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.{{sfn | Finch | Giaouris | 2008 | p=483}} Scalar control is based on equations valid for a steady-state operation{{sfn | Buja | Kazmierkowski | 2004 | p=744}} 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.{{sfn | Finch | Giaouris | 2008 | p=483}} Low cost and simplicity keeps the scalar control in the majority of low-performance motors, despite inferiority of its dynamic performance;{{sfn | Trzynadlowski | 2013 | p=43}} vector control is expected to become universal in the future.{{sfn | Bose | 2009 | p=11}}
Types
The variants of the scalar control include open-loop control and closed-loop control.{{sfn | Chan | Shi | 2011 | p=3}}
= Open-loop =
The most common approach{{sfn | Trzynadlowski | 2013 | p=43}} makes the voltage V proportional to frequency f (so called V/f control, V/Hz control, Constant Volts/Hertz, CVH{{sfn | Trzynadlowski | 2013 | p=43}}). 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.{{sfn | Finch | Giaouris | 2008 | p=483}}{{sfn | Bose | 2002 | p=340}}
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.{{sfn | Chan | Shi | 2011 | p=3}}
= 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).{{sfn | Chan | Shi | 2011 | p=3}} 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.{{sfn | Bose | 2002 | pp=342-344}}
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.{{sfn | Trzynadlowski | 2013 | p=43}} 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.{{sfn | Bose | 2002 | pp=345-346}} The increased performance comes at the cost of additional complexity and associated potential stability issues.{{sfn | Bose | 2002 | p=345}}
References
{{Reflist}}
Sources
- {{cite journal | last=Finch | first=John W. | last2=Giaouris | first2=Damian | title=Controlled AC Electrical Drives | journal=IEEE Transactions on Industrial Electronics | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=55 | issue=2 | year=2008 | issn=0278-0046 | doi=10.1109/tie.2007.911209 | pages=481–491 | url=https://eprints.ncl.ac.uk/file_store/production/76397/769727EB-8AC3-4F66-9F78-974A2B1D5700.pdf}}
- {{cite journal | last=Buja | first=G.S. | last2=Kazmierkowski | first2=M.P. | title=Direct Torque Control of PWM Inverter-Fed AC Motors—A Survey | journal=IEEE Transactions on Industrial Electronics | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=51 | issue=4 | year=2004 | issn=0278-0046 | doi=10.1109/tie.2004.831717 | pages=744–757}}
- {{cite book | last=Trzynadlowski | first=A.M. | title=The Field Orientation Principle in Control of Induction Motors | publisher=Springer US | series=Power Electronics and Power Systems | year=2013 | isbn=978-1-4615-2730-5 | chapter-url=https://books.google.com/books?id=bZ0MBwAAQBAJ&pg=PA43 | chapter=Scalar Control of Induction Motors | access-date=2023-10-29}}
- {{cite book | last=Chan | first=T.F. | last2=Shi | first2=K. | title=Applied Intelligent Control of Induction Motor Drives | publisher=Wiley | series=IEEE Press | year=2011 | isbn=978-0-470-82828-1 | chapter=Scalar Control | chapter-url=https://books.google.com/books?id=alORU2FNvowC&pg=PA3 | access-date=2023-10-31}}
- {{cite book | last=Bose | first=B.K. | title=Modern Power Electronics and AC Drives | publisher=Prentice Hall PTR | series=Eastern Economy Edition | year=2002 | isbn=978-0-13-016743-9 | url=https://eee.sairam.edu.in/wp-content/uploads/sites/6/2019/07/Modern_power_electronics_and_AC_drives.pdf | access-date=2023-10-31}}
- {{cite journal | last=Bose | first=Bimal | title=The past, present, and future of power electronics [Guest Introduction] | journal=IEEE Industrial Electronics Magazine | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=3 | issue=2 | year=2009 | issn=1932-4529 | doi=10.1109/mie.2009.932709 | pages=7–11, 14}}
{{electric-stub}}