DC Motors

DC Motors

Goals

1. Discuss the different types of motors used in small electronic devices

2. Introduce DC motors

3. DC motor power considerations

4. DC motor applications

Key Principles

• Quick reminder: Motors are really cool but this is not a physics course. More discussion about how they works at the end
• DC Motors convert electrical energy into mechanical rotation
• Apply more voltage and the motor spins faster (to a point)
• Electric current in a magnetic field p roduces force—this is what keeps the motor spinning

DC Motor Applications

• Hobby motors
  • Small electronics
  • Low-torque applications
• Large DC motors
  • Electric scooters
  • High-torque applications
• Motors can also generate electricity (generators / alternators)
• We cover brushed motors which are simpler to understand, but brushless motors are more efficient and powerful (used in high performance applications like drones)

Three Main Parts of DC Motor

• Permanent / stationary magnets (stator)
• Rotating wire coils (armature / windings)
• Switch that automatically reverses current to keep spinning (commutator)

Inside Simple DC Motor

Exercise

• Attach fan blade to motor
• Connect red wire on motor to 3v3; connect black wire to ground
  • What happens?
• Reverse the wires
  • What happens?
• Was the speed the same in either case?

Motors

• Spin is controlled by current direction
• If direction is reversed, the motor spins in the opposite direction
• It is not practical to have to reverse the wires to change motor direction
• But…we have no way to control speed
• And…motors could draw too much current for Photon 2 or damage pins in motor moves in reverse

Motor Controller

Motor Driver

• Motor (motor controller) provide greater control over motors
• Motor drivers don’t give a (+) or (-) connection
• Just have an IN1 or IN2 because we can change polarity
• Motor drivers also provide circuit protection
• Motor drivers can control two different motors independently

Motor Controller Wiring Guide

Motor Controller	Explanation
PWMA	Motor A speed (PWM)
AI1, AI2	Motor A direction control (connect to Photon 2)
AO1, AO2	Motor A output (connect to motor)
PWMB, BI1, BI2, BO1, BO2	Controls for motor B
VCC	Power for chip (3v3)
VM	Power for motors (3v3, or higher for powerful motors)
STBY	Enable motor (3v3)
GND	Ground

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Controlling the Motor Direction

• Setting the direction is done by changing the two input pins to HIGH and LOW separately
  • Ex: AI1 = HIGH and AI2 = LOW is one direction
  • Ex: AI1 = LOW and AI2 = HIGH is other direction
• Setting them both to LOW means stopping the motor
• Setting them both to HIGH can damage the motor

Controlling the Motor Speed

• The PWM method of controlling motor speed operates on this range: [0-255]
• Typically the PWM must be somewhat greater than 0 before it starts to spin
• Reminder: only certain pins support PWM
  • D1 (SCL or A4)
  • A2
  • A5
  • MISO (D16)
  • MOSI (D15)

Wiring Diagram

Wiring Guide

Motor Controller	Photon 2
PWMA	A5
AI2	D4
AI1	D3
AO1	Motor wire (either)
AO2	Motor wire (either)
VCC	3v3
GND	GND
VM	3v3
STBY	3v3

Exercise

• Attach the fan blade to the DC motor
• The theoretical PWM values are 0 to 255. Write a sketch to determine the min and max values.
• Write a sketch that powers up the fan from the practical minimum (the value we established in the previous exercise) to the maximum
• There should be a ramp up in speed, getting progressively faster until it maxes out

Bonus Notes on Motors

How Do Brushed Motors Work?

• Electromagnetism!
• VERY brief physics aside

Electromagnetism

• The guiding principle behind electric motors is the Lorentz Force

• Electrical current in a magnetic field produces a physical force
• Force direction is given by the “right-hand rule”
  • Current direction along your index
  • Magnetic field along your middle
  • Force direction is along your thumb

DC Motor Construction

Illustration

DC Motor Types

Brushed	Brushless (synchronous or electronically commutated motor)
Physical contact points in the commutator to switch current direction	Inverter to electrically switches the DC power supply to AC power supply
Pro: Relatively inexpensive	Pro: Less wear on moving parts
Con: Parts wear out over time	Con: More expensive

Credits

• Images created with Fritzing
• Original slides created by Ray Kim
• Sparkfun
• For more in-depth discussion of motors, see this page