Scalar control

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 orientation inside the motor. ^[1] Scalar control is based on equations valid for a steady-state operation ^[2] 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. ^[1] Low cost and simplicity keeps the scalar control in the majority of low-performance motors, despite inferiority of its dynamic performance; ^[3] vector control is expected to become universal in the future. ^[4]

Types

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

Open-loop

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]

Closed-loop

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]

References

1. Finch & Giaouris 2008, p. 483.
2. Buja & Kazmierkowski 2004, p. 744.
3. Trzynadlowski 2013, p. 43.
4. Bose 2009, p. 11.
5. Chan & Shi 2011, p. 3.
6. Bose 2002, p. 340.
7. Bose 2002, pp. 342–344.
8. With the current feedback in place, the motor can be driven using either a voltage-fed inverter or a current-fed inverter.
9. Bose 2002, pp. 345–346.
10. Bose 2002, p. 345.

Sources

• 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.