BEHAVIOUR OF SCHRAGE MOTOR UNDER ABNORMAL CONDITIONS OF OPERATION

Abstract

In this thesis, a systematic and detailed analysis of the behaviour of a Schrage motor under the following abnormal conditions of operation is described: (i) Supply voltage sinusoidal but unbalanced, (ii) Single phase supply, (iii) Supply balanced but injected voltage to secondary unbalanced, and (iv) Non-sinusoidal but periodic supply. As a prelude to subsequent analysis, the steady state operation of the motor under balanced conditions has been investigated using quaderature component approach (Two axis method). Through the use of appro priate connection matrices and a few derived parameters of time constant nature, comprehensive expressions for performance equations have been obtained. The resultant expressions are explicit functions of slip, brush separa tion and brush axis shift, and brings out easily, the dependence of a desired quantity on any operational adjustment. Methods are given to determine experimentally all the machine parameters. The method of symmetrical components is used to analyse the performance with unbalanced supply voltages. The nature and variation of positive, negative and zero sequence impedances for various mode of operations have been discussed. The technique of power flow concept has been used to provide physical interpretation of the behaviour of the motor under different operating condi tions. It is found that the negative sequence operation is characterized with many abnormalities compared to an induction motor and depends upon the brush adjustment. In the light of the aforesaid investigations, a criterion has been suggested to predetermine the allowable output of aSchrage motor under unbalanced supply voltage condi tions. The approach is based on the concept of conduction coefficient for thermal considerations. The commutation aspect is also explored to find its possible contribution to limit the output of the motor. It has been shown that the commutation of a Schrage motor deteriorates under voltage unbalance but it is the heating of the secondary winding which restricts the output. It is found that this motor is capable of sustaining more severe unbalances with normal brushes than with crossed brushes. The generalized performance equations of a Schrage motor reveal an interesting quality of the motor, i.e. it may develop starting torque with single phase supply under favourable operational adjustment of brush separa tion and axis shift. A detailed mathematical analysis is carried out for single phase operation. The speed and direction of motor rotation with single phase supply or on occurrence of single phasing while running with 3-phase supply, depends upon the combination of brush separation, axis shift and load torque. The starting behaviour is similar to that of a single phase repulsion motor but running characteristics conform to shunt nature. The experimental results corroborate the theoretical deductions. If the voltages injected to three phases of secon dary through commutator are unequal due to faulty brush gear settings, the secondary circuit becomes a source of negative and zero-sequence voltages. Amethod to analyse secondary unbalance has been outlined. The equa tions obtained are, however, not amenable to usual method of solution and physical interpretation. It is shown that the behaviour of a Schrage motor supplied with non-sinusoidal voltages may be predicted with the help of performance equations deduced earlier. The machine with normal brush settings behave similar to an induction motor, but the effect of harmonics on torque in this operating condition/ expressed as percentage of fundamental voltage torque, increases with increase of brush separation. For crossed brush setting, the effect is more pronounced but once more it is a function of brush separation. The copper losses with non-sinusoidal supply are less than with purely sinusoidal supply of the same r.m.s. value but it is shown that there is little difference in overall heating. The entire analysis is general, and the induction motor performance under the above stipulated conditions may be deduced as a boundary value case.