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

[adamaw11]

I want to use a variable resistor to control the amount of voltage an electric motor is getting, as a way of varying its speed. I'm aware that some motors are used like this all the time (eg radio controlled cars), but are some motors made to work like this and some not designed for it?

Say if a motor's intended application was to be either on, getting 12v, or off, getting no power at all would there be any bad effects on the motor to have it getting anything up to 12v, being adjusted from time to time by hand (variable resistor) and even giving it the minimum amount of voltage to just make it turn really slowly? Could this shorted the life of the motor or anything?

Its a 12v DC motor I'm talking about.
(I'm actually working out methods to contol a Davies, Craig EWP)

[Squid]

A straight DC motor with no fancy shennanigans going on wouldn't be effected. Think about what a motor is, it is a series of electromagnets, if in fact one does get too little current thenit won't blow up or anything, might not work tho!

[bobob]

As far as I know, It is a good idea!

I was planning on using something like this.

http://www.jaycar.com.au/productView...Max=&SUBCATID=

To control the opperation based on temp. Although this would be an ON / OFF method. Maybe some further thgouht into using that would combine both methods!

[JustCallMeOrlando]

What do you think the blower resistors for your cars ventilation system do?

[Squid]

As far as I know, It is a good idea!

I was planning on using something like this.

http://www.jaycar.com.au/productView...Max=&SUBCATID=

To control the opperation based on temp. Although this would be an ON / OFF method. Maybe some further thgouht into using that would combine both methods!

Variable voltage with PID (inverter) algorithm ftw!

Depends of course on what you want to do!

[ndgcpr]

Pulse width modulating motors are heaps better for them, or you get better low speed torque out of it or something, I believe it is better for some reason to PWM them.

For those that don't know this is where you would still produce a 12V square wave output, for low speeds the signal is 12V for only 20% of the time. Higher speeds the 12V signal would be present alot more.

I think why certain really low voltages are bad is because the spinning of the motor helps keep it cool, so at very low turning speeds it will get hot, but i may be wrong, makes sense to me

[adamaw11]

What do you think the blower resistors for your cars ventilation system do?

well yeah, good example. They have about 4 or 5 steps of resistance yes? I just wasn't sure whether an electric motor had to be designed to operate like that properly, and run on, say, 3v 6v 8v etc. And if im just stuffing round with a resistor dial thing from dick smiths it might be getting all sorts of in-between voltage amounts 3.4837 volts? 0.27134 volts? So if the voltage was too low to make the motor turn, its not going to do something odd with magnectic fields or anything is it?

[adamaw11]

I think why certain really low voltages are bad is because the spinning of the motor helps keep it cool, so at very low turning speeds it will get hot, but i may be wrong, makes sense to me

yeah that may be worth taking into consideration

[Ben Wilson]

The Davies Craig controller uses PWM as far as I know to control the pump speed and works very nicely FWIW.

[bobob]

I can see the point there for a motor in 'normal' conditions.

These motors are submersed in the coolant so I figure they will mainly opperating around 86 degrees or what ever your coolant temp sits on.

I like the idea of PWM anyway but is it needed?

This kit is a bit pricey and you need a special controler:

http://www.jaycar.com.au/productView...Max=&SUBCATID=


Is there a cheaper way than this kit, pricing up some of these options may be closer..

http://homepages.which.net/~paul.hil...enerators.html

[The Witzl]

PWM is the best way to go for reducing the speed of an electric motor.

When reducing the voltage by way of passive resistance, not only are you still using the same amount of power to run the lower speed, you are also generating heat in the resistors (lots of it) and if you reduce the voltage too much you can start to damage the internals of the electric motor.

With PWM, if you are running the motor at 80%, you are only using just a little over 80% of the power (taking into consideration efficiencies and other losses).

This is the preferred kit from Jaycar:
http://www.jaycar.com.au/productView...Max=&SUBCATID=

The alternative kit from Jaycar is:
http://www.jaycar.com.au/productView...Max=&SUBCATID=

[The Witzl]

also - this belongs in the Tech section.... i'll PM a mod of this section to move it

[adamaw11]

yeah at first I thought it wasn't exactly car related but I guess it is more or less anyway.

When reducing the voltage by way of passive resistance, not only are you still using the same amount of power to run the lower speed, you are also generating heat in the resistors (lots of it) and if you reduce the voltage too much you can start to damage the internals of the electric motor.

using same amount of power still, really??? I thought more resistance slowed the power flow, reducing the current etc? I guess if its generating heat then.... I should probably take this seriously.

Thanks for all the good info people, keep it coming.

[bobob]

Why do you want to reduce the speed?

Slower water flow means the water will stay in the block longer and heat up more, but also stay in the radiator longer and cool more! There would be a balance at some point in these exchanges where it will stay failry constant. I have no idea if that is around the desired temp for your purpose.

There is a manufactured supplied controler as mentioned above that might make this easier but it may not achive what you are trying to achive. And where is the fun!!

I guess the idea of the temp switch for our implementation was that it would leave the water in the block (no flow) until a set temp has been reached. Then pump. Then shutdown after it has cooled to a certian temp. It could be used while the car has been shutdown allowing for more gentle cooling...

The still water in the block may lead to hotspots though...

[Hen]

Witzl is correct (as usual).

Trying to control the motor by varying the voltage (Analog) means a large amount of power will be wasted heating a resistor or op-amp or other device trying to produce some voltage between 0 and 12V.

Using digital control (PWM for example) requires no analog electronics and so is much more efficient (therefore smaller parts and less heat generated).

Also, given that the motor coil is an inductor, if the PWM is switched fast enough, the digital voltage supplied will be "smoothed" into a relatively constant current flow.

But to answer the original question, running a 12V motor at a lower voltage won't damage it.

Hen