Eddy currents in induction motors While powerful generators working at approximately permanent rotation speed depending on electric power system frequency three phase induction motors are characterised by a wide range of their speed. When a motor starts its speed increases from zero to a steady value which is determined by the motor load and its rated slip depending on motor parameters. Assume that a motor is not operated by a frequency inverter that are widely used in modern drive systems. So when the motor gets connected to a voltage source at first the slip “s” is equal to 1. The induction motor design ensures high initial rotating torque. It is necessary to reach its rated speed in a rather short time, besides very often the motor gets loaded after the start. In order to improve motor starting characteristics the machine is designed in such a way as to obtain higher ohmic resistance at low speed. Figure 1 represents the initial induction motor circuit diagram. It should be noted that rotor resistances r2 and x2 depend on the slip s. The initial diagram may be converted to the equivalent circuit (Figure 2) in conformity with . In this case the rotor is represented by several (the number is equal to k) contours having the ohmic and inductance resistances that do not depend on the slip.
Eddy currents in DC machines When changes the magnetic flux passing through iron non-laminated areas of a d.c. machine there arise eddy currents which affect the machine electromagnetic transient characteristics. These eddy currents arise both in longitudinal (d) and lateral (q) axes of a machine. For the machine that contains a single pair of poles (p=1) the d-axis coincides with the axis of the main poles while the q-axis is perpendicular to it and goes along the axis of additional poles if those are available. This is always the case in powerful machines whereas toy-sized machines don't have such poles.
1 V.F. Sivokobylenko, V.B.Sovpel. About the design of induction machines equivalent circuits with the aid of their frequency characteristics. Electricity, 1975, № 7.