FAM control of induction motor

Lore & Background

The field acceleration method was developed from an analysis of electromagnetic transients in three-phase induction motors. Initial attempts derived equivalent circuits valid for both steady state and transient states. The largest inductance in the motor is the exciting inductance, and any transient in the exciting current would cause a very large delay time constant, so maintaining continuity of amplitude and phase of exciting current is essential.

FAM control has two types: current control, where the primary current is the control input, and voltage control, where the primary terminal voltage is the control input. Constant exciting current produces good linearity between torque and slip frequency. Two transient phenomena are associated with induction motor operation: mechanical transients of the dynamic system and electromagnetic transients within the motor. These can be treated separately, with the instantaneous motor torque fed into the analysis of the dynamic system.

In FAM, torque can be controlled by the vector of secondary current. The method provides very superior steady state performances based on its T equivalent three circuits, which behave the same when viewed from primary terminals but differ due to different phenomena occurring in the secondary side.

Reader's Guide

The significance of FAM control lies in its direct approach to torque control without coordinate transformation, avoiding the delays caused by electromagnetic transients. By keeping the exciting current constant and making the air gap flux speed adjustable, torque becomes a function of slip frequency only, enabling quick response. The method separates mechanical and electromagnetic transients for analysis, allowing the instantaneous torque to be used in dynamic system studies. Its two control types—current and voltage—offer flexibility in implementation. The T equivalent three circuits, though identical from the primary side, reflect different secondary phenomena, contributing to the method's superior steady-state performance. The legacy of FAM is its demonstration that induction motor torque can be controlled rapidly and linearly by manipulating the rotating field speed while maintaining constant flux amplitude, a principle that avoids the large time constants associated with exciting inductance transients.

Did You Know?

• FAM does not involve coordinate transformation.
• The largest inductance in an induction motor is the exciting inductance.
• FAM has two types of control: current control and voltage control.