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Circuit Owner
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Discussion Starter · #1 ·
I am looking for a way to measure current draw on motors for an open class at our club.

I am setting out my thoughts below but am VERY HAPPY to be corrected!!!

We need to set a maximum current draw limit as we are using Scalextric Digital with standard SSD chips and an APB with standard twin PSUs.

The PSUs deliver 8 amps in total which, allowing for the APB, processor and lane changer operation, means we probably need to limit current draw to 1.0 amps per car. I realise the SSD chip will handle 1.5 amps but if all 6 cars push the 1.5 limit - every time Murray shouts Go Go Go Go Go we will suffer brownouts!

I believe current draw is better than a rev limit because it is possible to get a 23k motor that draws less than an 18k motor simply because it is better engineered and maintained.

The class is magless.

As I understand it - peak current draw is when the motor is stalled. I appreciate that, due to windings and brush position - you can't just lock a spindle and assume the current draw is at its peak.

I would like to construct a device or means of measurement that measures peak current draw on a car fully assembled and about to be placed on the track.

If I used a short straight with a 13.8v power supply (the APB track voltage), an on/off switch and an ammeter connected in series - could I place and hold the car on the track and move it very slowly forward say 50mm in the space of 2 seconds whilst watching the ammeter for its peak reading?

Would this work and would it avoid damage to the motor?

Peak current draw, a plastic chassis and hard plastic body are likely to be the only restrictions placed on this class so an accurate peak current draw figure is important.

All advice gratefully received
 

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An open class with a 1A limit. Youre not going to be able to use much more than a Scaly 18k motor then. Just grab the wheels and hold them still and power up for 1 second and take a reading. Rotate wheels say 90 degrees and repeat, do this 3 or 4 times. Doing this for about 1 second wont harm anything.

The alternative is to by tripable fuses, one for each lane, and set the limit to 1.5A. Easy.

cheers
rick1776
 

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Living the Life!
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That's simple enough ........ on the old PB-Pro there is a menu setting that shows you what the current draw is. Get yourself one of these venerable old duchesses and place a block on the track so that the car can not move and voila (or Wallah as I have seen it
....gotta lurve those guys)
 

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Peak current in a standard Scalex 18k S can motor is over 2 amps at 13.8 volts.

Rick is probably right in saying holding it at stall for about 1 second wont harm anything. (It didn't do any harm when I repeatedly tried it.) However, if somebody's motor happens to go slower or even fail after the test, there's a good chance they'll blame the test. It would be difficult to be absolutely certain that the test hadn't contributed to the problem.

An alternative way of testing is to do it at a much lower voltage than 13.8. I tried a a standard Scalex 18k S can motor at 5 volts and the maximum current is just under an amp. As Rick says, there is a need to rotate wheels say 90 degrees and repeat, do this 3 or 4 times - it doesn't produce the maximum reading in all positions. (There is a good reason for that to do with brush overlap, but there's probably no need to go into that sort of depth here.)
This is a fair way of testing motors at lower current draw (and hence less risk of damage).
The stall current draw goes up roughly in proportion to voltage (so for example it takes roughly twice the current at 10v as it does at 5) but you cannot depend on the current being precisely proportional to voltage (There is probably no need to go into the reasons why here.). So if you are doing it this way, it's best to have a standard test voltage and stick to it.
 

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Circuit Owner
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QUOTE (rick1776 @ 10 Sep 2012, 22:53) <{POST_SNAPBACK}>An open class with a 1A limit. Youre not going to be able to use much more than a Scaly 18k motor then. Just grab the wheels and hold them still and power up for 1 second and take a reading. Rotate wheels say 90 degrees and repeat, do this 3 or 4 times. Doing this for about 1 second wont harm anything.

