Making Wind Power: How to Choose the Right Motor

It’s obvious that the motor you use is the most important part of your wind power generator. If you’re new to building a small wind turbines, then you’ll find that this can be one of the most confusing (and controversial) aspects to the process. Motors, generators, alternators, oh my!? You’ll find a lot of words out there that seem to be referring to the same things.

So why's it called a motor?

Many industrial motors make great and very affordable wind generators. In a wind turbine, the motor is used to create electricity. Technically, the “motor” would no longer be called a “motor”; it would be a “generator” or an “alternator.” This article focuses on potential motors that can be purchased online inexpensively as surplus items and can be used to build your own custom wind generator.

Obviously, it’s important you choose an appropriate motor for your generator. Choose the wrong one and you may discover that:

  • Your wind generator will make no electricity.
  • Your wind generator will make electricity, but will never reach a voltage high enough to produce usable electricity.
  • Your wind generator will initially be working, but after a few days or weeks, it overheats and stops working.

But don’t be discouraged. There are hundreds of motors that will produce several hundred or possibly even thousands of Watts of useable energy. And even better, we’ll give some tips on how you can track one down for a reasonable price.

There are three ways that generators make electricity: either by something called induction; by using an exciter; or by using PERMANENT MAGNETS.

Magnets, Magnets, Magnets!

Do-it-yourselfers build wind power generators almost exclusively with Permanent Magnet Motors, because they are widely available, reliable because of the nature of their construction, and start generating electricity at almost any RPM. The same cannot be said of some of other types of motors.

Inside a permanent magnet motor is a coil of wound copper surrounded by permanent magnets. These motors rotate using electromagnetic induction, which means electricity is supplied to wound copper wire which creates a magnetic field. The magnetic field created by the electricity flowing through the copper wire opposes the permanent magnets in the motor housing. As a result, the copper wire that is attached to the shaft of the motor tries “to push” itself away from the permanent magnets. So your motor starts spinning!

The same reasoning is applied when considering a permanent magnet motor as a generator. Spinning the copper wire by using the energy from the wind in the presence of the magnets creates a voltage difference between the two ends of the copper wire. The difference in voltage causes the electric charges (electrons) to flow in the copper wire, generating electric current.
So you now understand the basic principles of a generator.

So now what should you be looking out for in selecting your motor?

Volts-to-RPM Ratio

The Volts-to-RPM Ratio is one of the most important specifications to look out for in selecting your motor. Most DIYers use their motor to charge a 12-Volt battery, because of their cost and widespread availability. Charging a 12-Volt battery requires that the permanent magnet motor be generating at least 12 volts. If it doesn't, then it can't overcome the impedance of the 12V battery and the motor will never charge the battery. How do you know if your motor is capable of producing more than 12 volts when powered by the wind? Read on.

The volts-to-RPM ratio of a permanent magnet motor is defined as the volts required to spin the motor at a given RPM (rotations per minute). So let's assume you have a permanent magnet motor that says the following on its label: “100 Volts, 2500 RPM.” This simply means if you supply the motor with 100 volts, it will spin at 2500 rpm. Its volts to RPM ratio is 0.040 V/RPM (100 divide by 2500).

This number provides a rough estimate of how many volts the motor will generate at a given rpm. Now let us assume our 100 Volt, 2500 rpm motor is spinning at 450 rpm. How much voltage will it produce at that rpm? The calculation is as follows:

(450 rpm) x (0.04 Volts/rpm) = 18 Volts

Now, there is one more step to do. We must multiply 18 Volts by 80%. Why? Because the 18 Volts is the number only if the motor is being used as a motor. This motor is not being used as a motor. It is being used as a generator, and it is not 100% efficient as a generator. It is roughly 80-85% efficient as a generator.

Therefore, 18 Volts x 0.8 = 14.4 Volts

We know how many Volts our motor will produce at 450 rpm: 14.4 volts. We must next consider the realistic RPM’s of a wind generator. You are most likely building a “small” wind generator that will be in the range of 100-500 Watts. Putting some well-constructed, 50-to-60 inch diameter blades on that motor will easily produce 450 rpm in wind speeds of 8-10 mph when the motor is under load (under load means the motor is connected to your battery bank. A generator has to work harder when it is under load and thus it will spin a little slower compared to when it is not under load). So this motor will begin charging a 12V battery bank in wind speeds of around 8-10 mph.

This is about what you are aiming for, and so we can conclude that this permanent magnet motor could work well for a wind generator.

A Volts-to-RPM ratio of AT LEAST 0.035 is the minimum requirement when looking for a permanent magnet motor. If the number is higher than 0.035 that is perfect. If the number is lower than 0.035, it will likely be insufficient unless it is located in an area with high winds.

Amperage Rating

The next item is the amperage rating of the motor. This provides information regarding how much current the motor will put out as a generator. From our experience, it is very difficult to predict what type of current your motor will put out as a generator. We’ve seen motors that expel more amps than that for which they are rated. However, one thing remains true: The higher the amperage rating, the better. You should be looking for a motor with a minimum amperage rating of at least 5 Amps. Anything above 5 Amps and you are good to go.
The power that a wind generator produces is directly proportional to the amps and voltage:

In fact, Power = Volts x Amperage

Remember, the more amps and volts the wind generator creates the more power it is producing!

So remember these three critical points:

  • Keep it simple: Only purchase a permanent magnet motor
  • Look for a minimum Volts to RPM ratio of 0.035
  • Look for a minimum amperage rating of 5
  • This article is just an introduction, and we overlook some details in order to keep things simple and concise. But this information is all you’ll need to shop confidently for a wind generator motor.

    If you have more specific questions about a motor or motors that you have found, feel free to email us or post a questions on our User Forums. Our staff or one of our forum members will be happy to answer your specific questions.

    And please check out the selection of quality WindyNation products available right here on our website. Compare them to the competition and see if anyone will beat our 90-day Money Back Guarantee!

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