DX 33 Starting Problem Fixed

[Jerry/MT]

And of course as volts go down, current goes up. QUOTE]

Can you explain what you mean by this statement?

 

[ENTS]

Jerry,

I too am curious. In a pure resitive circuit, E=I*R. With R constant, and E goes down, I goes down. RE: 12V and 4 ohms = 3 amps. If 12V goes to 10V and R is still 4 then I = 2.5. Maybe it's the battery that delivers more current as the volts go down:confused2: I'm sure someone will weigh in here soon especially if I'm wrong.

 

[thclimer]

I'm not electrically astute but I believe this is the equation which applies in this situation:

P = I * E

Where P = Power (watts), I = Current (amps), E = Voltage (volts).

For example, if you have a 12 volt battery in a circuit with a device which uses 96 watts then the current running threw the device would be:

P = I * E or I = P / E

96 watts = I amps * 12 Volts or I amps = 96 watts / 12 Volts

I amps = 8

So if your power requirement (watts) does not change but the battery voltage drops to say 10 volts, then the current required would be:

96 watts = I amps * 10 Volts

I amps = 96 watts / 10 Volts

I amps = 9.6 or an increase of current by 20%

This why fuses tend to blow when the battery voltage drops.

 

[jinman]

So Tim, by that logic, if the voltage drops to 1 volt, you'd be drawing 96 amps.:shocked:

When the voltage drops in a circuit of a given resistance, the current also drops and the overall power is less. That's the way it works. For a given resistance, changing the voltage also changes all the other values.

 

[thclimer]

Hey Jim,

I was hoping someone with a better understanding of electricity like you would read this post.

So are we talking about V (volts) = I (amps) * R (ohms) like in Fred's example

12 volts = I amps * 4 ohms

I amps = 3

And if the voltage drops to 10, then the current will drop to 2.5 with the resistance being constant.

So in our case of blowing fuses when we are trying to start our tractors it not really because our batteries are weak, but because the resistance in the circuit has increased, thus drawing more current?

 

[jinman]

I think it's possibly the additional current by the inductive load of the starter motor and the interaction of the armature and fields. An inductive load initially has high resistance until the magnetic field is maximumized and then effectively low resistance. If the motor doesn't turn, the load goes to maximum and stays there. Essentially, because of lower voltage, the motor doesn't have sufficient torque to turn the engine at normal speed. Starting current is high for a longer period of time, causing even a slo-blow fuse to pop. If the battery is hot enough to get the starter and engine spinning rapidly, the full torque requirement of the motor is reduced, allowing it to spin at higher rpm but not in a steady high current state. This is a bit of an oversimplification, but I don't think it's too far off.

 

[djradz]

I think it's possibly the additional current by the inductive load of the starter motor and the interaction of the armature and fields. An inductive load initially has high resistance until the magnetic field is maximumized and then effectively low resistance. If the motor doesn't turn, the load goes to maximum and stays there. Essentially, because of lower voltage, the motor doesn't have sufficient torque to turn the engine at normal speed. Starting current is high for a longer period of time, causing even a slo-blow fuse to pop. If the battery is hot enough to get the starter and engine spinning rapidly, the full torque requirement of the motor is reduced, allowing it to spin at higher rpm but not in a steady high current state. This is a bit of an oversimplification, but I don't think it's too far off.

Nope. Not an oversimplification at all Jim. Right on, except for the use of the term "high resistance" in the second sentence, as the correct term is high impedience which reflects both the resitive and inductive loads. As you note, the high current stays for much too long (due to not turning, or the operator continuing to hold the key) and the fuse heats and blows. The fact that the current is initially slightly less due to the low voltage (as you and others have noted - this is Ohm's Law) is negligible and far outweighed by the substantial length of time. In fact, if the fuse didn't blow, there's a good chance the motor would overheat and fail.

 

[jinman]

Nope. Not an oversimplification at all Jim. Right on, except for the use of the term "high resistance" in the second sentence, as the correct term is high impedience which reflects both the resitive and inductive loads. As you note, the high current stays for much too long (due to not turning, or the operator continuing to hold the key) and the fuse heats and blows. The fact that the current is initially slightly less due to the low voltage (as you and others have noted - this is Ohm's Law) is negligible and far outweighed by the substantial length of time. In fact, if the fuse didn't blow, there's a good chance the motor would overheat and fail.

Thanks Duane, and you are exactly correct that impedance is the correct term when referencing inductive loads.:thumbsup:

 

[SteveInMD]

The fuse blowing problem with these tractors from the OP is much worse when the battery is even slightly below full charge (and I don't mean near dead, just slightly off peak). Also the problem did not show up for a few years after the tractors were new. I believe the original circuit also results in a somewhat slow crank.

The theory was that over a few years the safety switches became less than perfect connections resulting in less than 12 volts at the solenoid. Perhaps this caused it to throw out slowly??? I know my tractor would even blow a 15 amp fuse.

I don't know what the answer is. I do know that adding a relay solved the problem. (I replaced the key switch before doing this mod. and I still had the issue.) The starting circuit now draws less than 7 amps, the crank speed is faster, and it always starts. Jiman's explanation sounds reasonable. I am curious however why after the modification the tractor has much less trouble starting with a low battery than before.

 

[Fanlo2300]

My 2003 DX-33 did blowed many 10 amp, fuses after I bought it last summer.

I never happened again after I installed a new battery.

It even started A-1 last Saturday after sitting for a week in an unheated garage at -20 C (with a 15 min. preheat with the block heater).