Equivalent scheme of AM

Static mechanical characteristics of asynchronous motor (AM)

Asynchronous (induction) electric motors (especially three-phased) have the wide application among all electric motors. It is specified by simple construction, high reliability of operation, comparatively low cost and better mass-dimensional characteristics. It has, for example, active material costs on manufacturing in 1.5-2 times less than in DCM. Asynchronous motor has not commutator and in many case for its supplying converting devices are not needed – it is supplied directly from the main of alternative current of industrial frequency.

Asynchronous motors, as DCM, have invertibility property (can operates both in motor and generator modes) and can realize all three known methods of electric braking:

- recuperative;

- counter switching;

- dynamic.

The certain limits of sliding change are respect to every mentioned above modes.

Particularly the asynchronous adjustable electric drive with vector control on a base of modern static converters with microprocessors controllers is widely spread. Such a drive of a 3^rd generation in Ukraine is created in Zaporozhye, Kharkov and other industrial centers. The famous foreign plants create such a drive, this is firstly: SIEMENS, ALAN BRENDLI and others.

The basic method of analysis of AM stationary modes is using of equivalent circuits.

Taking into account that three-phase AM has electrical and magnetic phase symmetry, equivalent schemes are constructed on one motor phase with further connection of winding phase of stator and rotor in star or triangle.

The most convenient for operation analysis of AM is so-called L-shaped equivalent scheme with magnetizing contour, which taking out to the terminals of primary circuit.

a – T-shaped equivalent scheme;

б – L-shaped equivalent scheme.

AM is a little differ from transformer by physical processes, particularly when AM in the modes with stationary rotors, but T-shaped equivalent scheme of transformer is better then L-shaped. This explained by that fact that the electric energy incomes into transformer in outcomes from it as We, but in AM the electric energy We is incomes in it and consumed therein (transforming into mechanical one).

For obtaining of analytic expression of AM mechanical characteristic with certain approximation it should to use the simplified L-shaped equivalent scheme of replacement of AM one phase in such a view

Figure 3.32 – Computational L-shaped equivalent scheme

This scheme is known from the course of electric machines, let`s remind only some peculiarities.

3.10.1.1 Parameters of equivalent scheme:

- primary phase voltage (supply of stator phase);

- phase current of stator;

- reduced phase current of a stator;

- primary leakage reactance;

- reduced secondary leakage reactance;

- primary active resistance;

- reduced secondary active resistance;

- magnetizing current;

- reactance of magnetizing contour;

- active resistance of magnetizing contour;

- AM sliding,

where - synchronous angular velocity of a motor (Si-system);

- synchronous rotation speed of a motor (in practical unit system).

3.10.1.2 AM equivalent scheme is the AM mathematical model in which the mechanical, magnetic and electric processes of a real motor are equivalent only as electrical processes, that is real magnetic system, mechanical part and electrical stator and rotor windings non-connected with each other is only given as electrical circuit:

- magnetic system – magnetizing contour of a scheme (, , );

- rotor and stator windings are connected electrically, to this effect all parameters of one of a windings (for example, rotor winding) are reduced to parameters of a second winding (stator);

- mechanical energy of a motor and character of its changing is compensated on electrical power losses on non-linear circuit resistance .

In this case there is no need to execute the complex calculations (of electrical, magnetic and mechanical processes), but it is enough to execute the calculations on a base of its model (equivalent scheme), that is to calculate only the electrical circuit, applying the law of electrical contour (Kirchhoff law).The calculation results are being transferred on a real motor, applying the same reduction parameters, that were set when transforming real motor to equivalent scheme.

Дата добавления: 2014-01-05