About this page

This page is a brief treatise on motorized art mobiles. It focuses on internal motors and solar-powered operation.

The previous page discusses art mobiles in general. You might read it first.

Key takeaways

History of motorized and solar-powered mobiles

Art mobiles saw their heyday in the early 20th century, in the era of Duchamp and Alexander Calder. Both made motorized mobiles. At the time, electric motors were a consumer item, but large and powered from mains. They used motors external to the art mobile. Solar cells had not been invented. Batteries were large and heavy, commonly lead-acid batteries.

Advances in technology in the late 20th century made it possible to build art mobiles with internal motors. Advancements came in motors, electronics, batteries, solar power, and miniaturization.

External versus internal motors in an art mobile

A mobile can be driven by an external or internal motor.

This page focuses on internal motors in art mobiles.

External motor

Early motorized mobiles had external motors. One external motor is attached to the ceiling or floor. The motor body does not move. The motor can be powered from mains.

The external motor turns the top level of the mobile. The other levels of the mobile are turned by apparent wind induced by the motor.

This arrangement is also convenient for solar-powered mobiles. The motor can be powered from a stationary, large, and heavy solar panel, also external to the mobile.

Using an external motor, there are fewer limitations on motor weight, power, efficiency, and voltage.

But the motor is constrained to power the top level. It is a complex problem to transfer its power to other levels, other than through apparent wind.

Internal motor

Internal motors are in one or more levels of a mobile. They must be lightweight and efficient since their weight is part of the mobile. The motor body itself moves.

The power supply for each internal motor can be in the same level as the motor. This obviates the need for slip rings to transfer power between levels.

Considerations for internal motors in mobiles

Motors have a body and a rotating shaft or axis. A motor body is usually fixed or attached to a supporting structure. The shaft turns a load.

Choice of orientation and position of motors yields different aesthetic and subjective qualities in the mobile’s motion.

Orientation and position of internal motors in mobiles

A motor in a level of a mobile is conveniently oriented with its shaft vertical. Then the motor serves a dual purpose: motive force and pivot.

The motor can be oriented with its shaft up or down.

When oriented shaft-up in an art mobile, the motor primarily turns the level supporting the motor body. The motor is positioned at the balance point of the level.

When oriented shaft-down in an art mobile, the motor primarily turns the level below the level supporting the body of the motor. The motor body is positioned offset from the balance point of the level, where the level beneath is suspended.

Arrangement of internal motors among levels of a mobile

Not every level of a multi-level art mobile needs a motor to make all the levels rotate as expected in mobiles.

You can make a three-level motorized mobile that moves as expected for a mobile, using only one motor.

To strongly drive motion in each level might require more motors.

Using more motors allows more varied motion and more direct control of the mobile’s behavior.

Negligible effects of equal and opposite torques

A motor on one level torques and spins that level, and also provides an opposite torque on the connected level, at the point of connection. The opposite torque has negligible effect on the motion of the connected level.

When the motor is oriented shaft-up, the motor primarily torques the same level as the motor. The torque to the level above is negligible.

When the motor is oriented shaft-down, the motor primarily torques the level beneath. The torque to the level with the motor is negligible.

The lowest level requires no motor

The lowest level of a mobile requires no motor. Instead it can be constructed so that the rotation of the level above causes the lowest level to rotate by weathercocking. This can be done using a constrained torsional connection between the two levels that keeps the levels in alignment at rest, and a vane on the lower level that makes it weathercock to rotate orthogonal to the level above.

Rowing between levels above in a mobile

In a wind-driven mobile, a vane weathercocks into apparent wind.

In a motorized mobile, a vane acts as a paddle or oar, rowing in the air. This makes a motorized level not only spin, but push on the arm of the connected level. When a motor is oriented shaft-up, the push is translated to the arm of the level above. In a two-level mobile where only the bottom level is motorized but has a vane, the motor still indirectly causes motion in the top level.

