Machine inductances on variable frequency supplies

The use of modulated high frequency rectangular waves in voltage fed inverters to supply motors facilitates control of the speed of the motor and even control of the angular position of the rotor. However the steps in voltage from the supply and the associated harmonics can lead to difficulties if care is not taken. With the increase in the modulation of carrier frequency that is now possible high frequency variations on mechanical torque are small enough to be acceptable though there is still scope for reducing acoustic noise from the motors. However electrical failure can result either by straightforward dielectric breakdown or by breakdown after weakening of the insulation by corona discharges and by excessive heating in the dielectric and in the conductors. Resonance and reflected waves in the supply cables and in the windings of the machine itself may give rise to excessively high currents and voltages. A side effect arises from the flow of currents induced by capacitive coupling between the stator windings and the rotor especially if the stator slots are open and not electrically shielded. These currents to the rotor return through the bearings. Capacitive coupling across an oil film may be harmless but if arcing occurs this will damage the faces of the bearing elements. This paper describes some of the work we have been doing to try to compute the way windings respond to imposed voltages and currents based on information about the physical structure of the motor, the winding geometry and the core construction and the properties of the materials