US1746445A - Compensated induction motor - Google Patents

Description

Feb. 11, 1930. R. RICHTER: 1,746,445

COMPENSATED INDUCTION MOTOR Filed May 24, 1921 IHVENTOR BY 7 Rudolf Richter HTTORHEY Patented Feb. 11, 1930 UNITED STATES PATENT QFFICE RUDO LF RICHTER, OF DUBLACH, GERMANY, ASSIGNOR TO RADED PATENTS CORPORA- TION, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK COMPENSATED IIDUUTION MGTOR Application filed May 24, 1827, Serial hi0. 193,331, and in Germany September 30, 1924.

My invention relates to improvements in induction motors and more particularly to such motors having compensating windings. One object of my invention is to reduce the current during the starting period by means of a special type of winding and a short-circuiting arrangement.

Another object of my invention is to suppress excessive sparking in a compensated motor, at the same time securing a large starting torque and an improved power factor during operation.

Another object of my invention is to provide a compensating winding with such a pitch that the volta e generated during starting bet-ween the short-circuited segments is negligibly small without at the same time reducing appreciably the voltage generated by the working field.

A further object of my invention is to reduce the starting current of an induction motor and at the same time enable the motor to develop a high starting torque.

Still another object of my invention is to afford transition from the starting connection to the working connection in an easy manner without interruption of current in any electric circuit.

All the aforesaid objects of my invention tend to improve the starting and working conditions of induction motors with shortcircuited rotor winding and to render such motors, on account of their simple design and const uction, adaptable for extensive practical use.

The nature of the invention is set forth in the following description, taken with the drawings, which illustrate the preferred form of the apparatus by which my invention is practiced and in the appended claims the novel features of the invention are fully pointed out, but I of course reserve the right to make changes in the arrangement and connection of the parts or manner of windings that come within the principle of the invention and matter embraced within the mean ings of the broad terms in which the appended claims are expressed.

On the drawings Ifigure 1 illustrates the connections of an ordinary type of induction motor with a compensating winding such as is known in the present-day practice.

Figure 2 shows diagrammatically one type of winding, arranged for operation according to the principle of my invention; and

Figure 3 shows a diagram of connection for carrying out one form of embodiment of my invention as that of Figure 2.

Figure 4 shows the arrangement of compensating and rotating windings in a slot.

The use of machines compensated for power factor is limited at the present time by the excessive space necessary for slip rings and commutator, while squirrel cage motors (with short-circuited rotor winding) used until now have the disadvantage of excessive current during starting.

The problem of economy, which becomes more important from day to day, demands as a solution a construction of compensated motors which would require only a slight modification of the normal construction that uses slip rings and commutators, and would leave all the mechanical details unchanged.

It has been established that a squirrel cage motor with a reduced starting current can best beconstructed by increasing the number of poles during working and diminishing this number for starting, especially if the rotor possesses such a resistance that it comparatively high for the number of poles used in starting.

It will be noticed that the improvement in power factor is then attained by providing an additional windin in the rotor which A b winding is connected to the supply.

The connections for such a machine before the application of the herein disclosed improvement are shown in Figure l, where P indicates the windings of the stator connect-ed with the supply, S is the secondary winding on the rotor, and K indicates the compensating winding. The brushes are connected to convenient points on the stator winding or to a special winding on the stator.

The use of this method of compensation of power factor conforming to Heyland but without the herein disclosed improvements has several disadvantages. The great velocity bet-ween the rotating field and the additiona-l or compensating winding cause a high tension to exist between sections which are short circuited by the brushes, especially during starting. This causes excessive sparktained by providing a wave winding, otherwise each brush would short-circuit its corresponding turns.

If, for instance, the special starting winding is chosen for four poles and the working winding for six poles, and the widths of the turn of this latter compensating winding is equal to double the polar distance of the starting field, then the induced electromotive force due to this starting winding becomes zero while as working windings they behave as diametral winding and their pitch is three times the polar spacing of the working field. However, it is not desirable to make the pitch so great. If on the other hand a pitch is chosen which is equal to or less than the pole spacing of the working winding, then the electromotive force induced by the starting winding is reduced only a little and is small duringtheworking. Forexample,ifthe pitch is equal to the working pole spacing, then the winding factor would correspond to .87 of the four pole, or 1 of the six pole. lVith the pitch half of this, the winding factors will become correspondingly .5 and .71. r

In Fig. 2 the row of NS poles marked ST represents the spacing of the starting win-dings in the stator and these ST windings are re resented b windin s A in Fiq 8.

