However, when I tried it again today, I ramped up the speed a bit quicker - and the dimmer circuit popped. Oh, well - back to the drawing board.

Common problem with not using things designed for the working conditions at hand.

More than likely it had a triac that was not rated for the load of the motor.

 

1. The switch arrangement to stop the motor quickly by shorting it doesn't work. It seems to take about the same amount of time to stop when it is shorted.

I feel in my bones that this would be a more elegant solution, but I simply can't envisage a simple electronic replacement for the pot, which has fairly high voltages on its terminals.
.

The reason is a universal motor does not regenerate.

The Picmicro link also shows how to power a Triac controller via a Pic using a direct mains powered supply. See AN954 on the Pic site.
Max.

 

The reason is a universal motor does not regenerate.

Yes, I found that by experiment :-( The disk brake idea I had is a bit clunky. Do you know of an electronic way of doing it?

The Picmicro link also shows how to power a Triac controller via a Pic using a direct mains powered supply. See AN954 on the Pic site.

Thanks for that, Max. I looked up the reference, and I agree - I'd be better off with a triac or SCR (if the mains is rectified) than an IGBT, because it's a lot easier to drive the gate. I'll let you know how I get on - but don't hold your breath. Like Lawrence Oates, I may be some time.

 

Evidently there is a patent out that does it via:
A dynamic braking system for universal electric motors operating on alternating current is disclosed which utilizes the discharge of a capacitor to momentarily energize the series field of the motor after the power circuit to the motor is broken and the field connected across the terminals of the rotating armature to initiate dynamically braking of the armature to bring it to a stop within a few revolutions.
Max.

 

So how much time and money do you plan on spending to try and make a cheap low quality saw work 'sort of like but still not as well as' a basic quality one?

Any idea how my buddy can turn is geo metro into a cadillac other than by just accepting he has a geo metro and not a cadillac and the only fix would be to buy a cadillac?

 

Evidently there is a patent out that does it via:
A dynamic braking system for universal electric motors operating on alternating current is disclosed which utilizes the discharge of a condenser to momentarily energize the series field of the motor after the power circuit to the motor is broken and the field connected across the terminals of the rotating armature to initiate dynamically braking of the armature to bring it to a stop within a few revolutions.
Max.

I had thought of energising the field coil, but I hadn't thought of using a cap. I'll do some research on that. Thanks again, Max.

 

Your input is appreciated, Ian. I also saw soft start for amplifiers - but I think the requirements to soft start a TV or amp will be quite different from soft-starting a strongly inductive load (a motor).

The astable triggered SCR buck regulator popular in Pye and Philips sets would be just right.

 

So how much time and money do you plan on spending to try and make a cheap low quality saw work 'sort of like but still not as well as' a basic quality one?

I'm going to make a low-cost solution at any expense!

Seriously, though, tmtech, this is a project. It's not just to accomplish an outcome - I'm doing it largely for the project itself. Yes, you're right - I'll probably spend more on it in the end than if I had simply bought a commercial soft-start module - but I'll have a lot of fun along the way. I'm retired, and living alone, so I have time on my hands. I have to fill it somehow.
The way these motors start is a problem that quite a few people seem to have, so I would like to come up with a relatively simple-to-make, low cost solution for the simple pleasure of offering it to others of a like mind. I'm not looking to make modules for sale - just offer a write-up for anybody with a maker mentality.
I have had a little bit (a very little bit) of experience of the AVR processors, so I'd like to go further down that road, as I have other projects in mind for them.

 

I have rounded up the parts for the idea I have, just have to see if it works!
Max.

 

Hello, I have previously made a soft start with a dimmer type circuit replacing the pot with a ldr tubed together to a led fed by an emitter follower tracking a slow rising rc circuit. It worked well enough.

Regarding universal motor electric brakes: mitter saws usually have a second thinner winding in the field connected to the brushes in the opposite polarity as the main field winding. During normal operation the brake winding is open circuit and when the switched is released it short circuits the break winding.

 

I'm going to make a low-cost solution at any expense!

So what ramp up delay time are you aiming for?
Max.

 

Hello, I have previously made a soft start with a dimmer type circuit replacing the pot with a ldr tubed together to a led fed by an emitter follower tracking a slow rising rc circuit. It worked well enough.

That's a good solution. If you had suggested it earlier in the thread, I would have given it a go - though the dimmer I was using was too light, so it would have blown anyway. Right now, I have ordered parts for my project, and I don't think I'll change my mind again. This is just the first step down a path I want to follow after my soft-start works.

