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metalmagpie
11-26-2017, 12:40 AM
Please take a look at my little schematic and tell me what you think of it. I didn't design it - I believe it is the way the old Victor track torches were speed-controlled. I was impressed with how simple it is, basically a rheostat, a couple rectifiers, some switches and a fuse. Of course, it depends on the motor having separate field windings, and it doesn't have any feedback. I believe they got away without feedback because the track burner rolls with constant low losses. Also, if you need a little more speed you can just move the rheostat handle a little. In this application precision isn't really required.

Unless I made a mistake transcribing this circuit, it is time-tested. I don't think it is one of the basic DC motor control schemes I have seen not that I'm some kind of expert. I'm just a tinkerer jack of all trades type of guy.

Here's the schematic: http://nwnative.us/Grant/images/panelSchematic.pdf

metalmagpie

PStechPaul
11-26-2017, 12:59 AM
It looks like the field coil is powered from a fixed rectified 120 VAC source and the armature is fed a variable voltage from a potentiometer (not actually a rheostat, which is a variable resistor). The symbol looks more like a Variac, or variable transformer, which is much more efficient. The simplest motor controls, usually for universal or series-wound motors, is a series variable resistor (rheostat). Actual speed depends on the load (or "windage" which is air resistance of the spinning armature). Series resistance also limits the torque by regulating current. Some motors accomplish speed control by adjusting field current, where less current results in higher speed to cancel BEMF.

GNM109
11-26-2017, 01:03 AM
It looks good.

What is the purpose of the first bridge rectifier - the one connected to VCI, VC2? Is that a ballast or coupling resistor connected to it at the bottom of the drawing? It's not identified. Just curious.

metalmagpie
11-26-2017, 01:52 AM
It looks good.

What is the purpose of the first bridge rectifier - the one connected to VCI, VC2? Is that a ballast or coupling resistor connected to it at the bottom of the drawing? It's not identified. Just curious.

That's the motor's field winding.

metalmagpie
11-26-2017, 01:53 AM
It looks like the field coil is powered from a fixed rectified 120 VAC source and the armature is fed a variable voltage from a potentiometer (not actually a rheostat, which is a variable resistor). The symbol looks more like a Variac, or variable transformer, which is much more efficient. The simplest motor controls, usually for universal or series-wound motors, is a series variable resistor (rheostat). Actual speed depends on the load (or "windage" which is air resistance of the spinning armature). Series resistance also limits the torque by regulating current. Some motors accomplish speed control by adjusting field current, where less current results in higher speed to cancel BEMF.

Oops. In actuality I think it is a Variac and I screwed up in my nomenclature. My bad ..

metalmagpie

GNM109
11-26-2017, 02:24 AM
The problem with variacs is that they are rather expensive and they are also fragile. There are solid state units now that are variac-based and are inexpensive. The circuit looks fine though.

MattiJ
11-26-2017, 02:45 AM
The problem with variacs is that they are rather expensive and they are also fragile.

And heavy. Smallest ones commonly found are something like 2 amps and 6lbs

Paul Alciatore
11-26-2017, 03:15 AM
Does it work? Sure it does.

Is the speed regulation good? It will be between poor and just plain OK, depending on the motor and the load. It probably was OK for dragging a torch along a weld as there was little resistance to it's movement if the rails were kept clean and the cables or hoses kept properly dressed.

Will it be compact? NO!

Will it run cool? NO!

The main problems will be a lot of wasted heat/power and it will be expensive. That variac will have a lot of steel and copper and they cost $$$. Solid state components are only pennies by comparison.

And variacs will wear out if adjusted over and over. And then they need to be replaced. Been there! Done that! Expensive replacement cost. Solid state switches have no moving parts and if the circuit is designed properly they can last a long, long time.

I don't think any decent EE would design a circuit in that manner today.

Forrest Addy
11-26-2017, 04:22 AM
Your circuit would certainly work but the speed regulation would poor if T! was indeed an actual "rheostat: that is a variable resistor.

In your drawing legend, you call out T1 in your diagram a "rheostat," a specific type of high wattage variable resistor used in lighting and motor control many years ago.

The diagram symbol in your diagram is used to represent a variable transformer - that is a variac or equivalent. If it is a variable transformer, then in my estimation your speed control diagram if transmuted into hardware would work very well and even have pretty good speed regulation. The rectified armature voltage would be little affected by armature current demand and thus be stiff in RPM response to varying mechanical load.

The old track burners I've seen had a big knob that adjusted a spring on motor shaft driven centrifugal governor. The speed regulation was excellent: you couldn't hurry it or slow it down by hand without skidding the wheels.

metalmagpie
11-26-2017, 08:40 AM
Your circuit would certainly work but the speed regulation would poor if T! was indeed an actual "rheostat: that is a variable resistor.

In your drawing legend, you call out T1 in your diagram a "rheostat," a specific type of high wattage variable resistor used in lighting and motor control many years ago.

