Commercial Power Source Switching - Hitachi SJ300 Series Instruction Manual

Commercial
The commercial power source switching function is useful in systems with excessive starting
torque requirements. This feature permits the motor to be started "across the line," sometimes
Power Source
called a bypass configuration. After the motor is running, the inverter takes over to control the
Switching
speed. This feature can eliminate the need to oversize the inverter, reducing cost. However,
additional hardware such as magnetic contactors will be required to realize this function. For
example, a system may require 55KW to start, but only 15KW to run at constant speed. There-
fore, a 15KW rated inverter would be sufficient when using the commercial power source
switching.
The following block diagram shows an inverter system with bypass capability. When starting
the motor directly across the line, relay contacts Mg2 are closed, and Mg1 and Mg3 are open.
This is the bypass configuration, since the inverter is isolated from the power source and motor.
Then Mg1 contacts close about 0.5 to 1 second after that, supplying power to the inverter.
Power source, 3-phase
MCCB
L1
L2
L3
Switching to inverter control occurs after the motor is running at full speed. First, Mg2 relay
contacts open. Then about 0.5 to 1 seconds later, relay Mg3 contacts close, connecting the
inverter to the motor. The following timing diagram shows the event sequence:
Mg2
GFI
Mg1
Mg1
Mg2
Mg3
FW
Set to 0.5 to 1 sec. typical
[CS]
B003 (Retry wait time
Inverter
before motor restart)
output
Frequency matching
Mg3
R
U
SJ300
S V
T W
R0
T0
AL1
FW
[RV]
AL0
[CS]
AL2
CM1
H
O
L
Mg2/Mg3 delay time 0.5 to 1 sec.
Normal operation
4–23
SJ300 Inverter
Thermal
switch
Motor