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Abstract



Starter Motor Protection


Starter motors are sensitive for overheating. By estimating the temperature and preventing cranking in time, there is an option to avoid the dangerous temper- atures. The truck manufacturer Scania CV AB proposed a master thesis that should evaluate the need of an overheating protection for the starter motor. The aim is to evaluate any positive effects of implementing an algorithm that can estimate the brush temperature instead of using the available time constrain, which allows 35 seconds of cranking with a following 2 seconds delay, allowing the crank shaft to stop before a new start attempt is allowed. To achieve high load on the starter motor and high temperature in the brushes, tests were performed under -20 degrees Celsius. Initial testing on truck, under normal temperatures, showed that the batteries could not run the starter motor long enough to reach high temperatures in the brushes. This is believed to be caused by the voltage drop between the batteries and the starter motor, causing the starter motor to run in an operating area it is not optimized for. There are several other problems which gives a higher load on the engine, for example oil viscosity, resulting in higher currents, but those are not mentioned in this report. Three different models are compared, Two State Model, Single State Model and a Time Constrained Model. Tests and verifications show that the Two State Model is superior when it comes to protecting the starter motor from overheating and at the same time maximizing the cranking time. The major difference between the Two State Model and the Single State Model are the cooling characteristics. In the Single State Model the brush temperature drops quickly to the outside temperature while in the Two State Model the brush temperature drops to a second state temperature instead of the outside temperature. With the currently implemented time constrain it is possible to overheat the starter motor. The algorithms are optimized under cold conditions, due to problems in reaching high temperatures under warmer conditions.

Daniel Gerhardsson

2010

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