US1031802A - Hysteresis starting device for induction-motors. - Google Patents

Description

B. MQGOLLUM. HYSTERESIS STARTING DEVICE FOR INDUCTION MOTORS.

APPLIOATION FILED DEG.12, 1911.

1,031,802, Patented July 9, 1912.

WW awn/vent 1 1 g I BURTON MOCOLLUM, OF WASHINGTON, DISTRICT OF COLUMBIA.

IHYSTERESIS STARTING DEVICE FOR INDUCTION-MOTORS Specification of Letters Patent.

Patented July 9, 1912.

Application filed December 12, 1911. Serial No. 665,383.

To all whom it may concern.

Be it known that I, BURTON MoCoLLUM, a citizen of the United States, and a resident of Washington, District of Columbia, have invented a new,and useful Hysteresis Starting Device for Induction-Motors, of which the following is a specification.

It is well known that alternating current induction motors, in order to have good starting characteristics, should have a high resistance in the secondary circuit, while to have good running constants the secondary resistance should be low.

My invention has for its object to produce a motor without mechanical complications of any kind in which the apparent resistance of the secondary circuit shall automatically vary so as to have a very low value at near full load speeds, and gradually increase with decreasing speed to arelatively high value at standstill, thus enabling the motor to operate most satisfactorily under both starting and running conditions.

My invention applies particularly to the type of motor having a rotor known as the squirrel cage rotor, and consists essentially in surrounding the short circuiting rings of the squirrel cage with auxiliary rings'hereinafter called hysteresis rings composed of a magnetic material having certain characteristics specified below.

It is well known that when an electric circuit carrying an alternating current links with a magnetic circuit there will be a dissipation of energy in the magnetic material which gives rise to an apparent increase in the resistance of the electric circuit, which increase of resistance can be expressed approximately by the formula 1.6 R I I 7 where R is the increase of resistance, commonly called hysteresis resistance, f the frequency of the alternating current, B the maximum flux density reached by the magnetic' material each cycle, I the current flowing'in the electric circuit, or and 'v the hysteresisconstant and volume respectively, of the magnetic material. It will be seen from this equation that the increase in resistance tends to increase with increasing frequency, and since the frequency of the secondary currents in the short circuiting rings of the squirrel cage is very low at high speed and relatively high at starting, it follows that the hysteresis resistance will tend to be hi h at starting and low under running con itions. Further, in order that a sufliciently large resistance may be had to give the required starting torque without using too lar e a volume of magnetic material, it is desirable that the material used should have a high hysteresis constant. These conditions alone are, however, not sufiicient to insure a satisfactory rise in' resistance with decreasing speed. It is well known that the current flowing in the short circuiting rings of the squirrel cage rotor increases greatly under ordinary conditions with decreasing speed, and since the factor I appears in the de nominator of the equation for hysteresis resistance, this tends to cause a decreasing resistance with decreasing speed, and if B remains practically constant, which is substantially the case provided the normal full load currents in the short circuiting rings are suificient to magnetize the hysteresis rings practically to saturation, the increase in I may more than counterbalance the increase in f and thus cause an actual lowering of resistance with decreasing speed. In order to overcome this difliculty, it is necessary to so design the hysteresis rings that the factor B in the equation will greatly increase with increasingcurrent. In order to do this it is necessary to make the hysteresis rings of a material of such nature and of such proportions that the normal full load currents in the rotor will magnetize them but feebly, but so that the starting currents will magnetize them to a relatively high value. Since, with every magnetic material, the permeability tends to riseat a certain critical value of magnetizing force, it is possible by selecting a material one proper nature to make the factor B increase more rapidly than I throughout the range from full load to standstill, and in this way a large increase in resistance may be secured.

Since the normal load currents in the short circuiting rings of the squirrel cage are necessarily very large ordinary magnetic ma-' low since in this case the value of B wouldbe too small even at starting and a very large volume of material would be necessary tosecure the desired increase in resistance. Best results are obtained when the hysteresis rings are so designed that the starting currents will magnetize them to a de ree just approaching saturation, while the ull load currents magnetize them to a degree far below this point. I have found that pure metals cannot be used for the hysteresis rings because the magnetic metals in their plure state have too high a permeability.

umerous alloys of metals, however, and particularly those alloys containing manganese, when properly designed, give the desired results. Of these, I have found that an alloy of iron and manganese is most suitable, it being ada ted for use in motors of any size. that by varying the proportion of manganese from zero to approximately twelve percent, any desired permeability from the maximum of pure iron to ractically unity can be obtained, and at te same time, a

hi h hysteresis constant is secured.

y invention is further described below, reference being made to the accompanying drawings.

