Let's Make Robots!

DC motors: "altered brush timings"

In this thread, under Chris' noise question, Cliff suggested that motors sometimes are optimized to run in one direction. To the detriment of the motor's performance when running in the opposite direction. Ken observed the same thing in car-window-wiper-motors. Now I want to know more about this.

Have a look at the motor I use for my Theo Jansen Leg experiments.

dcmotor-commshift500.jpg

It is scavenged from a cordless power drill (about 18 years old). Notice the three slots in the metal case and the one (green) plastic tab in the plastic cover holding the brushes in place. The markings on the motor are mine. 

Is this motor "optimized" for use as a power drill? Presumably, more power is required to run forwards (drilling, driving screws in) than backwards (occasionally unscrewing screws). Is this motor case prepared for three different commutator settings? Did I assume correctly when naming them "R"everse, "0" Neutral and "F"orward?

 

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Hi Rik,

advanced brush timing is used by the RC car and buggy guys and gals to go a little faster. The motor you have looks like it has provision to do that, it's hard to Tell from the photo but what Diameter is the motor? 540's are a common RC car motor and these are availabe all the way from performance crap to oh my god, rare earth Magnets, 5 turn with water cooled brushes capable of handling 70 amps.

Changing the brush timing just alters the point at which a particular motor coil is energised, the effect becomes more noticable as the RPM goes up, current does not flow through and inductor instantly, in an inductor current lags Voltage, although the inductances is not high there is an effect. The other part is the actual internal geometry of the motos (magnet size, number and size of the poles...).

With Electric motors the way to go these days is sensor less brushless.

In interesting point, in your car there are a lot of little DC motors these days and they don't use encoders, they use the commuter noise to measure the RPM and Torque and detect stalls. People like Microchip have come out with a range of excellent chips which are good at this, the DSPics for example. The DSPic can be used to do the complete motor control.

 

42 mm

Never saw any toys though: it's from a cordless drill.

Thanks for the in depth explanation.

 

As a kid I used to "Twist" the endcaps of my motors so my toy cars would go like a bat out of hell in forwards but be gutless in reverse. Twisting the endcap changes the angle of the rotor when power is applied to a winding. This also affects the back EMF and will therefor make the motor noisier in one direction compared to another.

I use to file off any tabs that locked the endcap into position. This is certainly the best way to correct a motor that goes faster in one direction compared to the other.

From memory the motor goes faster in the direction you twist the cap.

could you explain a little more that "Twist" thing?   could I use that method to give extra power to the Ratazana bot?
That would seem like a reasonable guess, and the markings do appear as reminders of timings. THere is a webpage on re-timing an EV Warrior motor to neutral that might be intereting to look over. I had a couple of these but Ebayed them before performing the mod.