Gadge-Jammit!

I HATE back EMF and its spawn!!
I always forget they exists...

And I now know that the welding is at opening time too.
Scrap that idea.

Thanks muchly guys
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"The future is not set. There is no fate but what WE make."
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I forgot to mention that the relay contacts on my controlers switch on and off at the lowest pwm period so there isnt a lot of current flowing any way unles under heavy reversing but i have never used any metering to check only watched the contacts for arcing whislt switching and trialed and errored with diferent caps until i couldnt see arcing

The regen current surge problem is why I am in the process of obtaining the Roboteq from Nick.
Probably Overkill for what I will be using it for but after blowing up 5 or 6 IBC’s I am desperate enough to spend some money.

The trick that the Roboteq uses is to dump the current surge back into the batteries and then go back to dynamically breaking the motor by shorting the brushes together when the voltage has dropped to a safe level.

It monitors the peak voltage and if the batteries cant absorb the amps without the voltage across them going too high it shuts itself down for protection.

Apart from that it should just be big enough and tough enough to cope with the motors I have been having trouble with even without the fancy regenerative features and it can be programmed to slow the transition from full forward to full astern so everything doesn’t have to happen at once.

Its going in Bane 1 as soon as I get my hands on it for some testing.
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quote:

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Originally posted by Valen:
I don't think there is much need for tuning. Really what you are trying to do is make a low pass filter with the lowest frequency possible. Basically I think the goal is to provide the smoothest power with the fewest kicks to and from the motor.

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Ummm... I thought that was the ultimate aim of PWM, for the load to see the equivalent DC voltage?

Not AFAIK. I might be wrong, but IIRC, Short Duty Cycle high voltage PWM pulses result in higher startup torque than DC ramping.

Perhaps it depends on where the PWM->Dc filtering is done ? If the PWM pulses are actually hitting the motor coils, then the magnetic fields should have a higher peak flux than if you prefilter them to equivalent-DC before they get to make any magnetism ?

Maybe I should focus on the binary-bit-level magic- I always forget my left and right hand rules when it comes to magnetic fluxes, permeability and permissivity etc..
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Yeh, what your saying hits something in my memory banks, I just can't quite remember the details.

Right hand rule... I find it's easier to do the right handed screw method with the vector cross product . Yes, I'm currently studying electromagnets, and yes, it's hard!

Got to phrase this carefully.

We all agree that a motors output will increase to the square of the voltage. Double = quadruple the power etc.


I thought that PWM was the best way to control motor speed without sacrificing torque.

Dropping the voltage will slow the motor by reducing its power as in torque X RPM.


OK I can’t really explain why but I will make a prediction.

If you compare 2 controllers running the same motor at identical torque and RPM the PWM one will use less battery and the motor will run cooler than the controller that smoothly adjusts the voltage applied to the motor.
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Satisfaction is proportional to effort and results.

if the PWM is < the time constant of the motor then yeah you will get more torque with high power short pulses. That would likley be around the 3-4Hz level though.

The torque is produced by the current flow in the coil, the more electrons moving the greater the force produced. If your PWM frequency is high enough then the natural low pass filter the coil makes will act to average the current flow. Meaning you wind up with a steady flow of electrons past the magnets. IE constant force.
More volts = more amps = more force.

With the slow PWM mode (where PWM freq < motor time constant) then you don't get that averaging and you effectivley run the motor stalled. IE the motor will pull 100% power.

That was the problem nick was having with the thingaps. Their inductance is so low that the time constant was < the PWM period. Meaning the controller was trying to source 3000A (for a short period of time (ie the throttle position say 10%)) and the motor was doing the same.

By adding his inductors he increased the TC of the motor and got the motors seeing low voltage DC again.

From my understanding its mainly the ChooChoo mob who like the low frequency pulse mode of operation. The problem they have is static friction is greater than dynamic, IE if they slowly ramp up the power going to the motors it will build to the point where it breaks all the sticking force then lurches off at some decent speed. With the low freq pulses they can break the friction and use the mechanical averaging of the train to stop the lurch off the line. For them the excess heating isn't really a worry ;->

In terms of nice to motors you should run the highest PWM freq possible, you will minimize voltage ripple and magnetostriction which can wear through your windings.
In terms of nice to FET's you want the lowest PWM freq possible. Fets heat up with increased switching speed (basically).

So really for an ideal system you need to find the balance point between where your motors blow up and where your ESC blows up, and make them both happen at the same time.

Or you say ~3khz will do most jobs and don't worry about it too much ;->

Smarmy gits with thingaps should know better than to run them off victors which sound like they PWM at 150Hz or so anyway.

In short, the ideal ESC provides a smooth DC supply to the motor.
Torque is a function of power, you won't get more torque without applying more power.
Long PWM is just a way of applying more power, however it uses the mechanical time constant of the motor to average out the pulses of power it receives so it appears that the end result is the same. It will pull a whole bunch more amps doing that job though and as heat lost is I^2*R where I = Amps and R = resistance it will make things hotter.

Amps are bad, use less amps.
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quote:

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Smarmy gits with thingaps should know better than to run them off victors which sound like they PWM at 150Hz or so anyway.

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or stick a bunch of inductance in with them or switch at ~20khz or something ;->
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Just to be sure, what happens to an SSR if they would get an overload? do they pwer the motor without being able to shut it down or do they stop completely?

So in normal relay terms, do they stay open or closed?

i know fets can fuse shorted like a relay so i would think either way would could happen.
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i think generally they are likley to turn "on"
unless you really get lots of current then you can blow them apart (aka off)
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Mechanical engineers build weapons, civil engineers build targets

question - am hacking a servo to run an SSR.

the radio will have 0% and 100% foward control on the 3rd channel to which the servo will be attached and i was basically just going to stick the servo motor output to the SSR. anyone think there will be a problem doing it with regards to the PWM of the servo output and what not?

cheers
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www.demon50s.com - Minimoto parts
http://www.youtube.com/user/HyzerGlen - Videoooozzz