Figure 7-18.—Clamp-on ammeter.
jaws are actually part of a laminated iron core. Around
this core, inside the instrument enclosure, is a coil
winding that connects to the meter circuit. The
complete core (including the jaws) and the coil winding
are the core and secondary of a transformer. The
conductor, carrying the current to be measured, is like a
primary winding of a transformer. The transformer
secondary is the source of power that drives the meter
movement.
The strength of the magnetic field
surrounding the conductor determines the amount of
secondary current. The amount of secondary current
determines the indication of current being measured by
the meter.
All ammeters will have an adjustable scale. The
function and range of the meter are changed as the scale
is changed.
To take a current measurement, turn the selector
until the AMP scale you wish to use appears in the
window. To take measurements of unknown amounts
of current, you should rotate the scale to the highest
amperage range. After taking the reading at the highest
range, you may see that the amount of current is within
the limits of a lower range. If so, change the scale to that
lower range for a more accurate reading.
After choosing the scale you want, depress the
handle to open the transformer jaws. Clamp the jaws
around only one conductor. The split core must be free
of any debris because it must close completely for an
accurate reading.
To measure very low currents in a small flexible
conductor, you may wrap the conductor one or more
times around the clamp-on jaws of the meter. One loop
will double the reading. Several loops will increase the
reading even more. After taking the measurement,
divide the reading by the appropriate number of loops to
determine what the actual current value is.
The clamp-on ammeter is convenient and easy to
use. To measure the current of a single-phase motor, for
example, simply rotate the selector until the desired
amp scale appears, clamp the jaws around one of the
two motor conductors, and take the reading.
Some clamp-on instruments are capable of more
than one function; for example, they are designed for
use as an ohmmeter or a voltmeter when used with the
appropriate adapter or test leads.
VOLTMETERS
The meter component (or voltage indicator) of a
voltmeter is actually a milliammeter or a micrometer.
This instrument is series-connected to a resistor (called
a voltage multiplier) to operate as a voltmeter. The
series resistance must be appropriate for the range of
voltage to be measured. The scale of an instrument
designed for use as a voltmeter is calibrated (marked
off) for voltage measurements.
Panel voltmeters are similar in appearance to the
ammeters shown in figure 7-17, except for the
calibration of the scale. Examples of typical panel
voltmeters are shown in figure 7-19. Voltmeters are
Figure 7-19.—Typical panel voltmeters.
7-17
