electric motors: varying their voltage supply, bad for them?

[mrshin]

Use an AC motor

[river]

Hi,

Use an AC motor!!!!! Hehehehe, onya shiny!!!!

If its a multi-coil motor you can just switch on/off the extra coils. This is how your wiper motor works. It runs on a couple of coils (stator coils) and you switch in an extra coil and it goes half the speed.

If you want a few different speed/power settings, rather than use PWM you can use an LM138 5amp adjustable regulator. All you need is to switch a few different resistors for the adjustor and you can select any votlage you want, depending on the resistor. These are not power resistors - just your stock 1/4-1/2W resistor 'cos the LM138 regulator is doing the work and taking the load.

The spec sheet for the LM138....
http://mechatronics.mech.northwester...ata%20sheet%22

If you want a smooth and fully variable speed range then PWM is the best solution. If you are happy with three or four set speeds then then LM138 circuit would be fine.

Mind you, a 5A regulator means your motor has to rated at 60W or less. If it's more then you need to add a power trannie to help the LM138 deliver more than 5A. Not a big or complicated thing to do - but it adds a few dollars to the final circuit.

So, you have to decide if your application requires PWM or an adjustable regulator.

seeyuzz
river

[cambelt]

Toyota started there fuel pump control using a resister but then changed to a ecu type device which i think varies the pump voltage anywhere between 8 nad 12 volts by PWM
Dave

[mrshin]

Resistors do a job, as suggested by heater fan, injector and fuel pump resistors, but they definately aren't the most efficient way to control anything DC. But then that's the difference between a Motec and a Microleb!

[JustCallMeOrlando]

Indeed. Look at how the blower resistors look like after 20 years of use They get brittle from all the heat cycling and they're shaped to get rid of all the heat they produce.

[The Witzl]

an AC motor would also work... but lets not get too silly here!!


Just as a side note regarding the blower resistors..... i found that in the UZS131 it didnt use resistors to vary the fan speed, it actually used active componentry. Instead of a couple of cooked out chunky resistors i found a nice T03 packaged FET mounted to a small heatsink, both in perfect condition.
..... so what does that mean?
That means that even Toyota recognised that resistors are ghey and went for PWM to control the blower fan speed - and they probably bloody needed to with the amount of air that fan would put out (front and rear climate control aircon system in the UZS131 )

[adamaw11]

ok, so far it seems the best option is to go for PWM and I might as well look at getting the DC motor control from Jaycar.....

I think I might have to learn a bit more about PWM though - its not like its rapidly switching a relay on and off constantly is it?

[river]

Hi,

No, it switches power transistors on and off - all electronic.

Some PWM info for you....

http://www.netrino.com/Publications/Glossary/PWM.html

http://www.cpemma.co.uk/pwm.html

The 2nd site is good and has some circuits. The Jaycar kit is good, but you can build one cheaper by reading the 2nd site. Only a few parts - easy peasy!!!!

seeyuzz
river