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Application – Single-Phase Motors
Buck-Boost transformers are power transformers, not control transformers. They may also be used to lower voltage
when the available power supply voltage is too high.
When the available power supply voltage is not within
the proper range, a buck-boost transformer is often
used to adjust voltage to match the motor. The most
common usage on submersible motors is boosting a
208 volt supply to use a standard 230 volt single-phase
submersible motor and control. While tables to give a
wide range of voltage boost or buck are published by
transformer manufacturers, the following table shows
Franklin’s recommendations. The table, based on
boosting the voltage 10%, shows the minimum rated
transformer kVA needed and the common
standard transformer kVA.
(1) Do not add running capacitors to 1/3 through 1 hp control boxes, which use solid state switches or QD relays.
Adding capacitors will cause switch failure. If the control box is converted to use a voltage relay, the specifi ed
running capacitance can be added.
Table 15A Buck-Boost Transformer Sizing
Table 15 Auxiliary Capacitor Sizing
MOTOR RATING
NORMAL RUNNING
CAPACITOR(S)
AUXILIARY RUNNING CAPACITORS FOR
NOISE REDUCTION
S.F. AMPS WITH RUN CAP
HP VOLTS MFD MFD MIN VOLTS FRANKLIN PART YELLOW BLACK RED
1/2 115
0 60(1) 370 TWO 155327101 8.4 7.0 4.0
1/2
230
0 15(1) 370 ONE 155328101 4.2 3.5 2.0
3/4
0 20(1) 370 ONE 155328103 5.8 5.0 2.5
1
0 25(1) 370
ONE EA. 155328101
155328102
7.1 5.6 3.4
1.5
10 20 370 ONE 155328103 9.3 7.5 4.4
2
20 10 370 ONE 155328102 11.2 9.2 3.8
3
45 NONE 370 17.0 12.6 6.0
5
80 NONE 370 27.5 19.1 10.8
7.5
45 45 370
ONE EA. 155327101
155328101
37.0 32.0 11.3
10
70 30 370 ONE 155327101 49.0 42.0 13.0
15
135 NONE 75.0 62.5 16.9
MOTOR HP 1/3 1/2 3/4 1 1.5 2 3 5 7.5 10 15
LOAD KVA
1.02 1.36 1.84 2.21 2.65 3.04 3.91 6.33 9.66 11.70 16.60
MINIMUM XFMR KVA
0.11 0.14 0.19 0.22 0.27 0.31 0.40 0.64 0.97 1.20 1.70
STANDARD XFMR KVA
0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.75 1.00 1.50 2.00
Auxiliary Running Capacitors
Buck-Boost Transformers
Added capacitors must be connected across “Red” and
“Black” control box terminals, in parallel with any existing
running capacitors. The additional capacitor(s) should
be mounted in an auxiliary box. The values of additional
running capacitors most likely to reduce noise are given
below. The tabulation gives the max S.F. amps normally
in each lead with the added capacitor.
Although motor amps decrease when auxiliary
run capacitance is added, the load on the motor
does not. If a motor is overloaded with normal
capacitance, it still will be overloaded with auxiliary
run capacitance, even though motor amps may be
within nameplate values.
15