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Power to the motor

5177 Views 17 Replies 7 Participants Last post by  Gone Racin
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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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!
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!
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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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


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

many many thanks for sharing
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