Page 151 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 6 Options for Creating and Controlling Motion 131

• NO LOAD The next column is split into speed and current. No load means
this is what the motor is going to do when there is nothing attached to the
shaft. Under the no load condition, this little motor is going to spin at 23,000
rpm! That’s fast. And it’s going to take only 0.07A to do it.

NOTE Sometimes you’ll see motor speed in revolutions per second (rps) or
radians/second (rad/s). There are 2 radians in one revolution, and 60 seconds
in 1 minute. To convert from rad/s to rpm, multiply the rad/s by (60/2 ) to get
rpm. Or just go to www.onlineconversion.com/frequency so you don’t need
to remember the conversion.

• STALL TORQUE Let’s skip to the last column. This tells us that the motor will
stall, or stop moving, when resisted with 0.34 millinewton-meters (mNm) of
torque. Think of this as the maximum strength of the motor. This measurement
of torque is in the familiar force × distance units, but if you can relate better
to imperial units, go to www.onlinecoversion.com/torque to change it to
something else. It turns out that 0.34 mNm equals about 0.05 oz-in. This is
very weak, so this tiny motor could barely spin a 0.05 oz weight at the end of
a 1 in stick glued to the motor shaft. You can feel how little torque this is by
pinching the shaft with your fingers. It stops almost immediately. You can
always stall DC toy motors with your fingers since the stall torque is so low.

• AT MAXIMUM EFFICIENCY This column contains a lot of numbers that are
useful to review. Efficiency describes the relationship of mechanical power
delivered to electrical power consumed. DC motors are most efficient at a
fraction of the stall torque (in this particular case, maximum efficiency is
around one-fourth of the stall torque). This torque corresponds with the EFF
label on the bump on the graph of torque versus current in the middle of
Figure 6-6. The motor uses power most efficiently at this torque. You can use
the motor at a torque closer to its full stall torque, but it will be slower, and
less of the electrical power will be converted to mechanical motion, which is
particularly draining if you’re running on batteries.

DC Gearhead Motors
The next step in motor complexity is the DC gearhead motor. This is just a standard
DC motor with a gearhead on it. A gearhead is just a box of gears that takes the
output shaft of the standard DC motor and “gears it up” to a second output shaft