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Variable speed drives that drive three phase motors are ubiquitous components of industrial machines that help save energy and optimize systems. Traditional scalar control techniques for variable speed operation of three phase electric motors offer simple implementation but limit the performance that can be achieved. With a scalar drive, algorithm limitations can mean that meeting dynamic response specifications requires choosing a larger motor and a larger drive that complements the larger motor.
This tends to drag down the efficiency while resulting in a more expensive system. Field oriented control (FOC) overcomes this problem by squeezing out more performance out of the same motor. This allows designers to properly size motors and drives, lowering cost and results in a more efficient system overall.
Limitations of traditional scalar control
Electric motors are a prime mover of choice, and account for over half of U.S. electricity consumption, so the potential cost and energy savings through increasing the performance and efficiency of electric motors used in industrial applications is significant. Most of the motors in variable-speed drives are alternating current (AC) induction motors. Scalar control is based on a very simple control strategy: the voltage and frequency applied to the motor is changed to change the speed of the motor.
To run the motor at various frequencies, the frequency of the three-phase sinusoidal drive is varied, and the voltage applied to the motor is varied in proportion. This changes the speed of the rotating magnetic flux in the motor, and in turn changes the speed of the machine. This results in a rotating magnetic field, rotating at the synchronous speed, typically 1800 rpm or 3600 rpm for 2 pole or 4 pole per phase machines with 60Hz excitation.
These motors operate relatively efficiently at the synchronous speed when the voltage drop across the stator is nominal. For example, to reduce the operating speed to say half the nominal speed for a 208V/60Hz, 1800 rpm machine, the frequency would change to 30Hz and the voltage to 104V. This would result in the machine running at 900 rpm.
A significant number of industrial applications benefit greatly from variable speed operation, and variable speed drives are increasingly adopted in a large variety of industrial equipment. Since in a constant V/F control there is no overt effort to maintain the alignment between the stator and rotor flux, oscillations and current spikes can occur during rapid transients. An aggressive speed regulator might accelerate the stator flux too quickly, disturbing the flux alignment in the machine.
This may actually reduce the instantaneous torque produced, and also temporarily reduce the back-emf in the motor windings, resulting in a current inrush. In an open loop operation scenario, the induction machine self aligns to a new equilibrium. With an aggressive closed loop speed regulator this mechanism results in torque and current oscillations.
One way to avoid these transients is to limit the transient demands. To do this the regulator may be detuned, limiting performance. Another option can be to use a larger machine, with a higher torque capability. The larger machine may indeed allow the system to respond to the transient though it follows that with scalar control driving a larger motor, the power converters may need to be oversized to handle the torque requirements of the transient and surge currents.
Field oriented control basics
The basic idea behind the field oriented control is to manage the interrelationship of the fluxes to avoid the issues mentioned above, and to squeeze out the most performance from the motor. To understand how this works, we first look at the structure of the motor. A three-phase motor incorporates windings that are displaced by 120 degrees (or a fraction of that) along the stator. Feeding the windings with three voltages separated in phase by one-third of a cycle produces a rotating magnetic field. A conceptual representation is shown in Figure 1.
Figure 1: Generation of a rotating magnetic field and torque in an induction machine.
The rotor in an induction machine consists of a closed circuit. Most often, a squirrel cage rotor is used, which has conductor bars shorted together with thick end rings. When the stator magnetic field sweeps the rotor, an emf is induced in the rotor circuit, and produces a current. The current produces its own magnetic field, and this induced magnetic field interacts with the stator magnetic field to producing mechanical force upon the rotor.
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