CHAPTER 17 ■ DC MOTORS
To be fair, the escap motor costs 10 times as much, weighs 2.5 times as much, has ¼ the RPM at 3 V, and
is about 25% longer than the toy motor. But, that low current draw of 4 mA to 7 mA is enormously appealing.
Examining Periods of Widely Changing Current Consumption
The amount of current consumed by a motor changes a lot during operation. As you’ll soon see, the amount
of current flow during start-up and stall is very different than no-load or load current.
Start-Up Current
The exposed motor guts presented early in this chapter showed that an ordinary brush motor doesn’t
contain any resistors or other similar parts inside of it. If you connect your multimeter in W mode (like when
you measure resistors) to the two leads of a motor at rest (not spinning), you’ll see there is a resistance.
■ Note It isn’t always possible to accurately measure the resistance of assembled motor coils using a
multimeter. If the brushes are dirty or aren’t making full contact or if the motor is under mechanical stress, the
resistance values vary wildly.
Although wire can usually be thought of as having practically no resistance, the lengths of wire in a
motor are long enough to act like a resistor.
Big motors have a thick wire inside, so big motors usually have tiny resistances, below 1 W. However,
small motors have an extremely skinny wire, which is more resistant. Small motors have resistances usually
between 6 W and 150 W.
At the moment power is applied to a brush motor at rest, the only things resisting the flow of electricity
are the coils of wire in the rotor. Here’s a worst-case formula:
(V battery / W motor coils) × 1000 = mA maximum current
escap example: (12 V battery / 16.4 W motor coils) × 1000 = 732 mA
Figure 17-20. Escap motor (left) and toy motor (right)