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. 1 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. © 1996–2004 Schneider Electric All Rights Reserved