Hello everyone!

We are working on a project with a scalextric car - where we have to make it automatic. It has to read the course and change its speed and brake in the turns.

We have made a really good brake which can be controlled by a PWM signal from our ATmega32A. It works great - but we can't figure out how it theoretically works. Why does it brake faster?

The motor is illustrated by the inductor.

The IRLML0030TRPBF mosfet is controlled by a PWM aswell but that is for the gas. Our brake circuit is within the marked area.

Can someone make a neat explanation on why this makes the motor brake faster?

PWM Motor and PB3 = 5V in a duty cycle.
The IRLML0030TRPBF and IRF9230 is not activated on the same time obviously.

Thanks for the help!

 

The PFET short-circuits the motor. The motor's kinetic energy is converted to heat energy dissipated in the PFET.
Welcome to AAC!

 

When the motor is free-wheeling, it generates a back EMF proportional to the speed.

The upper MOSFET effectively shorts the motor, causing this back EMF to force current through the motor coil windings, this current creates a torque opposing the rotation. This continues until the motor reaches zero speed, where the coil current also becomes = 0.

Kinetic energy is dissipated as heat generated in the coil winding resistance and in the FET.

 

Hey Guys !

You guys are quick !

That is why we couldn't understand the theory behind it. We are only 2. semester students as Power Engineers, and EMF is something we learn about in 3. semester

Thanks for the perfect answers !

And thanks for the welcome

 

I'm surprised that a slot car needs brakes. I'd have thought that with a small mass and a fairly inefficient drive, the motor would come to a stop very quickly as soon as the power is turned off. Does the car really coast for any distance if you disconnect the power?

 

I'm surprised that a slot car needs brakes. I'd have thought that with a small mass and a fairly inefficient drive, the motor would come to a stop very quickly as soon as the power is turned off. Does the car really coast for any distance if you disconnect the power?

One of the rules for the project is that we have to use strain gauges to detect the turns. So that combined with all the other circuits makes the car weight a lot more. Braking distance from full speed without brake is: 172 centimeters - and with the brake we could get it down to 58 centimeters

 

OK, you've made the observations, so I can't say the car behaves differently! I seem to have imagined something much smaller than it really is.