Three Phase Inverter

Background

Three phase brushless control is optimal for DC brushless, non-stepper motors. Stepper motors are not considered here, as that is a different construction.

Brushless, brushed, AC, induction, and DC - all these are different

Brushless DC

• Sensorless control is possible - [1]. Good overview of characteristics.
• These limitations of using position sensors combined with the availability of powerful and economical microprocessors have spurred the development of sensorless control technology. Solving this problem effectively will open the way for full penetration of this motor drive into all low cost, high reliability, and large volume applications. - [2]
• To reduce cost and improve reliability such position sensors may be eliminated. To this end, many sensorless schemes have been reported for position (and speed) control of BLDC motors - same as above
• A PM brushless drive that does not require position sensors but only electrical measurements is called a sensorless drive [4]. - ibid.

Sensorless Brushless Motor Control

Most so-called ESCs (electronic speed controllers) measure the back-EMF and control the motor based on this EMF. [3]

Great Scott shows DIY make of VESC (just buy it) - [4]

Or you can make a small one to do the same - with limited current, though. - [5]

The full open source version is available in the VESC Project.

Induction

1. Field is rotor is induced via AC. These are not permanent magnet.
2. Induction motors are 10% less efficient than permanent magnet motors
3. Note that in extreme cases, electromagnets can have higher magnetic fields than permanent magnets, so induction motors can have higher energy density, but still lower efficiency.

Brushless AC (Induction)

Induction motors are brushless - [6]

3 phase motor control has significant advantages over 1 phase in AC induction motors:

1. Self-starting
2. More efficient
3. Higher power factor - better aligment of applied power to used power - better coordination of windings with power application.

Brushed AC Motors: Universal Motors

• Can run on AC or DC. [7]
• Only 30-75% efficient, with 30% for small ones.
• Starters for engines are universal motors, as are many power tool, appliance, vacuum cleaner motors.

More

• At each point of time, 3 phases are active, but offset by 120 degrees:

https://www.youtube.com/watch?v=l60p4G7Bdi0

There is a 60 degree dead spot in the 120 degree (instead of 180 degree) mode of operation, allowing time for commutation. Makes sense - that could allow for energy saving in transition as commutation is occuring (by turning power off)

• In the Colombian paper, the coils are run 60 degrees out of phase.

https://www.emworks.com/ckfinder/userfiles/files/Axial%20Flux%20Motor.pdf

• How do you control a motor with 3 phase power?