SCR Motor Controller

[adamq]

My lathe controller is pooched, so I'm going to build a new one.

AKA, I'm trying to control the speed of a 110VDC 250W motor.

I'm thinking SCR because it seems simple and affordable.

I'm going to rectify but not filter the AC power, thereby giving me DC pulses.

I'll have the SCR gate controlled by a cap and pot resistor.

The rate of firing will depend on the charge time of the cap, made adjustable by the pot, yeah?

I'm looking for suggestions and explanations on cap/pot size, and any other advice you have.

I'm aware I could use a pwm control system with MOSFET using a 555 timer or something, but I honestly don't understand the pwm/MOSFET system as well.

I do understand it's Superior as it provides full voltage (torque) in pulses rather than reduced voltage based on the wave of the rectified AC.

 

[Minder]

The simplest method would be to use a Triac instead of a full SCR bridge.
With the Triac you just use place the bridge on the output and feed the motor from the DC side.
There are plenty of Triac designs out there on the web, just that you do not have the refinements that a SCR bridge type has such as the KB or T.M. controllers for accell and current limit etc.
M.

 

[adamq]

Is a triac controlled the same manner as the SCR then?

Basically you recommend this:

120vac > rectified > triac with gate controlled with pot/cap > motor?

No danger running the 110VDC on rectified AC, because it's average received voltage will be much lower, correct?

Apologies for the crude description.

I'll look up triac controller designs

 

[adamq]

So I read a couple articles / schematics and it looks like although the triac can be triggered by both halfs (+/-) of the waveform (convenient), it also allows AC to pass through the terminals, which is undesirable for a DC motor, no?

Although this will all be rectified anyway, so it won't matter. Then why use a triac vs an SCR in this case? Are there heat/speed advantages to the triac vs the SCR?

Again, apologies if I am not understanding this. Just want to use the best system.

 

[Minder]

This idea, it is easier to build a Triac controller than a SCR style which requires the SCR's to be inside of a bridge.

M.

 

[adamq]

Thanks Minder. I'll study that and look up part numbers and try and understand.

 

[adamq]

Seems straightforward enough.

Isee the ht-32 is discontinued but any diac with current handling and voltage rating will work, right?

I see the 250k pot.

I imagine the 15k resistor is for reducing trigger voltage to the gate.

May I ask what is the purpose of the 3.3k resistor in series with the pot?

 

[Minder]

Yes the components are not super critical, the 3.3k resistor is to limit the max. current through the cap.
You may occasionally see a common mode toroidal choke in series with the output lead of the Triac.

M.

 

[adamq]

Excellent minder, thank you. Indeed there is a choke present on the old board, must be why.

Off to the parts store to buy three of everything

 

[adamq]

One more thing actually. I don't understand the motor inside the diodes. Is it just drawn this way?

My plan was to go ac > 8a rectifier bridge > switching components > motor...the reason being I could use a dpdt switch to reverse polarity and direction

The schematic uses 4 diodes to rectify, and then the switching components close the circuit, correct?

I suppose suppose it's no different. Just curious why it's drawn this way, there must be a good reason

 

[Minder]

it is just easier way to draw it that way, the motor could just as easily be drawn 'outside' the bridge.
The triac control is in series with the input to the bridge, AC side.
BTW, ensure you buy a linear pot, not logarithmic.
M.

 

[adamq]

Linear pot. Thanks.

Otherwise a logarithmic pot would exponentially increase/decrease, making it hard to control?

 

[adamq]

Speaking of buying a pot, is there a reason I couldn't salvage the pot that is on the lathe already? It is a 4.7k pot. Ohms out fine.

 

[Minder]

If log pot then the speed control would not be linear, obviously.
I suspect that the 5k pot would provide control that would prove a little coarse.
Nothing lost by trying it.
M.

 

[adamq]

HT-32 was unavailable. Clerk recommended 1n914b in its place.

Datasheet describes it as a small signal diode, not a DIAC. https://www.fairchildsemi.com/datasheets/1N/1N914.pdf

Thoughts?

 

[adamq]

OK. It's assembled. Before I go ahead and fire it up, I'm hoping you guys can look it over and give some advice.

Save some smoke, just maybe

I'm still not sure about that signal diode vs a DIAC.

Attached are the labeled pic with connections in blue, and without labels, and the original schematic.

 

[Minder]

I don't see that signal diode working for you, it does not perform the same function as the Diac which is a bi-directional device. .
DB3, DB4 etc.
A

 

[adamq]

OK. I'll pick up a DIAC somewhere before i power it up then. Thanks Minder.

Looks like a shop around here has an NTE6408 which is a DB3 equivalent..

 

[adamq]

Happy to report it is working with the NTE diac... However the speed control is ineffective with the 250k pot.

I tried a 4.7M resistor for fun and it's probably still 3/4 speed or better (I jump the resistor to compare).

Any pot value recommendation? Or advice on how to size the pot?

Edit: it occurred to me that maybe I had it wired up incorrectly. I'm using it as a variable resistor/rheostat with two terminals: center and right. That's how it makes sense to me, but maybe I'm wrong.

Could increasing the capacitor sizes help?

 

[Minder]

The circuit does show a variable resistance and not using it as a pot.
There are also many other examples out there that are very close.
The one shown is a very common circuit been around for decades, so not sure why it would not work.
M..