MVRT 115 Mechanical

1
MVRT 115Mechanical Training Motors
PowerPoint by Ashwin Mathur Team 115
2
What is a Motor?

• A motor is a device that converts electrical
  power into mechanical power
• Learn to love motors, they make the robot come to
  life

3
WHAT IS A MOTOR?
A motor generates rotational motion.
Conventional motors use chemical or
electromagnetic and electric PE.
ie Car engines (motors) burn gasoline to turn
wheels.
4
ROTATIONAL MOTION
Rotational motion has two components.
TORQUE
SPEED
Torque is the force of the rotation.
Speed is the rate of rotation.
5
Fundamental Motor Basics

• The greater the load on the motor, the greater
  resistance it applies on the circuit
• To overcome the resistance of the motor, the
  current, or amount of electrons, is increased
• However, resistance converts electric energy to
  heat, thus leading to overheating of motors
• Todays lesson Dont overload your motor! It
  makes magic smoke, and magic smoke BAD!

RPM Rotations Per Minute
6
Physics Review

• Torque is the rotational force
• Torque Force X Radius
• Power Force (N) X Velocity (m/s)
• Power Torque (N-m) X Angular Velocity (Rad/Sec)
• Electrical Power Voltage X Current

7
POWER
Power describes energy over time in Watts (W).
There are two power equations that deal with
electric motors
ELECTRIC
ROTATIONAL
PIV
PTV
Power Current times Voltage
Power Torque times Speed
8
ELECTRIC POWER
Power Current times Voltage
PIV
Voltage is difference in charge, or potential.
V
Volts
Current is the quantity of electrons moving over
time.
I
Amps
9
ROTATIONAL POWER
Power Torque times Speed
PTV
Torque is the force of the rotation.
T
Newton-meters
Speed is the rotations per unit of time.
V
Rotations per Minute
10
THE ELECTRIC MOTOR
An electric motor uses electric energy.
Electromagnets
Magnet
Magnet
Armature
A simple DC motor. Current flow generates an EMF
around the armature.
The armature continues to rotate.
11
Once it has reached the zero position, the
current is reversed and the process repeats.
12
Motor Characteristics

• Free Speed How fast the motor spins when not
  under a load
• Free Current How much current the motor is
  drawing when it is under no load
• Stall Torque How much resistance is needed to
  fully stop the motor from turning (stalling)
• Stall Torque How much current the motors draw
  when it is stalled

13
MOTOR CHARACTERISTICS
Electric motors behave with certain
characteristics.
As the armature of the motor turns, it generates
backward current.
With no load and constant voltage, motors output
a certain rpm.
With maximum load and constant voltage, the motor
outputs a certain torque.
The graph of the relationship between torque and
rpm looks like this.
Thus, the faster the motor turns, the greater its
resistance to current flow.
Even as you put in a huge amount of power, the
graph begins to flatten out.
At these high power inputs, most of the energy is
converted to heat.
This is the maximum speed of the motor at this
voltage.
Here is the graph of power vs. speed.
At low speed, you either have low power or high
power.
At those points, you have either low torque or
high torque.
To find the best power output of a motor...
We take our real torque vs. speed graph...
And find when their product is greatest.
This is always halfway between the two extremes.
The torque at this point is called the max power
torque.
The speed at this point is called the max power
rpm.
This is the maximum torque at this voltage.
Ideally, the graph should look like this.
At any point, the product of T and V is the same.
This is the graph of speed versus power.
However, in reality, the power is not constant.
At this point, the motor is most efficient.
This is called the free load rpm.
This is called the stall torque.
14
Combined motor curves

15

• Speed vs. Torque The harder the motor is
  pushing, the slower it gets.
• Current vs. Torque When the motor is pushed
  really hard, current get higher and higher.
• Power vs. Torque Power Torque X angular
  velocity. When motor power approach near zero, or
  at least not maximum, the extra electric power
  actually turned into heat. Maximum power happens
  at the peak of curve, when torque pushing the
  motor is ½ stall torque, and when speed is ½ free
  speed.

16
Different Motors

• Chipahua/CIM Motor
• Window Motor
• Van Door Motor
• Fisher-Price/Mabuchi Motor
• Globe Motor

Yes, Van Door Motors are the motors that make
your Van Door move! And Yes, Window Motors are
the motors that make your car window move up and
down!
17
Chipahua or CIM

• 343 Watts of power
• 252.98 ft-lb/sec stall torque
• 5310 free load rpm
• Efficiency does not reduce much w/ heat
• Useful for prolonged running (drivetrains)
• Most commonly used

18
THE CIM
Power Output
343 Watts
Free RPM
5310
Stall Torque
343.4 oz-in
Ideal for long running times.
19
Window

• 22 Watts of power
• 16.22 ft-lb/sec stall torque
• 90 free load rpm
• Gearbox does not back drive
• Useful for attaining and maintaining a position
  (arms)

20
THE WINDOW
Power Output
22 Watts
Free RPM
84
Stall Torque
10.6 Nm
Useful for preventing back drive.
21
Van Door

• 69 Watts of motor power
• 37 Nm stall torque clockwise
• 34 Nm stall torque counter-clockwise
• 75 free load rpm
• Considerably powerful, useful for medium
  actuations (arms)

22
Fisher - Price

• 140 Watts motor power
• 103.25 ft-lb/sec stall torque
• 24000 free load rpm
• Good for medium-load actuations

23
THE FISHER PRICE
Power Output
387.5 Watts
Free RPM
126
Stall Torque
51.79 Nm
Hi power, but suited for medium loads due to
plastic gears.
24
Globe

• 47.16 Watts of power
• 3194 oz-in stall torque
• 78.9 free load rpm
• Also useful for small-medium load actuations
• W/ gearbox, torque x 90, speed x 1/117

25
THE GLOBE
Power Output
55 Watts
Free RPM
81
Stall Torque
150 in-lb
Low power motor for low-medium load applications.
26
THE MABUCHI
Power Output
16.8 Watts
Free RPM
4700
Stall Torque
60.8 mNm
Counterclockwise rotation with additional spur
gear.
Sadly, the Mabuchi was not around when motor
photos were being taken.
Nobody missed it.
27
Picking the right motor

• Keep these in mind
• Ease of mounting
• Size
• Power output of the motor
• Stall torque
• Free load rpm
• Effects of prolonged use
• Limited of motors per year
• Efficiency
• Does it Back drive?
• Is it the best motor for the job?

28
Gear Ratio

• Using gears allows you to change the torque and
  speed output
• Gear Ratio - Ratio of the numbers of teeth on
  mating gears.
• Found by dividing the number of teeth on the
  larger gear by the number of teeth on the smaller
  gear.

36 teeth
18 teeth
Ratio 21
29
Gear Ratios

• The smaller gear will go faster, while the larger
  gear will go slower
• The faster it goes, the less torque it has

30
Motor and Gear Efficiency

• The more extensions you use on your motor, the
  more power you lose
• Typical spur gears lose 90-95 EACH
• Thus, after just 2 spur gears, youre left with
  1-0.25 of the original power
• So, if youre gearing down a motor, do it with as
  few gears as possible to conserve power
• Effective Power motor power (Watts) X component
  efficiency ()

31
Efficiency

• Spur Gears 90-95
• Worm Gears 10-60
• Nut on a Tread 10-60
• Twist Cables 30-70
• Chain 85-95
• Cable 0-98
• Rack Pinion 50-80

Worm Gear Chain and sprocket



Cable Rack and Pinion