No-Load Test - SD-255 Starter Motor

No-Load Test:

Connect a voltmeter from the motor terminal to the motor frame and an RPM indicator to measure armature speed. Connect the motor and an ammeter in series with a fully charged battery of the specified voltage, and a switch in the open position to the switch terminal. Close the switch and compare the RPM, current, and voltage readings with the specifications. It is not necessary to obtain the exact voltage. If the voltage is slightly higher the RPM will be slightly higher, with the current remaining basically unchanged. However, if the exact voltage is desired, a carbon pile connected across the battery can be used to reduce the voltage to the specified value. Compare the test results as follows:
1. Rated current draw and no-load speed indicates normal condition of the starter motor.
2. Low free speed and high current draw indicates:
^ Too much friction. Tight, dirty or worn bearings, bent armature shaft or loose pole shoes allowing the armature to drag.
^ Shorted armature. This can be further checked on a growler after disassembly.
^ Grounded armature or fields. Check further after disassembly.
3. Failure to operate with high current draw indicates:
^ A direct ground. in the terminal or fields.
^ Seized bearings. This should have been noted by turning the armature by hand.
4. Failure to operate with no current draw indicates:
^ Open field circuit. This can be checked after disassembly by inspecting internal connections and tracing the circuit with a test lamp.
^ Open armature coils. Inspect the commutator for badly burned bars after disassembly.
^ Broken brush springs, worn brushes.
^ High insulation between the commutator bars or other causes which would prevent good contact between the brushes and commutator.
5. Low no-load speed and low current draw indicates a higher internal resistance due to poor connections, defective leads, dirty commutator and causes listed under Step 4.
6. High free speed and high current draw indicates shorted fields. If shorted fields are suspected, replace the field coil assembly and check for improved performance.