The alternative is to by tripable fuses, one for each lane, and set the limit to 1.5A. Easy.

cheers
rick1776

Hi Rick,

I'm not so sure about the 18k limit - if you look at the slotcarnews motor list where they have tested a large number of motors - they give peak rpm at set voltages and a wattage reading based on a test rig that stalls the motor at a dozen points around a full turn. The NSR Shark 25k motor is reported as "NSR Shark (FC-130) 25,000 RPM/12v 176 gcm/12v, 11.0W/12v".

w=va or a=w/v so 11/12=0.917amps as far as I can work it out. The NSR is obviously more efficient than the Scalextric SP25 motor as that apparently blows chips so must draw at least 50% more current than the NSR.

This is why I want a good means of testing (with the motor in the car - not on a test rig) - I believe there is a wider choice of motors out there for standard SSD chips than we have been lead to believe.

Hi Greg,

I don't have a PB PRO. From what you describe it measures stall current. So does that validate my approach of an ammeter in line on a test block? Simply placing a car against a stop is probably not as accurate as a slow, braked roll forward. Think about riding a bike - if you tried to push a heavy object with a bike with the pedals vertical - it would be a lot harder to do than if the pedals were horizontal - available torque varies with where the pedal is placed. As I understand it this is the case with motors with regard to the position of the windings with relation to the magnets. Placing a car randomly against a block is likely to produce a different current draw every time you do it.

At least that's how I understand it - I'm seeking confirmation or re-education!!!

edit: I notice the Slot.It orange endbell V12 motor was tested at 23,500 rpm and 9.8 watts at 12v - so that is also a viable alternative to the 18k mabuchi - and I have one of these (I was going to have to buy an NSR shark and gamble it draws under 1 amp) so I will test the Slot.It motor and see if my approach gives comparable results to the slotcarnews results.
 

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Looking at the slotcarnews motor list , it says
"The last data given for each motor is maximum power output, in Watts (W), at 12 volts. Power is derived from both RPM and stall torque. Maximum power is produced at around half the max RPM, where torque is also halved. The formula used is:
((Max RPM/2)*(max torque/2))/100,000 = Watts"
So this is a calculation of the estimated mechanical output power, NOT a measure of the electrical input power.
The input power will be larger than the output - that's why motors get hot when running.
So in the above example of an NSR Shark, all the figures tell you is that the stall current must be over 0.917amps - it doesn't tell you how much over.

If you want to know how much current a motor takes, the way to do it is measure how much current the motor takes. Measurements of something else will at best be less reliable.
 

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Living the Life&#33;
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The PB-Pro provides feedback on whatever the maximum current draw for the time it is switched on, including normal type of usage.

Other than that, you could uprate the diodes and for the Rolls Royce job .... do the MOSFETs as well or you could take a chance and just go for it; I pay quite good rates for fried chips ......
 

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Discussion Starter · #10 ·
Hi Greg,

I guess we could discuss the upgraded chips at the club - it depends if all the other racers are happy to do the upgrade or pay for it. However - this doesn't get over the power sharing issues seeing as we only have twin standard Scalextric PSUs. Yes I know there's a power upgrade but that's not an option at the moment as the club is using my track and I want to keep things standard so I can swap-in spares if I get failures when it's out commercially.

When the club gets its own track then a power upgrade is on the cards - we can all run upgraded chips or Slot.It chips and kick-ass motors. But until then there has to be a power limit per car to keep things fair.
 

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Circuit Owner
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Discussion Starter · #11 ·
QUOTE (300SLR @ 11 Sep 2012, 10:31) <{POST_SNAPBACK}>Looking at the slotcarnews motor list , it says
"The last data given for each motor is maximum power output, in Watts (W), at 12 volts. Power is derived from both RPM and stall torque. Maximum power is produced at around half the max RPM, where torque is also halved. The formula used is:
((Max RPM/2)*(max torque/2))/100,000 = Watts"
So this is a calculation of the estimated mechanical output power, NOT a measure of the electrical input power.
The input power will be larger than the output - that's why motors get hot when running.
So in the above example of an NSR Shark, all the figures tell you is that the stall current must be over 0.917amps - it doesn't tell you how much over.

