Page 164 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

If a reading is noticed on the meter, turn the armature so that the next commutator bar is in the same position as
the earlier one, and test as before. Continue in this manner until all the bars are tested. Where the meter gives no
deflection, it is an indication that the grounded coil is connected to this particular bar.
Growler test for shorted coil : The procedure to test for short circuits in an armature is as follows.
The armature to be tested is placed on the growler and then the growler is switched on. A thin piece of metal, such
as a hacksaw blade, is held over the top slot of the armature as shown in Fig 7. In case of short in the winding,
the blade will vibrate rapidly and create a growling noise. If the blade remains stationary, it is an indication that
no short exists in the coil under test. After several top slots have been given the hacksaw blade test, turn the
armature so that the next few slots are on top. Test as before and continue this procedure for the entire armature.

An armature having cross connections or equalizers cannot be given the hacksaw blade test. This type of armature
will cause the blade to vibrate at every slot, which would seem to indicate that possibly every coil is shorted.
Test for open coil : Growlers are also provided with meters (milli-volt or ammeter) on the panel with variable
resistance. In this case an open in the armature coil can be found out as follows.
Growler test for an open coil : To locate an open coil with a growler, set up the armature on the growler in the
usual manner. Test the top two adjacent bars with an AC milli-voltmeter as shown in Fig 8. Rotate the armature
and continue testing the adjacent bars. When the milli-voltmeter bridges the two bars connected to the open coil,
the meter pointer will not deflect. All the other bars will give a deflection. This test for an open coil can be made
without the meter by shorting the two top bars with a piece of wire. Absence of a spark indicates that the coil is
open. The open may be either at the commutator bar or in the coil itself. The procedure may be used to determine
the location of the leads of a shorted coil. However, the hacksaw blade test is the most satisfactory method of
determining a shorted coil.

Fig 7 Fig 8

Drop test : The most accurate method of testing the armature for correct resistance, number of turns, short and
open and reversed coil connection is by the drop test. Connect a low voltage DC supply across the commutator
segments at a distance of pole pitch. Insert a variable resistance in series with the circuit. Switch ‘ON’ the DC
supply and connect a milli-voltmeter to the adjacent segments as in Fig 9a and b.
Adjust the readings to a specified value, by using a variable rheostat. Record the milli-voltmeter readings on the
consequent commutator segments by rotating the armature in one direction. The position of the segments and
the connection should be the same as in the first set up. The result could be concluded as enumerated below.
• If all the readings are the same, the winding is correct.
• If the milli-voltmeter reads zero or low voltage, the coil connected to the segment is short.
• If the milli-voltmeter reads high voltage, the coil connected to the segment is open.

• If the milli-voltmeter deflects in the reverse direction as shown in Fig 9b, the coil connected with the segment
is reversed.
Generally armatures are tested as a routine for insulation resistance and for shorted coils. Only when a fault in
the armature winding is suspected, a drop test is conducted.

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CITS : Power - Electrician & Wireman - Lesson 86-92