30-05-2020, 02:30 PM
Hi John P
When a starter motor is first turned on, the armature must speed up rapidly. This high acceleration needs high motor torque. Because starters are series wound, their torque is proportional to the square of the current. If things are working properly, when the battery is connected to the starter there will be an initial very high current surge of hundreds of amps giving a burst of high torque. Only a tiny part of this is needed to overcome friction, and there is not yet any load from the engine, so nearly 100% of the torque goes into acceleration of the armature. The pinion lags behind and is thus wound up its thread. If the starter is off the car, the torque reaction manifests itself as a "kick" if the starter casing isn't firmly held down.
Once the armature has sped up, it generates a back EMF in opposition to the battery voltage, so the current diminishes to a lower steady running value. At this point the motor torque is balanced by friction plus the load from cranking the engine. If the starter is off the car the torque is balanced by friction alone and the steady running current won't be very much. The speed when off the car will be more or less proportional to the battery voltage.
The upshot of all the above is that if there is any unwanted extra electrical resistance somewhere in the starter, its wiring or the battery it will have a MUCH more significant effect during the initial acceleration than it will during the steady speed running state.
To put some figures on it, say you have a total resistance of battery, starter and wiring of 20 milliohms. With 6 Volts the initial current will be 300 Amps. If you were to then add 10 milliohms of extra resistance the total would rise to 30 milliohms and the initial current would drop to 200 Amps. The torque would be only 2/3 x 2/3 = 4/9 of the previous value.
I don't know if new brushes are contoured to fit the armature. If they are not, then you should be able to get a closer fit by the use of thin abrasive paper. The increased effective brush area will reduce the resistance.
When a starter motor is first turned on, the armature must speed up rapidly. This high acceleration needs high motor torque. Because starters are series wound, their torque is proportional to the square of the current. If things are working properly, when the battery is connected to the starter there will be an initial very high current surge of hundreds of amps giving a burst of high torque. Only a tiny part of this is needed to overcome friction, and there is not yet any load from the engine, so nearly 100% of the torque goes into acceleration of the armature. The pinion lags behind and is thus wound up its thread. If the starter is off the car, the torque reaction manifests itself as a "kick" if the starter casing isn't firmly held down.
Once the armature has sped up, it generates a back EMF in opposition to the battery voltage, so the current diminishes to a lower steady running value. At this point the motor torque is balanced by friction plus the load from cranking the engine. If the starter is off the car the torque is balanced by friction alone and the steady running current won't be very much. The speed when off the car will be more or less proportional to the battery voltage.
The upshot of all the above is that if there is any unwanted extra electrical resistance somewhere in the starter, its wiring or the battery it will have a MUCH more significant effect during the initial acceleration than it will during the steady speed running state.
To put some figures on it, say you have a total resistance of battery, starter and wiring of 20 milliohms. With 6 Volts the initial current will be 300 Amps. If you were to then add 10 milliohms of extra resistance the total would rise to 30 milliohms and the initial current would drop to 200 Amps. The torque would be only 2/3 x 2/3 = 4/9 of the previous value.
I don't know if new brushes are contoured to fit the armature. If they are not, then you should be able to get a closer fit by the use of thin abrasive paper. The increased effective brush area will reduce the resistance.