Motors will
run at the voltage variations already mentioned. This does not imply that such
operation will comply with industry standards of capacity temperature rise, or
normally anticipated service life.
Figure 19-3
shows general effects. Such effects are not guaranteed for specific motors.
The
temperature rise and performance characteristics of motors sealed within hermetic
compressor shells constitute a special case. These motors are cooled by return
suction gas of varying quantity and temperature. Thus, Fig. 19-3 is not
necessarily applicable to this specialized type of equipment.
The chart
shows the approximate effect of voltage variations on motor characteristics. The
reference base of voltage and frequency is understood to be that shown on the
nameplate of the motor.
Some of the
terms used in the chart are explained here.
Normal slip
= synchronous speed – the rating plate speed.
Slip in the
graph indicates the change in normal slip.
Synchronous
speeds for 60-hertz motors are:
2-pole—3600
r/min or rpm
4-pole—1800
r/min or rpm
6-pole—1200
r/min or rpm
8-pole—900
r/min or rpm
Table 19-2
indicates the voltage drop that may be anticipated for various ampere flow
rates through copper conductors of different gage size. Figure 19-2 provides
the same data in graphic form.
These data
are applicable to both single-phase and three-phase circuits. In each case, the
wire length equals twice the distance from the power distribution panel to the
appliance terminals, measured along the path of the conductors.
This is
twice the distance between B and C in Fig. 19-1, measured along the path of the
conductors. For motorized appliances, particularly those that start under
loaded conditions, the voltage at the appliance terminals should not drop more
than 10% below rating plate values unless approved by the manufacturer.
Thus, the
voltage drop permissible in the load leads must anticipate any reduction below
rated voltage that may be suffered under full load conditions at the point of
power source connection (point A in Fig. 19-1).
Troublesome
voltage losses may also occur elsewhere if electrical joints or splices are
mechanically imperfect and create unanticipated resistance. Such connections
may exist in the distribution panel, the meter socket, or even where outdoor
power drops are clamped to the feeder lines on poles.
Where there
is a wide variation between no-load voltages and operating voltage, sources of
voltage drop can be determined by taking voltmeter readings at various points
in the circuit. These points might be ahead of the meter, after the circuit
disconnect switch, at the appliance terminals, and at other locations.
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