The generic term “Growler” describes a specialized tool used to test generator and motor armatures. Any generator or motor repair shop will have one of these tools. All Growlers have the basic “Growl” test feature and some have two additional test features depending upon the manufacturer. All three capabilities are desirable to comprehensively test an armature.
1. Basic “Growl” test function is the capability to place the armature windings adjacent to high strength magnetic field and check for shorted windings.
2. A Growler may have a set of two probes with a low wattage light bulb in series to make continuity test for low resistance shorts.
3. A Growler may have a single probe with two tips at a fixed distance apart. It can be placed on two adjoining commutator segments with the growler operating. This can be used to measure the current in a particular winding. An Ammeter will indicate the amount of current in the winding.
The “Growl” Test
We all know that by moving a loop of wire through a magnetic field a current is produced in the wire. The growler has a transformer with a set of open circuit magnetic laminations (a cradle) where you can place the armature.
See Figure 2.
A 60 Hertz alternating current power source is applied to the transformer core via an off/on switch providing an alternating magnetic field in the transformer laminations. When power is applied the laminations in the transformer vibrate with a loud humming sound; hence the name “Growler”. This magnetic field is impressed upon the armature windings when the armature is present in the cradle. In a good armature there is no closed circuit (loop) in any of the windings i.e. both ends of each winding are terminated to two adjacent commutator bars that are insulated from each other by Mica. Hence there is no current in the windings and no magnetic field from the windings is produced. In this condition when the hacksaw blade is placed parallel to the top of the laminations while rotating, the armature the blade will not vibrate. The armature must be rotated in the transformer cradle without moving the hacksaw blade to check each armature winding.
If there is as an intra or inter winding short in the armature windings, an alternating magnetic field will be created in the laminations associated with the closed-circuit loop winding causing the hacksaw blade to vibrate rapidly and noisily.
Winding Ground Short Test
If the growler is equipped with this test capability there will be a set of two probes that are used to make continuity tests. See Figure 1 and 3. These probes have a low wattage test lamp wired in series which will light when the tips of the probes are connected to each other. By placing the Black probe on the armature shaft (ground) and the Red probe on any of the commutator segments (remember they are all connected together in series) if there is a winding short to ground the lamp will light indicating a short is present. If the lamp does not light there are no windings shorted to ground. Caution: The two probes are connected directly to 120Volt AC power at all times when the growler is plugged into a wall outlet. If the growler does not have this continuity test capability this same test can be conducted using a Volt/Ohm meter.
Winding Current Test
This test – in Figure 4 below – measures the relative current flowing in each individual armature winding. The amount of current flowing in each of the windings should be approximately the same. It is important to measure the current in each winding with the probes connected AND the armature in the same position on the electromagnetic cradle. The electromagnet will create different Ammeter readings because the magnetic strength differs based on position of the armature windings to the electromagnetic cradle. The two fixed probes are placed touching two adjacent commutator segments (one winding) with the growler turned on and the armature mounted in the electromagnetic cradle. There is an Ammeter wired in series with the probe tips. The magnetic field produced by the growler will create a current in the armature winding under test. The amount of current flowing in each the winding will be shown on the Ammeter scale.
The Growler is a specialized tool that all generator and motor repair shops will have. The tests described above take very little time and most repair shops will complete them for a small charge. There is one armature fault the Growler tests above will not detect. It is referred to as a “flying short” This is a condition where winding short is only present when the armature is spinning and the high centrifugal forces upon the winding creates the short. This situation is not very common fault, but does occur.
Copyright Ron Patterson 12-5-2018