If looking at motors with English units, power will be
specified in horsepower.
Php = 6. 72 watts 1 hp = .009 hp 746 watts
Powerper motor = .0045 hp
These values are in a perfect world. Most robots do not
get to operate in a perfect world, so you will need to
account for the losses in the DC motor and inefficiencies in
the gearhead. I usually assume the losses are 50% or
greater. Find a motor that will deliver at least twice your
requirements or greater; this is a good rule of thumb.
I went to Grainger’s website ( www.grainger.com) to
see if I could find a motor to meet our specifications. The
motor specifications usually include the full load current,
rpm, and torque. These are the maximum values the motors
are designed to operate at. I was able to find one motor
that looked like it would meet all of the requirements; the
full load torque of 25 in-lbs and rated for 41 rpm. I then
noticed it would cost more than $150 for each motor.
Spending $300 for drive motors would bust my robot
budget, so I decided to keep looking. I was able to find two
more motors that were close to our calculated
requirements: one with an output torque of 10 in-lb at 50
rpm and the other with an output torque of 20 in-lb at 25
rpm. These both used the same motor with a different
gearhead and these motors were less than $50 each. This is
a much more reasonable price. Our ideal robot would have
at least 16 in-lb of torque per motor at 38 rpm. I decided to
go with the motor that would supply 10 in-lb of full load
torque at 50 rpm. This would give the robot plenty of
speed, but might require the final weight to be reduced.
Trading a little weight to save $200 seemed like a good
deal to me. I also know that my specification is for a “worst
case” situation and my robot will probably handle a 25 lb
robot just fine.
Once you have selected your motors, you can plug
them back into your equations and see how your choice
FIGURE 2. Torque oz-in, rpm graph.
Tips For Selecting DC Motors
36 SERVO 01.2010
may affect your final robot design. Again we start with
T = M (a + gsinθ)r
A little more algebra takes us to:
M= T =
(a + sinθ) r
(.254 m + ( 9. 8 m sin( 10)) .0762 m) s2 ( ) s2 ( )
= 7. 58 kg
If we multiply this value by two for each drive motor
and divide by two for our safety factor, then convert our kg
value to pounds, we end up with a 17 pound robot. You
can probably exceed this value by four or five pounds
without much concern, but you know that you will not be
able to build a 70 pound robot using these motors and
meet your performance requirements.
A better way to look at the performance you can
expect from a DC motor is the speed-torque curve. One
nice thing about DC motors is that most of the relationships
are linear. So, a graph that shows the relationship between
speed, current, and torque is easy to develop. You only
need to have to a few values. These are No Load Speed, No
Load Current, Stall Current, and Stall Torque. You already
know the stall speed is zero.
Figure 2 is an example of the graph from a motor for
a larger robot. The following values were supplied by the
manufacturers specification sheet. The No Load Speed is 65
rpm, the No Load Current is . 63 amps, and the Stall Torque
is 8,500 oz-in. The motor has a maximum efficiency of
66%. We would like the motors to operate at about 1,000
oz-in and around 55 rpm. I was able to quickly generate a
speed-torque curve in a spreadsheet. From this graph, I
could see the robot will need to supply about five amps per
motor. If possible, I like to stay on the lower 30% of the
speed–torque curve. Stalling a DC motor can result in a
much shorter operational life. If you are using a gearhead
motor, it can also destroy the gearhead.
During the initial selection of the motor, we assumed
that there was enough friction between the wheel and the
surface so there is no slip. This is actually a bad assumption.
Most wheels will spin when the robot is started at full
speed on a slicker surface. In some cases, this can be a real
problem. One solution to this situation is to select different
wheels. Or, a better solution is to ramp up the motor speed
instead of starting at full speed.
When selecting motors, another key consideration is
the fine print on the specification sheet. A motor may be
capable of supplying much more torque than the gearbox