Last Active: 6 Years Ago
Starter Motor Performance Curves
Do you see how similar these graphs look? All series wound dc motors no matter the horsepower or voltage have the same characteristics. There is much that is perhaps counterintuitive here and information about starter motors that will not come easy. Did you know that starter manufacturers actually specify available cold cranking amps (cca) that you should NOT exceed?! If you look at these curves and think about it you start to understand what is going on electrically (and thermally.) Where is the most power developed? At approximately 2/3 the nominal voltage, around 9 volts for 12 volt systems and 16 for 24 volt systems. In our battery ratings we find that those cca ratings also have a voltage factor, that is the cranking amperes have to be at a USABLE voltage, or about 7 volts in a 12 volt battery. The current goes off the chart as the voltage drops and the rpm lowers until the starter stalls and we have what is called locked rotor current. For all real world practical purposes it is as close to a dead short as we can get without being a dead short...if that makes any sense? When cranking voltage in a 12v system falls below 7 to 9 volts we should start troubleshooting. The batteries could be failing, there could be mechanical problems with the engine causing increased drag, starter problems, bad connections, cold weather and thick oil or? Starter too small for the job? Will have to continue this later, need sleep.....