[SOLVED] FOC vs Sine Wave Control : What's the difference? [SOLVED]

[james@N52E01]

A few months ago, Johannes did an EV fundamentals video explaining the concept of Field Oriented Control. In simple terms, what is the difference between Sine control and Field Oriented Control in how they function and what are the advantages/disadvantages of each?

[johu]

Both control methods output a 3-phase sine wave with 120° offset at a frequency (almost) equal to the rotor rotation speed. More on "almost" below. They mainly differ at how they arrive at the amplitude of those sine waves. The "sine" method calculates the amplitude from frequency (called V/Hz control) AND throttle input. The FOC method has a true current control loop, so it controls the amplitude to arrive at a certain phase current.

Now, the "almost" frequency. For asynchronous motors the sine method adds or subtracts a few Hz from the rotor frequency to generate something called "slip".
For synchronous motors that is not appropriate but here the requirement is for the sine waves to have a certain, but variable, offset from the rotor magnetic field. This offset translates to d and q current in FOC world.
Also for synchronous motors the amplitude control is much more difficult to get right. For an async motor 0V means 0 torque, more volts, to some degree, means more torque. For sync motors there is a very fine line where the motor generates no torque, that is when the voltage is equal to the so called back EMF, i.e. the voltage inherently generated by the motor. You can see that very well in Damiens last video when he puts a volt meter on disconnected MG1. This fine line is virtually impossible to hit with V/Hz control. Below it the motor regen, above it accelerates.

For that reason I decided that synchronous motors require FOC to operate smoothly.

The advantage of "sine" control is its simplicity and low hardware requirements. You can run it with only a motor speed feedback device. You don't need phase current sensors. For that reason, and because I'm too lazy to implement FOC for asynchronous motors, the sine software is still maintained.

[james@N52E01]

Thank you Johannes that clears a lot up.

[celeron55]

FOC for a permanent magnet AC motor almost works like an electronic commutator for the motor windings, if you're familiar with DC motors. Actually it's fancier than that, but that's the most important part of what it does.

Non-FOC control basically comes from the need to soft start industrial 3 phase motors. You want to avoid running them with stupid frequencies and voltages compared to measured motor speed as would happen if you just plugged them to 3 phase mains power. This way you save your breakers and as a bonus get more efficiency and also fine control of speed and torque. As a result, you have fine enough control to connect a throttle pedal and drive.