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Discussion Starter · #1 ·
On a earlier thread on motor lead cable sizes the question was posed on how much current do slot motors actually use, and from this logically follows into what current rating of power supply is really needed.

Trying to find this out with just a meter is only effective as a snap shot of a point in time i.e. taking the highest reading, during acceleration or waiting until the reading stabilises with a steady RPM & load. The following is actual measurement of current over time taken from a digital sampling oscilloscope, which shows graphically the current peak on acceleration and how this reduces as the motor reaches max RPM simulating the slot car reaching top speed.



In the picture above is a voltage trace against time where each square going vertical is 0.2 volts (200mV) and squares going across is 0.25 seconds (250ms). This signal has been created by adding to the power lead from the motor return, a 0.2 ohm resistor across which is the test leads for the oscilloscope. The trace shown above is the volt drop across this resistor so for each 200mV square going vertical now can represent 1 amp.

(0.2 volts divided by 0. 2 ohms = 1 Amp ) Therefore as current in a series circuit is equal across all points then this is a direct representation of the current through the motor.

The motor by the way is a standard Scalex/Fly Mabuchi 130 black stripe driving a 60 grm flywheel supplied from a nominal 4 amp PSU at a stabilised 12 volts. The trace clearly shows the MAB130 is pulling 2A+ initially dropping to 0.5A after 2.25 seconds. The Mabuchi under this load was struggling simulating a heavy car or magnet fitted.



The second picture is of the same setup but with a Ninco NC 5 motor in jig. Here we can see the initial current is a lot higher at 4A peak dropping to @0.5A at 2.25 seconds.



The third picture is of a Scaleauto Yellow boxer motor and the initial current has gone off the screen i.e. greater than 8A. It can be seen that the 8A peak is of short duration and falls rapidly to @6A then declines in a similar curve as before. Reason for this is the limitation of the PSU unable to sustain the 8+ amps required by the motor the output voltage has reduced from 12 volts.



Picture 4 has the same Scaleauto motor but the PSU is now a 17A rated unit which as can be seen above will sustain the 8A plus requirement of the motor during acceleration as the voltage follows a curve similar to the lower rated motors on the 4A supply.



The final picture is the Scaleauto motor again but this time the scale on the screen in the vertical has been changed to 500mV per square to find the actual current peak which from the new setting is 2.5 amps per square = 10A.

Points to note are that the initial peak current of a motor even from what may be considered the low powered hard bodied end of slot racing can still pull large current peaks during acceleration.

Secondly a PSU which at 4A should be sufficient for these motors will not supply the power the motor needs to sustain acceleration at design limits. From re-running the NC5 & MAB130 motors on the 17A supply it can be demonstrated that even these motors will benefit from the extra power.
 

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Interesting measurements, thanks for sharing them.

The measurements show the highest current only occurs when starting from rest, as speed increases the current decreases.
So the maximum current is only needed when starting from rest.
In normal running this only happens at the start of a race and when a car is marshalled after a deslot.

Once the car is up and running, the maximum current needed would be when accelerating out of the slowest corner on the track.
That requires less current than the maximum current for starting from rest.
So this lower current is all that is actually needed for minimum lap times.
The full "start from rest" maximum current wouldn't make any difference to lap times, just allow maximum torque from rest.

So an interesting question is putting a figure on this "lower current for minimum lap times"?

Anybody want to discuss??
 

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Rich Dumas
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I have measured the resistance of a Scalextric armature at 5.6 ohms. At 12 volts the motor would pull 2.14 amps at stall, actually a little less due to the resistance of the motor brushes. It is nice to know that the measured value agrees with the theoretical value. Of course a DC motor is not a resistor, it is actually an inductor and it generates a reverse EMF once it is turning, so the effective resistance increases with the speed. Having enough amps available to cover the stall value is mostly only necessary if you are drag racing. Even a well regulated power supply will sag if it does not have enough amps to cover the stall value of all the cars on the track.
 

