Is this potential problem of runaway controller limited to DC controllers? Should this be a consideration for a system using the open frame Mars motor? It seems like a DC to AC convertor won't fail in a full on condition.
I can understand the issue with a DC system which is typical in EV cars.
Eric
Marina del Rey
--- In electricboats@
>
> I have been reading this thread (and related one) now and want to put in my two cents worth.
>
>
> One area of debate here is around the issue of a controller failure and a run away motor. It is important to understand how this can happen and design safety features to deal with it.
>
> In order to understand this, it is important to understand the common failure modes of the MOSFET switching elements used in most of the controllers on the market.
>
> Without getting into detail, it is fair to say that they can either fail open (no current flow) or shorted (high current flow). In practice, it is most common that they fail shorted, at least initially.
>
> For the most part, the run away condition are only likely in systems using the simple low side switching controllers often used in golf cars, such as the Curtis controllers many are using. Reverse is obtained by mechanically reversing the current flow direction to the motor. Typically, these controllers don't regulate current or have much in the way of shut down features (don't flame me if the one you are using does).
>
> In these systems, one side of the motor is connected to the battery bank (+) connection. The controller switches one or more MOSFETs on and off to control the voltage applied to the motor. If the MOSFET(s) fail, they create a short circuit such that full battery voltage is applied to the motor. Since the current is limited by the motors internal impedance, the device will often remain in this state, hence the run away motor condition.
>
> In these systems is is wise to have a manual disconnect that is reasonably within reach of the helm. Also, a high quality circuit breaker may save fuse replacement, should a locked rotor condition cause it to trip. BTW, the same condition can occur if a short circuit bypasses the controller. Again, the manual disconnect is your most readily available way to shut down the motor.
>
> More sophisticatd systems use a 4 quadrant controller. This is the case with virtually all brushless motor controllers and the brush motor controllers that electronically reverse direction. Internally, there are MOSFETs in an H-bridge arrangement (three half bridges in the brushless motor case), meaning there is a switch from both + and - to each motor lead. In normal operation, the high side and low side switch in each leg are NEVER turned on at the same time. To do so causes a direct short circuit across the battery. Since the batteries can supply very high current under this condition, the fuse or breaker should trip.
>
> In almost any failure mode, the internal MOSFETs will quickly fail open. Initially, they may fail short, but the high current surge will usually cause catastrophic failure that ends in an open circuit. In many cases, this may occur before the fuse or breaker trips. Typically, when we have tested this condition with our systems (not always on purpose), the MOSFETs fail open. On our smaller systems with 200A fuses, the fuse also trips. On our large systems with 350A fuses, the fuse does not trip.
>
> It is much less likely that a run away motor condition will occur. In fact, it is nearly impossible with an electronically commutated (brushless) motor. For this reason, these systems are safer in failure. Of course, the same fire and burn hazards are possible. A manual disconnect and fuse are still mandatory. Also, care must be excercised when working with battery cabling.
>
> On the subject of fuse vs circuit breaker:
>
> I am in the fuse camp. Proper circuit breakers designed to interrupt high Dc currents are expensive. The inexpensive kind available at West Marine, etc WILL weld, if called on to interrupt high dc currents. A blown fuse should be a very rare event in a well designed system. Fuses are reliable and will not weld. It is easy to carry a spare or two. If you are having issues with your fuse or breaker tripping, you need to address the root cause, rather than relying on reseting a breaker.
>
> Cheers,
>
> Scott McMillan
> Electric Yacht
>
No comments:
Post a Comment