Improving the Solenoid Motor

Having built a number of these motors, my thoughts have turned to improving the solenoid motor.

Solenoid Engine - Crank, timing, connecting rod and homemade solenoid

The single piston solenoid motor runs very smoothly using a single 9V battery.

However, when I moved to the next motor I made some force measurements with the armature and stator. The force was just 0.22N for this small motor. A commercial solenoid of this size can develop 4N and so we have a long way to go.

This is a very easy motor to make and the plans are available on our website.

How do I increase the impulse force?

Solenoid Motor Circuit

solenoid motor circuit

The solenoid circuit is very simple and consists of 3 parts:

  1. Coil around the stator
  2. Battery
  3. Switch operated by the crankshaft
solenoid motor operation

When the switch is closed a current [I] flows and a magnetic field [B] is generated. This magnetic field pulls the armature towards the centre of the coil.

The force on the armature will be in a direction that reduces the inductance.

In simple terms the magnetic field is calculated as:

solenoid equation

Where N/l is the density of turns, I is the current through the coil, and μ is the permeability of the core.

When the current is switched off the magnetic field collapses.

One significant issue is that the coil is inductive and we are switching a battery across the coil. This means the current is varying with time. Hence this equation is very simplistic, Paul Schimpf [ref 1] derives the equations and hence gives a great explanation of what is supposed to be a simple solenoid.

This paper goes on to look at coil length, resistance, packing density and hence looking at optimising the force. My next step is to generate a spreadsheet with these equations so that I can optimise the design for this application.


Often people think that the direction of the magnetic field in the coil determines the direct of force that acts on the core. This is not true. The core is ferromagnetic (and not a permanent magnet with defined poles), therefore the core will always be attracted to the centre of the magnetic field. Both coils for AC and DC pull the core towards the centre. It doesn’t matter how the coil is connected to the power supply (the poles can be connected both ways).


The armatures have until now used permanent magnets. There are some papers that look at the design and optimisation of solenoids using permanent magnets [ref 3]. Using a smaller solenoid stator, they generate 3x the force generated by the single piston solenoid motor.

Changing magnetic fields will generate eddy currents in the armature. Therefore, it is good practice to make this from laminated steel.

Flyback Diode

When the battery is disconnected from the coil there will be a voltage spike. This will generate noise and might be effecting the life of the switch. One option is to place a flyback diode across the coil.

flyback diode

Design Direction

I will run some experiments with different options. My current list of things to do are:

  1. Generate mathematical model using Paul Schimpf’s paper.
    1. Optimise the stator length and diameter.
    2. Optimise the coil wire diameter.
  2. Make a laminated armature.
  3. Compare laminated and permanent magnet armatures.
  4. Optimise the stator laminations.
    1. Outer magnetic cage – this really works: Confining the Magnetic Field
    2. Laminated bobbin.
  5. Optimise cam profile and switching.
  6. Test with and without flyback diode.


  1. Paul H. Schimpf, “A Detailed Explanation of Solenoid Force“, Int. J. on Recent Trends in Engineering and Technology, Vol. 8, No. 2, Jan 2013
  2. Tameson, “Understanding Solenoid Design
  3. Chang-Woo Song and Seung-Yop Lee, “Design of a Solenoid Actuator with a Magnetic Plunger for Miniaturized Segment Robots“, Appl. Sci. 2015, 5, 595-607; doi:10.3390/app5030595
  4. Flyback Diode – Wikipedia

single piston solenoid motor plan

Single Piston Solenoid Motor

Here we have a pdf download of the plan for the Single Piston Solenoid Motor. An electric motor with a difference. The power output is low, but the action and the sound is fantastic. You can use this as a starting point for a more optimised design or for a multi-piston design. I built and tested the original version of this before moving onto a V-twin.

solenoid v twin motor

V-Twin Solenoid Motor

This really is a strange motor, a 4 stroke v-twin with a common crank pin – a sort of Harley-Davidson configuration, but driven by homemade solenoids rather than petrol.

This engine took quite some time to produce with plywood made out of hardwoods and gears cut on a Hobbymat lathe for the 2:1 reduction.

The resultant sound when it runs is rather fun and evocative of the Harley!!!

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