Regarding universal motor electric brakes: mitter saws usually have a second thinner winding in the field connected to the brushes in the opposite polarity as the main field winding. During normal operation the brake winding is open circuit and when the switched is released it short circuits the break winding.

Thanks - good info, but I don't think it likely that my motor has that feature.

 

So what ramp up delay time are you aiming for?
Max.

Hi Max, A second and a half would be OK, and although 0.5 secs would be a lot longer than it takes to start at the moment, I'd aim for rather more than that. I've been working on the idea of one second, but the application doesn't call for any precision.

BTW, I used a BT137 600 triac in another project (speed control for a washing machine motor (also a universal motor), using the schematic on the TDA1085C datasheet, but I was surprised that it popped immediately. I may have made an error in constructing the circuit, but in your opinion, would that triac be too light? The motor was on the bench, and not loaded, and the Triac blew so fast that the motor didn't move. I didn't have it on a heatsink, but it popped before it had had time to warm up.

BT137 600E triac
V(DRM) : Repetitive peak off-state voltage : 600V
IT(RMS) : on-state current : 8A
I(TSM) : Non-repetitive peak on-state current : 65A

Do you have a recommendation for what triac to use for 240V non-industrial universal motors, such as in tools?

 

cgallery - this thread seems to have died the death, but I'd like to ask if you ever resolved your problem, or what you eventually did. I have exactly the same issue - also primarily with a table saw. My previous (low cost) table saw had gear teeth cut into the motor shaft (1/2" diameter), which meshed with a 2" diameter gear wheel on the saw arbour, and after a couple of years of very little use, it stripped all the teeth off the 2" gear. I have bought another saw table, again low cost, though adequate for my needs - but it suffers exactly the same problem - the saw blade comes up to speed in what seems like about a mirosecond. I'm sure a soft-start of some kind would greatly extend its life. I'd really prefer to build something rather than buy an off-the-shelf solution, and I have a couple ideas (one triac-based, one IGBT-based), but I foresee problems with them both. How did you get around it?



Moderator's Note:
Please don't hijack other member's thread, now you have your own.
This thread was split from --
Modifying router speed control for soft start
https://forum.allaboutcircuits.com/...ed-control-for-soft-start.98637/#post-1100017

Just buy a slow start 120$ approx. ( one phase )
no way you can build a proper one for less.
Picbuster

 

Several triacs available....


http://cpc.farnell.com/webapp/wcs/s...true&aa=true&pf=110136080,110136082,110193965

 

Several triacs available....


http://cpc.farnell.com/webapp/wcs/s...true&aa=true&pf=110136080,110136082,110193965

Just buy a slow start 120$ approx. ( one phase )
no way you can build a proper one for less.
Picbuster

Thanks, Picbuster. If my sole objective was to make my saw start slowly, that would certainly be the easiest / quickest way. In fact, if my objective was simply to have a reliable table saw, I could buy a more expensive one and be done with it. I knew that when I bought the low-cost one I have, but frankly, I don't do enough woodwork to warrant high quality tools. I actually want to make something.
Thanks for the pointer all the same. Helpfulness and friendliness are always appreciated.

 

BT137 600E triac
V(DRM) : Repetitive peak off-state voltage : 600V
IT(RMS) : on-state current : 8A
I(TSM) : Non-repetitive peak on-state current : 65A

Do you have a recommendation for what triac to use for 240V non-industrial universal motors, such as in tools?

I would use at least a 14amp such as MAC16DGOS, or larger.
Max.

 

Several triacs available....


http://cpc.farnell.com/webapp/wcs/s...true&aa=true&pf=110136080,110136082,110193965

Thanks Dave, I looked at the link, and I see that the triacs shown are all heavier than the BT137 - but you say "Several triacs available". The BT137 was also one of several available. I figured that at 240V RMS, 8A, it should be capable of driving about 1920W, which is 2.5HP - and as the plate on the motor says "1.72A; 370W", I thought I'd be OK with that triac. I therefore repeat my question - would you have expected this triac to instantly pop? If so, can you explain why it was a bad choice?
I'm not disputing what you say, Dave - for me, it's a learning experience, and I came here because it's a room full of people who know better than I.

 

I would use at least a 14amp such as MAC16DGOS, or larger.
Max.

Thanks Max, your reply appeared while I was writing to Dodgydave, where I explained my choice of the BT137 (8A). I can't argue against direct experience (it didn't survive), but I don't understand why it was a bad choice.

 

Although in the slow start the in-rush will be alot less, the start current of a Universal motor is quite considerable, but for the price difference between a 8amp and a 15a-20a is minor.
Max.