Scalar control

The variants of the scalar control include open-loop control and closed-loop control.^[5]

The most common approach^[3] makes the voltage V proportional to frequency f (so called V/f control, V/Hz control, Constant Volts/Hertz, CVH^[3]). Advantage of the V/f variant is in keeping the magnetic flux inside the stator 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.^[1][6]

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.^[5]

If sensors are utilized (closed-loop configuration) for better/faster transitional response, the common approach uses a rotational speed sensor (so called closed-loop V/Hz control).^[5] 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.^[7]

In a torque-control variant (TC, not to be confused with the direct torque control a.k.a. DTC), the motor torque is held constant in the steady-state, this requires a current sensor.^[3] Frequency and flux (voltage or current, depending on the type of the drive^[8]) 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.^[9] The increased performance comes at the cost of additional complexity and associated potential stability issues.^[10]

• Finch, John W.; Giaouris, Damian (2008). "Controlled AC Electrical Drives" (PDF). IEEE Transactions on Industrial Electronics. 55 (2). Institute of Electrical and Electronics Engineers (IEEE): 481–491. doi:10.1109/tie.2007.911209. ISSN 0278-0046.
• Buja, G.S.; Kazmierkowski, M.P. (2004). "Direct Torque Control of PWM Inverter-Fed AC Motors—A Survey". IEEE Transactions on Industrial Electronics. 51 (4). Institute of Electrical and Electronics Engineers (IEEE): 744–757. doi:10.1109/tie.2004.831717. ISSN 0278-0046.
• Trzynadlowski, A.M. (2013). "Scalar Control of Induction Motors". The Field Orientation Principle in Control of Induction Motors. Power Electronics and Power Systems. Springer US. ISBN 978-1-4615-2730-5. Retrieved 2023-10-29.
• Chan, T.F.; Shi, K. (2011). "Scalar Control". Applied Intelligent Control of Induction Motor Drives. IEEE Press. Wiley. ISBN 978-0-470-82828-1. Retrieved 2023-10-31.
• Bose, B.K. (2002). Modern Power Electronics and AC Drives (PDF). Eastern Economy Edition. Prentice Hall PTR. ISBN 978-0-13-016743-9. Retrieved 2023-10-31.
• Bose, Bimal (2009). "The past, present, and future of power electronics [Guest Introduction]". IEEE Industrial Electronics Magazine. 3 (2). Institute of Electrical and Electronics Engineers (IEEE): 7–11, 14. doi:10.1109/mie.2009.932709. ISSN 1932-4529.