"230V" 120W DC Carbon Brush motor speed control?

[Coper]

Hello,
I have scroll saw with 120W DC motor, speed is controlled by triac regulation in the range of 500 - 2000 any as see pic (in my version for 230V)
The original electronics are destroyed. So I'm thinking about how to do speed control again?
Should I stick with this simple triac regulation or recommend something better?

 

[schmitt trigger]

In my opinion, the simpler the better.

The circuit that you show is very simple and almost foolproof. But it is open loop, meaning that the speed will change with both load and line changes.

If that is ok with you, keep the simple circuit.
If you require a tighter speed regulation via back-EMF feedback, while maintaining an all-discrete circuit, the following circuit courtesy of GE SCR Manual 5th edition, may be used.
If your motor is a permanent magnet type and has no shunt field connection, you can eliminate D3 and D4.

From there, the sky is the limit with respect to complexity and features.

 

[Coper]

2N4987 Silicon Unilateral Switch (SUS) to this day, I had no idea of the existence of such a component .

 

[schmitt trigger]

SUS are not very common but still can be found on the web. Or you can roll your own as shown in the datasheet’s equivalent circuit.

 

[Coper]

We'll see, said the blind .
So far, I have redrawn the schematics according to another piece of the same saw.
Speed is controlled by a 500k potentiometer, control range 500 to 1700, which corresponds to the voltage at no motor load 45 -130V.
The engine is 120W with carbons, I did not find out anything more about it.
From the logic of things (voltage control) it delivers 120W power only at high speeds .
Maybe I drew it well
I miss the meaning of WR1(+R2+C4+R1) a bit

 

[schmitt trigger]

The circuit with two capacitors is the well known double time constant to avoid hysteresis.
Also explained on the same GE book.

WR1 and WR2 are te minimum and maximum speed set points.

 

[Javert]

Thank you for pointing out the book GE SCR Manual 5th edition , interesting book, although I just did not find a connection with two C in it, but it is discussed her.
I have a question about this.
I have a grinder that has a similar speed control and I would like to add a foot switch .
My first idea.
Parallel to PT1 and WR2 add optotriac for example MOC3052.
if LED in MCO3052 is On current over R3 ids ground and voltage on diac is below its switching voltage and the big triac is open .
If LED is off the triac in MOC3052 is turned off and the device is operating normally .

Any reason not to do it like this or a better idea?

 

[schmitt trigger]

I am not following your description.

Why don't you draw a schematic with your ideas?

 

[Javert]

OK, I will try it
with permission, I borrowed the original scheme

First option

Device is blocked if LED in MOC3052 is ON because the triac in the MOC3052 short current from R3 and the Diac D7 never switch and open BTA12 triac.
It is perhaps simple and functional

Second option

Her is triac MOC3052 insert between Diac D7 and Triac BTA12 . If LED is OFF is not Gate of BTA12 powered from diac and do not switch, If LED is on is Gate powerd and device , the question is how?
On small triac in MOC3052 there will be some losses that will affect the operation of the equipment.
Perhaps this should be compensated by adjusting the resistance of WR1 and WR2 or am I wrong?

 

[schmitt trigger]

Use a small relay instead.

 

[Javert]

Big, expensive, impractical . for amateurs
but seriously , MOC30xx is actually small Solid State Relay

 

[schmitt trigger]

A Triac behaves like a relay, if it actually latches.
I will allow you to determine whether the short trigger pulse generated by the Diac can achieve that.

 

[Javert]

I tried a simple simulation in Multisim
the question of course is how accurate is the SPICE model MOC3051, but in the simulation it works and the voltage drop across the triac in the MOC3051 is below 200mV and has no problem with the pulse width behind the diac

the upper inserted oscillogram is the signal before and after the triac in MOC3051.
According to the simulation, it should work
How it will be in reality will be revealed, as soon as I find an optotriac, I will try it

 

[KlausST]

Hi,

You need to add a load for a more realistic simulation ... especially voltage drop and timing

Klaus

 

[Javert]

Are you right of course, multisim can simulate a DC motor, but I have no idea what R and L has a typical 120W motor with PM.

 

[BradtheRad]

I guess your DC motor is brush-type? Then it has numerous coils of wire wound on an armature. Each coil's impedance is some portion ohmic resistance and some portion inductance (Henry value). You can read ohmic resistance with an ohmmeter. Inductance is measured with more sophisticated equipment.

The waveform through the motor is sort of like a switched-coil converter. Say you have 20 coils going at 10 rps (=600 rpm). Each coil receives voltage for 1/200 second. In that time the coil draws 120 Watts average. Calculate the RL time constant which yields that response, to get an idea of Henry value.

There's the observation that a motor draws greater current at slower speeds. At first it seems unexpected, although it makes sense when we realize more current flows in an inductor the longer voltage is applied.