"Why aren't my motors working?"

Usually one of two reasons. What happened?


"One or two motors stopped working, but started working again after a few minutes."

The VEX 2-Wire Motor 393 contains a HR30-090 positive temperature coefficient self-resetting fuse. (PTC) This device will cut power to the motor if it draws too much current for too long or gets too hot, then reset itself once it cools down.

However, like most fuses, the relationship between current draw and time-to-trip is quite gradual. It is not the case that a PTC rated for indefinite operation at 1 amp will instantly trip at 1.01A, as we can see here: (The HR30-090 is curve A)

The HR30-090 has a rated hold current of 0.9 amps, the highest current at which is guaranteed not to trip. Its rated trip current is 1.8A, which is then the lowest current it is guaranteed to trip at. As you might guess from the graph above, it takes quite some time to trip at that current-- more than a thousand seconds. (This makes the datasheet's specified trip current not very relevant to a VEX competition robot, which rarely runs for more than 120 seconds at a time) It trips faster at higher currents: only 5.9 seconds to trip at 4.5 amps. And most importantly of all, stall current of a 393 motor at full power is 4.8A-- which will cause the PTC to trip in seconds.

(Another lesson in trusting datasheets here: VEX specifies the rated stall current at 7.2 volts. The NiMH battery is 7.2 volts nominal, a fully charged battery can top out a 8.4V, which, per Ohm's Law, should cause the motor to draw 5.6A.

So, this is why motors temporarily stop working if you run them hard.

It is critically important to note that a PTC is still hot after recovering from trip: a PTC that's been tripped is very easy to trip again. Tests have shown a motor generally takes 30 minutes to fully cool down to ambient after a PTC trip. If your motor stops during a match, you will want to swap it with a cold spare immediately after. Some teams will remove the back of the motor casing and shoot some freezer spray inside, which is effective, but expensive.

Given the behavior of the PTCs, it can be easy to build a robot that works fine in testing, but fails at a competition, where you play several matches in quick succession. If your robot is heavy, or the drivetrain is overgeared, then the motors can be partially overloaded: working for a while before overheating.

Note that plastic bearings wear out over time, as the shafts grind away material: a drivetrain that turns easily by hand can become noticeably stiffer after a few months of wear. A robot that worked fine at the start of a season can start tripping drive motor PTCs by the end of it.


"Half of all my motors dropped out and we lost the match."

The VEX Cortex Microcontroller has two HR16-400 PTCs for motor ports 1-5 and 6-10, something like this:

+----+----+
|  1 | 4A |
+----+    |
|  2 |    |
+----+    |
|  3 |    |
+----+    |
|  4 |    |
+----+    |
|  5 |    |
+----+----+
|  6 | 4A |
+----+    |
|  7 |    |
+----+    |
|  8 |    |
+----+    |
|  9 |    |
+----+    |
| 10 |    |
+----+----+

This PTC is larger, and is rated to higher currents: 4A hold, 20A fast trip. Once that PTC goes, all the motors on that set of ports stops working. This is harder to do, generally requiring stalling multiple motors, or an electrical short.

A VEX Power Expander contains another 4 amp PTC, so if you add a power expander, it brings your current budget up to 12A, as well as increasing your battery capacity to 6000 milliamp-hours. To spread current draw evenly, the power expander is generally plugged into ports 4-7, like so:

+----+----+
|  1 | 4A |
+----+    |
|  2 |    |
+----+    |
|  3 |    |
+----+----+
|  4 | 4A |
+----+    |
|  5 |    |
+----+    |
|  6 |    |
+----+    |
|  7 |    |
+----+----+
|  8 | 4A |
+----+    |
|  9 |    |
+----+    |
| 10 |    |
+----+----+