Of the drawings: Figure 1 is an end elevation of the rotor to which my invention applies. Fig. 2 shows a modified construction of the hysteresis rings. Fig. 3 is a sectional view of a portion of the rotor includin one of the shortcircuiting rings.

Iteferring to Fig. 1, 1 is the laminated core which may be of the usual type, 2 the conductorsof the squirrel cage winding, 3

the short circuiting ring, and 4 the hysteresis rings of magnetic material, which latter constitute the essential feature of my in vention. It will be seen that between each pair of bars 2 there is placed a hysteresis ring 4 surrounding the short circuiting ring 3. When the alternating current flows in the short circuiting ring, alternations of magnetism are produced in the ring 4 which give rise to the hysteresis resistance mentioned above. The relation of the hysteresis rings 4 to the other parts of the structure is more clearly seen by reference to Fig. 3 which shows a section through the ring 3, a portion of one of the conductor bars 2, and the laminated core 1.

In some cases, especially in large motors. where the h steresis rings 4 are large, eddy currents in uced therein may give rise to unequal distribution of magnetism which may be objectionable, and this can be avoided by the structure shown in Fig. 2. Here 4 shows the hysteresis rin s made up of a number of thin sheets, w ich are e ective in eliminating trouble from this source.

In putting the hysteresis rings 4 on the It is c aracteristic of this alloy short circuiting ring 3, the ring 3 is cut open and spread slightly at one point while the rings 4 are being put on, after which the ring 3 is closed by soldering, Welding, or by other suitable means. The rings 4 are then uniformly spaced and the ring 3 connected to the bars 2 as shown in Fig. 1. Connection of the ring 3 to the bars 2 may be accomplished by soldering as shown, or by screws, bolts, or other suitable means.

It is obvious that the hysteresis rings hereinabove described can be used on either or both of the short circuiting rings of the squirrel cage, and may be grouped on either ring in any desired manner without departing from the generic principle of my invention.

I claim:

1. In an induction motor with squirrel cage rotor, short circuiting rings connected to the bars of the squirrel cage, rings of magnetic material surrounding said short circuiting rings, said magnetic rings being composed of a material having a hlgh hys teresis constant, and being of such composition and so pro ortioned as to be magnetized to a high ensity by the starting currents in the short circuiting rings, but magnetized to a relatively low density by the normal full load currents.

2. In an induction motor with squirrel cage rotor, short circuiting rings connected to the bars of the squirrel cage, rings of magnetic materialsurrounding said short circuiting rings, said magnetic rings being composed of a material having a high hysteresis constant, and being of such composition and so proportioned as to have a high hysteresis reslstance at low speeds of the motor, and a much lower hysteresis re sistance at high speeds of the motor.

3. In an induction motor with squirrel cage rotor, short circuiting rings connected to the bars of the squirrel cage, rings of magnetic material surrounding said short circuiting rings, said magnetic rings being of such composition and so proportioned as to have a higher permeability when subjected to the magnetizing force due to the starting currents than when subjected to the magnetizing force of the normal full load currents in the short circuiting rings.

4. In an induction motor with squirrel cage rotor, short circuiting rings connected to the bars of the squirrel cage, rings of material surrounding said short circuiting rings, said magnetic rings being of laminated structure substantially as shown, and being of such composition and so proportioned as to have a higher permeability when subjected to the magnetizing force due to the starting currents than when subjected. to the magnetizing force of the normal full load currentsin the short circuit ing rings.

5. In an induction Inotor with squirrel to the bars of the squirrel cage, said short cage rotor, short circuitlng rings connected circuitlng rings being surrounded by rings to the bars of the squlrrel cage, sa1d short composed of an alloy of H011 and manganese. 10

circuiting rings being surrounded by rings BURTON McCOLLUM. 5 composed of a magnetic alloy of manganese. Witnesses:

6. In an induction motor with squirrel MINNIE LESTER, cage rotor, short circuiting rings connected JEssIE C. MGCOLLUM.

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