As we are using our wind turbines to generate electricity you should have some basic understanding of what is happening and how to measure the electricity.
In most of the wind turbines used for classrooms and science fair projects we are using small DC motors as generators.
If you spin the shaft of small DC motor it will generate a small amount of electricity. As these motors are designed to spin very fast, using relatively small amounts of energy, when we spin them using our wind turbines the power output will not be very robust as we cannot spin them super fast - unless you construct a geared model!
Depending how the DC motor is constructed, namely how many poles it has and how it has been wound, there can be great differences in electrical output.
Measuring & Understanding Electrical Output
Students who build and test small wind turbines want the electrical output to do something: Light a bulb, make a noise, move an object, etc. Unfortunately with the smaller DC motors and the lower RPMs of the wind turbines it can be hard to power these loads. A meter is helpful for measuring the power output.
We sell a great multimeter in our store for just $9! You can also buy your own, but know that you only need a cheap one, so do not spend lots of dough on a multimeter! It is useful to have one that has alligator clips on the leads or make some leads that have clips as it makes your life easier.
Select a low voltage range when starting to record voltage on your wind turbine. Typically most of the DC motors we use have voltage output that ranges from 0.1 - 6 volts.
When you record the voltage output of a small wind turbine you are basically recording how fast the shaft is spinning. Because you have not attached any resistance or load to the motor for this measurement it is easy for the motor to spin fast, but this can lead to some misleading results.
For example, the blades that will typically spin the fastest and give the highest voltage are the shortest blades. This makes sense if you think of a rotating skater...when does she spin the fastest - arms in our out? Well this data will not jive with the wind power equation which says that power output must increase as the blades get bigger. It does make sense if you understand that by measuring voltage you are not measuring power just a component of that calculation.
You may be wondering how voltage and amperage relate to RPM. Click here for a comprehensive chart on the relationship between voltage and RPM from the KidWind Wind Turbine Generator.
To properly measure the amperage or the current that your wind turbine is generating you must place a load on your generator while measuring. This can be in the form of resistors, light bulbs, or other motors. Select a low range as the typical output from these DC motors is around 50 - 1000 milliamps (1000mA = 1 A).
When you measure the amperage of your motor you are basically measuring how much torque force your blades are making. Depending on how much resistance you place on the motor it can get very difficult for the motor to spin unless your blades generate a great deal of torque. Short blades will not generate lots of torque, but longer blades can. What this means is that a set of blades that generates the highest voltage may not generate much current because they generate little torque force.
While it's challenging to understand the full power characteristics of your wind turbine, you must measure voltage and amperage, and then do a simple power calculation.
Power = Volts x Amps
Make sure you do the measurements in the same units. Don't multiply volts times milliamps, because it makes for messy math and incorrect results. Multiply volts times amps to get watts, and then convert that to milliwatts. You may have to first convert milliamps to amps.
Factors that Affect Power Output
The amount of power your wind turbine produces is dependent on a number of variables. Explore these factors and see how they affect the power output of your turbine
- Wind Speed
- Generator Type
- Turbine Shape & Design
- Gear Ratio