If your project involves any sort of movement, you’re most likely going to use some sort of DC motor to accomplish said movement. And if you’re into building or experimenting with robots, the use of DC motors is a necessity.
There is a lot to say about DC motors and complete books exist on this exciting subject. This post is going to cover the ones that are most likely to cross your bench. Also, this will be an introductory article where we’ll take a look at each type of DC motor from a bird’s eye view and cover the basics.
The various DC motors will get a more in-depth treatment each with their own article in the future.
Enough said, let’s acclimate ourselves with some common DC motors!
Intro to DC Motors
DC Motors and AC Motors
Some of you may be wondering about AC motors and asking yourselves why we’re only covering DC motors in this article.
While a few of us may work with AC motors, most of us will gravitate towards DC motors unless we’re working on a project involving movement that plugs into an AC outlet. As far as robotics goes, it is definitely possible to build a robot with an AC motor, but if you do the length of the power cord will place a limit on your creation’s movement. Sure, you could use batteries, but you’d need some pretty hefty batteries and more circuitry to convert DC into AC which increases cost and complexity.
In reality, AC motors are an essential part of modern life. Some AC motors have huge horsepower ratings of 100,000+. They find use in many commercial and industrial applications and are the workhorses of our society. And some of the appliances in your home use them, though brushless DC motors are starting to creep in and replace the AC versions in many home appliance applications. Needless to say, I’ll probably cover AC motors in a future post.
One last thing to note about motors in general is that an electric motor and an electric generator are one in the same. If you spin the shaft of an electric motor, a voltage will appear at the terminals. And, as we already know, if we apply a voltage to the terminals of a motor the shaft will spin.
Brushed DC Motors
Brushed motors are the oldest type of DC motor (around since late the 1800’s) with origins that trace back to Michael Faraday, so we’ll take a look at them first.
This type of DC motor consists of three main parts: a stator, armature and commutator. Figure 1 depicts a DC motor of this type. Figure 2 shows the insides of a smaller motor, one similar to the type you may use in a project. Note that brushed DC motors also find their way into commercial and industrial applications.
Figure 1: a brushed DC motor for use in commercial and industrial applications.
Figure 2: the insides of a small brushed DC motor.
A different view can be seen in figure 3 below.
Figure 3: another view of the inside of a DC motor with brushes. Here we can see the stator which is made from the magnets covering the inside of the case.
As we can see from figure 3, a series of permanent magnets comprise the stator. Figure 2 shows us that the rotor of this particular DC motor has windings on it with the commutator at the end.
The brushes (not shown in figure 2 or 3) are usually carbon or copper. They transfer electricity to the commutator and therefore make physical contact with it. This ultimately wears them down which is a draw back. Bigger, more expensive motors have replaceable brushes. Regardless, the brushes also cause a lot of electromagnetic interference due to sparking and arcing, which is another downside of this type of DC motor.
The image in figure 4 gives a good view of how it all fits together with the brushes. In figure 5 we get a close up of what brushes look like.
Figure 4: cutaway view of DC motor with brushes visible.
Figure 5: close-up of typical DC motor brushes.
Even though brushes present some disadvantages to the motor, these devices are easily reversible, plus they sport high starting torque and high speed with easy control.
Brushless DC Motors
Brushless DC motors (a.k.a. BLDC motors or BLDCMs) are one of the newer kids on the block as far as motors go.
As their name implies, they have no brushes. This makes them more efficient — brushes making contact with the commutator causes friction losses. This lack of brushes also makes them easier to maintain, and BLDC motors produce much less electromagnetic interference than their brush-bearing cousins because brushes cause arcing and sparking. This also makes them suitable for use in environments with flammable vapors and dust.
On the downside, they tend to be more expensive pound for pound and control is more complex requiring speed controllers or other special circuitry.
Internally, they are similar to the brush-toting variety, but with no brushes they need no commutator. Many brushless DC motors contain a Hall effect sensor or Hall effect IC to help with control. Figure 6 depicts the insides of two different types of BLDCMs.
Figure 6: inside a typical brushless DC motor. The one in part (a) is the inrunner type meaning the rotor is on the inside. Part (b) shows the outrunner variety with the rotor on the outside.
Stepper motors are also brushless, but we’re going to treat them separately later in this post.
As we can see from figure 6, the BLDC motor’s rotor supports permanent magnets (an even number of them). It’s the magnets that rotate in this type of motor while the windings stay put. This helps keep the windings in tact as they don’t suffer from the physical forces involving fast rotation.
The more magnetic poles the rotor has the smaller the rotational steps are. This results in less torque ripple. Don’t get confused by the term “rotational steps.” Unlike the stepper we’re going to learn about in a minute, the brushless DC motor is meant for continuous rotation. But, like a stepper, the BLDC motor commutates (in this context “commutates” means certain coils turning on and off) according to a predetermined coil activation sequence. That’s about the only similarity between the two. Reversing this sequence reverses the motor. One thing to note is that brushless DC motors are not great at precise rotor alignment.
