high power PWM project/ideas needed??

[TKS]

Hi all,

i wanted to control the power wich goes to a StarterMotor...

i wanted to control its RPM/Power by using a Pic Micro with PWM output..

because içant use relais.. i was thinking on hooking up some Mosfets in parralel to be able to cope with the Amps....

i was thinking of using a 12volt DC motor ..lets say 900watts?? (75Amps??)

for example a BUZ11 can have 30amps..(with cooling offcourse)

so i will need 3 of them in parrallel is that true?? (90Amps, bit over powered??)

any one ideas??

the reason is that i need to built a reverse on a car that weights +/- 500Kgs the output speed isn't important as long as its moveing...

TKS

 

[Nigel Goodwin]

Hi all,

i wanted to control the power wich goes to a StarterMotor...

i wanted to control its RPM/Power by using a Pic Micro with PWM output..

because içant use relais.. i was thinking on hooking up some Mosfets in parralel to be able to cope with the Amps....

i was thinking of using a 12volt DC motor ..lets say 900watts?? (75Amps??)

for example a BUZ11 can have 30amps..(with cooling offcourse)

so i will need 3 of them in parrallel is that true?? (90Amps, bit over powered??)

No! - that's VERY underpowered!, starter motors take HUGE!! currents, 90 amps of FET's would only be sufficient for a medium powered radio controlled car controller!.

any one ideas??

the reason is that i need to built a reverse on a car that weights +/- 500Kgs the output speed isn't important as long as its moveing...

You should look elsewhere than starter motors, they are designed for one specific job, and aren't much good for anything else - as they are series connected they are also difficult to reverse!.

Try looking at the robot wars sites, most use Bosch motors, or wheelchair motors, and commercially built speed controllers.

 

[audioguru]

This manufacturer of PWM motor controllers for golf carts talks about the problems with hundreds of amps of current required to start a vehicle moving, and even what happens when the motor is a generator when coasting downhill:
https://www.4qdtec.com/pwm-01.html

 

[TKS]

mhhh

2 Things Nigel:

1) Radio controlled cars use an 7.2 Voltage pack..and the engines they use have many many watts.. soow there is with less watts higher currents...

2) i have to build it soow i don't need to reverse the engine..(i can use his normal drive position...), maybe i only should connect it to the pinion different..


Regards and thanxs for the help..

TKS

 

[Nigel Goodwin]

Re: mhhh

2 Things Nigel:

1) Radio controlled cars use an 7.2 Voltage pack..and the engines they use have many many watts.. soow there is with less watts higher currents...

A car battery is only 12V, not even two times the 7.2V you mentioned!, and they are only a tiny fraction of the wattage of a starter motor. I would expect a car starter motor to take well in the hundreds of amps, and probably require a speedcontroller able to withstand over 1000A surges?.

2) i have to build it soow i don't need to reverse the engine..(i can use his normal drive position...), maybe i only should connect it to the pinion different..

If you can arrange a mechanical reverse it's going to be a LOT easier, also starter motors are only designed to rotate one way.

 

[TKS]

ok

the starter motor of an engine isn't maybe not suitable here

what we should mention is that

an cold engine has allot of resistance.. due to his compression

i one time cranked an engine round with the starter but the engine didn't had any plugs in it..the starter motor reved much much more and reacted much much quicker then with plugs in the engine

what i want to say is maybe in my app because we use pwm we make a 'soft' start...

if we change the starter motor for an wiper motor...

what would be then the case??

We also should mention that my differential has an working order of 4 that means 4 turns input 1 output you increase with this the force of the engine by 4
then we have a pinion set wich is 65 / 9 i guess soow allot of force isn't needed from the engine to move the car
if the car finally would reverse with 8Km/h top
or 5 i'm happy

Tks

 

[Nigel Goodwin]

Is it a real car you're trying to move?, a starter motor can just about do it (very, very slowly), a wiper motor would be no where near powerful enough.

 

[zevon8]

I think you may be well served to rely on someone elses engineering for your PWM controller. Moving a 500 Kg load using a large DC motor and FET's is not a simple task.

I have worked on a handicapped bus wheelchair ramp controller using FET's. It ran variable PWM on start, then fully on once moving, and used current sense to shut off once the ramp touched down ( the travel of the ramp varied depending on terrain. ) We spent most of a summer detonating FET's and other components before getting a working design.

All sorts of messy noise and current spikes generated by the motors were a nightmare for the design. International Rectifier has some very good application notes, many relating to H-Bridge design, other makers have similar info available on their websites, you should check them out.

Audioguru posted a good link, it too points out the complications involved.