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Discussion Starter · #4 ·
Interesting comment from 300SLR on the difference between standing start/race start and exit from bend acceleration and the potential current pulled. Tried some quick experiments tonight with the Scalaeauto Yellow Boxer, by running up to full power then coasting down then hitting the power button again for the oscilloscope to capture.



For trial run 1 the motor was run up to 25,000 rpm then allowed to drop down to 6,000 rpm before hitting the power button. As you can see the current is still high @7A falling back to 5A then reducing down. Standing start was 8A peak falling to 6A so not a great difference from the rolling figure.



For trial run 2 and using the 17A PSU, again the motor was run up to 25K rpm then allowed to fall, I miss-timed the power ON as the rpm had only dropped to 8000 but still it can be seen there is a high current profile of 7.5A (standing start was 10A) and a sustained current reduction due to the higher power available.

Net result is if the track bend forces a speed reduction down of 67-75% of full speed then the current only falls by @12-25% depending on exit speed so a 10A+ per lane supply is still needed if you want to run this motor to design limits.

Cheers
JCS100
 

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Very interesting John

No wonder your cars are getting great traction now out of the corners if you are using this data to fine tune your controller

But how long will a stabilized 4Amp supply be able to sustain that spike in draw ?

Michael
 

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Fascinating stuff....

Love what you are testing and the flywheel is a clever solution to estimating the current draw...

Any idea of what happens if you place some "drag on the flywheel" to represent what happens to a motor as you apply power and the car slides unable to turn the "push" into forward momentum placing a strain on the motor... I Guess the High amp peak would stay for considerably longer... guess the peak could last for half a second or more... gulp!
 

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QUOTE (hankscorpio @ 10 May 2011, 23:46) <{POST_SNAPBACK}>Any idea of what happens if you place some "drag on the flywheel" to represent what happens to a motor as you apply power and the car slides unable to turn the "push" into forward momentum placing a strain on the motor... I Guess the High amp peak would stay for considerably longer... guess the peak could last for half a second or more... gulp!
Yes I can tell you what happens in theory. It would be interesting to figure out a way of measuring what happens in a real motor and see how exactly (or not) it matches the theory.

The current taken by a motor depends on the applied voltage and the revs its doing at the time. Nothing else makes a differance (except small changes as the armature heats up.)

So for example a motor supplied with 13 volts doing 5000 rpm will take the same current whatever the drag or flywheel inertia.
(The current depends on the type of motor)
The drag or flywheel inertia will make a differance to how quickly in accelerates from 5000 rpm.
More flywheel inertia means it accelerates more slowly.
More drag means it accelerates more slowly, if there is just enough drag to balance the motor's torque it'll stay at 5000 rpm and if the drag exceeds the motor's torque it'll slow down.

Normally when cornering the controller isn't on full power, so the voltage and current is reduced.
If a motor were held on full power at low revs by drag for an extended period it would get hot. In the extreme that could be hot enough for the motor to fail.
 

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Discussion Starter · #8 ·
Mike,
Without giving to much away, then yes having access to a digital scope is a big help with understanding how the controller is translating the power.



Above is the response from the original analogue club controller design in the GT Raceways club forum

It can be seen that the controller goes from 0 to 6 volts as a vertical switch then steps to full power (wiper board steps can be clearly seen) as a relatively linear line. Drawbacks with this are high power supplies will kick the motor, promoting wheelspin or causing the car to slew if still exiting the bend. Low power supplies will cause the voltage to sag from the instantaneous demand.



In this pic the latest (unpublished) circuit modification allows a 'soft start' the voltage now rises from zero as a curve to full power which can be modified by a controller setting.



And this picture shows how the curve can be changed.

Numerous issues with this at the moment, as the current design has a mechanical full power stop, which overrides the curve.! The birds nest controller you saw Monday night is the solution to this with electronic full power rather than mechanical.

Finally did have a pic somewhere of the spike on an expanded timebase but can't find it, but from the pics in posts 1 & 4 the spike duration on the 4A supply is @25ms or 0.025 seconds.