Brushless DC motors are starting to replace their brushed predecessors, finding their way into cordless tools and home appliances like washing machines, DVD players and more. If you build drones you probably use BLDC motors.
Advances in semiconductor technology and Moore’s Law made replacing brushes with electronic control feasible. Figure 7 depicts a typical hobby BLDC motor. For the most part brushless DC motors look similar to the brushed variety from the outside.
Figure 7: a typical hobby BLDCM. My drone build includes motors similar to the one here.
The BLDC motor in figure 7 is an outrunner type, meaning the outside rotates rather then the inside. The windings don’t move but form the core of the motor. In other words, the rotor is on the outside. As we can see from figure 6, Inrunner versions also exist where the rotor dwells on the inside. The coils attach to the inside of the casing and as before remain stationary.
Stepper motors are another common motor you’ll run into if you deal with robotics or anything that moves. Steppers are a type of brushless DC motor but deserve a portion of the spotlight on their own. They’re similar to brushless DC motors (they have no brushes either) but have many poles per rotation. One of their main advantages is the ability to control them without closed-loop feedback which can be complicated to implement. Open loop systems are usually simpler and cheaper.
Two main types of stepper motors exist: unipolar and bipolar.
A unipolar stepper uses a coil with a center tap on each pole and requires current to flow in one direction. Only half the coil on a particular pole is driven at a time. A bipolar stepper uses a single coil per pole, usually with twice the turns as that of a unipolar stepper. They also require the current to reverse direction. Bipolar steppers generally need more complex circuitry to accomplish this which can increase the cost of the controller. However, the power to weight ratio of the bipolar type surpasses that of the unipolar type since the whole coil is driven at once.
Note that by pole we simply mean a north or south magnetic field that is generated by a permanent magnet or current passing through a coil of wire.
Figure 8 shows a typical stepper motor. Note that from the outside, it sort of looks like the DC motors from the previous sections of this article, with the exception of the leads.
Figure 8: a typical stepper motor.
Unlike the other DC motors we already know about, a stepper moves in discrete steps and does not rotate continuously, though they can run in forward motion or in reverse. For those who aren’t sure what I mean, imagine the second hand on an analog clock. It doesn’t rotate continuously, rather it “ticks” in small steps. Steppers are also very accurate in angular position which makes them great for robotics and anything requiring precise movement.
Like the BLDC, we can’t just hook one lead to ground and the other to some voltage greater than zero and expect it to work. They need special control, and depending on the stepper, they can sport four to eight connecting wires. Fortunately, one can build their own circuit with transistors if they know what they’re doing or grab a stepper driver IC. Since steppers are inherently digital, they are also compatible with microcontrollers though you can’t drive one directly from the pin of a microcontroller since they draw too much current.
A stepper needs to receive a rectangular pulse train to rotate its shaft a certain number of degrees. This is dictated by the number of pulses in the pulse train.
The standard stepping resolution (i.e. step size) for most common steppers is 200 steps for 360 degrees of motion which translates into 1.8 degrees per step. We can use half-stepping if we wish; this doubles our count to 400 steps per 360 degrees or 0.9 degrees per step. Some steppers are capable of micro-stepping and can obtain thousands of steps per revolution. Most hobbyists won’t need this level of precision though.
Figures 9 and 10 give a general idea of what the inside of a stepper motor looks like.
Figure 9: inside a stepper motor.
Figure 10: inside a stepper motor from a different angle.
You can find stepper motors in printers, various computer drives (like old floppy drives), robots, and CNC machines.
For most hobbyists, servo motors will be the go-to motor, especially for those into robotics.
There is another complete article that focuses just on servo motors on this site (https://www.circuitcrush.com/servo-motor-introduction/) so I’m not going to repeat too much of it here, but will quickly cover some servo basics to make things complete.
Figure 11 depicts a variety of servo motors.
Figure 11: a variety of servo motors.
A servo motor is a special subset of a continuous DC motor.
A servo motor has three leads. One is for power, one for ground, and the third is a control lead. Since a servo is a closed feedback system (unlike the stepper motor which is open loop), the control lead is needed to sense the position of the servo’s shaft and adjust it if necessary.
All servos have at least three main parts:
- A DC motor
- Reduction gears
- Control circuitry
Figure 12 shows the guts of a typical hobby servo motor and offers some explanation of the parts in the caption. A schematic drawing of a typical servo’s parts and how they fit together is shown in figure 13.
Figure 12: inside a typical hobby servo. We can see the DC motor, the control board, and the potentiometer (variable resistor; it’s the brown thing in the upper right next to the DC motor) that connects to the control board. The pot senses the position on the shaft. On the outside we can see some gears and a ‘horn’ attached to the end. The horn is the business end and can come in a variety of shapes and sizes depending on what it’s moving.
