What current does a starter motor use?

[Theo]

What current would you expect the starter motor to take? My son measured it this morning using a clamp meter and found it to be a little under 50A. I would have thought that this was very low but can't find any data in other threads.

 

I could hear the engine spinning as he measured the current. (He had the stop pull pulled so that the engine didn't start.) It was spinning freely with the batteries fully charged.

 

Edited to say that it is on a BMC 1.5 engine

 

TIA

 

 

Nick

Edited October 8, 2007 by Theo

[NB Alnwick]

What current would you expect the starter motor to take? My son measured it this morning using a clamp meter and found it to be a little under 50A. I would have thought that this was very low but can't find any data in other threads.

 

I could hear the engine spinning as he measured the current. (He had the stop pull pulled so that the engine didn't start.) It was spinning freely with the batteries fully charged.

 

TIA

 

Nick

 

Surely the answer must be dependent on many factors such as: the size of the engine, diameter of the flywheel and pinion (gearing), compression ratio and type/model of starter motor etc.

[John Orentas]

There is always some exaggeration about this, one of those things that people simply make assumptions based on what others have said, I have read reports of 250 and even 300 amps for narrowboat engines, I was always a bit sceptical.

 

I did the test last year with a clamp meter. On what was about the coldest day of the year, early in the morning, my engine is a heavy 2.1 litre 3 cylinder diesel and had not been run for a couple of weeks, I manually held the 'stop' solenoid closed whilst engaging the starter to prevent it from starting.

 

I tried a few times, the highest current reading I could measure was 150 amps, of course after the initial acceleration the current dropped back well below 100 A

[Theo]

There is always some exaggeration about this, one of those things that people simply make assumptions based on what others have said, I have read reports of 250 and even 300 amps for narrowboat engines, I was always a bit sceptical.

 

I did the test last year with a clamp meter. On what was about the coldest day of the year, early in the morning, my engine is a heavy 2.1 litre 3 cylinder diesel and had not been run for a couple of weeks, I manually held the 'stop' solenoid closed whilst engaging the starter to prevent it from starting.

 

I tried a few times, the highest current reading I could measure was 150 amps, of course after the initial acceleration the current dropped back well below 100 A

 

 

That rather confirms the readings taken. The old BMC was warm and is getting decidedly loose in the bearing department.

 

Nick

[Sir Nibble]

50A Cranking? No offence, but I don't believe it. Beware clampmeters, they are magnetic devices and will miss read if too close to another magnetic device, like, say a starter motor. The solenoid alone will draw about 10A cranking. I would expect your BMC starter to take about 80-100A FREE RUNNING.

[John Orentas]

Snibble, why don't you try it for yourself, the bloke has done the science, it is no good just saying his clamp meter is faulty.. Try it then come back to us.

Edited October 8, 2007 by John Orentas

[Sir Nibble]

John, Ive spent the last 35 years of my life stripping, repairing or reconditioning, and testing starters. How much more

Try it then come back to us.

do you want? I have seen starters that typically draw about 50A, but nothing over pocket size, (Without special trousers).

 

the bloke has done the science,

No he hasn't, his son has used a piece of equipment that can be misleading in untrained hands and got a reading that defies all my years of experience and training. Supposing someone measured injection pressure at 10 bar, would you say "That must be wrong" or "I must be wrong".

[John Orentas]

John, Ive spent the last 35 years of my life stripping, repairing or reconditioning, and testing starters. How much more do you want? I have seen starters that typically draw about 50A, but nothing over pocket size, (Without special trousers).

 

No he hasn't, his son has used a piece of equipment that can be misleading in untrained hands and got a reading that defies all my years of experience and training. Supposing someone measured injection pressure at 10 bar, would you say "That must be wrong" or "I must be wrong".

 

 

 

Consider the possibility that you may be wrong! I have been using clamp meters for as many decades as they have been available and I have never observed the problem you describe..

 

Bad form too, to pull rank in that way, I would say again to you and anyone else reading this, put a clamp meter on your starter motor cable and report back to the forum with the readings you get.

[Sir Nibble]

Lucas M45 starter (as fitted to BMC) tested, no load current 100A, moving iron meter.

Oddly, this coincides nicely with the other tens of thousands of starters I have tested, which is a surprise.

Theo posted this 'cos he couldn't quite believe the reading, look at the subtitle to the thread. Does the idea that a BMC diesel is

spinning freely

on only 50A not strike you as at all out of the ordinary? Theo clearly was doubtful enough to enquire.

May I enquire what

Bad form too, to pull rank in that way,

means? Be careful that you don't by your answer invalidate training and experience or you may find you disarm yourself as well.

[Theo]

Consider the possibility that you may be wrong! I have been using clamp meters for as many decades as they have been available and I have never observed the problem you describe..

 

Bad form too, to pull rank in that way, I would say again to you and anyone else reading this, put a clamp meter on your starter motor cable and report back to the forum with the readings you get.

