Controls; Overview Of Speed And Torque Control; Control Principle - Danfoss VLT 380-500 V Design Manual

Controls

14 Controls

14.1 Overview of Speed and Torque Control

The frequency converter can control either the speed or the torque on the motor shaft. Setting parameter 1-00 Configuration
Mode determines the type of control.
Speed control
There are 2 types of speed control:
•
Open loop does not require any feedback from motor (sensorless).
•
Closed loop PID requires a speed feedback to an input. A properly optimized speed closed-loop control has higher
accuracy than a speed open-loop control. The speed control selects which input to use as speed PID feedback in
parameter 7-00 Speed PID Feedback Source.
Torque control
The torque control function is used in applications where the torque on the motor output shaft is controlling the
application as tension control. Torque control is selected in parameter 1-00 Configuration Mode, either in [4] VVC+ open loop
or [2] Flux control closed loop with motor speed feedback. Torque setting is done by setting an analog, digital, or bus
controlled reference. The maximum speed limit factor is set in parameter 4-21 Speed Limit Factor Source. When running
torque control, Danfoss recommends to make a full AMA procedure since the correct motor data is essential for optimal
performance.
•
Closed loop in flux mode with encoder feedback offers superior performance in all 4 quadrants and at all motor
speeds.
•
Open loop in VVC
Open-loop torque function works only in 1 speed direction. The torque is calculated from the current
measurement within the frequency converter. See chapter 17 Application Examples.
Speed/torque reference
The reference to these controls can either be a single reference or be the sum of various references including relatively
scaled references. For more information on handling of reference, see chapter 15 Handling of References.

14.2 Control Principle

A frequency converter rectifies AC voltage from mains into DC voltage, after which this DC voltage is converted into AC
power with a variable amplitude and frequency.
The motor is supplied with variable voltage/current and frequency, which enables infinitely variable speed control of 3-
14 14
phase, standard AC motors and permanent magnet synchronous motors.
The control terminals provide for wiring feedback, reference, and other input signals to the following:
•
Frequency converter
•
Output of frequency converter status and fault conditions
•
Relays to operate auxiliary equipment
•
Serial communication interface
Control terminals are programmable for various functions by selecting parameter options described in the main or quick
menus. Most control wiring is customer supplied unless factory ordered. A 24 V DC supply is also provided for use with the
frequency converter control inputs and outputs.
Table 14.1 describes the functions of the control terminals. Many of these terminals have multiple functions determined by
parameter settings. Some options provide more terminals. See chapter 10.5 Motor Terminal Connections for terminal locations.
128
®
VLT Parallel Drive Modules
+
mode. The function is used in mechanically robust applications, but its accuracy is limited.
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