Is it safe to overload LEGO train motors?

Question

When I was in Fana'Briques in 2008, there was a speed record competition for TGV-themed train. The setup used a section of custom-made straight track (aluminium rails if I remember correctly) and was powered by a custom transformer which could deliver higher voltages than the standard LEGO ones.

As ar as I recall, they went up to above 30v, and there seemed to be more damage to bricks (one train did derail dramatically, and pieces flew everywhere) than to motors.

Is it safe to overload LEGO train motors that way? I assume 30v is pushing it too far, but what could be considered a safe limit that motors can endure for an extended period of time?

I'm speaking of regular 9V motors, but answers for other LEGO motors can be interesting too. I know Philo pushed them to 12v when testing their efficiency, but with a big warning that he couldn't guarantee the motors would tolerate it for a longer period.

Answer 1

What could be considered a safe limit that motors can endure for an extended period of time?

Their rated voltage.

Motors are designed to operate at their rated voltage indefinitely, or until they wear out, whichever comes first. Exceeding this voltage means you shorten the life of the motor.

Your big enemy is heat. At some point, the amount of heat generated by the windings in the motor will exceed the ability of the motor to dissipate it, and the wire in the winding will melt and break the electrical connection (usually at the point where the wire is soldered to the commutator).

The point at which failure or excessive stress or wear occurs is influenced by many factors, including the amount of current driving the motor, the speed at which the motor is turning, the amount of load the motor is driving, the ambient air temperature, etc. These variables make it difficult to predict a safe voltage above the rated one.

In addition, over-driving motors in this way makes them inefficient; speed increases are offset by higher losses due to back-emf, limiting the additional torque, so driving motors this way eats batteries fast.

Last but not least, overdriven motors are a safety concern. The paint covering the windings can catch fire and burn down your house; you have to watch such experiments very carefully.

If you really want to increase the power, your best bet is to choose a motor with more torque, so that you can run it at its rated voltage.

Answer 2

Yes heat is the ultimate problem but the most likely failure will be from the commutator brushes. As the voltage is increased so does the heat of the sparks created as the brushes pass from one commutator segment to the other. This is a particular problem on a 3 segment commutator. At 30V even if the motors can stand the mechanical loads and heat generated I doubt the brushes would last for more than a few minutes however such a short life would be quite acceptable for a record high speed run, particularly if someone else was paying for the motors!