Scalar control
Low-cost, simple AC motor speed control using voltage and frequency.
Scalar control is a method for variable speed operation of AC electrical motors that manipulates the supply voltage or current (magnitude) and the supply frequency while ignoring the magnetic field orientation inside the motor. It is based on equations valid for steady-state operation and is frequently open-loop, with no sensing except for a current limiter. Scalar control has been largely replaced in high-performance motors by vector control, which better handles transient processes, but its low cost and simplicity keep it in the majority of low-performance motors.
- Variants
- open-loop control and closed-loop control
- Common open loop method
- V/f control (Volts/Hertz, Constant Volts/Hertz, CVH)
- Closed loop sensor type
- rotational speed sensor
- Closed loop controller type
- proportional-integral controller
- Torque control variant sensor
- current sensor
Lore & Background
Scalar control achieves variable speed by adjusting the magnitude of supply voltage or current along with the supply frequency, deliberately ignoring the magnetic field orientation inside the motor. Its equations are valid only for steady-state operation, and it is most often implemented as open-loop control with only a current limiter for protection. The most common open-loop variant is V/f control, which keeps voltage proportional to frequency to maintain constant magnetic flux in the stator, with a voltage boost at low frequencies to compensate for coil resistance. Closed-loop scalar control adds a rotational speed sensor, passing the speed error through a proportional-integral controller to generate a slip difference that combines with the sensor reading into a frequency control signal. A torque-control variant uses a current sensor to hold motor torque constant in steady-state, decoupling frequency and flux control signals, with flux driven by a flux estimate and frequency driven by a torque estimate and speed sensor data.
Reader's Guide
Scalar control's significance lies in its low cost and simplicity, which keep it dominant in low-performance motors despite inferior dynamic performance compared to vector control. The article notes that vector control is expected to become universal in the future, but scalar control remains widespread due to its ease of implementation and suitability for applications with near-constant load torque and gradual speed changes. The open-loop V/f variant is so common that controllers implementing it are sometimes called general purpose AC drives. The closed-loop and torque-control variants offer improved transitional response and steady-state torque holding, but at the cost of additional complexity and potential stability issues. The legacy of scalar control is as a foundational, cost-effective technology that continues to serve a broad range of low-performance applications while vector control gradually takes over high-performance roles.
Did You Know?
- Scalar control is based on equations valid only for steady-state operation.
- Closed-loop scalar control uses a rotational speed sensor and a proportional-integral controller.
- The torque-control variant requires a current sensor and decouples frequency and flux control signals.
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