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Figure 4-11.—Polarity tests. | Up
Construction Electrician Intermediate - Electricity handbook for electricians | Next
Figure 4-15.—Transformer size calculations. |
It is apparent that when the voltage indicated on the
voltmeter is greater than the impressed voltage. it must
be the sum of the primary and the secondary voltages:
and the direction of the two windings must be opposite.
as shown in figure 4-12. Likewise, when the voltage
read on the voltmeter is less than the impressed voltage,
the voltage must be the difference of the primary and
secondary voltages, as shown in figure 4-13. When the
terminal markings are arranged in the same numerical
order, H1H2 and X1X2 or H2H1 and X2X1. on each
side of the transformer, the polarity of each winding is
the same (subtractive). If either is in reverse order,
H2H1 and X1X2 or H1H2 and X2X1. their polarities
are opposite (additive). The nameplate of a transformer
should always indicate the polarity of the transformer.
Additive polarity is standard for all single-phase
distribution transformers 200 kVA and below having
high-voltage ratings of 9.000 volts and below.
Subtractive polarity is standard for all single-phase
distribution transformers above 200 kVA irrespective
of the voltage rating.
Subtractive polarity is standard for all single-phase
transformers 200 kVA and below having high-voltage
ratings above 9.000 volts.
Figure 4-12.—Polarity markings and directions of voltages
when polarity is additive.
Figure 4-13.—Polarity markings and directions of voltages
when polarity is subtractive.
Standard low-voltage terminal designations are
shown in figure 4-14.
CAUTION
When you are making such tests. voltage must
not be applied across the secondary side of the
transformer because the primary voltage
would then be equal to the applied secondary
voltage multiplied by the transformer turns
ratio. This voltage would be dangerously high
to personnel and would damage the voltmeter.
Some important transformer installation rules are
listed below.
1. One or more transformers may be hung on a
single pole if the weight does not exceed the safe
Figure 4-14.—Standard connections for low-voltage
distribution transformers.
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