Siemens EK 726 Series Manuel de service télécharger pdf (Page 17)

TECHNICAL DESCRIPTION

MICROMASTER

MICROMASTER Vector

MIDIMASTER Vector

SIEMENS DA 64 – 2002/2003

2/3 Version D

2.1.5 Vector control principles

What is vector control?

This is most easily explained by comparing with a DC motor.

In a DC motor, the magnetic field is

separately wound, so that the

armature current (torque) and field

current (flux) can be controlled

independently of one another.

)

If the currents, which generate the

flux and torque, are separately

controlled, the output is optimized,

i.e. torque at zero speed, fast

response to load changes etc.

In an AC motor, the stator winding

currents define the flux and torque

so that it is extremely difficult to

separately control these parameters.

The current cannot be separately

controlled so that the magnitude

and phase – "the Vector" – of the

current must be controlled.

drive inverter

AC motor

Encoder

Load

Supply

Position

feedback

In order to control (closed-loop) the torque and flux in an AC motor, the magnitude

and phase of the stator winding current must be controlled, i.e. the vector quantity.

In order to control the phase, referred to the rotor, the rotor position must be

known. This means that for a complete vector control, an encoder

must

be used,

which signals the drive inverter the exact rotor position.

2.1.6 Sensorless vector control

However, for many applications, it is neither necessary to

use a pulse encoder nor is this justified for cost reasons.

If a drive inverter is to simulate the characteristics of a pulse

encoder, the software algorithm must precisely calculate the

rotor position and motor velocity. This is realized by

mathematically modeling the fundamental properties of the

motor.

To do this, the inverter must:

· Monitor the output voltage and output current extremely

accurately.

· Calculate the motor parameters (rotor, stator resistance,

leakage inductance, etc.)

· Precisely simulate the thermal motor properties.

· Adapt the motor parameters, taking into account the

particular operating conditions.

· Carry-out mathematical calculations extremely quickly.

This has been realized using an ASIC which Siemens

developed:

· The so-called Flash Floating Point Processor (F²P²; Fast

Floating Point Processor).

Siemens, who pioneered this technology, has, for the first

time, integrated an almost closed-loop vector control without

pulse encoder in a standard product. This has been achieved

by using the Flash Floating Point Processor, explained

above, which can execute the millions of computations per

second in order to achieve the consistent high performance.

Thanks to this technology, a torque rise to 150% or more for

0.5 Hz and to over 200% for 2.5 Hz is obtained. A

consistently high performance is guaranteed over the

complete temperature range by using a model to adapt the

motor temperature.

The complete MICRO/MIDIMASTER Vector series has an

overload capability of 200% for 3 seconds. This means that

this drive inverter is especially suited for difficult applications,

for example cranes and lifts.

It is not necessary to compute the motor constants, as this is

realized automatically so that the user only has to set a gain

factor to adjust the drive inverter to a specific system inertia;

however, in many cases it isn't even necessary to change the

standard values entered in the factory.

2.1.7 Flash Floating Point Processor

The sensorless vector control is an extremely sophisticated

real-time control (closed-loop), which generally uses DSP or

RISC processors or multiple processors. The Siemens

solution relieves the microprocessor of time-consuming

routine tasks and the floating-point functions are

implemented in an customized ASIC. Control algorithms are

precisely implemented thanks to the floating-point function

without requiring ongoing re-scaling. Arithmetic overflows are

thus avoided, guaranteeing a consistent high accuracy. The

overall result is a reliable product with repeatable, dynamic

performance. The floating-point processor is implemented

using entirely combinatorial logic and achieves a

performance equivalent to 3 Mflops. The algorithm, used in

the MICRO/MIDIMASTER Vector is practically the same as

the algorithm that is used in the widely accepted

MASTERDRIVE series.

2.1.8 Benefits of sensorless vector control

· Excellent speed control with integrated slip

compensation.

· High torque at low speeds without any excessive boost

(breakaway torque).

· Lower losses, higher efficiency.

· Higher dynamic performance – improved response to

loads of different magnitudes.

· Stable operation for large motors.

· Improved performance at the current limit with improved

slip compensation.

2.1.9 Vector operating range

1 2 ... 12 13 14 15 16 17 18 19 20 21 22 ... 119 120

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Siemens EK 726 Series Manuel de service Page 17

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