Likewise, in Fig. 2 the row of N-S poles marked WK represents the relative spacing of the working windings in the stator and these IVK windings are represented by windingsA' in Fig. 3. v I

The compensating winding K of the rotor in Fig. 3 has a difierent action from K of Fig.1 inasmuch as it is connected to field windings A and there are no coils similar to A in Fig. 1. Moreover the connections are made to some mid-portion of the windings A. V

In order to reduce further the electromotive force induced by the starting field, and maintaining a small pitch over the compensated winding, and a good use of this winding with the working field, it is possible to arrange instead of a simple winding several windings connected inseries. The advance of this winding relative to the pitch of a single winding must be chosen in such a manner that it would difler very little from an odd number of working poles, i. e., it must be about 1, or 3, or 5, etc., number of working poles.

If we choose, for instance, the pitch and the advance of the winding equal to the pole spacing of the working winding with six poles, it would result for this number of poles in a winding factor equal to 1. Figure 2 represents a part of such a winding with two turns in series. It will-be seen that the windingturns are arranged to lie exactly under several of the working poles while two of the windings B and C both lie under the edge of the same starting pole, and windings A and D lie under poles of the same polarity of the starting poles. In this way, as illustrated by the arrows for the direction of winding, the voltages in A and D will be cancelled and the voltages in B and C will be partly cancelled, resulting in a low winding factor for the operation with the starting number of poles. In this way the winding is utilized at its best. For the starting winding of four poles, the winding factor would be only .216, which means that the starting field only excites an electromotive force which is .216 of. a diametral winding.

If we use a wave winding properly designed for a certain number of working poles, then according to thisinvention an arrangement of turns is produced so that between the adj acent commutator bars noelectromoti've force will be induced'due to the rotating field produced by the starting winding of a similar number of poles.

A wave winding is therefore specially suitable for a compensation winding for this type of motor. The parts of the winding which are short circuited by brushes have not only an electromotive force induced by the rotating field, but also by the leakage field of the winding, that is, the commutating electromotive force. In order to reduce this it is desirable to increase the number of conductors in series with the main rotor winding which are placed in the same slot as the sides of the turns of the compensating winding which are short circuited by the brushes as shown in Figure 41. In this way the commutating elec tromotive force will be reduced by the presence of short circuited turns.

In this manner the short circuited windings of the rotor will neutralize the leakage field of the slots which are induced by the compensating winding. This condition is'secured according to this invention it we choose a high winding factor for the working number of poles, a reduced factor for the starting number of poles. 7

Referring to a further feature of my invention, it is obvious that during starting the conditions are different from the working conditions. Itis desirable during the starting to obtain a larger resistance of the rotor so that the starting current may be reduced while the torque is great, on the other hand during working the resistance of the rotor should below in order to reduce the losses and therefore the slip.

It is possible to satisfy these two conditions in accordance with my invention if we choose to have besides the two windings of the stator, a special winding on the rotor. A th oretical examinationconfirmed by tests has proved that the use of a short circuited rotor winding with a sufficiently small winding factor in respect to the starting number of poles satisfies this condition, provided that this factor does not differ much from unity in respect to the working number of poles. It is therefore similar to the compensating winding above referred to. The motor thus improved has a reduced current during starting as only a rotating field due to a starting winding is effective during the starting period, while a working field with a relatively increased number of poles is later developed as the speed of the motor is increased. Thus, the starting currentis reduced under all circumstances and the starting torque is increased at the same time. The starting winding, as soon as the motor has attained its proper working speed has a voltage drop which is only a small fraction of the total applied net voltage and may therefore be short-circuited by means of a simple switch. Thus, the transition from starting to working connection takes place without any interruption of a circuit and the method as specified presents important advantages over the hitherto known method of starting by means of a star-delta switch. Referring to Figure 3, B represents the working winding and SIV the switch which short circuits the starting winding after starting. For delta connection, the stator must be supplied with a special starting winding. All the connections conforming to Heylands system may have the above method of starting applied to them with reduced current consumption. In many cases of compensated motors, as for instance for cranes and elevators where a period of starting with large torque is followed by a period with reduced torque, it is possible to omit the winding with a reduced number of poles in the stator. The winding factor of the rotor must then be reduced (to, say, .9) so that the motor may start with large torque and a reduced current. The most simple form of rotor winding is such that the short circuited circuit consists of a single turn for which the relationis less than unity, or about twothirds (IV is the pitch of the winding, t is the pole spacing).