Conveyance of motive force in motorized mobiles

A motor can drive a propeller that turns levels of a mobile, or directly turn levels of a mobile.

Propeller-driven motorized mobiles

Propellers are inefficient. They require more power to achieve the same motion in a mobile.

Direct drive motorized mobiles

A motor can directly drive the rotation between elements of a mobile.

The rotational connection between levels of a mobile can be rigid or torsional. A rigid connection requires the motor to have high starting torque. A torsional connection lets the motor have lower starting torque.

Reduction gears can increase the torque of a motor, and reduce RPMs. But reduction gears are heavy and increase complexity. Reduction gears do not reduce the power required, only increase the torque.

Electric power sources for motorized mobiles

Mains

Slip rings are required to transfer mains power to internal motors of a mobile. Slip rings add complication and their contacts are mechanical and prone to wear.

Battery

Batteries are dense sources of energy. They have sufficient power to turn small motors. But rechargeable batteries eventually need recharging. Non-rechargeable batteries eventually need replacement. Since mobiles are often permanently installed in high places, it is difficult to replace batteries, or any other part.

Solar

Solar panels can provide enough power to turn a small electric motor. A solar panel in indoor light that can turn a small motor continuously is very large in comparison to the motor. Electric motors turn at a higher RPM than people expect a mobile to turn. Also, people do not expect a mobile to turn continuously in one direction, but to display intermittent, back-and-forth motion.

Consequently, it is convenient to intermittently pulse a motorized mobile. Then a smaller solar cell can store energy and deliver that power in pulses strong enough to turn motors.

For example, a small pager motor the size of a coin requires a tenth of an amp to turn, but a typical solar cell the size of a playing card only delivers a few hundredths of an amp. In this example, the energy from the solar cell must be stored for many tens of seconds before it can deliver a very short pulse sufficient to turn the motor only a few revolutions.

Forces on motorized mobiles

A motor imparts a rotational force on both its body and its shaft. This is the primary force in a motorized art mobile.

Motion of a motorized level of an art mobile also creates an apparent wind that imparts secondary aerodynamic forces on components of the level.

Any actual, external wind on a motorized mobile also imparts tertiary aerodynamic forces.

Location and orientation of solar panels on motorized mobiles

Solar panels can be positioned anywhere on a level of an art mobile and connected to the level’s motor by lightweight wires. The weight of the wires is small compared to the weight of the solar panels.

It is convenient to use a solar panel in a mobile for both its power, its aerodynamic effects, and its aesthetic effects.

Location of mobile solar panels for balance

Solar panels are conveniently used for their mass as balancing elements, and positioned at the end of the arms of a level.

Aesthetics of mobile solar panels

Solar panels are planar and most commonly rectangular. Traditional art mobiles use planar elements of many shapes, often rounded, for aesthetic and aerodynamic effects.

Orientation of mobile solar panels for aerodynamic effect

Solar panels are also broad in area and can be used as vanes on an art mobile. However, vanes need to be orthogonal to the plane of rotation, which is usually horizontal with the surface of the earth. Consequently, solar panels used as vanes are oriented with their face vertical. Then, when a level rotates, a solar panel faces varying directions. The light in a room usually comes from one direction. A solar panel with its face vertical may come to rest facing opposite the direction of most light, and then its power output is reduced. It may be convenient to forego the use of solar panels as vanes, and use other materials as vanes.

Orientation of mobile solar panels towards light

Solar panels are planar and produce the most power when light is orthogonal to the plane.

Solar panels in art mobiles oriented with their face up gather the most light, since artificial light indoors often comes from overhead, reflected off the ceiling. Sunlight through a window also comes from overhead.

But solar cells oriented face up near a window or skylight may sometimes receive direct sunlight. Sunlight is intensely strong, and may heat a solar panel, shortening its life. Sunlight provides much power that can overpower a mobile not designed for so much power.

Solar panels indoors with their face down gather light reflected from the floor. This light is more uniform, but weaker than light from directly overhead. This light is never direct sunlight.