DC motor not "suitable" for speed control

[tcmtech]

I was referring to the metal spiral wire that holds a kids cheap school notebook together. They are around .5 - 1.5 ohms depending on the size and length of wire and easily handle 100+ watts with air cooling.

Odds are you can steal one from your kids or someones kid for free if by some odd chance you don't have one laying around the house yourself!

 

[gabeNC]

Dude... you're my favorite redneck!

Yeah I know what you were talking about... i just didn't want to jury-rig it already.

 

[rumpfy]

NO. The one ohm job is only for a trial.
In the proper arrangement, a one ohm with 10 amp flowing will drop 1o volt and you only have 12 to start with.
Thats why tcm suggested a bit of old steel wire. DONT spend any cash until the pathway is clear.
for the final design a 0.1 or 0.05 ohm resistor will be necessary.

 

[gabeNC]

Alrighty gents, finally had a few minutes to test. I snatched metal from a spiral notebook, measurement came in a .5 ohm. The fan started just fine and didn't trip the supply, however voltage was down to 8.55 volts to the motor. Current was 6.5amps.

Moving up the spiral metal about 3 inches away from the other lead, ohm is .2, voltage is roughly 10.5 and amperage is close to 9. Power supply does not trip.

 

[tcmtech]

Now you know what I was talking about. A simple variable power resistor for test and tune purposes.

Is moving up and down the coil changing the fan speed as well now?

 

[rumpfy]

post #9 - dejection; post #24 - elation; thats progress.
So you can use this jury rig to decide your motor speeds then, how to implement the power supply and controls and switches and thermostats etc. You need to outline what EXACTLY you want to do, write it all down, then do it. Depending on how the speed controller is designed and how you want to change the motor set speed, you might have to consider some over voltage protection in the power supply if you suddenly switch to a lower speed from maximum speed. These are fine details.
If you need help than ask.

 

[gabeNC]

Is moving up and down the coil changing the fan speed as well now?

Yup! I'm going to connect it to the existing duct work and see what the friction loss / static pressure is like. Make sure a lower speed will still have enough velocity to throw some air. The blower will have two feet of pipe and then a 90 degree elbow before it hits the intake of the heat exchanger.

You need to outline what EXACTLY you want to do, write it all down, then do it.

I think at first, having a low/high/off switch would be enough and I'd like to get this installed before it gets crazy cold. My long suffering wife would appreciate it as well. After that, the next step would be tie in with thermostats. I'm all ears if you have any additional suggestions.

Thanks for all the help so far gents.

 

[gabeNC]

So far i've had the fan installed for a few days and it works fantastic. No redneck resister has been required because of twenty feet of 16 gauge wire and routing the power through the existing snap thermostats. Throws alot of air, heats the living room to a toasty 75+.

At some point I would like to have the power near the fan, use a relay or two and the thermostats for control signaling only. Been working through some ideas, found a pic 18f1220 and some optoisolators on my desk looking for something to do.

 

[rumpfy]

Seems like the better HALF now really happy and warm.
Actually I was thinking more about really complicated stuff like a full wave bridge around a thyristor and some opto isolators to set some voltage reference points to adjust speed. A UJT etc to drive the thyristor cos you have a 12 volt AC supply.

 

[gabeNC]

In your experience what would be the most efficient way to drive this? I don't mind spending a little bit more to get a power supply / controller extremely efficient and not be dissipating alot of energy through 100w power resistors. This is also turning into a fascinating design project.

 

[tcmtech]

I would say that if you are lowering the input voltage to slow it down the simplest thing would be to have a basic power supply you can adjust the output voltage on.

Given it's just a basic brush type DC motor I would think a common battery charger transformer with a few voltage taps to tweak the output voltage up or down a few volts would work just fine.

Either that or a dedicated SMPS unit with an adjustable output that you can swing +- two to three volts above or below 12 volts would probably do the job just as well.

 

[rumpfy]

tcmtech is on the ball. Problem with your question is that everyone will now want to give you advice and you'll have to choose between all the background noise what to do.
A tapped transformer is the easiest but you'll need a tapped transformer to get varying output voltage. Then you'll need a big switch to change speed or do it with big relays.
Or you can do it with an SCR and bridge and adjust the timing circuit with opto couplers for example.
Or you can buy a variable power supply and use that with appropriate voltage changing adjustment arrangements.
If it was me i'd have a look in the shed to see what's there, and then decide the approach; but in my case I'd be only able to make one type. If I needed two, then they would be different designs probably.
So to get started, it would be good if you go through your shed and see what you've got.
I dont know what you have in your shed or even if you have a shed, but it is well known that ;"All Australian boys have a SHED"

 

[tcmtech]

Actually with multi mode battery chargers the transformer tapping is done from the primary side not the secondary so the mode switch that came from the battery charger will give you multi speed function without problems.

The other option as I mentioned is to use a higher efficiency SMPS that has an output voltage adjustment. All that is needed for that is to rewire the adjustment pot to an external pot located someplace convenient and you have a infinitely adjustable speed control for the fan.