Bad Electrical Panel. A Rare Speed Commander Appears! A Danish 4kW Motor Drive

I got hold of two electrical panels with a bad wiring job, worst documentation and completely out of code (racing fast for illegal). The did …

AGFA ADC 5155 X-ray Scanner Teardown (Part 1 of 4)

Part 1: Teardown of a AGFA ADC 5155 x-ray image scanner / digitizer. Also known as Computerized Radiography (CR) it fulfills the job of scanning …

3 kW Yaskawa Servo Motor Teardown – Isolation, Day 38

I scavenged a old 4 ton stone polisher machine for the best components and got off with a 3 kW AC server motor + drive, …

3-Phase IGBT Module Teardown – Fuji 7MBP50RA120 From 3 kW Omron Servo Drive – Isolation, Day 36

I scavenged a old 4 ton stone polisher machine for the best components and got off with a 3 kW AC server motor + drive, …

Omron/Yaskawa 3 kW AC Servo Drive Teardown – Isolation, Day 34

I scavenged a old 4 ton stone polisher machine for the best components and got off with a 3 kW AC server motor + drive, …

3 kW AC Servo Drive From a 4 Ton Stone Polisher – Isolation, Day 32

I scavenged a old 4 ton stone polisher machine for the best components and got off with a 3 kW AC server motor + drive, …

Homopolar motor

What is a homopolar motor?

The name means that the electrical polarity of the motor never changes. The axis of the rotating part of the motor is parallel to the magnetic field from the permanent magnet. Lorentz forces is what makes the motor turn, a conductor conducting current through a magnetic field will respond to a external magnetic field and will gain velocity in what direction the fields happen to be aligned.

This restricts the motor to a single turn coil which limits it to small voltages around a couple of Volts and low torque.

Homopolar generator

The homopolar motor will also work as a generator if we remove the battery and apply the motion to the rotating disc ourself. They are only capable of supplying a couple of Volts but at very high currents, generators can be put in series for higher voltages.



Warning: this motor operates at high speeds up to 6000 RPM, the screw can at any time loose contact and fly through the air.


Michael Faraday demonstrated this motor concept in 1821 at the Royal Institution in London. The homopolar motor is the first electrical motor to be built. His construction was a large and crude concept that he himself describes as “This apparatus may be much reduced in size, and made very much more delicate and sensible.”

The original article in Quarterly Journal of Science:

How can you perform this experiment yourself?

You will need the following four items to make this experiment.

  • 1 1.5 Volt battery.
  • 1 iron wood screw.
  • 1 piece of wire.
  • 1 round neodymium magnet.

Follow these four steps to make it work.

  1. Place the head of the screw in the centre of the magnet.
  2. Let the screw attach itself to the positive pole of the battery.
  3. Hold the wire with your finger against the negative pole of the battery.
  4. Gently let the other end of the wire touch the side of magnet.
  5. Watch as the magnet and screw starts to spin faster and faster.



A quick experiment that can be made from common household items. It is a simple practical demonstration of the basic electro magnetic theory that would help many understand it better when having it hand on.

Coil winding machine


A coil winder is essential when it comes to making a beautiful coil on large forms.

It takes considerable less time to wind large coils and varnishing the coils while they are rotating gives a much better finish.


Control of speed and acceleration is key issues when building a coil winder.

The best choice is to get a Variable Frequency Drive meant for 3 phased asynchronous induction motors, especially if the motor have a gear.

With a VFD it is possible to control the speed with a potentiometer, control start and stop with a single push and hold button and control ramp up and down times.

Having both hands free to guide the wire onto the form is important


27th March 2010

With materials at hand the motor was secured to a horse with a custom drive wheel with a rubber band to get contact to the form. A gear is attached to the motor with the ratio of 1:14, this makes it possible to run the motor at higher speeds with the VFD. Running at speeds close to its native speed makes cooling more efficient as its self cooled from the rear fan.

The form is pressed against the roller wheels which keeps the form in place.

Controlling the winders motor is done with a Siemens Micromaster Vector 0.37kW VFD, a potentiometer is used to adjust the speed and a foot pedal is used for start / stop.


The machine was built without buying any new materials and is quickly put aside as the rollers are hold to the horses with welding clamps.

Further improvements could be a turn counter and automatic wire guide, but with the relatively low number of large coils I make there is other fields I like to focus on.


A time lapse movie of me winding 0.25 mm wire on a 160 mm diameter form.