What performance can we expect from a smoked Tetrix?

I applied a little motor theory to see what we would expect when a Tetrix is smoked. (See blog post) Maybe you motor experts can poke some holes in the reasoning.

On the assumption that a smoked motor has fewer windings that reduces kt,ke and R… the smoked motor shows a higher speed at a given torque, no change in the stall torque, higher currents at all torques . So long as you can supply the current, the motor will provide more power at a given voltage.

Huh? This seems counter to what we are seeing. We know the smoked motor climbs are a little slower. If the assumptions are correct, then the reason for the slower climb must lie with the series resistance in the motor circuit. The higher currents cause a corresponding voltage drop across the series resistance and the motor voltage is smaller.

See post for detail example.

There was a minibot at our 2nd regional that smoke every time, as they didn’t put any return mode on it, it just kept going until they turned off the switch on the back of it. it didn’t slow down, even after all of the qualifier matches.

If your motor has smoked then i would suggest replacing it.

When the tetrix motors smoke, its not because of the armature components or the brushes. Inside the endbell of the motors is a resistor. This is designed to burn at a very low amperage rating. When this resitor burns it usually smokes and breaks the circut. If your motor still works even after smoking then i would guess that the proformance has dropped and it will stop working with continued use.

If the resistor was not there then the motor would continue to get hotter until it would actually burn up. This is when the wire around the electromagnets in the armature gets realy hot and melt the thin insulation coating causing the wires to touch each other and short. When this happens the imediate result varies because the melting of the wire cannot be expected to occur equally throughout the motor. In this case (the tetrix motor) the three electromagnets on the armature are no longer putting out the same magnetic froce aswell as now pulling a different amount of amp draw. This causes the motor to be very unbalanced and create excess heat because the motor is now constantly trying to slow down and speed up thousands of times per-second. This will cause the speed and power output of the motor to decrease until the motor stops turning all together. Disproving the (untested) “theroies” you have posted. (which by the way are irrelavent when talking about the tetrix motors, the motors i think he was talking about are the standard DC motors that don’t have a resistor.)

If your motor has smoked then i would suggest replacing it.

When the tetrix motors smoke, its not because of the armature components or the brushes. Inside the endbell of the motors is a resistor. This is designed to burn at a very low amperage rating. When this resitor burns it usually smokes and breaks the circut. If your motor still works even after smoking then i would guess that the proformance has dropped and it will stop working with continued use.

If the resistor was not there then the motor would continue to get hotter until it would actually burn up. This is when the wire around the electromagnets in the armature gets realy hot and melt the thin insulation coating causing the wires to touch each other and short. When this happens the imediate result varies because the melting of the wire cannot be expected to occur equally throughout the motor. In this case (the tetrix motor) the three electromagnets on the armature are no longer putting out the same magnetic froce aswell as now pulling a different amount of amp draw. This causes the motor to be very unbalanced and create excess heat because the motor is now constantly trying to slow down and speed up thousands of times per-second. This will cause the speed and power output of the motor to decrease until the motor stops turning all together. Disproving the (untested) “theroies” you have posted. (which by the way are irrelavent when talking about the tetrix motors, the motors i think he was talking about are the standard DC motors that don’t have a resistor.)

Chris,
I think it is important to remember that it may only be one winding that is affected in the failure. While shorted windings will lower the resistance and therefore increase current, that additional current may not significantly increase magnetic field beyond or equal to the field in an undamaged motor. Damaged windings likely will affect the available current in the other windings due to voltage drop in the supply circuitry. We are also not considering shorts between adjacent windings. This phenomena may actually cause less currents to flow in both windings. In the Tetrix and several KOP motors, adjacent windings overlay each other.

as a student who has smoked a lot of those motors i would say replace the smoked motor or take the motor apart which is legal and check the resistor if it turned black that motor is dead but if you have more motors replace it

Marc, all the motors we have smoked are operational. The choke/resistor probably contributes to the initial smoke and in fact probably may short out and may decrease the series resistance while still allowing the motor to operate. The main observables are an increase in current and slight reduction in climb speed. As I mentioned in my post… a telling test was that ptc’s were usable before smoking and would not work afterwards. But, when the ptc’s were removed, the climb speed was almost normal. Now one could argue that the ptc trip levels changed but I think it more likely the motor current increased.

I am arguing that there is a reduction in kt,ke and probably R. How this occurs is certainly speculative and as Al and you point out there could be many ways the windings could short. We don’t know how the armature is wound… could be lap, wave…etc, but it probably will not be just a single coil affected if there is melting of the insulation.

I suspect you are right in arguing that the armature will be unbalanced. This would simply result in a reduced kt and not necessarily rebuke my assumptions. If an unbalance caused an increase loss in power in the motor due to additional bearing side loads then this would not be consistent with my assumptions and might deserve further consideration.

The main reason for my post was trying to explain why the climb speeds seems so good with smoked motors. If kt,ke were just reduced without a corresponding decrease in R then the motor peak power would be reduced and climb speeds would be significantly less than with a new motor even with higher currents.

So as a modeller, the assumptions seem to predict the performance we are seeing as well as any other explanation.

It’s an inductor to be clear.