1. DC Motors and H-Bridges
Presented By:
Sean Huang, Daniel Simon, and Scott Smith
[1]
[2]

2. Presentation Outline Electric Motors A (Brief) Review of E&M Physics
Brushed DC Motors
Controlling DC Motors
H-Bridges
Brushless Motors and Control
References and Further Reading

3. Electric Motors

4. What are Electric Motors?
Turn electrical energy into mechanical energy.
Useful in wide range of embedded applications from automotive to medical.
Vary from extremely complex to incredibly simple.
[3]

5. Many Different Types of Motors
Electric Motors
DC Motors
AC Motors
Brushless Motors
Brushed Motors

6. A (Brief) Review of E&M Physics

7. A Brief Review of Ohm’s Law
V = I * R
[4]
I = V / R = 9 / 3000 = A = 3 mA

8. A Brief Review of Magnets

9. A Brief Review of Magnets

10. A Brief Review of Magnets

11. A Brief Review of Lorentz Forces
In which direction is the force? N I S
Lorentz Force:
F = IL x B
[5]

12. A Brief Review of Lorentz Forces
In which direction is the force? N I S
Lorentz Force:
F = IL x B
[5]

13. Brushed DC Motors

14. How do Brushed DC Motors Work?
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15. How do Brushed DC Motors Work?
[6]

16. How do Brushed DC Motors Work?
Commutator Ring
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17. How do Brushed DC Motors Work?
Commutator Ring
Brushes
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18. How do Brushed DC Motors Work?
[6]

19. How do Brushed DC Motors Work?
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20. How do Brushed DC Motors work?
[8]
Adding additional coils increases motor stability.

21. Controlling DC Motors

22. Goals: Be reliable (Doesn’t blow up) Control Speed/Torque
Control Direction

23. One (Naive) Approach
Could work if you had a small motor with a beefy enough microcontroller to supply the power, AND you don’t get control of the direction. Ex. if the motor is 1 ohm, and the microcontroller goes 0 to 3.3v, then the motor try to draw 3.3AMPS. Most microcontrollers can’t supply that much current on a single pin.

24. One (Naive) Approach ??
How would you change the direction?

25. One (Naive) Approach Not enough power (usually) No directional control
Could work if you had a small motor with a beefy enough microcontroller to supply the power, AND you don’t get control of the direction. Ex. if the motor is 1 ohm, and the microcontroller goes 0 to 3.3v, then the motor try to draw 3.3AMPS. Most microcontrollers can’t supply that much current on a single pin.

26. Better Approach
We add an external voltage supply. This fixes the problem of supplied power. Now we have the GPIO pin controlling a MOSFET or transistor when to supply the motor with power and as long as we have a big enough supply there is no issue with the size of the motor. One problem here: Back EMF.

27. Better (Dangerous) ApproachI

28. Better (Dangerous) ApproachI
One problem here: Back EMF. steady state - when the switch is closed for a long time, and then turned off. If we open the switch there is nowhere for the current to go. This translates to a spark or arc at the switch. No good.
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29. Better (Dangerous) Approach
Henry I
[9]
Mr. Henry will not have a good time.

30. Better Approach Fixes the power supply problem, but Back EMF Sparks
Still no directional control
We add an external voltage supply. This fixes the problem of supplied power. Now we have the GPIO pin controlling a MOSFET or transistor when to supply the motor with power and as long as we have a big enough supply there is no issue with the size of the motor. One problem here: Back EMF.

31. Better Approach
[10] I
Flyback diode creates a path for the current to go after the switch is opened. This will slowly decay the current down.

32. Better Approach
[10] I I
Flyback diode acts as it’s not even there.

33. Better Approach
[10] I
Path for current to go now.

34. Better Approach Doesn’t blow up I Control Speed (PWM)
[10]
Doesn’t blow up
Control Speed (PWM)
Still doesn’t control direction I

35. H-Bridges

36. H-Bridge Components 4 switches centered around a load 4 flyback diodes
Power supply
Ground
[11]

37. Powering the Motor Create a path for current to flow through motor
Switches turned on/off independently using I/O
If switch is off (open), no current will flow through the switch
If switch is on (closed), current will flow through the switch
Current will flow through the load if the diagonally-paired switches are on simultaneously
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38. H-Bridge Hazards (Shoot-through)
If both switches on one side are on (closed), we’ll get a short circuit
Can happen when trying to reverse the direction of the motor
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39. Hazard Handling Delay is present when toggling the switches
Delay can cause both switches on one side to be on/off simultaneously
Perform “turn off” before “turn on” 1 1 1 1
So all practical bridge designs are biased in the other way, making sure that the two switches are never on at the same time, but as a consequence they will be both off for a short while during switching.
energy is dissipated through losses in the wire by the inductor to draw current from it in a continuous loop, the diode and the resistor.
The voltage across the inductor will be a function of the forward voltage drop of the Flyback diode. Total time for dissipation may vary, but it will last for a few millisecond
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[11]
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40. H-Bridge Hazards (Back EMF)
Sudden change in current creates large voltage on motor
I -> 0
V -> inf
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41. Flyback Diodes Diodes create a loop that reduces current change
Smaller current change means smaller voltage across the motor (no back EMF) 1 1
V = v
So all practical bridge designs are biased in the other way, making sure that the two switches are never on at the same time, but as a consequence they will be both off for a short while during switching.
energy is dissipated through losses in the wire by the inductor to draw current from it in a continuous loop, the diode and the resistor.
The voltage across the inductor will be a function of the forward voltage drop of the Flyback diode. Total time for dissipation may vary, but it will last for a few millisecond
[11]
I = i
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42. Controlling Motor Speed with PWM
[11] 1 1
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43. Brushless Motors and Control

44. How do Brushless DC Motors Work?
[8]
Switch the position of the magnets and coils.

45. Brushless Control
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46. Brushless Motors Brushed Motors
Long lifespan
Very high efficiency
Low noise generation
High power to size ratio
Cheap
Easy to use
Requires few wires
Simple control circuitry

47. Recommended Further Reading
Brushed DC Motors:
Brushless DC Motors: resources/engineer-school/brushless-dc-motor-01-overview.html
H-Bridges: the-basics/
Brushless Control: power-and-control-brushless-dc-motors

48. References
[1] DC Motor Photo:
[2] H-Bridge Hardware Photo:
[3] Toy Photo:
[4] Simple Circuit:
[5] Right Hand Rule Photo:
[6] DC Motor Still Image and Slow GIF:
[7] Alternate Fast DC Motor GIF:
[8] Better Brushed Motor Photo and Brushless Motor Photo:
[9] Shock photo:
[10] Flyback Diode Photo:
[11] H-Bridge Photos and Tables:
[12] Brushless Motor Control::
[13] Motor Spec sheet: