Growler (electrical device)

A growler is an electrical device used for testing insulation of a motor for shorted coils. A growler consists of a coil of wire wrapped around an iron core and connected to a source of AC current. When placed on the stator core of a motor the growler acts as the primary of a transformer and the stator coils act as the secondary. A "feeler", a thin strip of steel (hacksaw blade) can be used as the short detector.
The alternating magnetic flux set up by the growler passes through the window of the armature coil, generating an alternating voltage in the coil. If the coil is shorted, an alternating current induced in the coil will cause an alternating magnetic field to encircle the shorted conductors.
With the growler energized, the feeler is moved from slot to slot. When the feeler is moved over a slot containing the shorted coil, the alternating magnetic field will alternately attract and release the feeler, causing it to vibrate in synchronism with the alternating current. A strong vibration of the feeler accompanied by a growling noise indicated that the coil is shorted.