Trane R Series, Installation Operation & Maintenance

Page: 124 / 188

124 RLC-SVX07A-EN
Pre-Start Checkout
Voltage to the unit must meet the criteria given in Table 16 . Measure each leg
of the supply voltage at the unit's main power fused-disconnect. If the
measured voltage on any leg is not within specified range, notify the supplier
of the power and correct the situation before operating the unit.
CAUTION
Equipment Damage!
Inadequate voltage to the unit may cause control components to
malfunction and shorten the life of relay contact, compressor
motors and contactors.
Unit Voltage Imbalance
Excessive voltage imbalance between the phases of a three-phase system
can cause motors to overheat and eventually fail. The maximum allowable
imbalance is 2 percent. Voltage imbalance is determined using the following
calculations:
= phase with greatest difference from V
ave
(without regard to sign)
For example, if the three measured voltages are 221, 230, and 227 volts, the
average would be:
The percentage of imbalance is then:
This exceeds the maximum allowable (2%) by 0.2 percent.
Unit Voltage Phasing
It is important that proper rotation of the compressors be established before
the unit is started. Proper motor rotation requires confirmation 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”.
% Imbalance
l
x
l
ave
–
( )
x 100
l
ave
-------------------------------------
=
V
ave
V
1
V
2
V
3
+ +
( )
3
--------------------------------------
=
1V
x
221 230 227 + +
3
---------------------------------------
226 =
100 221 226 –
( )
226
--------------------------------------
2.2% =