SECTION
DRY-TYPE DISTRIBUTION TRANSFORMERS
ACME ELECTRIC
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9
one low voltage terminal unconnected. When the transformer
is excited, the resultant voltage appearing across a
voltmeter will be the sum of the high and low voltage
windings. This is useful when connecting single phase
transformers in parallel for three phase operations. Polarity is a
term used only with single phase transformers.
30. What is exciting current?
Exciting current, when
used in connection with transformers, is the current or amperes
required for excitation. The exciting current on most lighting
and power transformers varies from approximately 10% on
small sizes of about 1 kVA and smaller to approximately .5%
to 4% on larger sizes of 750 kVA. The exciting current is made
up of two components, one of which is a real component and
is in the form of losses or referred to as no load watts; the other
is in the form of reactive power and is referred to as kVAR.
31. Will a transformer change Three Phase to single phase?
A transformer will not act as a phase changing device when
attempting to change three phase to single phase. There
is no way that a transformer will take three phase in and
deliver single phase out while at the same time presenting
a balanced load to the three phase supply system. There
are, however, circuits available to change three phase to two
phase or vice versa using standard dual wound transformers.
Please contact the factory for two phase applications.
32.Can air cooled transformers be applied to motor loads?
This is an excellent application for air cooled transformers.
Even though the inrush or starting current is five to seven
times normal running current, the resultant lower voltage
caused by this momentary overloading is actually beneficial
in that a cushioning effect on motor starting is the result. The
tables on Pages 11 and 12 illustrate some typical transformer
requirements for use with motor applications.
33. How is an Acme Drive Isolation Transformer (DIT)
different than a General Purpose Tranformer?
DITs, as the name implies, are designed to be used with
motor drives (AC and DC) and to provide isolation from the
service line. They are specifically designed to withstand the
“short circuit like”duty imposed by the firing of the thyristors.
Harmonics generated by drives create added loads on the
transformer. Therefore, it is important that a transformer
of equal or greater kVA to that recommended by the drive
manufacturer be installed for a particular motor application.
34. How are transformers sized to operate Three Phase
induction type squirrel cage motors?
The minimum
transformer kVA rating required to operate a motor is
calculated as follows:
Minimum Transformer kVA =
Running Load Amperes x 1.73
x Motor Operating Voltage
1000
NOTE: If motor is to be started more than once per hour add
20% additional kVA.
Care should be exercised in sizing a transformer for an
induction type squirrel cage motor as when it is started,
the lock rotor amperage is approximately 5 to 7 times the
running load amperage. This severe starting overload will
result in a drop of the transformer output voltage. When the
voltage is low the torque and the horsepower of the motor
will drop proportionately to the square of the voltage. For
example: If the voltage were to drop to 70% of nominal, then
motor horsepower and torque would drop to 70% squared
or 49% of the motor nameplate rating.
If the motor is used for starting a high torque load, the motor
may stay at approximately 50% of normal running speed as
illustrated by the graph below:
The underlying problem is low voltage at the motor terminals.
If the ampere rating of the motor and transformer overcurrent
device falls within the motor’s 50% RPM draw requirements,
a problem is likely to develop. The overcurrent device may not
open under intermediate motor ampere loading conditions.
Overheating of the motor and/or transformer would occur,
possibly causing failure of either component.
This condition is more pronounced when one transformer is
used to power one motor and the running amperes of the
motor is in the vicinity of the full load ampere rating of the
transformer. The following precautions should be followed:
(1)
When one transformer is used to operate one motor, the
running amperes of the motor should not exceed 65% of
the transformer’s full load ampere rating.
(2)
If several motors are being operated from one transformer,
avoid having all motors start at the same time. If this is
impractical, then size the transformer so that the total
running current does not exceed 65% of the transformer’s
full load ampere rating.
35. Why are Small Distribution Transformers not used for
Industrial Control Applications?
Industrial control equipment demands a momentary overload
capacity of three to eight times normal capacity. This is most
prevalent in solenoid or magnetic contactor applications where
inrush currents can be three to eight times as high as normal
sealed or holding currents but still maintain normal voltage at
this momentary overloaded condition. Distribution transformers
are designed for good regulation up to 100 percent loading,
but their output voltage will drop rapidly on momentary
overloads of this type making them unsuitable for high inrush
applications.
Industrial control transformers are designed especially
for maintaining a high degree of regulation even at eight
times normal load.This results in a larger and generally more
expensive transformer. For a complete listing of ACME
industrial control transformers, refer to Section V.
50 100 150 200 250
20
40
60
80
100
SPEED (PERCENT OF
SYNCHROUS SPEED)
TORQUE (PERCENT OF FULL LOAD TORQUE)
SPEED vs TORQUE FOR A TYPICAL THREE PHASE
INDUCTION TYPE SQUIRREL CAGE MOTOR
STALL
ZONE
