Siemens SINAMICS G130 Engineering Manual page 32

Fundamental Principles and System Description
Engineering Information
Load-balance control
There are various methods by which the load or the torque can be balanced between different drives. The simplest
method is to transfer the torque setpoint of the master drive to the slave drive and to deactivate the speed controller
on the slave drive. This is possible, however, only if it can be safely assumed that the mechanical coupling between
the drives can never be interrupted. But this is rarely true because the mechanical coupling is generally created by
pressure (with calenders, for example) or by other connecting elements such as belts, wires or material conveyors
(webs). It therefore has to be assumed that this coupling can be interrupted in an uncontrolled manner when certain
boundary conditions are fulfilled. For this reason, it must be possible to maintain the drive in a safe state if the
coupling were to be interrupted. The drive must never be allowed to accelerate in an uncontrolled manner and
potentially reach or even exceed critical speeds. The best way to safeguard against this risk is to keep the speed
controller of the slave drive activated.
Simple load-balance control can already be implemented using BICO technology or the technology controller. The
load balancing controls described below can be created using DCC (Drive Control Chart) and are available as a
standard application "SINAMICS DCC load distribution" for SINAMICS S.
Torque coupling
A proportional-plus-integral-action controller is used to operate the master converter as a speed controller while a
simple proportional-action controller, i.e. without the integral-action component, is used on the slave converter. The
integral-action component of the master's speed controller is utilized as an additional torque in the slave. A drive-
specific torque pre-control can optionally be added to the torque value of the master. In this operating mode, the
torque setpoint is generated in part by the master and in part by the speed controller of the slave. The load balance
can be set with a load factor. Both drives have their own torque pre-control and both receive the same speed
setpoint.
Overload with torque limit
The torque setpoint of the master is added (preferably without pre-control) to the optional pre-control signal of the
slave. This signal is then applied as a positive torque limit and the speed setpoint is increased by an overload value
in order to ensure that the controller always operates at the upper torque limit unless the actual speed is higher than
the basic setpoint or is at the negative torque limit (if the master reduces the speed, i.e. the master torque setpoint is
negative). Should the connection between the two converters be interrupted for any reason, the slave cannot
accelerate to a speed higher than the speed defined by the overload factor.
A very precise model is required in order to calculate the correct torque values, particularly for the acceleration pre-
control. The master converter might otherwise become overloaded as a result of temporary load fluctuations or
acceleration.
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG