The Repulsion Start Induction Motor

To be presented at the 314JΔ Meeting of the American Institute of Electrical Engineers, St. Louis, Mo., October 19, 1915. Copyright 1915. By A. I. E. E. {.Subject to final revision for the Transactions.) THE REPULSION START INDUCTION MOTOR BY JAMES L. HAMILTON ABSTRACT OF PAPER The repulsion start induction motor for single-phase current has come into considerable prominence during the last 10 or 15 years as is fully attested by the large number of motors of this type now in operation, and by the increasing demand and pro duction. While there is considerable literature available on the subject of repulsion motors both compensated and non- compensated and on induction motors, there is comparatively little information available on the repulsion start induction motor which is really a combination of two distinct types of motors. The objects of this paper are: 1. To set forth the general characteristics of this type of motor and compare them with similar characteristics of direct- current motors and other alternating-current motors. 2. To outline a definite and commercially practicable method of studying the electrical design of existing motors and of pre determining the electrical design of new or proposed motors. 3. To discuss the mechanical design and construction. 1—GENERAL HE REPULSION start induction motor may be described T briefly as consisting of a field or stator of laminated toothed construction having a single winding, usually of the pyramidal type, wound thereon and connected to the supply circuit; a progressively wound armature or rotor with a com mutator and having brushes which bear on the commutator during the starting period, the brushes being removed from the commutator and the armature winding short circuited through the commutator after the armature has obtained sufficient speed. This type of motor therefore starts as a simple repulsion motor without compensating or auxiliary windings and operates as a simple repulsion motor until a predetermined speed has been attained at which time the armature winding is short circuited through the commutator and the brushes lifted. The motor then operates as a simple induction motor with an armature equivalent to the squirrel cage armature. Manuscript of this paper was received August 30, 1915. 2389 2390 HAMILTON: INDUCTION MOTOR [Oct. 19 The straight repulsion motor without compensation gives the most efficient starting torque possible and the single-phase induction motor with the squirrel cage armature or its equiv alent gives the simplest*and most efficient motor when running, hence the rapid development of this type of motor with its desirable characteristics throughout. The discovery that a single-phase motor with armature, commutator and brushes as described above would start and operate without any electrical connection to the armature was made by Elihu Thomson, and the experiments leading up to this discovery are described by him in the United States Patent No. 363,185 issued to him May 17, 1887 and later patents issued in that year and following years to Thomson and others. FIG. 1 FIG. 2 These early patents describe a motor which started and oper ated on the repulsion principle by short circuiting a number of coils in favorable position with regard to the field magnetism by a comparatively wide brush as is shown in Fig. 1. This plan, as is well known, has the disadvantage of using a part of the armature coils only at any instant. It was soon discovered however, that the entire armature winding could be used to advantage by employing comparatively narrow brushes and by connecting these brushes together as shown in Fig. 2. It may be .observed by referring to Figs. 1 and 2, the direc tion of rotation will be different in the two types of motors and that the coils short circuited by the brushes as shown in Fig. 2, exert a torque opposite from the direction of the torque 1915] HAMILTON: INDUCTION MOTOR 2391 of the armature as a whole, and therefore the number of coils short circuited in practise are kept to a minimum by using a comparatively large number of commutator segments and narrow brushes. Objection has been raised to the term " repulsion " as apply ing to this type of motor. The word " repulsion " motor will be used here however as it is a well known term and engineers in general understand that the fundamental principles causing torque in this type of motor are the same as in all types of direct- current or alternating-current motors ; that is, that a current in a wire at right angles to and situated in a magnetic field tends to move out of that field, the direction of motion being well known when the direction of the magnetic field and the direc tion of the current in the wire are known. Little was done on the repulsion type of motor in a com mercial way for several years after these fundamental patents were granted, due to the limited demand for alternating-current power motors and due more particularly to the fact that about the date of these fundamental patents on repulsion motors, it was discovered and patents* were granted showing that a single-phase motor having a squirrel cage armature could be started by a double winding on the field which gave a form of rotating field. This type of motor is now the well known split- phase alternating-current motor. This latter type of motor without the auxiliary winding is the kind of induction motor which we have in the type of motor under discussion when it is up to speed. Most engineers in this country and abroad, as is shown by the electrical literature following the dates of these funda mental repulsion patents considered the split phase motor with its substantial armature better and more satisfactory as a basis for developing a single-phase power motor. During the period of 1894 to 1900 the demand for a prac ticable single-phase power motor having good starting torque and efficiency, and at the same time good running torque and efficiency, caused some of the engineers in this country to take up actively the question of developing a commercially satis factory single-phase motor. They realized that the repulsion motor had the starting characteristics so much desired and that the single phase induction motor with the squirrel cage armature or its equivalent possessed the desirable running characteristics, that is, a definite limiting or synchronous speed 2392 ' HAMILTON: INDUCTION MOTOR [Oct. 19 and comparatively small slip or dropping off of speed under load and the absence of brushes on the commutator when the mptor is up to speed. The repulsion start induction motor came into commercial prominence about the period of 1895 to 1900 first as a hand start or manually operated motor when starting and later in genious devices for automatically performing this function were' developed. During the period of 1900 to the present date no radical improvements have been made with reference to this type of motor. However, it has been consistently im proved in all of its electrical and mechanical details until at the present time it is doubtful if any other alternating-current or even any direct-current motor is more satisfactory alike to manufacturer, central station and user. Torque Efficiency at Start. As the starting efficiency of an alternating current power motor is one of the most interest ing points for discussion, the starting efficiency of this type of motor will be considered and comparisons made with direct current motors, polyphase motors and other types of single phase motors. Table I gives the starting torque in per cent of the full-load torque, starting current in per cent of full-load current, per cent of full-load torque at start for 100 per cent full-load cur rent and maximum pulling torque in per cent of full-load torque. As may be noted from these data, the direct-current shunt or compound motor, the two- and three-phase wound rotor in duction motor with resistance in rotor at starting, and the repulsion start induction motor have very much the same starting efficiency, and all other types of alternating-current motors are inferior particularly the two- and three-phase squirrel cage of the larger sizes and the single-phase motor of the split- phase type, so that the repulsion start induction motor com pares very favorably in regard to starting efficiency with the best type of direct-current or alternating-current motors avail able. The central stations are favorably inclined toward this type of7 single-phase motor as it causes a minimum line dis turbance, and it is quite usual nowadays for central stations to limit the starting current of single phase motors to 300 per cent of full-load current for motors up to and including 5 h.p. and to limit starting current to 150 to 175 per cent of full-load current for 1\ h.p. and larger. While starters are not required for any size of repulsion start induction motor up to 50 h.p., as they take not over 300 per cent full load current at starting, 1915] HAMILTON: INDUCTION MOTOR 2393 however, it is quite usual to use a resistance or compensator starter for sizes of 7? .h.p. and larger so as to cause a minimum of line disturbance. TABLE I Starting tor Starting cur Per cent full Maximum Kind of motor. que in per rent in per load torque pulling cent of full cent of full- for full load torque in per load torque. load current current cent of full- load torque. Small d-c. comp. without] [starter. \ h.p.

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