High Performance Control; Low Speed Operation; High Starting Torque - Siemens 6SR41 series Product User Manual

Application and Operation

6.3 High Performance Control

6.3 High Performance Control
When applying the Harmony drives, applications requiring high starting torque or low speed
operation are considered as "High performance" control.
6.3.1

Low Speed Operation

In some applications, when stable, low speed (below 1 Hz) operation under high torque
conditions is required, an encoder may be used to provide speed feedback. Use of a shaft
encoder is recommended where the control's slip calculation block is disabled so that
encoder speed feedback is directly used as an input to the speed regulator.When an
encoder is used with the drive, the control loop type is required to be set to CLVC (for closed
loop vector control with an induction motor) or to CSMC (for closed loop vector control with a
synchronous motor). The VFD's Spinning load function should be enabled when this control
mode is enabled.
6.3.2

High Starting Torque

When OLVC or CLVC control is used, the harmony VFD can deliver approximately 85-90%
torque with 100% rated motor current. The V/Hz control mode can provide 95-100% torque
at 100% motor rated current.
This control mode is useful for obtaining high starting torque in applications with high series
resistance (> 10%). To enable this mode of operation, the following conditions should be
satisfied:
● Drive control loop type should be set to V/HZ
● Rated slip should be non-zero
● Minimum speed limit should be greater than rated slip
For long cable applications, the minimum speed limit should be set to be the same as the
percentage value of the total resistance in series with the motor. For example, if the total
series resistance (motor + cable) is about 30% of the motor base impedance (this is a very
long cable!), a minimum speed limit of 30% should be set.
The magnetization time or the Flux Ramp Rate should be set to at least twice the rotor time
constant of the motor.
Drive Operation in High Starting Torque Mode
Once the drive start is issued, the drive goes through the following stages to the minimum
speed setting, irrespective of the speed demand. (Refer to Figure 5-4.) During the initial
stages after the drive start is given, the drive operates in an open loop mode (i.e., both
speed and flux loops are initially disabled).
During the magnetization stage, the drive applies a magnetizing component equal to the no-
load current parameter. The drive then ramps up to rated slip speed while increasing the
torque component (Iqs) of current to its rated value. The drive maintains rated current for a
time duration equal to the magnetization time. During this time, the motor should produce
rated torque under locked rotor (or high stiction) conditions.
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Operating Instructions, Version AE 12/2009, A5E01454341C
Product User Manual