Trane RTWD Installation Operation & Maintenance page 81

Unit Voltage Imbalance
Excessive voltage imbalance between the phases of a
threephase system can cause motors to overheat and
eventually fail. The maximum allowable imbalance
is 2%. Voltage imbalance is determined using the
following calculations:
% Imbalance =
[(Vx - V ave) x 100/V ave]
V ave = (V1 + V2 + V3)/3
Vx = phase with the greatest difference from V ave
(without regard to the sign)
For example, if the three measured voltages are 401,
410, and 417 volts, the average would be:
(401+410+417)/3 = 410
The percentage of the imbalance is then:
[100(410-401)/410] = 2.2%
This exceeds the maximum allowable (2%) by 0.2%.
Unit Voltage Phasing
It is important that proper rotation of the compressors
be established before the unit is started. Proper motor
rotation requires confi rmation of the electrical phase
sequence of the power supply. The motor is internally
connected for clockwise rotation with the incoming
power supply phased A, B, C.
Basically, voltages generated in each phase of a
polyphase alternator or circuit are called phase voltages.
In a three-phase circuit, three sine wave voltages are
generated, differing in phase by 120 electrical degrees.
The order in which the three voltages of a three-phase
system succeed one another is called phase sequence
or phase rotation. This is determined by the direction
of rotation of the alternator. When rotation is clockwise,
phase sequence is usually called "ABC", when
counterclockwise, "CBA".
This direction may be reversed outside the alternator
by interchanging any two of the line wires. It is this
possible interchange of wiring that makes a phase
sequence indicator necessary if the operator is to quickly
determine the phase rotation of the motor.
RLC-SVX14H-GB
Proper compressor motor electrical phasing can be
quickly determined and corrected before starting the
unit. Use a quality instrument, such as the Associated
Research Model 45 Phase Sequence Indicator.
1. Press the Stop key on the Clear Language Display.
2. Open the electrical disconnect or circuit protection
switch that provides line power to the line power
terminal block(s) in the starter panel (or to the unit
mounted disconnect).
3. Connect the phase sequence indicator leads to the
line power terminal block, as follows:
Phase Sea. Lead
Phase A
Phase B
Phase C
4. Turn power on by closing the unit supply power
fused-disconnect switch.
5. Read the phase sequence on the indicator. The
"ABC" LED on the face of the phase indicator will
glow if phase is "ABC".
6. If the "CBA" indicator glows instead, open the unit
main power disconnect and switch two line leads on
the line power terminal block(s) (or the unit mounted
disconnect). Reclose the main power disconnect and
recheck the phasing.
CAUTION
Equipment Damage!
Do not interchange any load leads that are from the unit
contactors or the motor terminals.
7. Reopen the unit disconnect and disconnect the phase
indicator.
Pre-Start Checkout
Terminal
L1
L2
L3
81