CHAPTER 17 ■ DC MOTORS
Load Current
As force is added to the shaft of the motor, by attaching wheels or driving a robot uphill, the amount of
current increases. Even attaching an RPM measuring disc to a motor adds enough drag that the motor
consumes a bit more current than it did by itself.
The increase in current stays at that level as long as the load is there. When the load is removed, the
motor current decreases.
Stall Current
As more and more of the motor’s maximum torque is required to rotate more and more massive or forceful
loads, the motor’s current increases until the motor doesn’t have the twisting strength to turn anymore.
When the motor stops turning but is still receiving power, that’s called a stall.
Since power is fully applied but the motor’s rotor can’t turn, almost no inductance is generated. This
is because there isn’t a change in current like when the motor was starting to turn. Also, no back EMF is
generated because the motor would choose to go faster if it could. It’s physically stopped from turning; it’s
not settled into its natural speed-per-volt ratio.
A stall is the very worst state for a motor!
During a stall, the only thing resisting the current is the resistance of the coil windings. When the motor
shaft is held in place, the current flow in the escap motor rockets from 7 mA to over 600 mA.
That’s bad for battery life, but worse for the motor. None of the electricity is being converted to motion
through magnetism. Almost all of the electricity flowing through the motor coils is producing heat. The
motor could be destroyed by heat if allowed to stay in a stalled state for too long.
The dangers present in a stalled motor explain the phenomena of robot builders leaping to save their
creations when the robot gets stuck against a chair leg or a wall. If the wheels aren’t turning but the motors
have power, those motors are stalling.
Robots can be designed to watch their wheels and to pulse, reverse, or cut power if the wheels aren’t
turning. Fuses or self-resetting circuit breakers can turn off power in the event of large current drain due to a
prolonged stall. Adequate ventilation helps cool down the motors if they do stall briefly. By choosing strong
enough motors, the safety margin should provide more than enough torque to avoid stalling during expected
loads of operations.
Planning for Current Consumption
Here are some thoughts on planning for the electrical-current consumption of motors:
The robot’s chips must be able to continue working during the start-up current draw of the motors. This
usually means adding some capacitors. However, Sandwich has only one comparator chip whose datasheet
says capacitors aren’t necessary.
If a robot’s motors are turning on and off a lot, like they are in the line-follower, you can assume the
batteries will deplete faster due to start-up current. No big deal. However, if the motors stay on for longer
periods of time, you can almost ignore the start-up current’s effect on battery life.
No-load current tells you the absolute minimum current that the motors consume. This is one of many
criteria for comparing motors, but a “no-load” situation is unrealistic for calculating battery life.
Load current is a much more practical value to determine battery life and heat dissipation. Of course,
the robot will almost be complete, with motors and wheels installed, before you can use the multimeter to
get a load-current reading. You’ll also look a little weird following a robot around your house, bent over with
a multimeter.
Robust robots should be designed to be able to continuously provide more than the start current or stall
current of all motors. This takes care of worst-case scenarios. The impact of the design is negligible in that
the electronics and batteries need to be a little beefier and the motors must be properly ventilated. The robot
you spend tens of hours making (if not hundreds of hours) will be safe and capable.