Siemens 420 manual Open-loop/closed-loop control technique, Control

Issue 07/04 3 Functions

3.21Open-loop/closed-loop control technique

There are several open-loop/closed-loop techniques for speed and torque control for drive inverters with induction and synchronous motors. These techniques can be roughly classified as follows:

V/f characteristic control (briefly: V/f control)

Field-orientated closed-loop control technique (briefly: Vector control)

These techniques differ from one another both regarding the control quantity as also in the complexity of the technique, which in turn are obtained as a result of the requirements associated with the particular application. For basic applications (e.g. pumps and fans), to a large extent, V/f control is used. Vector control is mainly used for sophisticated applications (e.g. winders), where a good control and behavior in noisy conditions are required regarding the speed and torque. If these requirements are also present in the range from 0 to approx. 1 Hz, then the speed/torque accuracy without encoder is not sufficient. In this case, Vector control with speed feedback must be used.

NOTE

Vector control is only available for MICROMASTER 440.

3.21.1	V/f control
	Parameter range:	P1300
		P1310 – P1350
	Warnings	-
	Faults	-
	Function chart number:	FP6100

The V/f characteristic represents the simplest control technique. In this case the stator voltage of the induction motor or synchronous motor is controlled proportionally to the stator frequency. This technique has proven itself for a wide range of "basic" applications, such as

Pumps, fans Belt drives

and similar processes.

The goal of V/f control is to keep the flux Φ constant in the motor. In this case, this is proportional to the magnetizing current I∝ and the ratio between voltage V and frequency f.

Φ~ I∝ ~ V/f

The torque M, developed by induction motors, is proportional to the product (precisely the vectorial product Φ x I) of flux and current.

M ~ Φ ∗ I

In order to generate the highest possible torque from a given current, the motor must operate with a constant flux which is as high as possible. In order to keep the flux Φ constant, when frequency f changes, the voltage V must be changed in proportion so that a constant magnetizing current I∝ flows. The V/f characteristic control is derived from these basic principles.

MICROMASTER 420	Operating Instructions
6SE6400-5AA00-0BP0	143