Starter motor current draw?

[bedouin]

It’s a short run, maybe a metre from battery to switch to starter.

You need to factor in the return path as well - but I assume that will be equally short.

Calculations suggest that you could just about get away with 10mm2 (that is NOT a recommendation) so anything over that should be fine - 35mm2 certainly more than adequate and you could consider going down a size.

 

[rotrax]

Can we calculate the maximum possible current from the resistance of the motor measured with a multimeter? As I understand it, the motor's impedance (resistance when it's running) can only increase from there. I could be wrong though.

Dont forget the significantly increased current flow - AMPS - as the voltage drops when cranking.

Ohms law and all that.

A 6v bulb draws twice the amps of a 12v bulb for the same output.

 

[rogerthebodger]

Most if not all current carrying capacity of a particular size of electrical is based on a continuous current flow.

Generally a starter motor only runs for a very short time so the duty cycle is very low.

The current will be high so the volt drop will be an more important factor then the maximum current capacity.

The fuse should albe a slow blow type so the high inrush current does not blow the fuse but will blow is a short circuit occurs.

Also do remember than all starter motors are designed for high torque with a very short duty cycle so the cable and starter solenoid will also have a very low duty cycle.

 

[JumbleDuck]

Dont forget the significantly increased current flow - AMPS - as the voltage drops when cranking.

Ohms law and all that.

A 6v bulb draws twice the amps of a 12v bulb for the same output.

Only because it has been designed that way. If you apply 6V to a 12V starter motor it will simply take less current and produce less power/torque.

The reason current is higher at the start of cranking is because DC motors also act as generators and, as they turn, produce a voltage opposed to the supply voltage, often called the "back emf" (emf = electromotive force is and old name for voltage. When the starter engages initially the engine isn't turning, so the starter isn't turning either, so there is no back emf and the full battery voltage is applied. As the engine and starter motor start turning the back emf increases and a smaller current flows.

The internal resistance of the battery means that the voltage applied to the starter falls as current increases, but that's another story ...

 

[TernVI]

Only because it has been designed that way. If you apply 6V to a 12V starter motor it will simply take less current and produce less power/torque.

The reason current is higher at the start of cranking is because DC motors also act as generators and, as they turn, produce a voltage opposed to the supply voltage, often called the "back emf" (emf = electromotive force is and old name for voltage. When the starter engages initially the engine isn't turning, so the starter isn't turning either, so there is no back emf and the full battery voltage is applied. As the engine and starter motor start turning the back emf increases and a smaller current flows.

The internal resistance of the battery means that the voltage applied to the starter falls as current increases, but that's another story ...

Actually, if you apply 6V to a 12V starter motor (with an engine attached) it will quite likely draw quite a lot of current, fail to turn the engine and burn out. A duff battery is a major cause of death to starter motors.
The whole thing is designed around the stall current only happening for a very short time, before things heat up.
Some starter motors are designed to run from quite a small battery, as if they are really say 10V motors rather than 12V.
There is subtlety in the system learned from ~100 years of design experience.

 

[JumbleDuck]

If you apply 6V to a 12V starter motor it will simply take less current and produce less power/torque.

Actually, if you apply 6V to a 12V starter motor (with an engine attached) it will quite likely draw quite a lot of current, fail to turn the engine and burn out.

Absolutely. Your "quite a lot of current" will, of course, be the same as my "less current [than if 12V had been applied]" and your "fail to turn the engine" is because of my "produce less torque", so I am happy to say that we are in perfect agreement!

Some starter motors are designed to run from quite a small battery, as if they are really say 10V motors rather than 12V.

I think most car starter motors are designed to run on a fair bit less than 12V. It's one reason for having ballast resistor coils in cars with contact breaker ignition systems - when starting the whole electrical system is effectively a 9 or 10V one.

 

[rotrax]

In a previous life we had a starter test bench at LSUK Park Royal, the largest depot in the UK at the time.

I distinctly remember amps drawn climbing as the cranking voltage dropped to 10.5v or less.

If the amps are available in my experience - which is 'electricks' rather than electronics - the amps will go down the winding and cause heat. And will go up as voltage drops, but only to a certain extent.

Seen it, done it. On proper test equipment too.

 

[Birdseye]

Starter motor is 1.5hp
Engine is 4 cylinder Petrol 2.3 litres.
Battery is 12v

Anyone want to hazard a guess at the sort of current flow when the turning engine over?

Best I can come up with is around 90a based on 1.5hp being equiv 1100w at 12v, but I know that initially, at least, it will draw more. The question is, how much?

In case you are wondering why I need to know, I am thinking of fusing the circuit and trying to work our fuse size. Would 150a be too small?

starter motors are normally in the 1000w to 1200w range so 1000/10v = 100A

 

[VicS]

starter motors are normally in the 1000w to 1200w range so 1000/10v = 100A

No, because as already explained the power quoted is the mechanical power delivered ,. The electrical composition will be significantly higher

Based on the figures I postedearlier I would expect somewhre in the range 250 to 300 amps