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Sunday, March 2, 2014

Quality time with my daughter

My daughter this year choose to do something different from the usual "biology" science fair project.
She choose to build an electric motor and do some tests to see how is possible to improve the conversion of electricity into kinetic energy.
So it was natural my direct involvement in this.

The science fair project had some specific guidelines and "things to do", but from a more practical point of view, the first thing to do was  to build a basic electric motor.
We found  a lot of suggestions on the net about how to do so. In the end the choice depended about the availability of material and simplicity of building.
Here a brief video showing the motor running.

video

Oh ... she won the Third place for her category at school :)
She also participated to the regional science fair. Unfortunately the motor broke down during the presentation (see notes "Problems" at the end of this article)

Third place for the category

Building a motor

There are many ways to build an electric motor and there are many types of electric motors.
There are AC motors and DC motors.
The object of the experiment was to show how some components of the motor could affect it's performance, when changed.
So we picked up one of the basic-simple type of electric motor, a DC static magnet rotor motor.
The type of motor we wanted to build, had fixed magnet on the moving part (rotor) and an electromagnet on the base (stator).
Many commercial motors have the electromagnet on the rotor and the fixed magnets in the stator, or both electromagnets for rotor and stator, but they are more complex since it must exists a way to bring the electricity on the rotor, a moving part.

Principle


The type of electric motor we choose to build  is extremely simple.
The idea is to have two or more magnets on the rotor. The magnets need to be "paired", i.e. they need to be aligned to themselves.
This simple schematic can help to understand the principle.
When a magnet is close to the Reed switch (a switch activated by a magnetic field) it powers the electromagnet.
The electromagnet generates a magnetic filed with the same polarity of the magnet glued to the rotor, "kicking" it and thus forcing the rotor to spin.
As soon the rotor starts to spin the magnet close to the Reed switch is going away, thus the electromagnets cease to work.
The rotor however continues to rotate for mechanical inertia until the sequence restart, i.e. magnet close to reed switch, electromagnet activated, kick to the other magnet, and so on.

There are some constrains :
  • the magnets need to be glued on the rotor facing with the same pole
  • the position of the Reed switch must be as much as possible aligned with the position of the electromagnet
  • the distance between the Reed switch and the magnet on the rotor is critical. If too far the Reed switch is not activating.
  • the distance between the electromagnet and the rotor is critical too. If too close the magnet on the rotor "attach" to the electromagnet when it is disabled.


Rotor

The rotor is the part of the motor that "rotates" and, for the experiment, is the part we decided to change in order to evaluate the performance of the motor.
The rotors were  built using an empty thread spool, magnets and office pins.
Two or more magnets (always in pair) were  glued on the spools and two pins were glued on the spool central hole to form the axe.



Gluing the magnets on the spool

The spools with the inserted pins to form the axe

We chose to orient the static magnets on the rotor with the South pole facing out, mainly because the RPM meter sensor works on the South pole of a magnet.
We used a simple app for a smartphone to determine the magnet polarity.

Stator

The stator is, for our purposes, everything around the rotor.
We used a wooden base and wooden blocks to build the support for the rotor, the electromagnet, the Reed switch and accessories (main switch, RPM meter, ecc.)

The basic components of the stator, a wooden base and wooden blocks

The electromagnet was connected to a battery and a Reed switch.
The Reed switch is necessary because we need to turn on and off the electromagnet in order to generate a magnetic field only when a rotor's magnet is close by.

Electromagnet

We needed something metal and an insulated electric wire in order to build a coil around the metal part.
Since we used  using small voltage for the project (up to 4.5 Volt) we needed a lot of wire.
We used a nail as metal core of the electromagnet, with the wire coiled directly around it.
We used a drill to facilitate the winding of the wire around the nail

Electromagnet, Reed switch and wooden blocks

Prototypes

Two prototypes were built.
The first one was built mainly to prove the concepts and experiment positioning the components.
The second prototype was the one used to actually perform the test and it was built differently, since we learned from the first one what NOT to do.

First prototype


The first prototype was built positioning the electromagnet on the top of the rotor and the Reed switch fixed at the base.
The idea was to have the Reed switch easily glued at the base and position the electromagnet on the top, it was easier to adjust the distance between the electromagnet and the rotor.

The first prototype
The first prototype was working however it had many problems, like :
  • the electromagnet was moving, required more strong holding
  • the distance between the rotor and the Reed switch was fixed
  • it was difficult to change the rotor
  • the rotor was not strongly hold on the wooden blocks
  • the electromagnet was too weak
  • the voltage used was too high
In the end we realized it was better to rebuild it with different principles.

Second prototype

The second prototype was built with  the idea to be able to easily change the rotor and position the electromagnet/Reed switch more easily.
The second prototype layout

A new electromagnet was built, using a 3 inches nail and almost 250 feet of wire, held by a wooden block carved to the shape of  the electromagnet.

The electromagnet holder, a wooden block carved to the electromagnet shape

The new layout included also a better support for the rotor and the attachment of an RPM meter.


The second prototype used for the test. The RPM meter is shown on the  lower right corner

The RPM meter


In order to evaluate the motor performance, we decided to use the speed of the rotor.
To measure the rotor RPM we needed a tool, an RPM meter or 'tachometer'.
So I built an easy tachometer using one of the boards I had around.

The experiment

At this point we had everything ready for the experiment.
We had a motor base, capable to support a rotor and an RPM meter.
We built 5 different rotors, changing the type of the magnets and the number of the magnets.

  • Rotor 1
    Two ceramic 0.5 inch magnets - Weight : 12.5 g 
  • Rotor 2
    Two Neodymium 0.5 inch magnets - Weight : 20.9 g 
  • Rotor 3
    Two Neodymium 0.2 inch magnets - Weight : 12.7 g 
  • Rotor 4
    Four ceramic 0.5 inch magnets - Weight : 16.4 g 
  • Rotor 5
    Four Neodymium 0.2 inch magnets - Weight : 14.4 g
We decided to proceed in this way :

For each rotor, we choose to perform the test 4 times.
During each test, we set up the RPM meter, let the rotor run for about a minute to "stabilize" the system and then start a 2 minutes timer.
Every 10 seconds we took a reading from the RPM meter.
In this way we ended up with 5 different set of data, one for each rotor.

Capturing the data

Setting the RPM meter

Reading the RPM meter every 10 seconds
From the readings we created 5 graphs to better compare the performance using different rotors.
Here they are :






All the graphs were built using the same scale, so it was possible to compare them directly.
Looking at them resulted that indeed changing the type of the magnet and the number of magnets, the motor had different performances.

Problems

We also experienced a problem.
After building the second prototype we started to collect data for the experiment.
However some rotors had problems. They were spinning at very low RPM and often they were not spinning at all.
We checked the motor and apparently everything was OK .. only apparently.
In the end we discovered that the Reed switch was defective or damaged somehow and it was not always capable to react to the magnet on the rotor.
We decided to substitute the defective Reed switch with a new one, and all the rotors were behaving as expected.

The same problem happened later, during the regional science fair.
The Reed switch broke down again.  The very probably cause of the broke down is the spike of voltage generated by the electromagnet when the Reed switch opens.
Hooking up an oscilloscope we saw spikes up to 250V. On the long run, spikes like that can warm up the metal of the contacts, bending them and leaving the Reed switch unable to operate correctly.
i.e. on the long run the Reed switch mechanical characteristics are compromised by the sparks generated by the electromagnet generated spikes.

The spike in full view - each vertical block is 50 V

A detail of the spike

The poster


Here a couple of pictures of the poster my daughter prepared for the presentation.



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