The compensating winding may be fed from a part of the working winding of the stator. If the starting winding is short circuited during working, it may also serve for feeding the compensating winding if the latter is connected with the point of concatenation. Figure 3 represents an example with a connection in which the compensating winding K is fed by the starting winding A.

Having described my invention, what I believe to be new and desire to secure and protect by United States Letters Patent is:

1. In an asynchronous motor, a starting winding, a working field winding, compensating windings comprising a plurality of sections, said plurality of sections each comprising a plurality of turns connected in series and so arranged as to have a large winding factor relative to the spacing of said working field winding and so connected with respect to the said starting field winding that substantially all the voltages generated in said section by the field of said starting'winding are cancelled.

2. In a motor, means to produce a starting field, means to produce a working field, a compensating winding, a segmented commutator, said compensatingwinding being connected to said segments, a brush for alternately shortcircuitingturns of said compensating windings, said compensating winding turns having a winding factor small with respect to said starting field and large with respect to said working field.

3. In an alternating current motor, a starting winding, working winding connected in series with said starting winding, a source of energy, a commutator, brushes and a compensating winding for said motor, said compensa ing winding being connected through said commutator and brushes to said source of energy, means whereby said starting winding is effective during starting only and whereby said working winding is effective substantiat ly during operation only and further means for rendering said compensating winding non-operative to compensate during starting and rendering it effective to compensate during operation while said compensating winding continues to be connected to said source of power through said commutator during starting and working.

4;. In an alternating current compensating motor, a starting field winding, a working field winding, a compensating winding, the method of operating said compensating winding which comprises generating bucking electro-motive forces in the compensating winding during starting to prevent arcing and generating adding electro-motive forces in the compensating winding during opera tion.

. 5. In a compensating electric motor, an operating winding, a compensating winding, a commutator, said compensating winding being connected to said commutator, said compensating winding having a Winding factor less than 9/10 with respect to the poles of said operating winding. r

6. In an asynchronous motorhaving a commutator and brushes in contact with said commutator, a starting field winding for assuming substantially the full load of the motor during the starting, said starting field winding having a plurality of poles, a work ing field winding for assuming substantially the full load of the motor during working, said working field winding having a different number of poles than said starting winding, a source of energy, a compensating winding on said motor having a permanent connection during said motor operation to said source of energy through said commutator and brushes, said compensating winding being arranged to be effective for compensation only with respect to said working field winding and substantially non-effective with respect to the starting winding while said brushes are maintained in engagement with said segments.

7. In an asynchronous motor having a commutator and brushes in contact with said commutator, a starting field winding for assuming substantially the full load of the motor during the starting, said starting field winding having a plurality of poles, a working field winding for assuming substantially the ,full load of the motor during working, said working field winding having a different number of poles than said starting winding,

a source of energy, a compensating winding on said motor having a permanent connection during said motor operation to said source of energy through said commutator and brushes,

' said compensating winding having circuit 7 working winding having a difierent number of poles than said starting winding, a compensating winding on said motor connected to the segments of said commutator, said compensating winding having a'pitch with respect to said working winding such that a voltage is impressed across the terminals of said compensating winding connected to adjacent commutator segments by said working winding, said compensating winding having a pitch with respect to the starting winding such that no voltage is impressed across the terminals of said compensating winding connected to adjacent segments by said starting winding.

9.- In an asynchronous motor having a commutator and brushes in contact with said commutator, a starting field winding having a plurality of poles, working field winding having a difierent number of poles than said starting winding, a compensating winding on said motor, said compensating winding being arranged in sets, each set being connectedto adjacent commutator segments, the turns in each set being connected in series and having a pitch with respect to the working winding such that a voltage is induced in said turns across the terminals of said sets by said working winding and having a pitch with respect to the starting winding such that no voltage is induced across the terminals of said sets by said starting winding.

In testimony whereof he has afiixed his signature.

RUDOLF RICHTER.

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