Introduction to AC motors

1. Introduction to Motors
Understanding the CEENBot’s muscles
Presented by:
Herb Detloff
(Prepared by Alisa N. Gilmore, P.E.)
CEPA Summer Robotics Institute Summer 2009
at Central Community College, Columbus, NE

2. Robot Defined


3. Overview
 Motors in context of robotics, different types of
robots have different types of motors
 Overview of motor types / characteristics
◦ Motors convert electric energy to mechanical force /
motion
◦ Motor parameters: AC or DC power source, torque,
speed
◦ Industrial robotics: AC servo motor
◦ Mobile robotics & Hobby robots: dc motor, dc servo
motor, and stepper motors
 Principle of operation of a DC motor
◦ Inside a DC motor
 Principle of operation of stepper motors
◦ Performance advantages of stepper motor over DC motor
and DC servo motor
 CEENBot stepper motor operation/control

4. Motor Basics
 Motors convert electric energy to
mechanical motion.
 Either an AC or DC electrical energy
source serves as the input to the motor.
 The result is mechanical motion of the
output shaft, that is a rotation about or a
translation along the shaft, provided the
load carried by the shaft does not exceed
the maximum load the motor is designed
to carry.

5. Choosing a Motor
 There are numerous ways to design a motor,
thus there are many different types of
motors.
 The type of motor chosen for an application
depends on the characteristics needed in
that application.
 These include:
◦ How fast you want the object to move,
◦ The weight, size of the object to be moved,
◦ The cost and size of the motor,
◦ The accuracy of position or speed control needed.

6. Motor Parameters
 The level of performance a motor can provide
is described by its parameters. These
include:
 Rated Speed
◦ Speed measured in shaft revolutions per minute
(RPM)
 Torque
◦ Rotational force produced around a given point, due
to a force applied at a radius from that point,
measured in lb-ft (or, oz-in).
 HorsePower = Speed x Torque / 5252.11...
◦ A measure of work expended: 1 HP = 33,000 foot-
pounds per minute.
 Torque-Speed performance of a motor

7. Types of Motors
 The different types of motors possess
different operating characteristics.
◦ Heavy Industrial applications: AC motors
◦ Mobile robotics & hobby robots: dc motor, dc servo
motor, and stepper motors
 Brief overview of the operation characteristics
of:
◦ AC motors
◦ DC motors
◦ DC servo motors
◦ Stepper motors

8. AC Motor Characteristics
 When power is applied, AC motors turn in
one direction at a fixed speed.
 Both reversable and non-reversable models
available
 Usually high voltage (110V AC and up)
 Inexpensive and commonly available
 Optimized to run at a fixed, usually high
RPM.
 If the applied load is greater than the
capacity of the motor, the motor will stall and
possibly burn out.

9. DC Motor Characteristics
 When power is applied, DC motors turn in one direction
at a fixed speed.
 They are optimized to run at a fixed, usually high RPM.
 Torque is highest at the rated speed and lowest at low
speeds.
 Speed can be varied if a (pulse width modulation) PWM
controller is added.
 Almost all can be reversed.
 Inexpensive and commonly available.
 Available in wide range of speeds and power.
 Suitable for turning, spinning, etc.
 Not suitable for positioning unless some kind of position
feedback is added.
 If the applied load is greater than the capacity of the
motor, the motor will stall and possibly burn out.

10. DC Servo Motors
 Servo motor requirements may include control of acceleration,
velocity, and position to very close tolerances and allow for fast
starts, stops and reversals, and very accurate control.
 DC servo motors consist of a DC motor combined with feedback
for either position or speed.
 A system with a motor, feedback, and a controller which
constantly adjusts the position or speed to in reaction to the
feedback is called a closed-loop system
 Hobby Servos require a desired position signal to tell them where
to turn to.
 Once told where to go, a Hobby Servo uses its built-in controller
and feedback system to hold its position.
 When power is applied, in the absence of a signal, a hobby servo
goes to its central position
 The signal to control a hobby servo is non-trivial to generate.
 Hobby servos can also be modified to turn continuously, in which
case the control signal is speed instead of position

11. Stepper Motors
 Requires a separate controller circuitry or it will
not turn when power is applied.
 Inexpensive and commonly available, especially
in salvaged computer equipment
 Precise positioning is possible by keeping count
of steps requested, even without feedback.
 Torque is highest at the full stop and decreases
as speed is increased.
 If the applied load is greater than the capacity of
the motor, the motor may not step, thereby
making precise positioning no longer possible.

12. DC Electric Motors
 Electric Motors or Motors convert electrical energy
to mechanical motion
 Motors are powered by a source of electricity – either
AC or DC.
 DC Electric Motors use Direct Current (DC) sources
of electricity:
◦ Batteries
◦ DC Power supply
 Principle of How Motors Work:
 Electrical current flowing in a loop of wire will produce
a magnetic field across the loop.
When this loop is surrounded by the field of another
magnet, the loop will turn, producing a force (called
torque) that results in mechanical motion.

