Motors, The Finale

Motor terminology is super annoying. Let's fix that.

# Motors 101 - A Summary

Brushed DC Motor - Permanent Magnet, DC

BLDC - Permanent Magnet, AC (trapezoid or sine wave designs)

VR Stepper Motor - Variable Reluctance, DC

Hybrid Stepper Motor - Variable Reluctance + Permanent Magnet, DC

AC Induction Motor - Induction, AC

All need their own motor drivers:

- Simple H Bridge w/ Rotary Encoder for Servo Closed Loop Control

- ESC Inverter, uses back EMF (aka "sensorless") for Variable Speed "Sensorless" Control (technically closed loop)

- Stepper Motor Driver (same for both I think), high voltage to overcome high inductance issue, Open Loop Control

- Variable Frequency Drive VFD Inverter, turns DC or AC into desired frequency AC power, Open Loop Control, Sensorless Control (uses math on current/voltage fluctuations), or Servo Control

# Motors 101

People will tell you that there are 4 types of motors:

• Brushed DC Motor

• Brushless DC Motor

• Stepper Motor

• AC Induction Motor

Unfortunately, these popular motor names are incredibly opaque as to what they are, what they do, how they're constructed, and how they're powered. The pedantically correct nomenclature for the fundamental motor designs are as follows:

• Brushed Permanent Magnet DC Motor

• Permanent Magnet Synchronous AC Motor

• Variable Reluctance AC Stepper Motor

• Asynchronous AC Induction Motor

Important variants of these motors include:

• Hybrid Variable Reluctance Permanent Magnet AC Stepper Motor

• "PMSM" (Sine Wave Version) BLDC

(fix highlighting later, but the alternating shows the different descriptors)

These descriptors are based on the following:

• Construction/Geometry

• Motor's Input Power (AC vs DC)

• Method of Generating Rotation (Brushed, Reluctance, Permanent Magnet(? think harder on this one vs brushed), Induction)

• For AC: synchronous vs. asynchronous

# Nomenclature

• ALL of these motors are brushless, so the "Brushless" descriptor has been dropped from the Permanent Magnet Synchronous AC Motor.

• Instead of using "brushed" as an antonym for "brushless", I now categorize "brushed" as the "method of generating rotation".

AC vs DC - This describes what you need to feed the raw motor in order to get it to turn. All of them need 3 phase AC power because of the sets of electromagnetic coils used in all of them. How they use them is slightly different

Synchronous and asynchronous are for AC motors only, describing the relationship between the line frequency from the inverter and the mechanical speed of the motor rotor. If they are the same, they are synchronous, and vice versa.

A stepper motor is a motor that uses a doubly salient stator/rotor structure to create fine resolution "steps".

• Doubly salient = protruding teeth on both rotor and stator

• Salient = protruding

• Called Salient poles

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MOTOR DESCRIPTIONS

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# Brushed Motor

DRIVER: H-Bridge. This simply flips the polarity of the DC voltage, so your motor can run forward or backwards. The brush inside the motor is still what causes the polarity switching necessary to rotate the motor.

# BLDC

## Ordinary BLDC

## "PMSM" BLDC

These orient the copper and the magnets differently so that they are optimized to be controlled by a sine wave. Conversely, when the shaft is rotated, the output is a smooth sine wave.

# Stepper Motors

HIGH INDUCTANCE - Small teeth, lots of windings = high inductance, which is why stepper motor drivers are higher voltage than the motor is (meant?) for. 24V driver, 3V motor?

## Variable Reluctance

Iron is attracted to any magnetic field, regardless of polarity. That's why the driver uses pulsed DC.

## Hybrid

• Reversing AC (or "chopped DC") - Must flip polarity to push/pull the internal permanent magnets. 

# AC Induction Motors

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INCLUDE GRAPHS OF TYPICAL TORQUE SPEED CURVES

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# Controllers

Brushed DC Drive

• Output: PWM Pulsed DC

• Primary Control Variable: Voltage

ESC Inverter

For traditional BLDC:

• Output: Trapezoidal 3 Phase AC

• Primary Control Variable: Commutation Timing (via Hall/Back EMF) 

• NOT a servo, but a "variable speed drive" because back EMF cannot provide absolute position feedback or any feedback at all at 0 speed.

• Can sometimes use FOC to improve torque and speed efficiency (CHECK SOURCE)

For sine wave PMSM:

• Output: Sinusoidal (3-Phase) AC

• Primary Control Variable: Field Oriented Control (Id​,Iq​)

• Brushless AC Drives. Uses FOC to produce smooth sinusoidal currents. Pedantically, a Servo drive is a high-performance AC inverter with a feedback loop.

• Usually uses an encoder, making this a servo motor.

Stepper Motor Driver

• Output: Stepped / Chopped DC 

• Primary Control Variable: Current and Pulse Frequency

• 
  

VFD Inverter

• Output: Variable Frequency AC

• Can take in AC (grid) or DC (battery)

VIDEOS:
Stepper Motor https://www.youtube.com/watch?v=eyqwLiowZiU

Brushless DC Motor https://www.youtube.com/watch?v=bCEiOnuODac

AC Induction Motor https://www.youtube.com/watch?v=AQqyGNOP_3o

IPM-SynRM https://www.youtube.com/watch?v=esUb7Zy5Oio

"PMSM" https://www.youtube.com/watch?v=cmRGhoF_ZEM

Other Nice Links

• Stepper vs Brushless https://www.motioncontroltips.com/comparing-stepper-and-brushless-dc-motors/

• Brushed Motor Animation: https://en.wikipedia.org/wiki/Brushed_DC_electric_motor

• 
  

A servomechanism, aka a "servo", is a system that uses closed loop control system to precisely control position, velocity, or acceleration. It is achieved by adding a sensor that provides the feedback necessary for closed loop control. It must contain a motor, a feedback sensor, and a controller that utilizes that feedback to correct for errors in position or speed.

Any of the motors above can be turned into a servomechanism motor, or "servo motor", by adding a feedback sensor. This is usually done with an encoder, a sensor that tracks precise angular position. The angle position of the encoder is then fed into the motor's control board and used in a closed loop control system.

The hobby servos you are used to seeing are usually brushed DC motors, paired with a potentiometer acting as a rotary encoder and a motor control board. The motor control board takes in 3 wires - positive, ground, and PWM signal. The PWM signal tells the board what position you want, and the board regulates the voltage going to the motor until it reaches the desired position. These cheap boards usually only do proportional (or maybe proportional-derivative) control, and are underdamped.

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One more thing to highlight is the IPM-SynRM AC motor. This is the motor used in most Tesla vehicles, starting from the 2018 Model 3 onwards.