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Small DC Motors

 DC Motors come in sizes from very small dc motors used in toys that run on as little as 1.5 volts to very large motors that can use 24 volts or more

For purposes of this tutorial a Small DC Motor is defined as small hobby type motors typically found in toys and rc vehicles that operate on from 1.5 to 6 volts DC and powered with typical consumer batteries such as A, AA, C, D and 6 volt lantern cells. They can also be powered with wall transformers. This section does not cover specialty DC motors such as Servos or stepper motors.

For smaller DC motors that come out of toys current draw is not an issue. However, as you purchase more powerful motors you need to be aware of the current requirements of the motors. You can find small DC motors that require more current than can be output from a battery pack containing a few household type batteries. Some motors may have current requirements (in amps) listed on a name plate. For others you may find it on the packaging. The motor may have two current ratings - one for no load and then a loaded (or a stall) rating. For other motors you may have to measure the amperage requirements yourself.   Video: Using a multimeter to meaure amps.

It is important to remember that the current use when the motor has no load on it is less than the current load when the motor is under load or in a stall (no longer turning) condition.

If you shop for dc motors at a surplus store (local or online) you may find motors listed that are listed as running on from 3V to 24V. Do not purchase these motors thinking that you will be able to run them on a battery pack with 2AA batteries that outputs 3 volts of power. The motors will run on 3 volts but the current requirement at this low voltage is very high. It would be very difficult to put together enough 3volt batteries to provide the current requirements of this type of motor.

DC Motor Direction Control

You can control the direction of most DC motors simply by swapping the polarity of the power connections to the motor. A DPDT switch can be wired to work as a motor reversing switch. See the Switches tutorial for information. If you want to use a DPDT switch to reverse the spin direction on a motor you may want to find a special type of DPDT called a DPDT center off switch. A center off switch provides a position on the switch where there is no connection to either set of throws which shuts off the motor.

DC Motor Speed Control

There are 3 basic ways to control the speed of a DC Motors:

  • Use gears
  • Vary the Voltage
  • Pulse Width Modulation

DC Motor Speed Control with Gears

Gears are a good way to provide speed control on a motor. However, it can be difficult to find gears that will work with the motors that you might find. Tamiya (see Resources) sells several gear boxes that operate at a variety of gear ratios. For a discussion of gear ratios see the Mechanical section of the tutorials. For this section you should just understand that a low gear ratio (i.e. 8:1) provides higher speed but less power than a large gear ratio (216:1) An advantage of using gears is that you can control the speed of a motor without losing any power. With higher gear ratios you actually create a device that is much more powerful than a device powered directly by the motor.

You can purchase Tamiya products from their website but many hobby shops also carry their products. The gear boxes sell in a range of $6 to $20. Tamiya is not the only supplier of gear boxes for smaller DC motors; they are just the one I am most familiar with. Click the thumbnails to see pictures of some Tamiya products.

3 of the gear boxes that Tamiya offers. Most of the gear boxes offer the user a choice of 3 gear ratios.  On the side of the boxes you will find a list of the gear ratios for the particular product plus the ratios of other Tamiya products. Users have to assemble the gear boxes at the gear ratio they have selected.

It is important to note that these gear boxes can only operate at one gear ratio at a time. To change gear ratios the box must be disassembled and reassembled for the new ratio. There may be inexpensive hobby gear boxes that provide some way to switch gear ratios without rebuilding the boxes but I have never found them.

You will need to experiment and find the gear ratio that works best for your particular device. The inability to change gear ratios on the fly can be problematic for teams considering using a gear box to power a vehicle. They must find a compromise gear ratio that best meets both the power and speed requirements of the vehicle they are wishing to build.

DC Motor Speed control by varying voltage

You can also vary the speed of a motor by varying the voltage. As you reduce the voltage to a motor the motor slows down. The problem is that when you reduce the voltage to a motor you also reduce its power. You can vary the voltage to a motor by using fewer batteries - if it is designed to run on three AA batteries (4.5 volts) then try to run it on two AA batteries (3 volts). It may be that the motor still may operate acceptably for you. Another way to reduce the voltage is to connect a resistor in series with one of the leads to the motor. For smaller motors (like the hobby motors sold by Tamiya) you can experiment with 1/4 watt resistors with 5 ohms or less of resistance. Purchase several different sizes of resistors and experiment with different values to see how motor speed changes. The problem with using a resistor is that the current drawn by the motor increases as the load on the motor increases. More current means means a larger voltage drop across the resistor and therefore even less voltage to the motor. The motor now tries to draw even more current possibly resulting in the motor stalling.

DC Motor Speed control by Pulse Width Modulation

Pulse Width Modulation (PWM) is the method of speed control used in many higher end remote control vehicles. In simple terms Pulse Width Modulation means applying the supply voltage to the motor for a variable amount of time. The advantage of Pulse Width modulation is that the motor slows down without losing any power since the voltage supplied to the motor is not reduced. It is simply "pulsed" to the motor for a variable time period. PWM control circuit kits can be purchased for less than $20. Verify that the kit will handle the voltage and amperage of the motor that you wish to control before purchasing a kit.  Video: Pulse Width Modulation kit controlling a 12 volt motor.

It is also possible to create a Pulse Width Modulation circuit using a PIC such as the Basic Stamp II (by Parallax) with some additional electronic components. The Parallax website has downloadable information on how to do this. Search the web and you can find additional information from others who have built these circuits.

Teams may want to investigate the solid state "H" Bridge. This circuit works much like a DPDT switch wired for motor direction control. Research the web and you will find sample circuits using a PIC microcontroller (see advanced topics) and an "H" bridge to provide both directional control and speed control via Pulse Width Modulation.