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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.