Patent GB-385349-A (GB)
Title: Improvements in or relating to alternating current electric motors for driving liftsand the like
IPC: H02K17/14
Filed: 1931-08-13
Abstract: 385,349. Alternating-current induction motors. PUNGA, F., 8, Marienhohstrasse, Villenkolonie, Eberstadt, near Darmstadt, Germany. Aug. 13, 1931, No. 22891. [Class 35.] Motors, controlling.- In an alternating-current motor for lift &c. driving having a squirrel cage or short-circuited armature, an inching speed is obtained during braking as a crawling speed resulting from a higher harmonic of the rotating field, means being provided to cause the crawling speed to disappear during acceleration and normal running. If the stator has a single winding, a crawling speed in the negative torque range is utilized so that starting and normal running are not interfered with, and' for braking and inching, the connection of the winding to the mains must be reversed. The invention makes use of two species of crawling speeds. The first has a synchronous character and depends on the ratio Z1-Z2/2p, where Z1 = number of stator slots, Z2 = number of rotor slots, and 2p = number of poles. The difference Z1 - Z2 must be equal to the number of poles. For example, where Z1 = 24, Z2 = 20 and 2p = 4, the ratio is + 1 and there is a pronounced cusp in the speed-torque curve in the negative speed range at one-fifth of synchronous speed. Where Z1 = 24, Z2 = 28 and 2p = 4, the ratio is - 1 and a similar cusp occurs at one-seventh of synchronous speed in the positive torque range. The motor may have two windings, viz. a four-pole running winding and an eightpole winding which produces a crawling speed, or a single pole-changing winding may be employed. The second species of crawling speed employed is of an asynchronous character and utilizes a higher harmonic of the rotating field, particularly 'the 5th harmonic lying in the negative speed range and the 7th harmonic lying in'the positive speed range. In a normal winding, the torques arising from these harmonics are too small to produce crawling speeds, but the winding may be re-arranged so as to increase the harmonic fields and to diminish the main field. Fig. 1 shows a three-phase winding in which for normal running the sections of each of the phases I, II, III, are connected in the order 1, 3, 2, 4 producing a four-pole field. For the inching speed, sections 2, 4 are reversed so that the fundamental field disappears and the speed drops to a speed corresponding to the 7th harmonic, or, if the whole winding is reversely connected to the mains to make use of a negative crawling speed, the speed drops to that corresponding to the 5th harmonic. To increase the torque arising from the 5th harmonic, the effect of the 7th harmonic may be diminished by the use of a secondary winding having two bars per slot in which the bars are connected at one end to a short-circuiting ring, and at the other end they are interconnected by end connections of such a pitch as to eliminate the 7th harmonic. Alternatively, the effect, of the 5th harmonic may be so limited. The motor may have an unsymmetrically connected winding in addition to an ordinary winding the additional winding being preferably wound for a greater number of poles than the main winding. The auxiliary winding may be of the form shown in Fig. 1 so that it can be used also in the symmetrical connection for obtaining an intermediate step with regenerative braking. To reduce the current rush in changing over, mesh connection is employed for running and star connection for inching, or choking coils are connected in series. Fig. 4 shows an alternative connection of a three-phase winding by which a 120‹ phase shift is produced in one half of each phase. The changing over may be simplified by using a mesh-connected winding, the mains being connected to the points of the delta for normal operation and to the centres of the phases for the inching speeds. In the case of a motor having two 'windings, both windings may be used together for the crawling speed, or the principal winding may be used first and then the auxiliary winding. Two motors may be employed, namely a main motor with a double slot or eddy current squirrel cage motor and an additional motor with a normal squirrel cage rotor for producing the crawling speed. The additional motor preferably has a large number of poles. The dimensions of this additional motor mav be reduced if braking to the inching speed is effected mechanically or electrically by the main motor. For this purpose, the main motor may be designed to have a synchronous crawling speed in the negative torque range and the additional motor has an asynchronous crawling speed of the same value so that it will help to synchronize the main motor at the crawling speed and can be switched off immediately after synchronizing. In the case of a single-phase motor, a condenser is connected in series with the auxiliary phase or the motor may have a three-phase winding, two terminals being connected directly to the supply lines and the third in series with a condenser to one of the supply lines.