Full Voltage Bypass Starting

Bulletin No. 8638PD9605R8/04
August 2004
Raleigh, NC, USA
Data Bulletin
Replaces 8638PD9605R1/98 dated 1/98
AltistartīŖ¨ Soft Start Application
Full-Voltage Bypass Starting
To allow for a full-voltage bypass start, an isolation contactor and bypass
contactor are required when using a soft start. The term “bypass contactor”
implies that the contactor must be rated for full-voltage starting. Because the
Altistart soft start’s internal thermal-overload protection is inactive during
full-voltage bypass operation, this option must also include a separate
thermal overload relay. An isolation contactor is required to prevent voltage
from being applied to the motor through the soft start, in the event of an
silicon controlled rectifier (SCR) failure.
The following figure shows the recommended bypass configuration. Because
the soft start is still connected to the power circuit, the bypass setup is not
intended to allow maintenance on the unit while the motor is operating. This
bypass scheme allows a full-voltage start if either:
• Full torque is required - For specific applications, full-voltage torque may
be required to start the motor during abnormal loading conditions. For
example, with applications where material jams can occur, a full-voltage
starting option may be required.
• Process is critical - If the process is critical and requires a backup in case
of soft start failure, the non-isolated bypass arrangement allows the motor
to run until the process may be shut down for controller repair.
Bypass Contactor
with Overload Relay
When used in the configuration above, full-voltage bypass operation causes
the Soft Start to detect a fault, because current is flowing through the Soft
Start’s current transformers when it does not have a run Soft Start command.
Control power must be cycled to clear this fault condition. To avoid a fault we
recommend removing control power to the soft start during full-voltage
While a bypass contactor can also serve as a shorting contactor after the
motor is at full speed, the shorting contactors recommended in the Altistart
soft starts catalog are not sized for full-voltage starting.
Altistart Soft Start Application
Power Circuit Recommendations
Schneider Electric USA
8001 Highway 64 East
Knightdale, NC 27545
1-888-SquareD (1-888-778-2733)
Bulletin No. 8638PD9605R8/04
August 2004
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consequences arising out of the use of this material.
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