Specifications

Fig. 6.20

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 induction that no short exists in the coil under test. After

several to slots have been given the hacksaw blade test, turn the armature so that the few slots

are on to. 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 on the growler in the usual manner. Test the top two adjacent bars with an AC milli-

voltmeter as shown in Fig. 6.20.

Rotate the armature and continue testing the adjacent bars. When the millivoltmeter 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

tops 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 loads of a

shorted coil. However, the hacksaw blade test is the most satisfactory method of determining a

shorted coil.

6.6 DROP TEST: The most accurate method of testing the armature for correct resistance,

number of turns, short and open 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

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the DC supply and connect

a milli-voltmeter to the adjacent segments as in Fig. 6.21.