 

I know some of the science but have done no science on this one. I have not even done the measurements myself. I phoned no1 son and he did the measurements while I was listening to his running commentary. The first time he did it he got 45A and when I expressed amazement he realised that the awkwardness of his position meant that he had inadvertently twisted the meter and opened the jaws a fraction. He then did a few careful readings and repeatedly got a little less than 50A.

 

I cannot see why a starter motor free running should take anything but a tiny current. The friction losses can't be that big and other losses like hysteresis and eddy current losses will have been designed out as far as possible (?) or perhaps not. The intermittent nature of their use might mean that they have not been very carefully designed, but surely they would have used low permeability magnetic materials and laminated pole pieces like every other motor that I have ever seen.

 

I am sure that a magnetic machines expert is out there somewhere and will put me right.

 

On the subject of the accuracy of the clamp meter: I would have thought it reasonably accurate, certainly within 10%. I measured the inverter current at 100A while daughter in law's hair dryer was being used and that was about right at 100A.

 

Many thanks for the thoughts so far. I look forward to further ideas.

 

Chris W will be dropping in soon I hope. He will have something for us.

 

Nick

[John Orentas]

Lucas M45 starter (as fitted to BMC) tested, no load current 100A, moving iron meter.

Oddly, this coincides nicely with the other tens of thousands of starters I have tested, which is a surprise.

Theo posted this 'cos he couldn't quite believe the reading, look at the subtitle to the thread. Does the idea that a BMC diesel is on only 50A not strike you as at all out of the ordinary? Theo clearly was doubtful enough to enquire.

May I enquire what means? Be careful that you don't by your answer invalidate training and experience or you may find you disarm yourself as well.

 

 

But the measurements I made tend to validate Theo's findings.

 

I did the experiment originally because I read of some very high and unlikely figures, in fact I was a little surprised at the low currents I measured.. If I have time this week I will will try again in less extreme conditions.. I hope others do it too.

Edited October 8, 2007 by John Orentas

[Gibbo]

Horsepower ratings for typical starter motors here.

 

http://www.lucas-tvs.com/products/star.html

 

Multiply the hp figure by 80 to get a rough current on a 12 volt system (this accounts for motor inefficiency).

 

A typical diesel engine with normal starter gearing requires about 1/20th the engine power in order to start it. So a 50 hp engine needs about 2.5hp (200 amps at 12 volt) to start it.

 

A start motor pulling 50 amps is a result of:-

 

A. A 15 horse power engine.

B. A duff readings

C. An engine on its last legs as the compression has almost gone.

 

My 4LW pulls 340 amps on a cold day.

 

There is a patent somewhere for a device that indicates engine compression ratio from the starter current.

 

Gibbo

[Sir Nibble]

I cannot see why a starter motor free running should take anything but a tiny current

With respect theo, "I cannot see why" is not an argument, it is a statement of ignorance. Starter armatures are indeed laminated, but the pole shoes are not. The reading I just took is typical. Ever noticed how your car headlamps dim as the battery voltage falls when cranking? Do you think it would do that under a load of 50A?

I do not propose to go into a "whose starter takes the least current" competition, I am a qualified and experienced professional and I have made my contribution, I am not going to argue further to defend the validity of my training and experience.

[Gibbo]

Another way of looking at it.

 

Turn the fuel off and crank the engine for an hour. A single start battery should just manage that if the starter motor only pulls 50 amps.

 

Bet it doesn't last longer than about 3 minutes.

 

Gibbo

[smileypete]

What current would you expect the starter motor to take? My son measured it this morning using a clamp meter and found it to be a little under 50A. I would have thought that this was very low but can't find any data in other threads.

Hi,

 

I usually use a spiral of 1mm² bare copper wire, water cooled, as a shunt.

 

For a 1m length 1.7v across either end represents about 100A current.

 

It does introduce some voltage drop though.

 

cheers,

Pete.

Edited October 8, 2007 by smileypete

[Sir Nibble]

Bet it doesn't last longer than about 3 minutes

Long enough to make toast on the motor casing tho'.

[Gibbo]

Long enough to make toast on the motor casing tho'.

 

That's assuming the battery lasts that long. Which is doubtful.

 

Gibbo

[Theo]

With respect theo, "I cannot see why" is not an argument, it is a statement of ignorance. Starter armatures are indeed laminated, but the pole shoes are not. The reading I just took is typical. Ever noticed how your car headlamps dim as the battery voltage falls when cranking? Do you think it would do that under a load of 50A?

I do not propose to go into a "whose starter takes the least current" competition, I am a qualified and experienced professional and I have made my contribution, I am not going to argue further to defend the validity of my training and experience.

 

Thank you for the respect, Snibble. It was, indeed a statement of ignorance. By displaying what I know, people will be aware of what I don't know relevant to the subject and will, I am sure with kindness, put me right.