Hank & 300SLR
Another interesting point should be able to add a mechanical brake (like car disc brake) to flywheel and then measure the extended current draw. Will have to car park this as off to China on business Sunday and not back to end of the month. Time permitting might try pinching the flywheel by hand at different points of the acceleration.

Cheers
JCS100
 

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Nobby Berkshire
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I often wondered why some folks put such high amps into their track. 10 amps for a tiny slotcar seems so high!

Does anyone have any stats on what actual laptime differences running at 1 amp and 10amps at the same voltage actually translates to under real conditions?
 

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QUOTE (Screwneck @ 11 May 2011, 21:56) <{POST_SNAPBACK}>Does anyone have any stats on what actual laptime differences running at 1 amp and 10amps at the same voltage actually translates to under real conditions?

Its really down to the characteristics of YOUR psu... the overall effect will vary from PSU to PSU but the principle is never the less as discussed above...

A 12V 1Amp PSU faced with a 5 amp peak demand will redcuce the voltage to maybe 5 or 6 volts for a few 10ths of a second.... or overload!

A 12V 10Amp PSU faced with a 5 amp demand will output 12v... What you see with a high amp PSU is a much "crisper" response when driving... but it can make cars harder to drive with more wheelspin!
 

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QUOTE (hankscorpio @ 11 May 2011, 21:20) <{POST_SNAPBACK}>A 12V 1Amp PSU faced with a 5 amp peak demand will redcuce the voltage to maybe 5 or 6 volts for a few 10ths of a second.... or overload!
Agreed, that what can happen (including overload = power supply turns itself off so all the cars stop)

Where only one car is running from the power supply, a reduction in voltage will make it slower than it should be - at least it's the same every time - a driver can deal with that.

Where two car are running from the same power supply, the reduction in voltage will caused by one car will slow the other car , then when the first car stops taking all that current, the other car suddenly speeds up - and quite frequently ends up crashing. Surges of power completely outside the driver's control is not the sort of thing a driver can deal with.
 

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Another point to consider with higher currents is the resistance of the track wiring.
While the tracks designed for high current motors will have perhaps a tenth or two of an ohm wiring resistance, the tracks used for lower current motors often have considerably more resistance. That adds up to a serious voltage drop at 7 amps +.

QUOTE (Screwneck @ 11 May 2011, 20:56) <{POST_SNAPBACK}>I often wondered why some folks put such high amps into their track. 10 amps for a tiny slotcar seems so high!
One reason for higher current power supplies is that the quicker 1/32 cars need the higher currents.
Really quick 1/32 cars average about 10 amps round a lap and take considerably more on acceleration from slow speed. Even more modest motors won't run at all on 1 amp.

The sort of motors JCS100 take considerably less power than this and would commonly be run on rather less than 10 amps per car.
 

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Gone racing makes a good point about amps....

But before every slot racer throws their hands up and goes and buys some mains cable to wire up their layouts... a std 18k motor is designed and balanced on a typical 0.5 - 1.5 amp load and the hardware sold by major brands is designed to cope with that.... HOWEVER use hot motors with std controllers and PSU's and thats where the issues begin... You can smell a scaley analogue controller heating up when you run a slot it with magnets!

It is important to upgrade your power system "En bloc" want to step up from "box standard" to 21k motors... then use 3-5amps as your guide (per car) for wiring and the PSU and a use parma type controllers... However these cars will run on standard kit but will put all components under strain and never run to their best.... or worse fail.... This level will run hotter motors but again you will not get the best out of them unluess you step the PSU and throttles and therefore wiring up a gear again.

At the top of the tree (there are lots of cars that can pull these kinds of amps) and you are into the 13 amps per car per lane territory for PSU's and wiring and some serious controllers...

AMPS = Torque... Its the amount of "push" the motor can give on accelleration... its the Amps that cause the strain!

Volts = Speed... The amount of RPM's the motor will turn at...

Motors can be wound for torque or power generally high RPM motors (30k) pull less torque as the size of the can is the same so the windings are thinner but allow the use of lower gearing to compensate... But there are high torque 30k motors... these little buggers would melt a std scaley controller in seconds!
 

