DE566664C - Device for starting short-circuit three-phase motors with full load - Google Patents

Description

ISSUED ON JANUARY 3, 1933

REICH PATENT OFFICE

PATENT LETTERING

CLASS 21c GROUP 59 so

Patented in the German Empire on February 6, 1927

It is known to reduce the starting current in KurzscMuss three-phase motors to permissible Reduce values to use current displacement anchors. Have such anchors two concentric cage windings of different resistance with separate or common short-circuit ranks. Advantageous the cage windings are made from a single one as a coherent double cage Material executed. The inner winding has a lower resistance, the outer one greater. Immediately upon start-up of such a rotor, so when the mains frequency still prevails, the magnetic flux becomes in the rotor so pushed away from the inner winding that it is only in contact with the outer Chained winding and therefore reduces the starting current and increases the torque. With such an anchor it is

ao possible to adjust the starting current from the 6- »to 7 times the value of the nominal current to approximately 4.5 times the value at high torque to push down. If you then use star-delta connection, the starting current drops to 1.6 times the nominal current down. Despite these advantages of the current displacement motor over the ordinary Short-circuit three-phase motor it still has a considerable amount compared to the usual three-phase motor with slip rings Disadvantage, namely a lower starting torque.

One could now try to eliminate this disadvantage by removing the short-circuit motor lets it start empty and a centrifugal clutch ensures that the load is only is taken over when the motor 'has reached full speed. This measure leads but not to the goal. The number of revolutions of a three-phase motor is given by the number of poles in its winding and the network frequency imposed on it, but not, at least not directly, through the imposed frequency Tension. So if the motor with the first stage of the star delta switch 4-5 is started, it seeks its full speed despite the greatly reduced voltage to reach. A centrifugal clutch of the usual type that only depends on the speed is, will respond and take over the load if the motor is still in star connection in the much too low Voltage is present, so that a large drop in speed when the clutch is successful and therefore a strong current surge when switching to triangle is unavoidable.

These ratios of the interaction of an ordinary centrifugal clutch with Spring counter-torque with an ordinary short-circuit motor or a double-groove

*) The patent seeker stated as the inventor:

Dr. Lionel Pleischmann in Berlin-Grunewald.

engine show the diagrams of Fig. ι and 2 of the drawing. In these diagrams, Md _{k means} the torque that can be transmitted by the clutch, Md ^ and Md ± the torque of the motor in star or delta connection, / ,. the starting current in star connection, / _Δ the starting current in delta connection. If the motor in the star connection has reached its normal speed, so

ίο the clutch starts up and takes over the load. As a result, however, there is a drop in speed up to point P, at which the curve of the engine torque Md ₃ intersects with the torque curve of the clutch Md _k. At this speed, the motor takes up the starting current J \ ρ. When switching from star to delta, the current J \ p jumps to / λρ · In the example illustrated in the diagrams, / ap corresponds to the

ao normal short-circuit motor 3.1 times, in the case of the double-groove motor even 3.7 times the nominal value.

So you can see that when using a normal centrifugal clutch you can let the motor start idling, but that with a normal short-circuit motor there is still a high starting current of about 2.4 times the rated current and, when switching from star to delta, inadmissibly high current surges are also present appear. If the usual centrifugal clutch is used together with the current displacement motor (Fig. 2), the starting current is low, but the high current surges when switching from the low to the high voltage still occur in this case. However, this eliminates the advantages of the current displacement motor through the use of an inexpedient clutch.

To avoid power surges when switching from star to delta, the already proposed for common short-circuit three-phase motors with clutches Use clutch delays. These clutches cause the takeover the load by the motor only takes place when the motor is already through its switching element is due to the full mains voltage. The combination of an ordinary short circuit motor with a clutch with clutch delay but still has the disadvantage that the starting current of the short-circuit motor exceeds the permissible value of 1.6 of the nominal current (Fig. 3).

According to the invention, the Stromverdrän supply motor in conjunction with a centrifugal clutch used with clutch delay, whereby the advantages of the small starting current, the small over-switching current and the good tightening torque. The diagram of Fig. 4 shows the How the new arrangement works. As can be seen from this diagram, exceeds the starting current is 1.6 times the value of the rated current, and it also occurs when switching from star to delta, no current surges occur, as the takeover the load by the engine using the centrifugal clutch with "clutch delay is only effected when the motor is switched from star to delta by its switching device is switched, so is at full voltage.

Claims (1)

Claim: Device for starting short-circuit three-phase motors with full load, thereby characterized in that a current displacement armature in connection with a centrifugal clutch of known type is used with clutch delay. 1 sheet of drawings