i remember reading that magnet cars draw the most on startup, and i THOUGHT i remember they can draw up to an amp at that point, but I don't remember if that was a stock magnet car or like a Patriot or Modified...
At startup, the maximum draw for any car is the voltage divided by the total resistance in the circuit. The total resistance in the circuit includes the static resistance of the armature, brush resistance, all contact resistances (shoes, hangers, brush tubes, etc.), resistance of the wiring to the car, resistance of the rails from the tap points to where the car is on the track, and of course any residual resistance in the controller, including wiper and full throttle contact resistance. If you ignore everything except the static resistance of the armature windings it would be voltage divided by armature resistance. For a typical 6 ohm armature with an 18V power supply, the instantaneous current draw while the motor is not turning would be:
Current = 18/6 = 3 amps
With 4 lanes in perfect synchronization due to a power relay starting the race with everyone at max throttle the instantaneous current demand would be around 12 amps. This peak demand would last a few milliseconds and once the motors are turning the demand drops significantly. If the power supply cannot produce the peak demand, in this case 12 amps, it produces what it can and the cars get less current than they demand, with the amount of current each car gets being inversely proportional to its resistance. Lower resistance arms will get more of the available current.
For TJets with 15 ohm arms, the maximum instantaneous current under the same conditions would be 1.2 amps per car, or 4.8 amps total.
So that's the maximum possible demand and it only occurs for a few milliseconds. Once the motor starts turning, its current demand goes way down as the motor starts generating a voltage that opposes the power supply voltage. How far down does it go?
Ideally, the current draw for an operating motor would be zero. But since we all know that a motor's generated torque is directly proportional to motor current, it would generate zero torque. It would be highly efficient and totally useless. So this tells us that any current draw after the motor is turning is what is needed to produce torque.
How much current? Easy answer, how much torque do you need? The amount of torque the motor needs to produce depends upon the work you are trying to do with that torque. If you want to move a slot car that has big 'ol traction magnets trying to keep it from moving, you'll need more torque. If you have sloppy gear mesh and friction on your drive train bearings, you'll need more torque. If your motor is wobbly, you'll need more torque. If your tires are rubbing the body or chassis, you'll need more torque. Brushes too tight, need more torque. Anything that places a load on your motor increases the amount of torque you'll need to move that load. More torque equals more current. Plain and simple.
Based on empirical data, most box stock magnet cars only demand around 0.3 amps to 0.5 amps while running. TJets should demand even less, but until you get all of the friction out of them, they can demand more current than you'd think. If your TJet is pulling more than a half amp you probably have a friction problem.
Drag cars with 0.09 ohm arms and races lasting a few hundred milliseconds? Nothing short of a battery, supercap, or power supply with a robust stiffening capacitor is going to provide a power source that does not detract from the raw potential of the car.
- That is a nice little power supply for bench top tuning and break-in. The one thing it's missing is a built-in ammeter, but you can use your multimeter. The ideal break-in and tuning supply needs both a voltage and current readout. But for $30 I can compromise.
