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Power to the motor

5176 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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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.

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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.

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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……….

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