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Rich Dumas
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Where I come from tracks that experience a voltage sag when the cars are starting up are said to have soft power. If you run without magnets soft power is not always a bad thing, it amounts to the poor man's traction control. With a little less power on acceleration your lap times can be reduced in many cases. The problem is that you can also get voltage surges if the power supply is undersized. Here in the US big power supplies are relativly inexpensive, my club has eight tracks and none of them use wallwart power supplies. Most of our racers now use controllers with some sort of a "traction control" feature.
 

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Other things being equal, winding the armature with fewer turns produces more revs.
This frees up space in the armature which is often (but not always) used for thicker wire.
This is why higher revving motors are usually wound with fewer turns of thicker wire (going to thinner wire for a higher revving version of a motor is very unusual).

Other things being equal, torque is proportional to the number of ampere turns in the armature.
Higher revving armatures have fewer turns, so need more amps to produce the same torque.
Fewer turns have less resistance, so the current is higher. Does that balance out as more or less torque? Here's an example

Simplisticly 20% fewer turns = 10% lower armature resistance (assuming the same gauge of wire is used).
Neglecting resistances elsewhere in the circuit, 20% lower armature resistance would give 10% more current so you'd end up with the same number of ampere turns. In practice the resistances elsewhere in the circuit are important, so fewer turns of the same gauge of wire usually produces less torque, but not as much of a reduction as the 10% reduction that might at first be assumed.
It is normal practice to wind fewer turns of thicker wire for higher revving motors, as a starting point 10% fewer turn would mean around 10 % (by area) thicker wire, so the armature resistance would be reduced by around 20% (OK I knoe its npot exactly 20%). This usually adds up to higher revving motors producing more torque as long as the track is up to providing the necessary current.

Taking for example the Scaleauto range of motors, for the same size motor higher revving motors generally produce more torque.
 

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Discussion Starter · #16 ·
On the question of power supply versus lap times, and voltage sag here is a comparison of three power supplies and the Scaleauto Yellow boxer motor again.

Power supply 1 is roughly 1A rated or transformer rating of 16VA which is broadly equivalent to some track set wall PSUs, the second is the 4A regulated PSU used in the previous testing the third is a 17A ATX PSU. The PSUs have all been set to 12 volts loaded with a 60 grm flywheel.



Using each PSU in turn on my Dyno linked to the PC and collating the data gives the graph above. The 1A PSU is totally inadequate as the time to full speed is @4 seconds and off the chart, the 4A and 17A PSU results are closer especially on initial acceleration then as the 4A PSU voltage sags the 17A PSU gives better acceleration up to the same top speed for both.

The 1A RPM curve is so poor due to the voltage collapsing then slowly rising as the current demand falls, on the track this would look like excessive magnets in the car slowing it down.

The 4A PSU has a large reservoir capacitor fitted which can be seen discharging in the current traces previously (sharp initial peak up to 8A) which helps maintain the voltage until discharged.

The 17A PSU has no problems supplying the 10A maximum current required continuously throughout the RPM range and would be running the motor close to its design limits.

Now if we changed the motor to an NC1 all three lines would be much tighter together&#8230;&#8230;&#8230;.

JCS100
 

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perfect illustration of what was said earlier about not getting the best out of the cars with a low power PSU... and high power wheelspin

superb...


Great work... so nice to see the theories in easy to understand pictures...

many many thanks for sharing
 

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QUOTE (Gone Racin @ 12 May 2011, 16:08) <{POST_SNAPBACK}>Simplisticly 20% fewer turns = 10% lower armature resistance (assuming the same gauge of wire is used).
Neglecting resistances elsewhere in the circuit, 20% lower armature resistance would give 10% more current so you'd end up with the same number of ampere turns.
Sorry, I should have checked my typing more carefully
What I should have said was
Simplisticly 10% fewer turns = 10% lower armature resistance (assuming the same gauge of wire is used).
Neglecting resistances elsewhere in the circuit, 10% lower armature resistance would give 10% more current so you'd end up with the same number of ampere turns.
 
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