amperage for starters

[Reg]

Yes. Motor resistance is very low. If you applied Ohms law to the actual motor resistance you would predict extremely high currents. This resistance doesnt change as the motor spins, but as the motor spins it generates a counter V [opposite from the batt V] called Back EMF. As the motor goes faster so rises BEMF until current limits. - So the resistance of the motor appears to vary with speed. Slow the motor with a load, BEMF drops and current rises. As current rises there is more V lost due to the internal resistance of the batt so the V remaining to feed the motor goes down. The current to a starter is way high at low cranking speeds in cold engine starting applications regardless that the battery may be consuming 2V internally and only delivering around 10V to the starter.

Very few tractor sized 12V batteries will hold up to deliver 12V to a 100+ Amp load-NONE will deliver 13.2V . Thats why I used the 11.5V figure for predicting current consumed by a 2.2kW starter.
larry

Yeah, yeah, mumble, mumble, back emf, rising internal resistance of the battery as it drains, falling of back emf as the starter slows, etc......
Which is why I used the word UNPREDICTABLE.

At the instant the switch is thrown ohm's law pretty much applies, self and mutual inductance will follow, soon, but not at the instant of initial contact.
Have enough cable to take the full stalled load, not the load at free spinning speed.

 

[SPYDERLK]

If so equipped, pull the fuel cutoff while cranking, and put the DMM on peak hold.

//greg//

you dont want to use peak hold until actually cranking, cuz that will give you the instantaneous locked rotor current before the motor starts to spin. Interesting in that it will be the max current youll ever see in the system, but little relation to actual cranking current. Just pulling the fuel cutoff like you said will do good for a cranking current - and then during cranking you can do peak hold and get the peak caused by compression strokes.
larry

 

[SPYDERLK]

Have enough cable to take the full stalled load, not the load at free spinning speed.

No point in using the huge cables necessary to support a stalled load. The stall is over as soon as the motor spins up. Amperage drops to a fraction of the stalled Amps. Not UNPREDICTABLE.
larry

 

[greg_g]

you dont want to use peak hold until actually cranking,

good point - I guess I should have been more specific

//greg//

 

[Wayne County Hose]

I bought a set of these inductive ammeters (MT1009) quite some time ago for about 50 bucks and are capable of getting that reading your interested in.

I'll slip some shoes on and go see what my 790 draws.

These are the exact ones I use. I have the high dollar clamp-ons for my DVM's. Don't like them. Reaction time is too slow. My DVM's are not cheap ones either. I have 2 Flukes, a Beckman and a Greenlee. There is nothing like the real time reading of an inductive ammeter like the MT1009 style.

 

[Reg]

No point in using the huge cables necessary to support a stalled load. The stall is over as soon as the motor spins up. Amperage drops to a fraction of the stalled Amps. Not UNPREDICTABLE.
larry

The stall is only over IF the starter motor spins up.
If you ENJOY electrical fires by all means connect up with wire that is barely adequate. Your choice.

MY preference is to plan for the worst case, but hope for something less bad.

UNPREDICTABLE, as in too many variables to derive a reliable value.

 

[SPYDERLK]

The stall is only over IF the starter motor spins up.

When I hit the key I note that the engine is accelerated to cranking speed almost instantly. So the stall condition has ended even quicker than that. If the engine does not turn I dont spend much time in Duh mode - I let off the key. In either case the high current condition of around 1000A at stall ends plenty quick to obviate the need for cable that would support 1000A steady state. If I did have 1000A cable and became frozen in Duh mode with a locked engine or starter, there would still be starter fires and battery explosions to worry about. But I predict that standard cables, batteries, starters, and observation of whether the engine is turning will serve well enuf.
larry

 

[Soundguy]

At the instant the switch is thrown ohm's law pretty much applies, .

Ohms law applies no matter what.. just have to take the circuit into account. remember on an inductive circuit, you get to mess with stuff that looks an awfull lot like impeadance,.. vs plain dc resistance. you also get to play with inductive reactance.
In this circuit, the moment the switch is thrown, voltage will lead current / current will lag voltage.

soundguy

 

[Soundguy]

No point in using the huge cables necessary to support a stalled load. The stall is over as soon as the motor spins up. Amperage drops to a fraction of the stalled Amps. Not UNPREDICTABLE.
larry

the more conductor betweent he source and load the better.. at some point you hit diminishing returns.. but 'adequate' is never 'ideal' when it comes to battery cables.

12v auto's can get away with 4ga cables. diesel and 6v application like 0/00 thumb thick cables... more copper never hurts.

Rremember.. small copper between a battery and a starter = a shunt and a votlage drop

soundguy

 

[SPYDERLK]

Rremember.. small copper between a battery and a starter = a shunt and a votlage drop
soundguy

Dont forget cable length. You just want to limit Watts lost in the cable. Shorter cables can be smaller. When you have cables 5 or six or more feet long theyd better be big - especially where it gets cold.
larry