. If in the neutral side one probe was hot thru the bulb and would pass enough current to be dangerous.

Those were the days of live front switchboards using knife switches and such things. People had great respect for the dangers of electricity but accidents did happen. In order to be safe your test light needs to be operated from a low voltage transformer and use a low voltage bulb and/or be energized from a GFCI outlet.

Don Young

OK, Don. Yes, that would have been a dangerous shorts tester. If the purpose was a shorts tester I can do that with an ohmmeter. I thought perhaps the bulb would indicate voltage being induced in the windings. I am familiar with everything you say having retired from an Electrical/Engineering position (but without a degree). I now have it rewired.

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You won't see a couple shorted turns with your ohm meter.

The particular subject under discussion was a tester to detect shorts from windings to the armature frame, not turn-to-turn shorts. An ohmmeter will do this. The 120V test lamp might be better or worse at detecting partial high-resistance shorts but either will do a satisfactory job in most cases.

The sequence we used was check for grounded windings first, then for shorted turns using the hacksaw blade and growler, then for open windings or commutator joints using the growler and shorting the commutator bars to check for equal spark intensity. Some growlers did have ammeters connected to a two prong probe to use instead shorting the commutator bars. A low voltage bulb might work for this but I never used one.

Don Young

Thanks gentlemen. No, I wouldn't even try to find shorted turns with an ohmmeter -- would use to look for short to armature structure. If the windings were 18 guage (just to pick a wire gauge) the resistance would only be about 6.5 milliohms per foot. Low resistance measurements are difficult. Even moderately low resistance measurements require a 4-wire instrument for accuracy. Jim H.

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