How can we regulate the power going to our electric car motor?

[chris10230]

Hey everyone,

Just something for you to think about,
My school has just started a project do build an electric power car, (for more info please look at our website http://greenpower.aldenham.com ) and we have come up against the problem of how to regulate the power going to the motor. We are currently using a solenoid, but they keep burning out.
We would like to have it so that we can regulate the power instead of having it either on or off, but this is not essential.

Please let me know if you think of anything we could use, my email address is cdroese@gmail.com
Also any feedback on the website would be most helpful.

Many Thanks

N.B.
It must be able to cope with aproximatley 80 Amps at 24 Volts for more than 6 hours.

 

[brewnog]

Aha!

I've been working as an SEA on the Greenpower project with a school for the past year. Hence you can treat me as a competitor...!

Obviously I could help you out, and tell you how we got around the problem, but then I'd get hung by the kids for giving away our secrets!



Oh, twist my arm then...

The first thing to be sure of is that you're not asking too much of the motor. As you probably know, the motor isn't really capable of starting the car from a standing start; you're supposed to have some helpers push it. Obviously your gearing will affect this (but with the standard Seagull gear parts we started off with, this was still the case). If you're asking too much of the motor, you'll be drawing too much current for the motor anyway, and burn it out, regardless of whether the solenoid is man enough for the job.

Secondly, the original solenoids genuinely were pathetic. Pester Greenpower to send you a couple of the newer uprated ones. It doesn't solve the problem, but we only burnt out one during 2x 6 hour heats where previously we'd been getting through them every 10 minutes or so.

Thirdly, are you keeping your motor cool enough?

Finally, yes, it's possible to build a (very effective) electronic speed control for the motor, so you have something more akin to a 'throttle' than just a simple 'go/stop' switch.

Since this is a learning project, I'm hesitant to say much more than this until you go away and do some proper research on how you'd go about building the regulator. I'm not being arsey, but I've seen too many enthusiastic Greenpower students completely lose interest because of having people fix such devices to the car, with absolutely no understanding of how it works, or how it was designed.







On an unrelated note, make damn sure you rivet your chassis together properly! Make sure you have a spare set of relays, solenoids, tyres and wires ready when you do your first heat, keep your motor cool, and do some calculations to support your tactics regarding battery changes!

 

[russ_watters]

I've been working as an SEA on the Greenpower project with a school for the past year. Hence you can treat me as a competitor...!

Obviously I could help you out, and tell you how we got around the problem, but then I'd get hung by the kids for giving away our secrets!

Oh, twist my arm then...

Of course you are aware, chris, that anything brewnog tells you could cause your car to explode, right..?

 

[brewnog]

Of course you are aware, chris, that anything brewnog tells you could cause your car to explode, right..?

Industrial sabotage in the name of engineering education? Me?

 

[chris10230]

i hate this field

cheers for the help and i do hope that your planning on sabotaging my project, joke but still thanks a lot for the advice as it will be helpful

 

[Panda]

Have you considered using something a bit funkier than a solenoid? Not sure if you have material constraints that will prevent this but...

Using IGBTs you can get the same effect as the solenoid but much faster acting, therefore offering superior control.
Also with some clever wiring you can use the IGBT as a current limiter protecting your motor windings.

Speed control is very simple using a switch, think of how a 1 cylinder 4 stroke petrol engine works and see when power is actually applied, that may give you a hint on how to do it.

p.s. Don't tell Brewnog about the IGBTs

 

[chris10230]

cheers for the help now can you maybe explain a little more what an igbt is as i cannot relly understand it

cheers

 

[Panda]

An IGBT (Insulated Gate Bipolar Transistor) is a type of transistor that can handle a lot of voltage and current, much more than a simple bipolar or FET transistor.

They are designed to switch high power loads in and out and are used in high voltage power supplies and motor controllers. You could use and IGBT to replace tha solenoid or use it in a motor control circuit to actual precisely control the power flow to teh motor.

 

[dlgoff]

You might want to check out http://en.wikipedia.org/wiki/Solid_state_relay" .

Regards

 

[MedievalMan]

An efficient way to control the power to the an electric machine (for example, if it's a dc machine) is:

-Use a power supply of fixed voltage.

-Use an H bridge as your "switch": this allows the current in the motor armature (and thus the direction) to be forward, reverse, or brake (i.e. short the motor terminals, generator mode.)

-To vary the voltage to the motor, use a PWM (pulse width modulation) scheme or chip. Basically, PWM means you rapidly switch on-off the voltage applied to the motor, to get an equivalent average voltage. The current through the motor armature is smoothed out anyway, because of the coil's natural inductance (recall V=L d/dt (i)):the inductance resists changes of current.

I believe you can get chips that combine both the PWM chip and H bridge. This would be much simpler than trying to interface the chips or building them yourselves. They are off the shelf components used in industry everywhere, so they are cheap.

see www.digikey.com

 

[triden]

PWM and an H-Bridge is what you should defenitely take a look at like what MedievalMan suggested. They are easy to implement, and with PWM, you will be able to vary the speed of the motor without losing much power.