Using the Growler

Using the Growler

A growler is a useful piece of equipment used for testing armatures. It consists of a U shaped iron core which has a coil wound around the arms. Often, this coil is in two sections which can be switched to a series or parallel connection as required. The two ends of the U shaped core are angled in such a way that a wound armature can sit in the gap thereby completing a magnetic circuit. The whole arrangement works as a transformer with the growler coil acting as the primary winding and the armature coils acting as secondary windings When switched on, the growler coil is fed with mains electricity which, here in the UK, is AC at 50Hz resulting in a noticeable ‘hum’. This creates an alternating magnetic flux in the iron core/armature body which also passes through the armature coils. This arrangement allows the armature to be tested for both shorted and open windings.

Testing for shorted windings
Any armature coil which has a short will create a closed circuit and an alternating current will be induced in the coil. This will result in an increased load which will often change the tone of the ‘hum’ to a ‘growl’. It will also create an alternating magnetic field around the shorted armature coil. 
A flat, metal feeler containing iron, such as a hacksaw blade, can be used to detect the magnetic field generated by a shorted armature coil. The feeler is held lightly in line with each armature slot in turn. Normally, no pull will be felt on the feeler but when it is over a slot containing a shorted coil, the alternating magnetic field will alternately attract and release the feeler. This will cause it to vibrate in sync with the alternating current and this vibration, together with the growling noise, indicates that a shorted coil has been found.
Not all armatures can be tested for short circuits in this way. Some armatures produce excessive vibration of the feeler all around the armature during the test. These armatures can be tested for short circuits by using the growler’s probes and ac ammeter.
The two test probes are placed on adjacent commutator segments and the meter reading noted. Keeping the armature stationary in the growler’s jaws, the two probes are ‘walked’ around the commutator segments until the highest reading is obtained. If required, this reading can be adjusted on the meter by use of the growler’s variable resister.  Then, keeping the probes stationary, the armature is rotated in the jaws bringing each pair of segments under the test probes in turn. The readings should be nearly the same for each pair of adjacent segments. If a coil is short-circuited, the ammeter reading will drop to almost zero.

Testing for open windings
One method of testing for open windings is to use the probes and ammeter exactly as described above. A zero reading on the ammeter indicates a broken winding or open coil. 
A rather more crude but none the less, effective test for open windings involves placing the armature on the growler and using the feeler to short circuit the ends of the coils. This can be done by successively shorting adjacent commutator segments. Note that good coils will create sparks - no spark indicates that the coil is open circuit. In order that the smooth brush surfaces are not damaged by the sparking, it is important to place the feeler on the riser part of adjacent commutator segments, not on the brush surfaces themselves. 
When the test requires the armature to be rotated in the jaws of the growler it is not always easy to do so because the growler acts as an electromagnet and grips the armature quite tightly, particularly with larger armatures. For this reason, a foot switch is often used for turning the growler current on and off as the armature is being rotated. Use of a foot switch also leaves both hands free to hold the probes.

Testing for grounded windings
Most of the older growlers have a built in continuity test circuit consisting of a pair of probes and a bulb. Holding one probe on the armature shaft and the other on each commutator segment in turn, lights the bulb if a short to ground exists. However – this circuit is powered directly from the mains supply which is not really a very safe procedure and would probably be frowned upon by today’s Health and Safety people!
The picture shows where the probes would be placed but we choose not to use this method so did not wire up the probes when we rewired the growler. Note that, if this test is used, the growler is not switched on but is simply used as a convenient way to hold the armature while the test is carried out. These ‘high voltage’ continuity testers were common-place decades ago - there is such a circuit on our Octopus Test Bench but we don’t use that one either!
The continuity test setting on digital multimeters could be used instead. It would show up a definite ground but the test voltage is too low to pick up what might be called ‘near’ grounds where, perhaps, the insulation has degraded.  In our view, this test is best done with a standard insulation test meter. 

All the above notes refer to the use of the growler to test dynamo armatures. We also use a growler as part of the testing procedure for all of our rewound magneto armatures.
The growler acts as a transformer as before but here we have two secondary windings – the primary and secondary windings of the magneto armature. The actual voltages induced in the magneto windings will vary with the number of turns/layers and gauge of wire used as well as the size and composition of the metal armature core itself. The design of the magnetic circuit and the number of coil turns in the growler itself will also have an effect so different growlers will give different figures for the same magneto armature. We always use the same growler for these tests so some factors are kept constant. As an example, previous tests have shown that, for a Lucas K2F armature tested on our growler, we should expect a primary voltage of 18-19v and a secondary voltage of 1200-1300 volts – which explains the use of the HV probe shown in the picture. Much variation on these figures would indicate a possible problem that needs investigating further. 

CAUTION: DO NOT OPERATE THE GROWLER WITHOUT AN ARMATURE IN PLACE. Without an armature in the jaws, the current draw is greater and will cause overheating and possible damage to the growler’s coil. 
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