Scalar control of an AC

electrical motor An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate Laplace force i ...

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 (sometimes called B-field) is a physical 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 ...

orientation inside the motor. Scalar control is based on equations valid for a steady-state operation and is frequently

open-loop A control loop is the fundamental building block of control systems in general and industrial control systems in particular. It consists of the process sensor, the controller function, and the final control element (FCE) which controls the process ...

(no sensing except for the

current limiter Current limiting is the practice of imposing a limit on the current that may be delivered to a load to protect the circuit generating or transmitting the current from harmful effects due to a short-circuit or overload. The term "current limiting" ...

). The scalar control has been to a large degree replaced in high-performance motors by

vector control Vector control is any method to limit or eradicate the mammals, birds, insects or other arthropods (here collectively called " vectors") which transmit disease pathogens. The most frequent type of vector control is mosquito control using a varie ...

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 we ...

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 (such as bacterial flagella or ATP synthase). Energy flows through a stator to or from the rotat ...

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 control systems in general and industrial control systems in particular. It consists of the process sensor, the controller function, and the final control element (FCE) which controls the process ...

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 ''

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'' 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 In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

is held constant in the steady-state, this requires a

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (hydr ...

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

* * * * * * Electric motors {{electric-stub