Holy cow!

I was wondering what kind of top speed it can produce.

Sorry this got long, but I got going, and well....

It's a little hard to tell on the test stand. My optical tachometer can't get a good reading off that flywheel, even when

I painted it black and left one shiny spot (just too much other metal around it). But I can actually count the driver revs

at 12 volts, and I get about 65 mph for the top speed.

Some may quibble that a big-drivered Pacific should run at more like 100 mph at full speed. That may be true.

But this is one of those personal preference things. To me, an engine running at 65 mph looks like it is running really

fast on a model railroad. So a top end of 65 is just perfect for me. At 80 or 100, I think engines start to look

ridiculous.

The key is that this is only a 6v motor, driven up to 12v. How can we do this without destroying it?

Simple. The voltage actually is not the limiting factor. It is the

*current* and indirectly,

*the heat* that matters.

These motors have a *nominal* rating of 6v, but in fact, they can be run much higher than that. A Micro-Mo engineer I spoke to on the phone explained that the bearings and the armature (or rather,

*basket* since this is is a coreless motor) will likely fly apart from the excessive speed before the higher voltage will matter. So as long as the motor can physically hold together,

you're fine. The motor in my I1 only draws about 120ma with something like 50 cars behind it going up a 2% grade at 12v. It's not anywhere near its limits.

As a safety precaution, I do recommend putting something like a 22 ohm resistor in series with these to limit them a little,

but you don't really have to. The effect on the top speed is very slight, and that resistor can help limit the current.

Here are the calculations for the motor in my example, a 1016, with the optional ball bearings (this option allows

the motor to sustain extra shaft load and extra heat, by the way). This all comes from equations and tutorials

provided on the Micro-Mo website, and the datasheet for the motor (also on the website).

Maximum rotor temperature - Ambient temperature = Allowable temperature rise

Allowable temperature rise divided by thermal resistances (add up rotor-to-case and case-to-ambient) =

Continuous power that can be dissipated in Watts.

Set this power = to the current squared x armature resistance.

P = I x I x R , Solve for I

So here we go:

Max rotor temp = 125 C (from the datasheet)

Assume ambient temp of 40C (104 F, I don't know about you, but I don't run my trains in a room

that's over 100 degrees).

125 - 40 = Allowable rise of 85 C

Thermal resistances (From the datasheet)

26 deg C/watt and 56 deg C/watt (rotor-to-case and case-to-ambient) total = 82

Temp rise / thermal resistance = 85 / 82 = a continuous power dissipation of about 1 watt.

At this point, I pause. Realize how much power a full watt is!

Terminal resistance R = 20.1 (from the data sheet)

So, we get:

1 = I^2 * 20.1

1/20.1 = .049

I^2 = .049, so SQRT(0.049) = .221

Thus, this motor can sustain a 221ma load. That is a HUGE load for a coreless motor.

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A 221 mA load... and it runs on the rollers at about 25 - 30 ma, and my fully loaded-down I1 uphill draws about 120ma.

Clearly, these motors are not being pushed over their limits.

The key is the gearhead. That 4:1 reduction takes a ton of load off the motor.