Figure 13: drawing of the insides of a servo and how they fit together. As we can see, the pot senses the position of the shaft. The “electronics” in the picture translate to the control board we can see in figure 12.
You can’t just connect a battery to a servo motor and watch it go. This is because servos need special control signals to operate correctly. The good thing is there are plenty of ways to easily do this with microcontrollers, boards like Arduino, or even simple electronic circuits.
We’ll end our discussion on servo motors here, but for much more detail I urge you to see An Introduction to Servo Motors.
DC Motor Specs
An intro to DC motors wouldn’t be complete without a quick overview of some important specs.
The term operating voltage is self-explanatory. Generally, the more voltage you apply to a DC motor the faster it spins. However, there are limits to this and too much voltage can damage the motor.
Most DC motors will run at voltages either lower or higher than the specs call for but there are some caveats. For example, a 6 V motor will likely run at 3 V, but it won’t be as powerful and it’ll run slower. Also, many motors may not run (or run very poorly) at voltages below 40 or 50 percent of their rating.
Finally, try not to run a DC motor at more than 200% of its rating, or damage may result.
Current draw is another self-explanatory spec. It’s the amount of current the motor requires. Current draw for a motor with no load is going to be at its lowest. Once you apply a load, the current draw increases until it hits a maximum when the motor stalls. Keep this in mind when using motors in your projects. The power supply or batteries need to be able to handle the worst case scenario where the motor stops because it can no longer handle the load. Just imagine your small robot running into a wall. If it tries to keep going the wheels will either slip or the motors driving them will stall as they futilely try to push the robot through the wall.
We all know what speed is. It’s usually given in RPMs (revolutions per minute). Most motors spin too fast and need some sort of gears on them to slow them down. Gearing also affects the next spec — torque.
Torque is turning power. Simply put, it’s the force a motor exerts on its load. People measure it by attaching a lever of known length to the end of the motor shaft and using a scale or weight gauge. When a DC motor stalls, it puts out max torque. Torque and current draw are proportional, that is, the more torque the motor puts out the more current it will draw until it stalls where both are maximum.
How to Test the Current Draw of a DC Motor
Let’s get practical. It’s possible to obtain the current draw of a particular motor with various loads (free running, stall, everything in between) so you can size your power supply right. You’ll need a multimeter with a 10 A input to do this. Please be sure this input has a fuse in case your motor happens to draw more than 10 A, or you’ll fry your meter.
Here’s how to do it:
- Plug positive the lead (the red one) into the 10 A input. Leave the black or common lead alone; it should be in the common or negative position.
- Place the meter leads in series with the motor. If possible, try to determine if the motor will draw more than 10 A before you do this. The motor specs or the Internet may help with this. If it does, you can’t use this method, but you may be able to use the alternate method below. If you’re not sure how to hook your meter up, see figure 14.
- Dial in the 10 A setting, power the motor and observe the output.
- You can place various loads on the shaft. Your fingers may suffice for small motors. Be careful when using pliers; this can damage the shaft and/or be downright dangerous if your motor is powerful enough. Also, you should make sure powerful motors are secured to something stable before trying to stall them. For every reaction there’s an equal and opposite reaction…
Figure 14: here’s how to hook up your meter to test the current draw of a motor. The battery can be any given power supply. The fuse should be built into your meter. The switch is optional, and the load device is the motor.
So, what do we do if we suspect our motor draws more than 10 A?
Fortunately, there is a solution though it’s a little bit more complex (but still not very hard).
We can measure the current of a DC motor indirectly. We do this by using a power resistor and measuring the voltage across it. Using Ohm’s Law, we can then calculate the current. If you forgot about Ohm’s Law, see Simple Circuit Analysis Techniques You Should Know.
Here’s the steps:
- You won’t need to put the positive lead in the 10 A jack, so just leave the positive (red) lead of your meter alone. Of course, you’ll want to leave the black lead alone also.
- Insert a power resistor in series with the motor. Use anything between 1 to 10 Ohms, but make sure it can handle at least 20 watts of power, the higher the better especially if your motor draws more than 10 A.
- Power up the circuit and measure the voltage across the resistor. If you’re not sure how to rig the set up, see figure 15.
- Use Ohm’s Law to get the current: I = V/R or current equals voltage (across the resistor) divided by the resistor’s value.
- Vary the load and get current draw under different conditions.
Figure 15: alternate indirect way to measure current draw of DC motor.
DC Motor Wrap-up
That does it for our intro to DC motors.
There are a few other specialized DC motors (such as linear motors), but they’re not as likely to end up on your bench.
In future articles I’ll go into more detail on each type of motor.
Meanwhile, comment and tell me about your projects involving motors. What are you building? What’s your go-to motor? I’d really enjoy hearing about it!