starter amps

[Mister Moose]

Trouble is, a 12 v starter doesn't do the same job as a 6 v one. It spins the motor faster because it has more power, hence more than half the amps.

I don't follow this. Power and starter speed is not dependent on voltage, it's a function of design, the windings, and enough current at the battery to run it. Of course you could build a 6V starter to be more powerful.

The reason we build 12 volt systems, (and most modern aircraft at 24V, and tugboats at 32V) is to lower the current for the same power. Less resistive losses and as mentioned you can use smaller conductors. But it is not a power limitation.

 

[rotorwrench]

I always think of it as current draw. In other words, a load like a series wound electric starter motor will only draw the current it needs to function. The amount of current needed will vary depending on the torque it needs to turn and the condition of the motor. The voltage is just the electromotive force that the current uses to flow. A higher voltage will allow for less current being needed to do the same job even though the current changes same as the voltage so it's not quite proportional. The 6-volt starter will certainly turn faster if not modified to work with the higher voltage but it still won't have to draw as much amperage with twice the amount electromotive force and current available to it. It would have around 4X the power as on 6-volt.

 

[Synchro909]

I always think of it as current draw. In other words, a load like a series wound electric starter motor will only draw the current it needs to function. The amount of current needed will vary depending on the torque it needs to turn and the condition of the motor. The voltage is just the electromotive force that the current uses to flow. A higher voltage will allow for less current being needed to do the same job even though the current changes same as the voltage so it's not quite proportional. The 6-volt starter will certainly turn faster if not modified to work with the higher voltage but it still won't have to draw as much amperage with twice the amount electromotive force and current available to it. It would have around 4X the power as on 6-volt.

Give the man a prize!

 

[Patrick L.]

These threads always seem to take on a life of their own. The man asked a simple question and I think we chased him away.

 

[Bob Bidonde]

This starting motor data is from Dyke's Encyclopedia, and it applies to a starting motor in new condition:
Terminal Grounded (+)
Normal Armature Speed 1500 RPM
Amperage, Normal 175
Normal Running Torque 3 Lbs-Ft

 

[Patrick L.]

Just get the little do-hickey Katy shows in post 15 and try it. Only takes a couple seconds. Its a great opportunity to buy another tool.