Testing a Model T Generator Armature Using a Growler

Figure 1 – Growler equipped with all three options

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.

Figure 2. Growler Testing an Armature

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.

Figure 3 Ground Short Test Using the Growler

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.

Figure 4 Winding Current Test

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

Hand Crank Starting the Model T Ford Car on Magneto by Ron Patterson

Once again we are privileged to have another fine article written by Ron Patterson.



When the Model T Car was originally developed Henry Ford insisted that it have a self-contained power source for the Ignition System so owners would not have to rely upon battery power. All inexpensive cars at that time used dry cell batteries to power Trembler coils for ignition. A dead battery would disable the vehicle until it had been replaced. Many people carried extra batteries, but commonly forgot to keep a supply of charged batteries on hand. These batteries were not rechargeable and were a common nuisance to maintain.

While Ford’s idea sounds anachronistic today, it was a major selling point of the new Model T Ford in 1908.Henry Ford’s idea was implemented by Joseph Galamb and Edward Huff in the form of the internal flywheel driven alternating current generator (the Model T Magneto); sixteen permanent magnets mounted on the flywheel rotating (rotor) near a ring of sixteen fixed field windings (stator) to produce ignition current.

In this magneto system spark timing was controlled by a combination of Magneto current pulses that occurred every 22.5 degrees of flywheel rotation and the Timer which was connected to the driver manipulated spark control lever (advance and retard) on the steering column.

For those interested in more complete details of the entire system read our article entitled “The Model T Ford Ignition and Spark Timing” and may be found at the following link:

The Model T Ford Ignition Spark and Timing

Figure 1 Ford Recommended Starting on Magneto Procedure

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