EVALUATION OF THE ELECTROMAGNETIC TIME-CONSTANT OF SCIM ROTOR BASED UPON ITS RATED SLIP

Abstract

Based upon data of experimental researches with VFD controlled asynchronous motors with
rated power 11-315 kW, number of poles p=4-10, produced by leading manufacturers, there was established
the relationship between rated slip s of the motor and the rotor electromagnetic time-constant T2. As a result
of the researches there was received the analytical expression that uniquely relates the estimate of the rotor
electromagnetic time-constant of the asynchronous motor with its rated slip, and this expression depends
neither the motor power nor the number of poles. The relationship found is valid for both squirrel-cage and
wound-rotor induction motors when the rotor is shorted. Its approximation of the power function is
characterized by the reliability of estimate R2 = 0,98 for the whole range of considered values of the nominal
slip (from 0.0067 to 0.06). Due to the fact that the calculated values of T2 from the approximated expression
made for the lower part of the slip range differ from the measured values of the constant rotor with a
difference of up to 25%, the entire range of nominal slip s is divided into 2 sections: 0.0067 ≤ s ≤ 0.0107
(first) and 0.0107≤ s ≤0.06 (second). At both sections there is performed approximation of T2=f(s) with
power functions characterized by the reliability of estimate R2 = 0.98 at the first and R2 = 0.9736 at the
second section. Calculations of the T2 values performed for both sections showed that the calculated values
differ from the experimentally determined values by no more than 3%. The determination of the delay for the
successful automatic restart of the asynchronous squirrel-cage motor, feeding directly from the network,
necessary to attenuate its EMF to a level of 3-5% of the mains voltage after a sudden loss of line supply and
its recovery with automatic load transfer, is considered. There are defined preconditions under which it is
possible to implement VFD for the mechanism of the flying shear drum.