 

The point about the car headlamps is worth considering, though, but I don't know the magnitudes of all the factors that would contribute to the dimming. Factors such as battery cable csa, headlamp power, short circuit current delivery of the car's battery. (The data on the new batteries that I intend to fit says that the short circuit current is, IIRC, 1360A, not that I intend to try it out!)

 

 

Nick

Edited October 8, 2007 by Theo

[Theo]

Horsepower ratings for typical starter motors here.

 

http://www.lucas-tvs.com/products/star.html

 

Multiply the hp figure by 80 to get a rough current on a 12 volt system (this accounts for motor inefficiency).

 

A typical diesel engine with normal starter gearing requires about 1/20th the engine power in order to start it. So a 50 hp engine needs about 2.5hp (200 amps at 12 volt) to start it.

 

A start motor pulling 50 amps is a result of:-

 

A. A 15 horse power engine.

B. A duff readings

C. An engine on its last legs as the compression has almost gone.

 

My 4LW pulls 340 amps on a cold day.

 

There is a patent somewhere for a device that indicates engine compression ratio from the starter current.

 

Gibbo

 

A. I think that the BMC1.5 is about 30HP but someone will put me right if not.

 

B. Could be. I wonder if my son read the decimal point in the wrong place but 500A would be too high wouldn't it. I will do the HP calculations in a minute or ten.

 

C . The engine certainly spins very easily but it also starts very easily and I haven't even sorted out the glowplug problem yet. (They don't work). It burns a little oil but you don't often have to top it up. After a day's cruising it had hardly gone below the max mark on the dip stick. I am assuming that worn piston rings would cause the engine to burn oil as well as to lose compression.

 

Nick

Edited October 8, 2007 by Theo

[Theo]

50A Cranking? No offence, but I don't believe it. Beware clampmeters, they are magnetic devices and will miss read if too close to another magnetic device, like, say a starter motor. The solenoid alone will draw about 10A cranking. I would expect your BMC starter to take about 80-100A FREE RUNNING.

 

I suspect that the "other magnetic devices" error is not a problem here. the starter motor is on the other side of the engine from the batteries.

 

Nick

[dor]

My 1.2 litre thorneycroft mitsubishi drew 180 amps when cranking. Measured with a blue sea digital meter through a shunt and I have every reason to believe that is pretty accurate.

 

My 1.7 litre vetus mitsubishi draws about the same amount, but it is 20 years younger, so maybe starter motor design has improved. Certainly my 2.2 litre Honda car much have a more efficient starter as it is about the size of a baked bean tin.

[smileypete]

I suspect that the "other magnetic devices" error is not a problem here. the starter motor is on the other side of the engine from the batteries.

 

Hi,

 

If the meter uses a magnetic sensor then the field on an nearby return cable might lower the reading.

 

50A at 12v is 600w, about the same as a small mains drill. Doesn't quite sound right to me, especially for a diesel...

 

cheers,

Pete.

Edited October 8, 2007 by smileypete

[Gibbo]

I suspect that the "other magnetic devices" error is not a problem here. the starter motor is on the other side of the engine from the batteries.Nick

 

You don't perchance use a "1,2,both,off" switch do you?

 

Gibbo

 

My 1.2 litre thorneycroft mitsubishi drew 180 amps when cranking. Measured with a blue sea digital meter through a shunt and I have every reason to believe that is pretty accurate.

 

My 1.7 litre vetus mitsubishi draws about the same amount, but it is 20 years younger, so maybe starter motor design has improved. Certainly my 2.2 litre Honda car much have a more efficient starter as it is about the size of a baked bean tin.

 

That's more like. I'll believe those figures.

 

Petrol engines require quite a bit less because the compression ratio is much less than on diesels.

 

Gibbo

 

Edited: That bloody carriage return fairy is starting to get right on my tits.

Edited October 8, 2007 by Gibbo

[Theo]

You don't perchance use a "1,2,both,off" switch do you?

 

Gibbo

 

Yes.

 

Why?

 

Nick

Edited October 8, 2007 by Theo

[chris w]

Measure the battery voltage at the battery terminals. Have someone crank the engine and note the voltage reading, specifically the voltage drop.

 

This volts reading will be solely dependent on the volts dropped across the battery's internal resistance. If you are using a fully charged starter battery, then the volts drop divided by the battery's internal resistance will give the current.

 

For a fully charged battery, the internal resistance is of the order of 10 milliohms. Thus, if you get a 2 volt drop, the current is 200A. Of course, you won't know exactly what the internal resistance is, but this will at least give you an indication of current level.

 

I would also agree with others that the starter current is somewhere in the range 150A to 300A typically depending on engine, temperature etc etc.

 

If it were only 50A, the expected volts drop would only be about 0.5v. It'll drop this amount just on the glow plugs.

 

Chris