Three phase brake switch...safe?

[makenzie71]

 Short the three phases together to brake the turbine. Or that's what I've always thought you were supposed to do. That's an idea that I got from this forum. But I saw the video from a somewhat popular YouTuber today who says that this does damage to your coils. Has the consensus changed somewhere in the last 10 years?

[Adriaan Kragten]

If short-circuit to stop the rotor damages the coils, depends on the cooling capacity of the generator and on the characteristics of the safety system. Any PM-generator has a certain Q-n curve for short-circuit (see report KD 78 for measured Q-n curves). The highest Q-n curve is obtained for short-circuit in delta. Short-circuit in star is the same if the star point is short-circuited too. When the rotor has no safety system, there is always a wind speed for which the Q-n curve of the rotor is lying higher than the Q-n curve of the generator for short-circuit. So if you make short-circuit at these high wind speeds, it won't result in stopping of the rotor and then the winding will certainly burn. But if the rotor has a proper safety system and if the Q-n curve of the generator is lying high enough, short-circuit will result in rather fast slowing down of the rotor and finally the rotor will rotate only very slowly.

During short-circuit, all generated energy is dissipated in the winding as heat. It depends on the type of generator if this heat can flow away easily. PM-generators made from asynchronous motors have a steel stator stamping and often an aluminium hosing with cooling fins. This construction can guide the heat rather easy to the surrounding air. But for axial flux generators there is no contact in between the stator and the housing and generated heat in the stator is therefore radiated more difficult. If the braking procedure takes only some seconds, it will be no problem but if braking takes longer than a minute, the winding will probably burn.

[makenzie71]

Thanks that's the essentially the way i understood it to be...shorting the three phases would generate some heat but it should be pretty short lived while the rotor slows.  Guy on youtube said it needed to go through resistors to absorb the heat, but i don't understand how that would work differently from a dump load.  And if you're in so much wind that the brake doesn't slow it down and it's fixed (can't furl) then you're probably better hoping than braking anyway

[SparWeb]

Usually shorting phases is fine.

If you short the phases while the turbine is over-speeding, then that pulse of current can be very high.  But, then, you already have a problem if they're overspeeding, anyway. 

[Adriaan Kragten]

Usually shorting phases is fine.

If you short the phases while the turbine is over-speeding, then that pulse of current can be very high.  But, then, you already have a problem if they're overspeeding, anyway.

If the generator is connected in star and if the star point is hidden somewhere in the winding, you can only make short-circuit in star. The peak of the Q-n curve for short-circuit in star is lying about a factor 0.8 lower than for short-circuit in delta. The Q-n curve for short-circuit in delta is lying higher than for short-circuit in star because higher harmonic currents can circulate in the winding for short circuit in delta.

You can make short-circuit before or after the rectifier and close to the generator or at the position of the batteries. Make short-circuit after the rectifier is only allowed if the rectifier diodes can have the large short-circuit current. You get a voltage drop of about 1.4 V over the rectifier and also a voltage drop if there are long cables in between the generator and the short-circuit switch. This voltage drop results in shifting to the right of the Q-n curve and the P-n curve of the generator. This shifting may make that the P-n curve of the generator for short-circuit is no longer lying to the left side of P-n curve of the rotor for the maximum wind speed.

In figure 4 of my public report KD 484, I give the P-n curves of the rotor for different wind speeds, taken into account the effect of the safety system for higher wind speeds, together with the measured P-n curves of the generator for short-circuit in delta and in star. It can be seen that the P-n curve for short-circuit in delta is lying most to the left of the P-n curve of the rotor for V = 9.5 m/s and so braking in delta will result in a larger power and torque difference and so the rotor will slow down faster. The P-n curve of the rotor for V = 9.5 m/s is the highest rotor curve as the safety system turns the rotor out of the wind such that this curve is also valid for higher wind speeds.

Making short-circuit over a resistor creates a voltage drop over that resistor and this voltage drop also results in shifting to the right of the P-n curve of the generator. So you have the advantage that not all the heat is dissipated in the winding but the disadvantage that now there is a larger chance that the P-n curve of the rotor is lying no longer to the left side of the P-n curve of the rotor for the maximum wind speed. My advise would be to make short-circuit in delta before the rectifier and at a distance as close as possible to the generator. If you won't climb the tower, this position will be at the tower foot.

[SparWeb]

...Making short-circuit over a resistor...

That's not a short-circuit, then.  Sure, less power is dissipated as heat in the generator's stator coils, but less torque is applied.  The slow-down to a stop takes longer, and the heat in the stator is applied for a longer time.  This could result in a net increase in heat put into the stator, particularly if the resistors are not "stiff" enough to stop the rotor from turning.

...You can make short-circuit before or after the rectifier and close to the generator or at the position of the batteries. Make short-circuit after the rectifier is only allowed if the rectifier diodes can have the large short-circuit current...

Shorting on the AC side of the rectifier is much simpler.  Shorting the rectifier on the DC side is very complicated to do safely and reliably.  You would need to disconnect the battery at the same time, otherwise the battery would be short-circuited.  Even if the battery is disconnected, shorting after the rectifier exposes the rectifier to the short-circuit transient current spike too.  You would rely on having large safety margins to resist the transients, higher cost, and carefully design for the reliability of the battery disconnect at the same time.  What is the advantage to doing this?  There is a lot of risk.