13. Motor Basics
 Motors are powered by electricity, but rely
on principles of magnetism to produce
mechanical motion.
 Inside a motor we find:
◦ Permanent magnets,
◦ Electro-magnets,
◦ Or a combination of the two.

14. Magnets
◦ A magnet is an object that possesses a
magnetic field, characterized by a North and
South pole pair.
◦ A permanent magnet (such as this bar
magnet) stays magnetized for a long time.
◦ An electromagnet is a magnet that is created
when electricity flows through a coil of wire. It
requires a power source (such as a battery) to
set up a magnetic field.

15. A Simple Electromagnet
 A Nail with a Coil of Wire
 Q – How do we set up a magnet?
 A – The battery feeds current through the coil of
wire. Current in the coil of wire produces a
magnetic field (as long as the battery is
connected).

16. A Simple Electromagnet
 A Nail with a Coil of Wire
 Q - How do we reverse the poles of this
electromagnet?
 A – By reversing the polarity of the
battery!
+ -
S N

17. The Electromagnet in a Stationary
Magnetic Field
 If we surround the electromagnet with a stationary magnetic
field, the poles of the electromagnet will attempt to line up
with the poles of the stationary magnet.
 The rotating motion is transmitted to the shaft, providing
useful mechanical work. This is how DC motors work!
OPPOSITE
POLES
ATTRACT!

18. DC Motor Operation Principles
 Once the poles align, the nail (and shaft) stops rotating.
 How do we make the rotation continue?
 By switching the poles of the electromagnet. When they line up
again, switch the poles the other way, and so on.
 This way, the shaft will rotate in one direction continuously!

19. Brushed DC Motor Components

20. How the Commutator Works
 As the rotor turns, the commutator terminals
also turn and continuously reverse polarity of
the current it gets from the stationary
brushes attached to the battery.

21. Controlling Motor Direction
 To change the direction of rotation:
◦ Simply switch the polarity of the battery leads
going to the motor (that is, switch the + and –
battery leads)
+
-
-
+
CW CCW
Direction of
Rotation

22. Inside a Toy Motor
(Similar to TekBot Motor)

23. Advantages of Stepper Motor
 The DC motors on the TekBot offer limited
speed control and low torque.
 The CEENBot uses a stepper motor for each
wheel.
 The stepper motors on the CEENBot enables
accurate wheel positioning with high holding
torque and allows for open-loop speed
control (wheel position feedback is not
required).

24. Stepper Motor Operation
 A stepper motor consists of:
◦ A permanent magnet rotating shaft (or rotor)
◦ Electromagnets on the stator – the stationary portion that
surrounds the motor
 The stepper motor moves as the permanent rotor
magnet attempts to line up with the poles of the
electromagnets on the stator.
 The electromagnets are digitally switched to change
their pole orientation, which when done in a sequence
produces continuous rotation of the rotor, or can be
controlled to give precise rotation to a desired
angular position.
http://www.interq.or.jp/japan/se-inoue/e_step1.htm

25.  The smallest step of angular rotation a stepper motor
can make is called its resolution.
 Unlike the example, which had 90 degrees per step
resolution, real motors employ a series of mini-poles
on the stator and rotor to increase resolution.
 Surprisingly, the same sequence of the 4 stepping
phases is used to control this scenario.
http://www.interq.or.jp/japan/se-inoue/e_step1.htm
 CEENBot stepper motors have a resolution of 1.8
degrees per step.
◦ Q: How many steps are needed to make 1 complete wheel
revolution?

26.  Because the rotor is fixed by the magnetism
in the stationary condition as shown, the
stationary power (Stationary torque) is large.
It suits the use to make stop at some angle.
◦ The CEENBot can better hold its position on a ramp.
 Speed control is achieved by digitally cycling
through the phases at a desired speed of
rotation.
 A microprocessor is used to reverse the
current after each step, which changes the
poles of the corresponding electromagnets.

27. References
 “The Difference Between Stepper Motors, Servos, and RC
Servos” by Roger Arrick
http://www.arrickrobotics.com/motors.html
 Making Things – “General Information on Motors”
http://www.makingthings.com/teleo/products/documentati
on/app_notes/motors_general.htm
 “How Stepper Motors Work” by Images Scientific
Instruments
http://www.imagesco.com/articles/picstepper/02.html
 CEENBot Stepper Motor & PM DC Motor Testing Unit
Operations Manual by Ben Barenz, CEEN Student
 Hansen Corp. “Servo motors” http://www.hansen-
motor.com/servo-motors.htm
 Animated operation of a Unipolar stepper motor:
http://www.interq.or.jp/japan/se-inoue/e_step1.htm