SinusLeistungsSteller - Partial Load Safety |
Partial load safety: ... first: The SLS feels comfortable with partial load! Common block commutated ESCs are designed to achieve and hold the highest rotation speed possible (full load operation). This is the point were the motor reaches highest power. (Power is proportional to speed * torque, only the max. motor current is limiting the torque, therefore the higher the torque the more power is possible). At max. rotation speed the block commutated ESC works with relatively high efficiency, because (compared with operation in partial load) only a few switching cycles at the semiconductors occur. By that, switching losses in the semiconductors are minimized. If a block commutated ESC works in partial load, much more switching cycles appear (PWM-frequences for partial load operation are usually 8kHz, 16kHz or 32kHz) and therefore more switching losses. Block commutated ESCs are usually not designed for such high power losses and will overheat, if operating to long in partial load. The only way to prevent a block commutated ESC from "heat-death" in this situation is to switch the ESC off. In practise this problem is encountered by simply avoiding the partial load ("never between 70% -90%") or a time limit for a partial load situation, by "intuition". Some help provides additional heatsinks and fans, which are in most cases mounted by the user. By this the allowed partial load time can be extended in order to delay the controller switch off. Poor thermal connectivity between heatsink and semiconductor results in failure of this method - just a single overheated power semiconductor can result in failure of the entire ESC. Beside this, an additional heatsink and/or fan creates extra weight. Also must be mentionned: Some block commutated ESCs get rid of this problem, by simply switching up onto 100% above a certain (but yet non-critical) partial load. Part load does not exist for that kind of ESC in the upper area! The field oriented control, used by the SinusLeistungsSteller, forces the SLS to use a high PWM-frequency even at full load - the motor current needs to be shaped to a sine. The thermal capability of the SLS must be designed for the "worst case" full load . This also means that each operating point below full load (so every possible partial load operation) has less power losses and therefore is uncritical! Consequently the SLS is completely part load safe! In case of overheating the SLS is able to reduce the output power (goes in partial load) and automatically provide cooling, since the largest heat loss is produced at full load. In case of overtemperature the SLS does not turn off, but works with just enough reduced power, that no more temperature excess occurs. A safety aspect worth mentioning! OK,at full load the SLS has more losses than block commutated ESCs, because higher switch losses occur at the SLS. But on the other hand, losses in the motor can be avoided by the sine-shaped currents (keyword: harmonics), which compensate the additional losses in the SLS. When comparing a complete drive train "SLS + motor" with a "block commutated ESC + motor" the SLS is more efficient! ... also in the expected flight time! Just the fact, to be able to use partial load all the time,results in longer flight time! |