Abstract: The paper studies some aspects regarding

asynchronous motor electric drives. In connection with these categories of electric

drives, it is proposed to develop a modern electric drive scheme, based on a

converter made with the I.G.C.T technology. With such an approach, we aim to

increase the functional parameters of the electric drives so that we can

achieve a reduction of the electricity consumption and a higher reliability of

the installation.

Key words: Electric drives, asynchronous motor, automatic adjustment, converter, I.G.C.T. technology, power losses.

1. INTRODUCTION

The asynchronous motor with a squirrel cage rotor are

frequently used in electrical drives. The operation of the three-phase

asynchronous motor is based on the existence of a spinning magnetic field in

the machine’s air gap created by the stator windings powered by a three-phase

voltage system.

The rotation speed called synchronous

speed is determined by the stator current frequency, 5:

(1)

where:

f1

– frequency of stator currents;

p – number

of pairs of poles.

Defines

the sliding:

(2)

Frequency of rotor

currents:

(3)

The

electromagnetic torque will be, 5:

(7)

where:

and the total leakage reactance is:

The

critical torque is given by the equation:

(8)

The critical slip is:

(9)

From

the relation (3) result the speed adjustment methods: variation of rotor

circuit resistance, sliding variation, variation of supply voltage, frequency

supply voltage variation, etc. 5.

The most convenient and cheap method would be to

adjust the supply voltage. A very good method for speed adjustment a.m. is the variation

of the frequency of the supply voltage correlated with the variation of its

amplitude, U/f = const.

The induction motors with squirrel cage rotor are

classified according to IEC 60034-30:2008 in three efficiency classes:

a. Standard Efficiency IE1;

b. High Efficiency IE2;

c. Premium Efficiency IE3.

In the conductors of the stator winding appear

principal and supplementary losses. Starting of T equivalent scheme (fig1.1),

parameters are reported to the nominal impedance, and are expressed for slide s

through the below relations: so noting r1 – the stator resistance; r2

– the rotor resistance (reported to stator); r0 – the iron losses

resistance; x1 – the stator leakage reactance; x2 – the

rotor leakage reactance and x0 – the magnetization reactance; pw1- Joule power losses in

stator; pw2 – Joule power

losses in reported rotor; p0

– the iron losses;

pmv – ct.,

the mechanical and ventilation losses, 4.

Fig. 1.1. Asynchronous motor T scheme

(11)

(12)

(13)

Where:

(14)

(15)

(16)

The applied voltage is equal with 1 in the equivalent

electrical scheme, written in the relative units, and the current is i1(sn)

= 1 and the power p2(s) = 1 which involve the equality (9). The

equation solution drives the load of motor to a nominal load, 4.

The motor efficiency and the power factor can be

determined:

(17)

(18)

(19)

(20)

Usually, the asynchronous motor with a squirrel cage

rotor requires relatively big starting moments and low starting currents. This

requires a certain shape of the rotor notch, which must have a pronounced

refutation of the current from the rotor bar, in order to increase the apparent

resistance, 4.

The supplementary losses in the core appear both at no

load and full load status of the machine. According to the literature, the

supplementary losses can reach up to 8-25% from the total losses.