A common misapplication that is
encountered is the improper sizing of the six motor leads between the
still very popular wye-delta reduced voltage motor starter and the
motor. This is best demonstrated by an example.
Assume that you have a 500-ton
electrical centrifugal chiller operating at 460 V, three-phase, 60
Hz, with a nameplate rating of 588 full load amps (FLA).You would
normally apply the correct factor of 125 percent required by NEC
Article 440, to arrive at the required conductor ampacity: 588 ×
1.25 = 735 ampacity for each of the three conductors.
Because there will be six conductors
between the load side of the starter and the compressor motor
terminals, the 735 ampacity is divided by two; you would select six
conductors, each having an ampacity of not less than 368 A. Referring
to NEC Article 310, Table 310-16 for insulated copper conductors at
75°C would result in the selection of 500-kcmil conductors.
This wire size is incorrect when used
between the wye-delta starter and motor terminals. The problem is
caused by a common failure to recognize that the motor may consist of
a series of single-phase windings.
To permit the transition from wye-start
to delta-run configuration, the motor is wound without internal
connections. Each end of the three internal motor windings is brought
out to a terminal, as shown in Figure 8.5.
FIGURE 8.5 Wye-to-delta internal motor
windings brought out to terminals.
The motor windings are configured as
required for either starting or running at the starter as shown in
Figure 8.6, panels a and b, respectively. In the running-delta
configuration, the field wiring from the load side of the starter to
the compressor motor terminals consists of six conductors,
electrically balancing the phases to each of the internal motor
windings as described below in Figure 8.7.
FIGURE 8.6 Wye-start, delta-run motor
winding configuration.
FIGURE 8.7 Field wiring between starter
and motor in wye-start, delta-run configuration.
Note, for example, that motor winding
T1 − T4 is connected to the line voltage across phase L1 − L2. It
should be apparent that the windings within the motor are single
phase- connected to the load side of the starter. Thus, the
interconnecting field wiring between the starter and motor must be
sized as though the motor were single-phase.
Electrical terminology simply describes
this motor as being phase-connected, and the current carried by the
interconnecting conductors as phase amps.
To correctly size the conductors
between the motor starter and the motor, therefore, it is necessary
to calculate the ampacity with the 125 percent feeder-sizing factor
required by the NEC on a single-phase basis as follows:
Ampacity per terminal conductor =
three-phase FLA × 1.25/1.73
For the example given:
Ampacity per terminal conductor = 588 ×
1.25/1.73 = 424
Thus, it is clear that the current in
the conductors between the starter and the motor on a single-phase
basis is 58 percent of the three-phase value, not 50 percent as
originally assumed, because the current in one phase of a three-phase
system in the delta-connected winding is one divided by the square
root of three due to the vector relationship.
In the original example, the conductors
were sized for a minimum ampacity of 368 A.From the NEC, 500-kcmil
copper conductors at 75°C have a maximum allowable ampacity of
380.The preceding calculation discloses that the conductors should be
selected for not less than 424 ampacity.
Referring to the NEC again, 600-kcmil
conductors have a maximum allowable ampacity of 420. In many cases,
depending upon the interpretation of the local electrical inspector,
600 kcmil would be acceptable (usually within 3 percent is
acceptable).
Five-hundred– kilocircular mil wire
would not be. Almost needless to say, the conductors supplying the
line side of the wye-delta starter are sized as conventional
three-phase motor conductors.
No comments:
Post a Comment