# Help with calculating Back EMF from dc motor braking

Discussion in 'The Projects Forum' started by dumle29, Feb 18, 2012.

1. ### dumle29 Thread Starter Member

Jun 26, 2011
45
0
Hi, so im building an H-Bridge for a electric racer, and im now at the point, where i´m to decide how to break.

So how would i go about calculating the amps and voltages i will generate, when breaking from x speed?

It must be something with the mass and speed of the vehicle, combined with the engien specifications. (simple permanent magnet DC 500W engien)

The H-Bridge will use 8 IRF1405 (2 in parrallel), and i was wondering, if these would be able to carry the energy generated by a possible emergency break.

The other option, would be to have a dedicated break board, with either some power resistors, or some mosfets.

if going with the mosfets (which i think would be easiest,) i would PWM the gate with a microcontroller, dependent on how far a footpedal was pushed down.

if going with the power resistors, they would have to be variable in some way, and controllable by a foot pedal.

So all in all, how would i calculate the voltage and ampearge generated by an emergency stop?

2. ### John P AAC Fanatic!

Oct 14, 2008
1,638
225
I think the issue isn't primarily with the motor at all, but how much energy the vehicle has when the brakes are applied. That energy then has to be transferred into heat, either in the transistors themselves or in resistors. Then you have to work out the temperature rise based on the coupling from the MOSFETs to the heat sink, if that's what you're doing, and the thermal mass of it all--noting that the thermal coupling won't be perfect, and the transistors will be heating up more than the heat sink.

Next there is the question of how rapidly you really expect to slow the vehicle down. If it's near-instantaneous, you have one huge brief pulse of current to handle. If it's a slower deceleration, you'll be dissipating power at a slower rate over a longer period.

If you're sure that the electronics can handle the energy load, you can think about the actual voltages and currents that will be involved. Then you can also consider how much energy will be lost in the motor itself, and whether you risk damaging it.

And, uh, does a human being ride on this thing (you?) Are you willing to trust your life to this rig working properly?

3. ### dumle29 Thread Starter Member

Jun 26, 2011
45
0
Im young and stupid, so yeah im going to ride it

The brakeing capability should be the equivelant of "blocking" the breaks in an ordenary car. Though it would mostly be used to keep controll over the vehicle.

Im pretty sure the voltages would be too high for IRF1405's. Something like these FDP52N20 in parallel.

We havent built the vehicle yet, and therefore dont know the weight, but it will be welded together, from not too expensive metal. It will contain 3 95Ah batteries (in series to give 36v for the engien) two 500W or two 750W DC engiens. And i hope the others will agree on a rollcage and a 5 point seatbelt

It will hold one person, so if i should give it an estimate, i would say it would weigh around 250kg with a person, maybe a bit more.

4. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
If it a PM motor, it won't generate any higher voltage in braking mode than it takes to run it at the same speed. Basic K/v of a PM motor. That is how we once checked the magnet strength on industrial PM motors. We would spin them in generator mode and measure the voltage and compare to the manufacturer spec OR we would apply the spec voltage and measure the RPM.

5. ### dumle29 Thread Starter Member

Jun 26, 2011
45
0
Oh nice thanks

Then i think im okay, since the H-bridge i have designed, have been designed for 50A per channel, and at 50A, Tj would only reach 50°C at an ambient temperature of 40° in static air of a max of 175°C.

I think it can handle a bit of braking then

Last edited: Feb 21, 2012
6. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
You need to be aware that the current generated will be proportional to the amount of torque the motor is subjected to. You no doubt have current limit built into the acceleration mode of operation. It also needs to be taken into account on deceleration. Of course the internal resistance of the motor windings play a part in all of this.