I am trying to use a DC motor as a generator for a wind turbine but then there are two possibilities for the DC motor which are brushed and brushless. Which one is more efficient to use?

I know that if I use a brushless motor then I need to use a 3 phase rectifier which includes 6 diodes. Will the voltage drop in these 6 diodes affect efficiency? But if I use the brushed motor then I don't need to use the rectifier. Can this be the advantage over the brushless motor?

  • \$\begingroup\$ Not an answer to your question, but possibly relevant to your interests - here's how it's done in a car with an alternator; ie: a synchronous machine. \$\endgroup\$ – Peter Apr 16 '14 at 13:31
  • \$\begingroup\$ And - I think you're right, although intuition tells me that an alternator will be running at a high enough voltage that the diode losses aren't really that significant (since diode losses = diode current * abs(voltage drop)). It's not my area of expertise though so I'll let someone else chime in. \$\endgroup\$ – Peter Apr 16 '14 at 13:33

A brushed motor has the rectifier built in via the commutator. So you don't need the 3 phase rectifier. However, the brushes and commutator contacts do wear out and the brushes need to be replaced from time to time. There are still losses associated with the commutator, even though not as much as with a diode bridge. In a wind turbine I would think that maintenance could be painful so you would want to minimize it as much as possible.

The brushless motor is typically much more reliable and doesn't require periodic replacement of anything. Depending on the output voltage (Speed and back EMF constant) the rectifier losses could be significant. You could use a 3 phase FET bridge instead of a diode rectifier, but that becomes much more complicated.

  • \$\begingroup\$ FET bridge in wind turbine? Lol, I like this :) \$\endgroup\$ – Kamil Apr 16 '14 at 14:58
  • \$\begingroup\$ I have been to a students' project presentation where one student built a wind turbine generator using a 75W stepper motor as a generator. Stepper motors are likely to generate more EMF because the magnetic field alternates many times per turn, and the higher frequency makes it easier to filter out the ripple by itself. \$\endgroup\$ – motoprogger Apr 16 '14 at 15:10
  • \$\begingroup\$ Thanks a lot everyone for sharing your knowledge, it was really help full. Can anyone suggest me if I want to make a small wind turbine what kind of blade should I be using? I am thinking of making a small scale wind turbine with the Rotor Diameter as 10cm. How do i choose the motor which has the rotor diameter of 10cm? \$\endgroup\$ – Electronic Curious Apr 18 '14 at 9:32

Brushless motor is much better for few reasons:

  • efficiency
  • reliability
  • it can work in harder conditions (brushed motors have much worse tolerance for humidity, dust etc.)

Efficiency of brushless motor is better because:

  • no brushes friction
  • no brushes voltage drop
  • smaller rotor inertia (same power brushless motor is smaller)
  • generally better by design

Small motor inertia is very important. When wind stops or slows down, and turbine is still turning - you are wasting energy (kinetic) into... working as fan. Thats why turbines have 3 blades instead of 50.

Voltage drop on diodes can be reduced to 0.3V per diode (by using diodes with low drop, like Shottky diodes).

Typically (at least in my country) - generators for wind turbine have nominal voltage like 48V. Voltage drop on 2 diodes is no more than 0,6V. Thats just 1.25%.

In addition - low current diode voltage drop is smaller than nominal.


I'm guessing the machine in conjunction with a typical speed control (Darlington Inverter) will operate well as a DC generator even with commutation turned off, whereby the diodes provide natural rectification of 3-phase "DC" output, perhaps smoothed with an output capacitor before current is sent to the battery. Confession: I'm a mech. engineer. Elec. guys please verify or correct my assessment


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