Variable-speed drives for three-phase motors are ubiquitous components of industrial systems that help save energy and optimize performance. Traditional, scalar control techniques for variable-speed operation of three-phase electric motors offer simple implementation but limit the achievable performance. With a scalar drive, algorithm limitations can mean that meeting dynamic response specifications requires choosing a larger motor and a larger drive to complement it. That tends to drag down efficiency while driving up system cost.
Field-oriented control overcomes this problem by squeezing out more performance from the same motor. Thus, designers can properly size motors and drives, thereby lowering cost and optimizing system efficiency.
Electric motors account for more than half of U.S. electricity consumption, so the potential cost and energy savings achievable by increasing the performance and efficiency of industrial electric motors is significant. Most of the motors in variable-speed drives are ac induction motors. Scalar control is based on a simple control strategy: The voltage and frequency applied to the motor are altered 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 proportionately. That changes the speed of the rotating magnetic flux in the motor, in turn altering the speed of the machine. The resultant magnetic field rotates at a synchronous speed, typically 1,800 rpm or 3,600 rpm for two- or four-pole-per-phase machines with 60-Hz excitation.
Such 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 half the nominal speed for a 208-volt/60-Hz, 1,800-rpm machine, the frequency would change to 30 Hz and the voltage to 104 V. Those voltage and frequency changes translate into a 900-rpm speed for the motor. Variable-speed drives are increasingly adopted in a large variety of industrial systems.
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. That may actually reduce the instantaneous torque produced, and it may 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 such transients is to limit the transient demands. The regulator may be detuned to do so, but that limits performance. Another option is 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.