Page 20 - March 2015 Volume 9, Number 3

Ask the Expert
Starter Time Limits
Y ou memorized this during tripled, heat production goes up
During the initial stages of engine starting, when the starter is providing all of the power that is providing compressor rotation – not yet helped by any exhaust gas flow – we are faced with a double-whammy: The most heat production and the worst heat dissipation. The heat production comes from the high current flow at this time and the poor heat dissipation comes from the relatively low N1 – and cooling fan – speed.
The bottom line? We can burn up the starter by getting it much too hot if we do not abide by the starter time limits. To state the reasoning very simply, “Don’t use the starter for such a long time that it gets too hot and also give it enough time at rest to dissipate the heat before using it again.”
The 300-series engines are physically larger than those on previous King Airs and their starters generate more heat while spinning the bigger compressors. This explains why their limits are so much more conservative, requiring both shorter working times and longer cooling off times.
The starter will get hotter during a particular period of use when the ramp temperature is over 100° Fahrenheit on an August afternoon as compared to its operating cooler on a January ramp with the temperatures
by Tom Clements
your Initial King Air
training program, right? “Use of the starter is limited to 40 seconds ON, 60 seconds OFF, 40 seconds ON, 60 seconds OFF, 40 seconds ON, then 30 minutes OFF.” For the 300-series of King Airs, the limits are more restrictive: “Use of the starter is limited to 30 seconds ON, 5 minutes OFF, 30 seconds ON, 5 minutes OFF, 30 seconds ON, then 30 minutes OFF.” In addition to merely reminding you of these important limits, the intent of this article is to explain the reasoning behind the limits and to emphasize the real- world method of observance.
Engine starting subjects the electrical system to the highest current flow it ever experiences during normal operation. A peak flow in excess of 800 amperes is typically experienced in the fraction of a second that the starter motor receives power before it begins to rotate. The current demand rapidly decreases as the starter and engine increase speed, but when stabilized N1 or Ng speed is attained, usually there is still over 200 amps flowing to the starter motor.
nine-fold. Comparing the heating effect of 800 amps to that of 200 amps, this four-fold reduction reduces heat generation by a factor of sixteen! It is quite apparent then that the starter is building up a lot of thermal energy when it is being used, especially in the early stages of a start. Does this mean it is getting hot? Usually, yes, but not always.
The other factor that determines how hot the starter becomes is its cooling mechanism ... how, if at all, the thermal energy is being removed. Although it would be ridiculous to submerge the starter in a vat of ice water in the cowling, it could stay quite cool throughout the starting process if such a setup were available. Since it is not available, however, the only mechanism that is designed to help remove the thermal energy buildup is a fan that pushes air across the starter. This four- blade fan is attached to the rear of the starter’s armature shaft and rotates with it. Hence, the faster the starter turns, the more thermal energy dissipation capacity it has. That explains why when this starter device is being used as a generator, limits exist requiring us to use higher N1 speeds when experiencing
Current flow generates heat
energy in a squared relationship.
That means that when current is higher electrical loads.
18 • KING AIR MAGAZINE
MARCH 2015