Unbalanced magnetic pull in a 6-pole induction motor

A 6-pole 10kW induction motor has been specially constructed for a comprehensive series of steady-state and transient measurements of the magnitude of unbalanced magnetic pull resulting from an eccentric air gap. These measurements have shown that unbalanced magnetic pull is critically dependent on Saturation of the magnetic circuit, and that the magnitude in the cage-rotor motor is much less than in the wound-rotor motor. Comparisons between measurements and calculations show that most existing theories inaccurately predict the magnitude of transient unbalanced magnetic pull during starting, the effect of slotting and saturation, the effect of a cage rotor, and the action of load currents. A simple theory is developed which gives reasonable agreement with measurements for the wound-rotor motor over a range of supply voltages about the rated value.