Using HANDS-OFF-AUTO Selectors with Your Motor Protection System

To include HANDS-OFF-AUTO support for your MPS, motor protection system, specific input programming and menu selections are required because the MPS supports multiple start sources which may cause conflicting signals. To review, the MPS is responsible for monitoring voltage, current, and temperature. It is a modular system that can also be used to monitor multiple motors given certain configurations.

Littelfuse's tech report describes why the usual MPS installation is not appropriate for HANDS-OFF-AUTO. When using a start-stop station, the start command is internally fastened by the MPS up until a STOP is dispensed. Take note that if start is sustained then the stop signal will remain active only as long as STOP is pushed. The report mentions that relay chatter will occur in the event of an OPI stop since it is detected at a 250ms rate.

Use 2-Wire Start1 or 2-Wire Start2, not Start1 or Start2. 
The HANDS-OFF-AUTO requires the use of the selection 2-Wire Start1 or 2-Wire Start2 instead of Start1 or Start2 . The motor in a 2-wire operation is not latched; therefore, it must be maintained for a motor run condition. Once the start signal is removed, the motor will stop. A record is created by MPS indicating which source caused a start. A stop command will be issue, causing a latching trip, if the start command originates from a 2-wire input.
Littelfuse's full report may be found here.

By Maximilian Teodorescu