If you want to know how much current a motor takes, the way to do it is measure how much current the motor takes. Measurements of something else will at best be less reliable.

Thanks 300SLR - that has clarified things.

I will try my rolling stall test with an ammeter in the circuit to see if I can read peak current draw. I will also run the same motors freewheeling at various voltages to see what sort of results I get.
 

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Rich Dumas
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The Scalextric chip is barely able to handle a stock black stripe motor which can use nearly 3 amps at stall. That is a calculated value based on the armature being 4.6 ohms and the track being 13.8 volts. Once the motor starts to turn it no longer acts like a plain resistor and the power demand drops to well under an amp. Actually measuring the amp draw on the fly is difficult, an ordinary amp meter with a needle has too much inertia to measure amperage spikes. Digital displays are time averaged or they would flicker too rapidly to be legible. A digital meter with a peak hold feature would be the best thing to use if you want to take on track readings. In order to have a constant voltage on the track the power supply would have to be sized for the worst case scenario, that is for the maximum number of cars being at stall as they would be at the start of a race. For six regular Scalextric cars that would be 18 amps plus whatever the lange changers, etc. needed. In that case even if the power supply was regulated the voltage would drop considerably, which might actually be a good thing. If the voltage drops off it is less likely that the amperage spike would burn out a chip. The 8 amp power supply is perfectly adequite once the cars get moving a little. If you were to switch to a big aftermarket power supply you might notice that the cars had more snap when they were starting up. If you were going to use cars with more powerful motors the Slot.it chip would be the thing to use, it is more robust. With more powerful motors the protective circuit in the power supply might trigger from time to time.
 

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Discussion Starter · #13 ·
Interesting.

Are you saying that supply voltage / armature ohms = peak current?

Can the armature resistance be measured with an ohmeter across the motor terminals? Is it as simple as that?
 

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The simple supply voltage / armature ohms calculation gives a current somewhat higher current than the maximum measured stall current.
This is because there are other things such as the brush gear in circuit.

I've seen an ohmeter across the motor terminals tried as a scrutineering test. It didn't give precise enough readings to be any use.
The problem seems to be that the small non linear voltage drops in the brush gear prevent the armature resistance being measured reliably at the very small current and tiny voltage drop the ohmeter uses.
A solution to this is to increase the current / voltage drop being measured.
Which in fact is what a rolling stall test with an ammeter in the circuit is doing.
As I mentioned in an earlier post, you don't need to go all the way up to full operating voltage to measure something meaningful.
 

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Discussion Starter · #16 ·
Thank you 300SLR.

And thank you JCS100 - the SlotForum search engine didn't throw up this thread so thanks for the link.
 

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Rich Dumas
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I just checked a Scalextric armature and it is actually 5.1 ohms, so the maximum current draw would be 2.7 amps at 13.8 volts. In practice added resistance from the brushes, etc. would make it a bit less. You can't measure armature resistance across the motor leads, you really need to measure across the commutator segments.
 

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Why not turn the problem on its head. Its an open class right? So any motor goes, right? The caveat being that you must run the scaly chip. You want to run the gauntlet and put a big motor in, do it. But if it fries itself during the race, thats it your done and out of the race.

cheers
rick1776
 

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Discussion Starter · #19 ·
Yes Rick - you are right - that is what it boils down to.

I have at least 6 motors of various makes, speeds and sizes that I would like to try but I don't fancy blowing over £60 in chips finding out which ones work. I was looking for a way of non-destructive testing to save that expense - and when you consider there are 9 members of our club who could be racing in this class - that adds up to a potentially big pile of wasted chips.

The "blow it and and you've blown it" principle applies of course - I wanted a way to reduce the risk of it happening to me!
 

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hi
Rich why not just get the chips upgraded so then you could even use the scaley 30k motor if that's your thing.

Cost is very favourable and once it's done sit back and enjoy with no worries.
 
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