The diagram symbol in your diagram is used to represent a variable transformer - that is a variac or equivalent. If it is a variable transformer, then in my estimation your speed control diagram if transmuted into hardware would work very well and even have pretty good speed regulation. The rectified armature voltage would be little affected by armature current demand and thus be stiff in RPM response to varying mechanical load.

The old track burners I've seen had a big knob that adjusted a spring on motor shaft driven centrifugal governor. The speed regulation was excellent: you couldn't hurry it or slow it down by hand without skidding the wheels.

OK. If you revisit the link you will find everything the same except I renamed T1 from 'rheostat' to 'variac'. Senior moment. My bad. Mea culpa. Nuff said?

Forrest, I think you are talking about newer old track burners! :-)

metalmagpie

J Tiers
11-26-2017, 08:53 AM
Speed regulation should be "ok". Shunt motors have decent inherent regulation. If turned down to very low speed, it will be worse, due to resistance vs the low voltage applied.

MaxHeadRoom
11-26-2017, 10:40 AM
Do you have a wound shunt field motor? They are getting pretty rare now, the circuit will work the same for a P.M. field, just delete the field bridge.
This is the basic method of the low end KB DC motor controllers, just that they use a couple of SCR's in the bridge in place of the Variac control.
Max.

J Tiers
11-26-2017, 10:52 AM
....
This is the basic method of the low end KB DC motor controllers, just that they use a couple of SCR's in the bridge in place of the Variac control.
Max.

Yes, and many have an option of a low speed boost, to counteract the resistance problem. That improves speed stability vs a no feedback system like a variac. They are probably cheaper than a variac of the same capability, also.

As for the field, the permanent magnet (P.M,) field is exactly equivalent to a wound field with constant voltage on it.

lakeside53
11-26-2017, 11:09 AM
And heavy. Smallest ones commonly found are something like 2 amps and 6lbs

Nope.. the miniature panel mount Variacs are small - 2 inches or so across. They were very common up to the 70's.. even later. I still have some.

boslab
11-26-2017, 02:01 PM
I have a really old BOC bantam track cutter, so old that thieves didn't even bother to steal it, it's really heavy but when I looked inside it a long time back there was bugger all in it, couple of switches, wire wound pot thing, a bridge that I could see, seems this design got about a bit, speed control is good, I've actually welded with it, welding torch in the cutter clamp, dip dip dip dip...you get the idea, I was toying with sticking my plasma torch on it to see what happens, turn the pot up and it can fairly fast crawl along, going to have to dig it out now (good job I didn't cannibalise the 8' track, there's a rack gear all along it.
Mark

BCRider
11-26-2017, 02:36 PM
I know you said no semiconductors but depending on the current needs for this motor I wonder if your variac could be replaced with a lamp dimmer. It chops the waveform up where the variac just gives a lower voltage but you're rectifying and filtering it anyway so that shouldn't matter. And a lamp dimmer is WAY cheaper than a variac.

MaxHeadRoom
11-26-2017, 03:06 PM
As for the field, the permanent magnet (P.M,) field is exactly equivalent to a wound field with constant voltage on it.

I realize that, I have been working with motors of all stripes for the past 65 years, many technologies that have fell by the wayside, including a session of rewinding at one point.
Max.

Mark Rand
11-26-2017, 03:30 PM
It'll work perfectly. It's a classic design. My only significant comment would be that there's no need for S2 to be a double pole switch, since you are only using one side of it. :)

lakeside53
11-26-2017, 04:15 PM
I know you said no semiconductors but depending on the current needs for this motor I wonder if your variac could be replaced with a lamp dimmer. It chops the waveform up where the variac just gives a lower voltage but you're rectifying and filtering it anyway so that shouldn't matter. And a lamp dimmer is WAY cheaper than a variac.

Maybe it's not clear but he already has the burner, it's not broken, works fine so why replace it? Nobody even would know it has a variac inside unless they took it apart ;

BCRider
11-26-2017, 05:21 PM
Maybe it's not clear but he already has the burner, it's not broken, works fine so why replace it? Nobody even would know it has a variac inside unless they took it apart ;

Fair enough. I took it that he was looking to build a copy of that circuit.

J Tiers
11-26-2017, 05:46 PM
I realize that, I have been working with motors of all stripes for the past 65 years, many technologies that have fell by the wayside, including a session of rewinding at one point.
Max.

No worries, I was agreeing.......

GNM109
11-27-2017, 09:54 AM
I discovered the unit shown in the link below last year. It's called a regulator, temp controller or diimmer, but in operation it works just like a Variac . In operation, it opens at 3 or 4 volts and regulates Ac power nicely on up to 120 Volts.

Something like this with a proper bridge and isolation transformer might work to operate the OP's track burner.

