CHAPTER 17 ■ DC MOTORSLuckily, it’s not that bad. There’s a property called inductance that doesn’t like to see a sudden change
in current. So, although there is a nasty blast of current (see Figure 17-21) in the interval of start-up, it isn’t
quite as bad as the simple formula indicates.
The surge of electricity from the battery can be a problem for any electronics attached to the same battery.
It is possible for chips and LEDs to be temporarily starved while the battery feeds the engaging motors. There
are simple techniques, like adding capacitors, which provide local energy storage during the energy crisis.
Depending on the motor, the start-up current diminishes to no-load current in about one-tenth of a
second.
No-Load Current
As the motor comes up to the appropriate RPM based on the voltage provided, the amount of current
flowing through the motor declines. Why? Because it is easier to keep something spinning than it is to get the
thing spinning in the first place.
Let’s take the escap motor as an example. At first, a lot of power is applied to make the rotor change
from 0 RPM to 4000 RPM. But, thereafter the rotor has some inertia and is going the desired speed. The only
continued electrical investment needed at that point is to overcome a bit of friction, noise, vibration, and
sparking.
Let’s say the motor would drop down to 3900 RPM in a tenth of a second if power were disconnected.
So, in that tenth of a second, a motor with power needs only change from 3900 RPM to 4000 RPM, rather
than 0 RPM to 4000 RPM. It obviously takes less power to speed up 100 RPM as opposed to 4000 RPM, which
is why a spinning motor uses less power than a motor starting up.
When a motor is up to speed and the motor shaft is not connected to anything, the amount of current
flowing is called the “no-load” current. This is the least amount of current the motor uses.
No-load current is easy to measure. Simply connect your multimeter in amp mode (like when you
measured current in the LED Circuit) between the battery and the motor. When the number settles down
after a second or so, that’s the no-load current.
At 12 V, the escap motor has a no-load current of a mere 7 mA. It’s as though the coil resistance went from
16.4 W at power-off to 1714 W at no-load spinning. An almost magical property called back EMF or induced
EMF (Electromotive Force) resists the flow of more electricity than is needed to keep the motor going.
The important thing to keep in mind is that an already-spinning motor takes less power than getting a
motor to start spinning. That’s no-load current.
Figure 17-21. Oscilloscope trace of current flow through an escap motor