Stopping Time and Energy Loss or A-C Motors with D-C Braking

The d-c dynamic braking torque speed curve of a-c motors, dependent upon a magnetizing reactance which varies progressively throughout the speed range, is approximated by a discontinuous curve dependent upon the magnetizing-reactance values at standstill and top speed only. The former reactance corresponds to the full d-c excitation, while the latter is the unsaturated value. By this means, the torque speed curve is expressed by a simple mathematical relationship. The validity of this simplification is investigated by comparison with an accurate method of deducing the curve, published recently by the author, for any degree of magnetic saturation and value of the leakage reactance. Good agreement is obtained at all except the largest values of torque, which occur over a comparatively small speed range. In consequence, the stopping time and total energy loss of the motor, which are dependent upon the integration of the inverse torque throughout the speed range for an inertia load, are readily determined with considerable accuracy by the simplified theory, using derived mathematical relationships. Further, the value of secondary resistance which provides the minimum stopping time and energy loss can be readily deduced. Experimental confirmation of the theory is included.