Rectifier vs diodes

[Jerrywoo]

I recently built a wind turbine out of a bicycle frame and a 21 amp 90 volt 2.65 hp treadmill motor. I used a 50 amp battery isolator off of a motorhome in place of standard blocking diodes..my question is this, should I replace the isolator with blocking diodes or with a bridge rectifier.. which would work better

[Jerrywoo]

I really like the fact that the isolator let's the turbine have a little more rpm before the battery bank locks it down..It is geared 8.75 to one with sprockets and a chain..it has a home brew hub and blades..I went with a 5 blade configuration for the added torque and startup..the blades are 36 inches long..in 15 mph winds open circuit voltage is around 60 vdc..I am going to replace the hub and blades with a large set of aluminum blades with slightly more pitch and only 3 blade configuration..I've already got the gearing..but if anyone has a good idea for a decent charge controller I am absolutely all ears..this is my first DC motor turbine and I am slightly out of my comfort zone because I am used to alternators..any advice would be greatly appreciated

[MattM]

With longer blades you probably want less pitch not more.  Sounds counter-intuitive, but honestly it helps.

[makenzie71]

your motor produces DC voltage so you wouldn't need a rectifier or a blocking diode.  What job does the isolator do?  In an RV all an isolator does is allow both (or more) batteries to charge from one source while being discharged independently.

[Adriaan Kragten]

your motor produces DC voltage so you wouldn't need a rectifier or a blocking diode.

If the motor is a PM-DC-motor it can work as a generator for battery charging if the unloaded voltage is higher than the battery voltage. However, if the open voltage is lower than the battery voltage it will work as a motor and it will extract energy from the battery. So you certainly need a blocking diode to prevent this.

The load characteristic of a PM-DC-generator is that the power is rising rather fast if the generator is used to charge a battery with an almost constant charging voltage. This means that the P-n curve of the generator will be much steeper than the optimum cubic line of the rotor. The wind speed for which both curves intersect is called the design wind speed. The matching is optimal for this design wind speed as the rotor runs at the optimum tip speed ratio lambda and the maximum Cp for this wind speed. For lower wind speeds the tip speed ratio is too high and for higher wind speeds it is too low.

Matching is explained in chapter 8 of my free public report KD 35. To check the matching, you need a measured or estimated Cp-lambda curve of the rotor and measured generator characteristics for the given load. A way to estimate the Cp-lambda curve is given in chapter 6 of KD 35. However, this works only for a rotor which is designed according to the aerodynamic theory which is given in chapter 5 of KD 35.