Abstract :
This paper contains an account of tests made on a new-type of squirrel-cage induction motor with high starting torque. Each conductor of the rotor winding is a compound bar of which the characteristic property is that the resistance increases with the frequency of the alternating current flowing in it. The compound bars are each built up as follows: There is a central core of copper, which is surrounded by a sleeve of steel. Outside the steel sleeve is a skin of copper, the outer copper skin and the central copper core being brought into good electrical contact at each end. When alternating current flows through the compound conductor the portion passing through the central copper core produces in the steel sleeve a magnetic flux which induces a back E.M.F. tending to choke the current out of the core. The result is that the current will flow chiefly through the high-resistance copper skin outside- the steel sleeve. The greater the frequency of the alternating current the greater will be the back E.M.F., and the effective resistance of the compound bar will be correspondingly increased. If a squirrel-cage rotor winding is formed from these compound conductors, then, at starting, since currents of the full supply frequency will flow in the bars, the effective resistance and the starting torque will be high. When running normally, the currents in the rotor bars will be at the very low frequency of slip and hence the resistance will be correspondingly low; in fact, it will be practically the same as the resistance as measured by direct current. The paper contains the results of tests made on a squirrel-cage rotor built with compound conductors in accordance with the author´s proposals, and also the test-results on a standard rotor, the same stator being used in each case.
