Extech Instruments 380260 User Manual page 5

Applications
Measuring Power Tools and Small Appliances
Testing AC Motors
Testing DC Motors
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This section applies to any device under test that uses a line cord. For
double insulated power tools, the meter's leads should be connected to
the device's housing (chuck, blade, etc.) and the lines of the power
cord.
Disconnect the motor from the line by disconnecting the wires from the motor terminals or opening
the mains switch.
If the mains switch is opened, and the motor also has a motor-starter, then the starter must be held
in the ON position. With the mains switch opened, the measured resistance will include the
resistance of the motor wire and all other components between the motor and the main switch. If a
weakness is indicated, the motor and other components should be checked individually. If the motor
is disconnected at the motor terminals, connect one meter lead to the grounded motor housing and
the other lead to one of the motor leads. Refer to diagram at below.
Motor
Starter
GND
1. Disconnect the motor from the line.
2. To test the brush rigging, field coils and armature, connect one meter lead to the grounded
motor housing and the other lead to the brush on the commutator.
3. If the resistance measurement indicates a weakness, raise the brushes off of the commutator
and separately test the armature, field coils and brush rigging (one at a time). Leave one lead
connected to the grounded motor housing while testing the motor components. This also
applies to DC Generators.
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Mains
Line
Switch
Notes:
1. The STARTER must be in the ON position during the
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