winding is short. The pulling effect of the
shorted portion of the winding is lost. This,
in turn, places more load on the active
winding, causing the motor to draw higher
voltage and amperage with a concurrent
increase in winding temperature. In this
condition the motor either fails to start, or it
starts and continues to run, finally causing the
overload protector to open. The fuses may
also blow. The result is likely to be a burnout
where the insulating varnish deteriorates from
excessive heat. Ultimately a ground or short
An ohmmeter can be used to check
windings for shorts. For most applications a
low-range meter with a scale graduated in
tenths of ohms between 0 and 2 ohms is best.
However, to check motors throughout the
sizes normally encountered in hermetic
motor-compressor units, a range of 0 to 25
ohms is necessary. The meter is used to
measure resistance of the windings. The
r e a d i n g s a r e c o m p a r e d w i t h d e s i gn
resistances. A short is shown when measured
resistance is less than design resistance. The
ohmmeter connections are the same as those
shown in figure 14-30.
Often manufacturers data is not available
and the design resistances are not known.
Table 14-1 lists the approximate resistances
for fractional horsepower single-phase motors.
The following guidelines may also be helpful:
1. The starting winding of low-starting
torque motors usually has a resistance of
about seven to eight times that of the running
2. The starting winding resistance of
high- starting torque motors is usually three
to four times that of the running winding.
A ground is the result of an electrical
conductor in contact, either directly or
indirectly, with the motor frame or the metal
Table 14-1.Approximate Resistances for Fractional
shell of the unit. Either the starting winding,
the running winding, or both can be affected.
The ground is either one of low resistance or
one of high resistance. A low-resistance
g r o u n d i s i n d i c a t e d w h e n f u s e s b l ow
repeatedly and the motor fails to start. A
high-resistance ground is shown by an
occasional blown fuse, but more often, by the
opening of the overload protector.
Three methods of testing windings for
grounds are the ohmmeter continuity test, the
test lamp continuity check, and the resistance
measurement with a megohmmeter. The
procedure to follow in making each of these
tests is provided below.
Ohmmeter Continuity Test
To perform an ohmmeter continuity test,
proceed as follows:
Disconnect the power and remove the
wires from the motor terminals.
Scrape off paint and clean a spot on
the motor-compressor shell for testing.
Figure 14-33.Testing windings for
ground with an ohmmeter.