Adafruit Motor Array

Motor Basics

Main image from Adafruit.com

Anyone that has even dabbled in animatronics has come across a variety of motors.  They range from tiny vibrational motors that can make noise to giant motors that lift and animate heavy, elaborate props.  This prop-building staple is great when it works and easy enough to repurpose from an existing prop into a new concept, but buying one without context can be confusing.  Here, we’ll cover the most common motor types, the terms you’ll encounter, and where you might find them in the wild to incorporate into props for next year!

AC vs. DC

AC (alternating current) and DC (direct current) motors require different types of power because of how they’re constructed.  DC motors are the simplest, are based on magnets, and need a DC power source such as a battery or power supply that converts AC power from the wall to a usable DC voltage.  AC motors rely on the alternating highs and lows in AC voltage to spin, and will not work with a DC power source.

AC motors are typically application-specific and do not have characteristics that can be easily changed.  For example, the most common AC motor we encounter in the animatronic scene is a 5RPM motor that rotates or raises and lowers the head of a light-up deer.  This motor is designed to use 110VAC so that it can plug straight into any wall outlet (in countries that use 110VAC rather than 240VAC) and spins at about 5RPM under any load up to a maximum capacity.  Any AC motor you are likely to encounter will almost definitely use 110VAC since it is readily available to most consumers, and most are under 10 RPM. These low RPM, high-torque motors are more efficient at moving large and heavy props than DC motors, and are commonly found in holiday lawn animatronics for that reason.

FrightProps Deer Motor

DC Deer Head Motor from FrightProps.com

DC motors, on the other hand, come in a huge range of sizes, speeds, and power requirements.  In general, you will find an inverse relationship between how quickly they want to spin and how heavy a load they can move for the same electrical power.  Some hobby motors that spin very quickly (15,000-24,000RPM) cannot even overcome the resistance of a piece of paper taped the shaft when powered by a 9V battery, while I’ve encountered 30RPM motors that I cannot stop with my fingers when powered by the same source.  DC motors are typically found in anything battery powered, from RC cars to household robots like Roombas.

Specialty motors, such as stepper and servo motors, are just DC motors with additional hardware and circuitry that allow for finer position control based on a signal generated by a microcontroller.  We love these motors for many applications, and will cover them separately in the future.

RPM and Torque

There are two main questions to ask when evaluating a motor:

  1. What power does it want?
  2. How fast can it spin under the weight of what I want spun?

You should be able to tell whether the motor wants AC or DC power based on where you found it and/or the markings on the motor itself.  The second question is based on the RPM and torque of the motor.

RPM (revolutions per minute) is fairly self-explanatory.  A 5RPM motor’s shaft (the spinny bit sticking out the top) makes 5 full rotations per minute.  A 10,000 RPM motor makes ten thousand rotations per minute.  Minutes are not an especially intuitive measurement for most people, and I tend to have to divide the RPM number by 60 to get rotations per second.  I can visualize something taking 2 seconds to spin all the way around much better than 1 minute to spin 30 times, even though it’s the same speed.  The aforementioned 5RPM deer-head motor will appear to spin very slowly if you are only watching the shaft rotate, since it will take a full 12 seconds to make a single rotation.  However, the deer head that it is attached to appears to move much faster because it is further away from the center of rotation:

Deer Head Motion

This brings us to the other important motor parameter: torque.  Torque is a measure of how much weight a motor can move at a certain distance from the shaft.  You will typically see torque values in a combination of length (ft, in, or cm) and weight (oz or lbs), or even the SI unit of Nm, the Newton meter.  The conversion necessary to compare a 2.5 foot-pound motor to one rated at 480oz-in is not something I can do in my head, so I rely on quick online converters like this one.

It can be a little tricky to figure out how strong of a motor you need if you are not moving a single mass at exactly 1ft from the shaft.  Typically, we are doing something like moving a zombie’s arm up and down, which is moving a mass distributed more or less evenly along a motor arm that is probably longer than 1ft.  Calculating the exact minimum torque for an application like that involves integrals, so I tend to spec my motors as though all the weight were at the end of the arm, where it’s hardest to move.  For example, if I’m moving a 3’ arm that will be covered with 2lbs of padding and material, I do the math as if that entire 2lbs of padding were out at 3’ from the shaft. Thus, I look for a motor that’s rated at 6lb-ft.  That motor will be more powerful than absolutely necessary, but gives us some headroom and can even extend the life of the motor by not driving it at its absolute maximum rating.

Once we find a motor that can move what we need to move and at the speed we need it moved, we need to understand how much power it requires to do so.  All the math we’ve done so far assumes the motor will have access to all the power it can handle, but that needs to be practical for both our application and the motor.  With no limitation on current, a DC motor’s speed is proportional to the applied voltage, so a motor rated 4V-7V will spin much faster at 7V than 4V even with a load.  However, there is always some sort of limitation on current, which will limit the strength of the motor.  If you are trying to power a big motor with a little battery, it likely won’t be able to supply enough current to match up with what you calculated.  Always check the datasheets if possible, or measure the power yourself using a benchtop power supply and multimeter – two tools that will come in handy over and over again for any haunter. Don’t spend more than $20 on your first multimeter – the cheap models will tell you everything you need to know about most circuits. Trust me, if you use it regularly, you’ll know when it’s time to invest in a fancier meter. If you’ve got family members that never know what to get you for Christmas, this is a great request!

Motors are a key element in haunted houses and yard displays everywhere, and can be used to do anything from animating a prop’s arm to vibrating an entire floor.  Developing an understanding of how power, speed, and strength relate based on a motor’s specs is key to successful projects, and we hope this has been helpful!  We will share our motor-based projects as they come up, but feel free to leave a comment or contact us if you have any specific questions.

Happy haunting!