Short the AC.  Works great and it only needs a simple switch.

[Adriaan Kragten]

It isn't always true that less torque is replied if short circuit is made over a resistor. It depends on the rotational speed. I have measured Q-n curves for different voltages (see KD 78) and all Q-n curves have about the same maximum value as the Q-n curve for short-circuit (which is for U = 0 V). All Q-n curves also have the same shape but the Q-n curve shifts to the right if the voltage is higher. So the rotational speed for which the torque is maximal is lying at a higher rotational speed if the voltage is higher. So for high rotational speeds, the torque for a certain voltage can lay higher than for pure short-circuit. For low rotational speeds, the torque for short-circuit lies highest. So it depends on the rotational speed of the generator, which Q-n curve is highest. For most effective braking one can start to brake over a resistance and if the rotational speed has reduced, then switch to pure short-circuit.

I don't see it as a big problem to make short-circuit after the rectifier if the rectifier diodes are large enough. You have some voltage drop over the rectifier and over the cables to the battery but the rotor can stop almost completely if the generator is strong enough. If you rectify a 3-phase current in star, you don't use the full sine wave but only the part of the sine wave in between 30° and 150° and in between 210° and 330° (see KD 340). The power loss because of not using the full sine wave is only about 6 %, so the maximum torque level is also only about 6 % lower.

[kitestrings]

Electrical braking works well within the limits described earlier.  What I don't like about it is that it is generally a very harsh way of shutting things down, especially when used as a high-wind action.

We've taken a different approach, but one that has worked very well for us for several years now.  We added a furling line mechanical actuator at the base of the tower.  It has a built-in potentiometer feedback which allows for position control, but essentially has two preset positions; furled or unfurled.


When the winds are predicted to be unusually high, or rough (or both), or when we have ample solar production we can easily shut things down manually.  This is by far the softest method – furl the tail, and then close in the (resistive) load bank – now we're ready for most anything that comes our way.
The furling actuator is also controlled by the charge controller whenever the input voltage gets above a user-selected pre-set.  It is a simple dry-contact relay action.

[kitestrings]

This was the photo I'd intended.  We have a 4 mm dyneema line to furl the tail through a pivoting snatch block, like those used on many PTO tractor winches:

[DanG]

I remember DanB discussing a shut-down safety (failure?) versus cable gauge coming down the tower - IIRC rectifiers were not at tower base, at 8000ft elevation building sites are sheltered so best wind might be off a ways and keeping at 60VDC as long as possible reduced line losses... I think I remember they enjoyed some benefits & economy of lower cable size (higher RPM at cut-in?) right up until repeated gust surges overpowered the short-circuit while the offset-furl was being forced to repeatedly do its set up which duly softened the stator to allow coil circuits to mingle (eww) then the blades finally splintered from over speed &  turbulence.

Being a fan of heavy-copper and heat both - on a dump-load circuit I always wondered adding induction, charging a (large) reactor with paralleled bleeder resistors, add inductance to the circuit to force a more even ramp of RPM & voltage/current rises would better buffer a batteries system when automatically switched to a dump load situation.... a system can't be attended 100% of the time to do a manual shorting and/or mechanical furl, but that combination sudden wind and happy batteries equals a lot can happen in the 5, 10, 15 minutes it may take to reach the controls.

I'd think it could stall a proportion of RPM surges better versus a straight resistance dump-load and maybe protect hand-wound axial flux stators against too-quick coil temperature spikes a little better?  I mean with all that wind happening there is more cooling happening, try to increase time before things go critical and off-load some of the inductive work effort...

[bigrockcandymountain]

I am a chicken and have never shorted wires to stop the rotor.  I have a similar manual furling system to kitestrings.  Mine is just a hand lever at the tower base.

That being said, most people just short the wires and don't think twice.  If your generator has very little drag when it is shorted, it may not work for you though.  Trust your gut.  You seem to have a good feel for things. 

[Adriaan Kragten]

...Making short-circuit over a resistor...

  What is the advantage to doing this?  There is a lot of risk.

Short the AC.  Works great and it only needs a simple switch.

The switch you need to make short-circuit in the AC line isn't that simple especially if you have to short-circuit the star point too. In this case you need a switch with three contacts. If you don't short-circuit the star point, you need a switch with two contacts. It is true that for making short circuit in the DC line you first have to disconnect the batteries but you need a switch with only one contact to make short-circuit in the DC line. If you make short-circuit over a resistor first, you need only one resistor but you need three resistors if you make short-circuit over resistors in the AC line. So making short-circuit in the DC line can have certain advantages although the disadvantages are generally more important.

[kitestrings]

 If you don't short-circuit the star point, you need a switch with two contacts...

This is what Adrian is describing here

[Adriaan Kragten]

Yes, that is the right wire diagram for a switch with two contacts if you want to short circuit the three phases of a winding which is connected in star.
At my VIRYA-4.2, I have short-circuited the star point too to get a stronger braking torque. I have used a standard star-delta switch because such a switch has three heavy contacts in parallel. The three hinge points of the contacts are connected to each other and to the star point. The three open contact ends are connected to the three phases of the winding. If you turn the switch, all three contacts are closed and so all three phases are connected to the star point.