The Master slave configuration is basically used for load sharing purposes when 2 identical motors connected to two different drives are coupled to a common load e.g. pinch rolls in Cold rolling mills as well as some transfer cars.
The underlying concept is One drive is defined as the Master and is configured for running in the Speed Control mode whereas the other defined as Slave is configured for running in Torque control mode.
The external speed reference go to the Drive defined as the Master and from within the drive unit, the output of the speed controller section is sent in as the external Torque/Current reference fo the Slave.The Slave drive has only current controller section and no speed controller section.
It means the current controller input for both the Master and the Slave are identical and the Slave is not doing any Speed Control but just doing Torque/Current Control.
It is advisable to have speed feedback from the motors connected to the respective drives so that in case overspeeding of the motor connected to the Slave drive , the drive can be safely tripped at Overspeed Fault.This condition arises ,when the mechanical coupling between the two motors breaks off then the motor connected to the Slave drive will tend to run at real high speed because it is in current control mode.
Normally we can use tacho feedback or Overspeed switch in the hardware circuit to avoid such conditions.