Medium Voltage Application Guide

SOFT STARTERS
Multi-motor starting
This standard method of starting several medium voltage motors is often found in the water and mining industries.
Most multi-start control systems have 2 ~ 4 motors of the same kW size.
Each motor is started and stopped from the output of an electronic motor starter. The starter is usually a soft
starter (SST), providing the utility system has the capacity to supply the maximum required current, without any
significant disturbance. A guideline for maximum required current is [4+(n-1]) x motor FLC, where n = total number
of motors in the system. If supply capacity is limited, a variable frequency drive (VFD) may be used instead of a soft
starter.
Once a motor has reached full running speed, it is fed directly from an input bus. In this mode of operation, some
form of motor protection is required for each motor.
A master controller is required to control and supervise the entire multi-start system. This can be a PLC or an
integrated part of the starter.
There are typically two modes of operation.
In Auto mode, the start and stop sequence can be preselected and the master controller handles the entire
switching procedure.
In Manual mode, the starter is disabled and DOL control of each motor is provided by manual switching of
each motor bypass circuit breaker or contactor.
The entire system relies on critical time switching of circuit breakers or contactors, which are usually fixed switching
devices. Withdrawable switching devices are often used on the starter input and output to provide physical
isolation. This allows the starter's input and output to be isolated for servicing, in the event of a fault.
NOTE
The following example shows a typical configuration. There are many different control methods
available for multi-motor starting systems.
710-12280-00A Medium Voltage Application Guide Page 61