Electronic Design Help Please

[NoIdea]

Evening All – Just looking for a wee bit of help please, hoping someone on here knows more about electronics than me.

Back Ground: I am more effective at tearing electronics apart then I am making them. :lol:

So, with that said, find enclosed pictures of a DC power supply for running a 240VAC motor, the diagram came from the internet and I have adapted it for this project. It works well but I am concerned about the heat generated by the transistor. Transistor = 2SC3460

The four transformers are 240VAC to 16VAC, the three bigger ones power the motor and the littler one powers the base voltage of the transistor. The four bridge diodes are rated well over their current use.

My questions are thus:
Can I run this unit better?
Do I need a current limiting resistor between VR and Base?
Can I somehow reduce the amount of heat generated by the transistor?





Note: Every part used in this project has been scavenged from other electronic I have dismantled, and the reason for three transformers is because I don’t have a 70VDC on hand.

Please, any help would be muchly appreciated.

Regards

Deano

 

[Geo]

the heat sink is as good as your gonna get. thats probably over kill. :lol: a resistor will help if its pulling alot of amps. you will know if it pops a capacitor.

 

[NoIdea]

Thanks Geo, yep, pretty much over-kill on most of the parts, when it comes to me making electrical stuff its always better over than close. :lol:

Um, where would you put the resistor?

Cheers

Deano

 

[Geo]

in a battery charger, its between the out post on the transistor and the load. not sure about the setup you have but it shouldnt be that different.with multiple coils, it may need one for each coil.

 

[lazersteve]

If your goal is to drive an AC motor that operates at a lower AC voltage than 240Vac simply step the voltage down with one transformer (easily found on ebay) that gets you to the proper AC voltage that the motor likes. You can even add a properly rated (in wattage) rheostate to control the speed. Alternately the speed could be controlled on the primary side with a variac.

From the looks of the DC-to-AC motor drive circuit listed you are over complicating the solution.

Why not run the AC motor directly off of a stepped down AC transformer (240Vac to the appropriate motor Vac)?

Maybe I'm missing something with your goals or project requirements.

Steve

 

[macfixer01]

If your goal is to drive an AC motor that operates at a lower AC voltage than 240Vac simply step the voltage down with one transformer (easily found on ebay) that gets you to the proper AC voltage that the motor likes. You can even add a properly rated (in wattage) rheostate to control the speed. Alternately the speed could be controlled on the primary side with a variac.

From the looks of the DC-to-AC motor drive circuit listed you are over complicating the solution.

Why not run the AC motor directly off of a stepped down AC transformer (240Vac to the appropriate motor Vac)?

Maybe I'm missing something with your goals or project requirements.

Steve

Another option is buying a used Variac, then you can just adjust the output to the 70 volts needed. With what you have though, adding a bit of silicone heatsink compound between the transistor and heatsink is a good idea if you haven't already. You might also add a cheap muffin fan to help with the air cooling, especially if this unit is normally run with a cover on it.

 

[NoIdea]

Hi All - thanks for the help.

I am using what i have because thats all i have, would be so nice to just go out and buy what i need. :lol:

AC motors run quiter using DC and you need less voltage ta-boot, hence my choice to use it. I would rather play with DC than AC any day.

I would like to use what i have created, because thats really all i have, i am considering using a darlington pair and see if that improves control.

Thanks again

Deano

 

[lazersteve]

What are the specs on your motor and power source?

Steve

 

[NoIdea]

What are the specs on your motor and power source?

Steve

Hey Steve,

The three transformers are 240VAC to 16VAC @ 40VA, these are wired in series on the output end, after rectifying, giving a rectifyed value 70VDC, this value really depends on the AC mains supply, which today was 263VAC :roll:

As for the motor, well its got nothing written on it, it did come out of a 240VAC food processor, and i have a 3amp fuse on the 240VAC mains end of things.

Thanks

Deano

 

[element47]

I am unaware of how one can run an AC motor from a DC source. Never heard of it. Pulsed DC, that would be a different story. An AC motor (most of them) depend upon the phase of the incoming current "rotating" within the stator, and inducing eddy currents in the rotor. Most DC motors utilize brushes and pass actual current via a commutator (rotatable connection or "slip ring") = true electrical connection (vs pure induction) into the rotor and generate repulsion, thus turning the rotor and thus the shaft.

If your control transistor which I can only guess you have configured as a pass (series) element is overheating, it's passing too much current, period. If it is an NPN-type (arrow pointing outwards) it wants a positive collector (blank, angled terminal) relative to the emitter (arrow terminal) which it appears you have. But there can never be more than about .7 volts between the base (the non-angled terminal in the diagram, the one that hits the central bar of the transistor at a right angle) base being positive rel to the emitter, because the base to emitter junction is simply a diode and once you hit .7 volts, it is turned on, all the way on. Hence, there is no need to feed it 20 volts. The base needs to be a lot closer to zero volts than 20 volts in the scheme of things and you are probably over-currenting the base, eg; the base-emitter junction. The practical aspect is that the pot (VR) has to be turned very, very close to zero, and, unless it is a linear pot, you might have like 3 degrees of rotation before you overcurrent the base-emitter junction of the transistor. If it's an audio taper pot, you may have .3 degrees.

I am still very skeptical that this can work (running an AC motor on DC) but you say it sort of works. So I will proceed.

