[Mike] Shows Us How To Use An Armature Growler

[Mike] has put up a great video  on his [SmallEngineMechanic] YouTube Channel about a tool we don’t see very often these days. He’s using an armature growler (YouTube link) to test the armature from a generator. Armature growlers (or just growlers for short) were commonplace years ago. Back when cars had generators, just about every auto mechanic had one on hand. They perform three simple tests: Check armature windings for shorts to other windings, for open windings, and for shorts to the armature body. [Mike’s] particular growler came to him as a basket case. The wiring was shot, it was rusty, and generally needed quite a bit of TLC. He restored it to like new condition, and uses it to help with his antique engine and genset addiction hobby.

Growlers essentially are a transformer primary with a V-shaped frame. The primary coil is connected to A/C mains. The armature to be tested sits in the “V” and through the magic of induction, some of the windings become the secondary coils (more on this later). This means some pretty high voltage will be exposed on commutator of the armature under test, so care should be taken when using one!

Testing for shorts to the ground or the core of the armature is a simple continuity test. Instead of a piezo beep though, a short will trigger the growler to turn on, which means the armature will jump a bit and everything will emit a loud A/C hum. It certainly makes testing more interesting!

Checking for open windings is a matter of energizing the growler’s coil, then probing pairs of contacts on the commutator.  Voltage induced in the windings is displayed on the growler’s meter. Open windings will show 0 volts. Not all the armature’s windings will be in the field of the growler at once – so fully testing the armature will mean rotating it several times, as [Mike] shows in his video.

The final test is for shorted coils. This is where things get pretty darn cool. The growler is switched on and a thin piece of ferrous metal – usually an old hacksaw blade, is run along the core of the armature. If a short exists, the hacksaw blade will vibrate against the core of the armature above the shorted windings. We’re not 100% clear on how the coupling between the growler’s primary and two windings causes the blade to vibrate, so feel free to chime in over in the comments to explain things.

Most commercial shops don’t troubleshoot armatures anymore, they just slap new parts in until everything works again. As such the growler isn’t as popular as it once was. Still, if you work with DC motors or generators, it’s a great tool to have around, and it’s operation is a pretty darn cool hack in itself.

Click past the break for [Mike’s] video!

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