BECKWITH ELECTRIC M-4272, Инструкция

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Introduction – 1
1–7
Referring to Figures 1-6 and 1-7, a few
generalizations can be made regarding the inertia of
motor loads, motor size, and the mix of synchronous
and induction motors.
• Inertia — The higher the inertia of the
aggregate motor loads on the motor bus,
the more slowly the motor bus frequency
will decay during the disconnected
coastdown period. That has a direct
impact on how fast the phase angle
changes. Low inertia loads will cause
the phase angle to change quickly, as
the frequency of the motor bus decays
quickly, and the slip frequency between
the motor bus and the new source quickly
increases.
• Motor Size — The larger the motor size,
the longer the time the voltage will take
to decay on an induction motor.
• Mix of Synchronous and Induction
Motors — Voltage will tend to decay
much more rapidly on a motor bus with
all induction motors. On a motor bus
with a mix of synchronous and induction
motors, the synchronous motors will
attempt to hold up the voltage during the
transfer interval.
• Loading — The higher the load on the
motors, the faster the motor bus
frequency will decay.
These parameters are key to analyzing the motor
bus transfer issue and developing schemes to
accomplish transfers that promote process continuity
while causing no damage to the motors or the
loads.
An important value to decide the viability of MBT is
the resultant volts per hertz (V/Hz) across the
breaker. The resultant V/Hz is derived from the V/
Hz vectors of the motor bus and the new source at
the instant just prior to connection and must not
exceed 1.33 V/Hz [1]. Pursuant to phase angle and
voltage, and their effect on resultant V/Hz, some
generalizations can be made:
• Phase Angle — As the phase angle
increases between the two sources,
assuming the two source voltages are
the same, the V/Hz will increase.
• Voltage — As the voltage difference
between the two sources increases,
assuming the phase angle between the
sources remains the same, the V/Hz will
increase.
0
0 10
20
30 40 50 60
20
40
60
80
100 0
-72
-144
-216
-288
-360
V
ol
tag
e M
ag
ni
tud
e (
%
)
P
has
e (
D
egr
ee
s)
Time (Cycles)
960 HP Boiler Circulating Pump
(Operating at Full Load)
Phase (not shown for T>20)
Voltage
Figure 1-7 Coastdown of Low Inertia Load
On a Large Induction Motor
0
0 10
20
30 40 50 60
20
40
60
80
100 0
-72
-144
-216
-288
-360
V
ol
tag
e M
ag
ni
tud
e (
%
)
P
has
e (
D
egr
ee
s)
Time (Cycles)
6000 HP I.D. Fan
(Operating at 25% Load Prior to Transfer)
Phase
Voltage
Magnitude
Figure 1-6 Coastdown of High Inertia Load
On a Large Synchronous Motor