The first thing I would change is to put the motor on the other side of the transistor, in series with the emitter, not the collector. This is assuming you have the neg of the bias supply common with the neg of the 71 volt supply. You want the transistor to turn on. Once the transistor is turned on, the "big" current flows through the emitter-collector junction..the difference is the amount of bias current, which should be very low (but....it will be bigger than "usual" in a fat power transistor) This way the motor forms a proper load for the power transistor and also current-limits the base current. The transistor turns on, and the emitter tries to suck up to the positive rail. The emitter will stay .7 volts negative to the base, but will rise in voltage above zero as the transistor conducts more and more. That voltage difference drives your motor, if this thing can work at all.

 

[NoIdea]

Thank you all for your reply's, you have all been a great bouncing board.

The darlington pair(DP) worked and worked well, neither transistors got warmer than my touch, and that's at full throttle.

The changes i made were to remove the 1.8k resistor and make all leads that lead to that resistor go to ground, next was to replace the transistor with a DP, and putting a 500 ohm resister between the DP's base and the VR, next was to remove the capacitor from the power source, which dropped the output DC voltage from 70VDC to 45VDC.

Got the information from here: http://elektroarea.blogspot.co.nz/2011/01/darlington-pair-to-drive-dc-motor.html

And yes, an AC motor, with "brushes", will in-deed-dee work with DC voltages, you can even turn an car altinator into a DC motor, with a little bit of re-wiring, i.e. removing any voltage regulators and diodes and the rest is on the internet. 8)

Somewhere it states that the people whom designed the AC motor forgot to make sure it would not work with DC.

Like i wrote earlyier, AC motors running on DC are quiter and need less voltage.

Cheers

Deano

 

[Geo]

thats great.whats the application? when we were first discussing it, a 24V starter came to mind. ive used them for many things. ive built a wench that could lift an entire car with a 12V ford starter and a matching flywheel.i know a starter run time is limited to maybe 30 minutes before it overheats depending on load.so you moved the resistor between the coils and the voltage regulator? i was thinking that may be the case, but i was thinking it may have to have one for each transformer. im glad you got it figured out.

 

[NoIdea]

thats great.whats the application? when we were first discussing it, a 24V starter came to mind. ive used them for many things. ive built a wench that could lift an entire car with a 12V ford starter and a matching flywheel.i know a starter run time is limited to maybe 30 minutes before it overheats depending on load.so you moved the resistor between the coils and the voltage regulator? i was thinking that may be the case, but i was thinking it may have to have one for each transformer. im glad you got it figured out.

Hi Geo - it's to run my wet magnetic stripper, i was using the three transformers, output in series, with a straight rectifyier to the motor with no control, and it ran a bit too fast, had to use friction between the brush and magetic wheel to control speed, got tiried of that pretty quick. :lol:

So now i am a happy chappy, ready to process my small fines an get rid of that pesty magnetic stuff.

Ill get around to posting an update of the circuit diagram soon.

Cheers

Deano

 

[NoIdea]

Here Yah go.



Deano

 

[macfixer01]

Here Yah go.



Deano

I see you put a diode in parallel with the motor which is also a good addition. For an inductive load like a motor, solenoid, or relay that's being switched with a transistor it shunts any reverse EMF so it doesn't damage the transistor.

 

[NoIdea]

I see you put a diode in parallel with the motor which is also a good addition. For an inductive load like a motor, solenoid, or relay that's being switched with a transistor it shunts any reverse EMF so it doesn't damage the transistor.

Hi macfixer01 - it was recommended for that very reason, i was supprised how well the darlington pair worked, only problem is the weight of the blasted thing. :lol:

Cheers

Deano

 

[lazersteve]

Deano,

If the mains are 240 Vac and the motor is 240Vac why add all the extra circuitry? A power cord, ac switch, current limiting fuse, and the motor seem to be all you need. For speed control you could use a 240Vac transformer with multiple taps (240 Vac to 240Vac and 120Vac are very common types).

The circuit you posted would be useful in my opinion if you were running the AC motor on battery power, but you already have 240Vac from the wall, why rectify it to DC just to convert it back to AC to drive the AC motor?

Steve

 

[NoIdea]

Hi Steve - valid points ........Butt

Deano,

If the mains are 240 Vac and the motor is 240Vac why add all the extra circuitry? A power cord, ac switch, current limiting fuse, and the motor seem to be all you need. For speed control you could use a 240Vac transformer with multiple taps (240 Vac to 240Vac and 120Vac are very common types).

I dont have a multi-tapped transformer, i opted for a more control approach. Be nice to have one though.

The circuit you posted would be useful in my opinion if you were running the AC motor on battery power, but you already have 240Vac from the wall, why rectify it to DC just to convert it back to AC to drive the AC motor?

Steve

Ahh, but i am not converting it back to AC, it is unfiltered DC that is running an AC motor. Brushed AC motors will run on filtered or un-filtered DC and at lower voltages. With filtered DC, you do run into trouble at very low speeds with stalling, in my case, anything lower than about 1000 RPM with load, with unfiltered DC i have been able to dropped it down around 200 RPM with load.

Regards

Deano

 

[Geo]

since the normal voltage where you live is 240v, couldnt you just have used a dimmer switch for a light fixture? or maybe a canister type rheostat?

 

[nickvc]

Geo I think with Deano it's more about using what he's got than ease of getting there, you know how inventive he can be, I feel Rusty might have a competitor in that field now.