Abstract: INTRODUCTION The asynchronous motor with a squirrel

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.

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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.