Scalar control of an AC electrical motor is a way to achieve the variable speed operation by manipulating the supply voltage or current ("magnitude") and the supply frequency while ignoring the

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

orientation inside the motor. Scalar control is based on equations valid for a steady-state operation and is frequently open-loop (no sensing except for the current limiter). The scalar control has been to a large degree replaced in high-performance motors by vector control that enables better handling of the transient processes. Low cost and simplicity keeps the scalar control in the majority of low-performance motors, despite inferiority of its dynamic performance; vector control is expected to become universal in the future.

Types

The variants of the scalar control include open-loop control and closed-loop control.

Open-loop

The most common approach makes the voltage V proportional to frequency f (so called V/f control, ''V/Hz control'', ''Constant Volts/Hertz'', CVH). Advantage of the V/f variant is in keeping the

magnetic flux In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ( ...

inside the

stator The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric mot ...

constant thus maintaining the motor performance across the range of speeds. A voltage boost at low frequencies is typically employed to compensate for the resistance of the coils. An open-loop V/f control works well in applications with near-constant load torque and gradual changes in rotational speed. The controllers implementing this method are sometimes called ''general purpose AC drives''.

Closed-loop

If sensors are utilized (

closed-loop A control loop is the fundamental building block of industrial control systems. It consists of all the physical components and control functions necessary to automatically adjust the value of a measured process variable (PV) to equal the value of ...

configuration) for better/faster transitional response, the common approach uses a rotational speed sensor (so called ''closed-loop V/Hz control''). The speed error is passed through the proportional-integral controller to create the accumulated '' slip'' difference that is combined with the direct reading of the speed sensor into a frequency control signal. In a torque-control variant (TC, not to be confused with the

direct torque control Direct torque control (DTC) is one method used in variable-frequency drives to control the torque (and thus finally the speed) of three-phase AC electric motors. This involves calculating an estimate of the motor's magnetic flux and torque based ...

a.k.a. DTC), the motor torque is held constant in the steady-state, this requires a current sensor. Frequency and flux (voltage or current, depending on the type of the driveWith the current feedback in place, the motor can be driven using either a voltage-fed inverter or a current-fed inverter.) control signals are decoupled, with the flux control driven by the flux estimate, and the frequency control driven by the torque estimate and speed sensor data. The increased performance comes at the cost of additional complexity and associated potential stability issues.

References

Sources

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