Data Bulletin
Bulletin No. 8638PD9602R8/04
August 2004
Raleigh, NC, USA
Replaces 8638PD9602R1/98 dated 1/98
Altistart Soft Start Application
Overvoltage Protection
OVERVOLTAGE PROTECTION
Protection of a soft start from voltage transients that exceed the peak inverse
voltage rating (PIV) of the SCRs should be provided where damaging line
voltage transients are expected. The Altistart soft start is designed to
withstand normal power disturbances. The minimum PIV rating for SCRs
used on 480 VAC systems is 1200 V. However, to provide additional
durability, the SCRs used on the Altistart soft start are rated for 1800 PIV.
Further protection is provided by an RC snubber network to prevent improper
gating of the SCRs. The control module is optically isolated from the line
power to protect the microprocessor from line transients.
Metal Oxide Varistors (MOVs) can be used to prevent voltage spikes, but they
do not offer the best protection for soft starts. MOVs deteriorate over time and
when they fail, they do so in an open mode and without indication, leaving the
user with a false sense of security.
If MOV protection is provided, the most common method is to connect them
across each input phase of the soft start. This method protects the motor, not
the soft start, by offering a current path phase-to-phase. The correct
connection for SCR protection from line transients is to connect the MOVs
line-to-load. This scheme clamps the voltage drop across the SCRs when the
PIV rating of the SCR is approached. While many of our competitors offer
transient protection devices, most do not offer MOV protection for use in this
manner.
Using isolation and shorting or bypass contactors in conjunction with a soft
start provides superior protection from damaging voltage transients for the
SCRs. An isolation contactor coordinated to open when the soft start is not
in use protects the SCRs when they are most susceptible to damage. When
power is applied to the line side of the soft start and the motor is stopped, the
voltage drop across the SCR is roughly equal to the line voltage. Without an
isolation contactor, a 1000 V spike on a 480 V system could damage a typical
1400 PIV-rated SCR when the motor is idle.
To complement the isolation contactor, a shorting or bypass contactor
clamps the voltage across the SCRs to zero when the motor is at full speed.
When a shorting or bypass contactor is used, the SCRs are only susceptible
to damaging voltage spikes while accelerating or decelerating the motor. See
the Altistart Soft Start catalog for circuit diagrams and recommended
isolation and shorting contactors.
NOTE: When mechanical contactors are used with the Altistart soft start, the
mechanical wear on these contactors is minimal because these contactors
are not required to make or break the motor-locked rotor amps under normal
operation.
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Altistart® Soft Start Application
Overvoltage Protection
Schneider Electric USA
8001 Highway 64 East
Knightdale, NC 27545
1-888-SquareD (1-888-778-2733)
www.us.SquareD.com
Bulletin No. 8638PD9602R8/04
August 2004
Electrical equipment should be installed, operated, serviced, and maintained only by
qualified personnel. No responsibility is assumed by Schneider Electric for any
consequences arising out of the use of this material.
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