shorting phases vs dump load

ws9876
ws9876 Solar Expert Posts: 450 ✭✭✭
in Wind Power Generation #1
with regards to wind gen. blade stop/slow down

are these the same thing or not..???
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #2
    Re: shorting phases vs dump load

    Not as I understand it...

    Normally, a small wind turbine is permanently attached to the battery bank. Which provides the standard 12 volt / 24 volt / etc. fixed voltage load. This keeps the horizontal axis wind turbines from over-speeding.

    If the battery becomes fully charged and the wind turbine output exceeds the load--A Dump/Shunt controller will turn on (typically) a resistance heater and draw current from the battery bank until it falls enough to turn off--then let the turbine recharge the bank again (dump energy to heat the air, hot water, run a well pump, etc.).

    Ideally, you want two dump controllers/loads--if one fails, you want a backup to keep the battery bank from overcharging (driving out hydrogen gas, exposing plates, etc.) and becoming a possible fire hazard.

    Shorting phases... With a PMA (permanent magnet alternator), the torque required to turn the rotor is a function of current flow through the windings (and loads/battery bank etc.).

    A simple way to stop the turbine is to just short out the windings (before or after the rectifier). You do need to make sure that when the windings are shorted, that you don't short the battery bank too (bad things can happen :cry:).

    Some turbines simply place a dead short across the PMA... Some folks here have used a resistor bank to place a heavy, but controlled, current on the turbine to slow it down without drawing too much current.

    Remember, I2R heating--If you draw 2x the current, you get 4x the heating effect. The concern is that you may overheat the stator windings or the slip-rings/wiring back to the controller while the turbine slows down (heavy wind/momentum). Then you can throw the dead short on once the turbine has slowed down a lot (as I understand from reading posts here--I do not have a wind turbine).

    Be warned, some wind turbines have overheated when shunted. Whether they were under designed or subjected to too much wind--I can't say.

    In any case, having several methods to control turbine speed (battery load, shunt load, mechanical brake, feathering blades, etc.) is the key to having a long lasting/safe installation. Remember the power in wind increases with the cube of the velocity... 20 MPH vs 60 MPH is 16x more power. Get into the 100 MPH vs 20 MPH--25x more power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #3
    Re: shorting phases vs dump load

    Those smaller PMA turbines that are designed to be "stopped" by shorting the output, will very likely burn out the stator windings if you short only one or two of the phases. Why? Because a total short puts such a load on the PMA that except for hurricane force type winds, the blades are slowed to the point where they aerodynamic stall, thus hugely reducing the efficiency of the overall system, likewise greatly reducing currents flowing in the coils and thus heat generated within said coils. In most wind conditions that would require reducing turbine output, shorting just one phase would not "stall" the blades, therefore near full power would still be available at the PMA shaft, high currents would be produced within the shorted coil, followed by smoke.
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  • 65DegN
    65DegN Solar Expert Posts: 109 ✭✭
    #4
    Re: shorting phases vs dump load

    There are several ways to deal with controlling a wind turbine's blade speed. Some manufacturers are using PWM pulsing that is engaged when rotor speed hits a certain level. I understand that Midnite Solar is working a scheme that counts alternator RPM's and engages a PWM pulse at a programmed speed. The PWM pulse becomes longer and loads the alternator heavier as RPM's attempt to increase. This system relies on a stout alternator. I suspect a puny one will not hold up.
    Even with this scheme at some point loading the alternator with pulsed loads or even pulsed shorts will not be able to control blade speed when winds exceed a certain speed. At that point the machine should be brought to a stop at a lull in the wind.
    Shorting all three windings should bring a turbine to a near complete stop if done at the appropriate time. It will turn but very slowly. Loading the turbine with a dump load will not slow it anywhere near as much. With a dump load connected in high winds (~>35 MPH ) a turbine can still spin at full rated RPM's.
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