Starting performance of synchronous motors with solid salient poles

Abstract

The asynchronous performance of a synchronous motor, at a given slip, may be estimated from the 2-axis operational-admittance frequency functions Y_d(jsω₀), Y_q(jsω₀). The functions are commonly depicted as frequency-response loci. The frequency-response loci of a laminated-pole motor are shown to be analogous to the traditional induction-motor circle diagram. An accurate theoretical method is given to determine the asynchronous performance from the 2-axis loci, allowing for the effect of armature resistance. In addition, an approximate graphical method is given for the simplified condition when armature resistance is neglected.New equivalent circuits for the solid-pole motor are derived. The new circuits allow for the distribution of flux entering the rotor surface, and for the possibility of complete pole-tip saturation. Impedances, representing the parts of the magnetic circuit containing solid iron, are based on the rectangular magnetisation characteristic, and therefore have a magnitude, determined by the voltage across them, and a constant angle of 26.6°. The impedances are inserted into the equivalent circuits and the operational admittances are calculated by an iterative method. It is shown that the angle of the single effective ‘solid iron’ rotor impedance is found to lie between 26.6° and 45°, depending on the rotor frequency and flux. The method is also applicable to machines in which adjacent pole shoes are connected by end rings.Comparisons are shown between calculated and measured operational-admittance loci and between calculated and measured starting-performance characteristics for the solid-salient-pole micromachine at Imperial College, London, and for ten large solid-salient-pole machines of widely different dimensions and numbers of poles.A method of measuring, without attenuation, the component of oscillating starting torque of a synchronous motor, by measuring the total instantaneous input power, is demonstrated.