GB715605A - Cascade set of two asynchronous motors - Google Patents

Abstract

715,605. Control of A.C. motors. SIEMENSSCHUCKERTWERKE, AKT.-GES. Dec. 31, 1952 (Jan. 3, 1952], No. 33090/52. Class 38(3) [Also in Group XXXV] In a cascade set comprising at least two asynchronous motors which are mechanicaly coupled and electrically connected, the resistance of the intermediate circuit, formed by the secondary winding of the first machine and the primary winding of the second machine, and the resistance of the secondary circuit of the second machine are arranged to provide a series characteristic with the torque decreasing continuously throughout the range from zero speed to the synchronous speed of the first machine. The effect of placing suitable resistance in the intermediate circuit is to increase the positive torque when the set is running at synchronous speed (that based on the total number of poles of both machines), this resistance being of the same order of magnitude as the main reactance of the second machine transferred to the intermediate circuit and related to the synchronous speed of the set. The resistance in the secondary circuit of the second machine is arranged to decrease the slope of the speed-torque curve in the neighbourhood of the synchronous speed of the set and is chosen to be of the same order of magnitude as the main reactance of the second machine transferred to its secondary side in the stationary position. The second machine of the cascade may be operated with a high magnetizing current by increasing the air gap. This has the same effect as increasing the resistance of the intermediate circuit. The slope of the speedtorque curve in the neighbourhood of the synchronous speed of the set is reduced by arranging that the main field of the second machine becomes saturated at low speeds. One machine may have twice the number of poles of the other, the two machines being accommodated in one housing with common rotor and stator cores, the windings being arranged so that there is no unwanted magnetic interference between the two machines, Fig. 5 (not shown). The windings of the machines may be so connected that their torques are added below the synchronous speed of the set and subtracted above it. The resistances of the intermediate circuit and of the secondary circuit of the second machine are obtainable by forming the windings of resistance material, or separate variable resistances are employable. As shown, the resistances 10 for the intermediate circuit formed by the rotors of the two machines are mounted on the common shaft, a heat insulator 19 and cooling fins 20 are provided. Metallic protecting tubes for the resistances can be supplied, together with separate cooling means. The resistances for the secondary circuit of the second machine are connected to the stator terminals u, v, w. Speed regulation is afforded by means of pre-magnetized chokes connected in series with the primary winding of the first machine, or by varying the leakage reactance of that winding. The winding 21 has, for example, U-shaped armatures 22 at its ends disposed at variable distances from a magnetic member 2'3. In another arrangement, a laminated iron ring 34 is displaceably arranged in the bore of the stator 31 to vary the tooth leakage. To reduce the losses at low speeds, capacitors, which may be mounted on the common shaft, are connected in parallel with the resistors in the intermediate circuit of the machines. The resistors may also be of the type the conductance of which increases with increasing temperature.