I use two of them in a home-built dual power pack for my model railroad. They are very sturdy and well built.


https://www.amazon.com/Yeeco-Regulator-Thermostat-Temperature-Controller/dp/B00MKU4W3Y/ref=sr_1_10?ie=UTF8&qid=1511733647&sr=8-10&keywords=Yeeco+supply+voltage

MaxHeadRoom
11-27-2017, 10:13 AM
I discovered the unit shown in the link below last year. It's called a regulator, temp controller or diimmer, but in operation it works just like a Variac . In operation, it opens at 3 or 4 volts and regulates Ac power nicely on up to 120 Volts.

Something like this with a proper bridge and isolation transformer might work to operate the OP's track burner.

I use two of them in a home-built dual power pack for my model railroad. They are very sturdy and well built.




That actually specifies AC regulation, if so yes, it would need a bridge rectifier to run a DC motor.
The Chinese are fond of using 'SCR' when in fact it is a Triac controller.
There are DC motor versions on ebay, cheap, but are PWM/Mosfet style and much smoother control.
Max.

GNM109
11-27-2017, 10:57 AM
That actually specifies AC regulation, if so yes, it would need a bridge rectifier to run a DC motor.
The Chinese are fond of using 'SCR' when in fact it is a Triac controller.
There are DC motor versions on ebay, cheap, but are PWM/Mosfet style and much smoother control.
Max.


They work nicely with a bridge rectifier and an isolation transformer to provide smooth DC. I built a dual 10 amp, 12 vdc power supply for my G Scale Model Railroad a while back. It uses two of the Yeeco units with bridge rectifiers and 10 amp, center-tapped transformers for isolation and output.

With a proper transformer, you could make very nice DC in any voltage desired. I used a 6,800 mf cap on each side to filter and the output is very smooth on my vintage, second hand store oscilloscope.

I found these right after my last Variac died of old age. LOL. I hate those things, but I mean that in a nice way. LOL.

J Tiers
11-27-2017, 11:17 AM
The variac and the "light dimmer type" are fundamentally different, in ways that can affect the operation of whatever it powers.

The variac provides an AC output which is exactly the same as the AC input, but at an adjustable lower voltage. The output is every bit as smooth as the input AC, and the controlled device will generally work as smoothly on that as it would on full voltage (there may be a minimum voltage it will operate on, of course). The output is smooth sine wave AC.

The light dimmer type is different. It does not change the actual voltage. What it does is to allow the original line voltage through, but controls how much of a full cycle of AC gets through. This is done by delaying the switching "on" of the triac.

To provide full output, the triac switches "on" at the beginning of each "half cycle", and lets through almost the whole half cycle. When a lower output is wanted, it will delay until later to turn "on", so that instead of a full smooth sine wave AC voltage, the output is partial sine waves, as if you took the smooth sine wave and cut out the first part (exactly what happens, actually).

The "dimmer type" control depends on the load, whatever it is, to smooth that chopped-up sine wave out so that it "imitates" an actual lower voltage. They also make a huge amount of radio interference, and need a noise filter on the input.

Incandescent lights, and heating equipment really are not bothered by that. Motors can be affected. Motors tend to be rougher in operation at low speeds with the light dimmer style than with a variac. The basic "dimmer" circuit can be modified to work better, and any such device advertised for use with motors should be made that way. But they rarely have the smoothness of the same type motors using a variac or a higher frequency PWM setup.




I discovered the unit shown in the link below last year. ..... They are very sturdy and well built.


https://www.amazon.com/Yeeco-Regulator-Thermostat-Temperature-Controller/dp/B00MKU4W3Y/ref=sr_1_10?ie=UTF8&qid=1511733647&sr=8-10&keywords=Yeeco+supply+voltage

I like the fact that even in the pic for the advert, the heatsink is mounted in a twisted position.... you can see the PCB outline, and the heatsink is clearly askew. Perhaps that is truth in advertising...... ;):rolleyes:

The solder leads of the triac may be the only mechanical connection holding the heatsink to the PC board. If here is any other support, it must not be very steady.

GNM109
11-27-2017, 11:38 AM
The variac and the "light dimmer type" are fundamentally different, in ways that can affect the operation of whatever it powers.

The variac provides an AC output which is exactly the same as the AC input, but at an adjustable lower voltage. The output is every bit as smooth as the input AC, and the controlled device will generally work as smoothly on that as it would on full voltage (there may be a minimum voltage it will operate on, of course). The output is smooth sine wave AC.

The light dimmer type is different. It does not change the actual voltage. What it does is to allow the original line voltage through, but controls how much of a full cycle of AC gets through. This is done by delaying the switching "on" of the triac.

To provide full output, the triac switches "on" at the beginning of each "half cycle", and lets through almost the whole half cycle. When a lower output is wanted, it will delay until later to turn "on", so that instead of a full smooth sine wave AC voltage, the output is partial sine waves, as if you took the smooth sine wave and cut out the first part (exactly what happens, actually).

The "dimmer type" control depends on the load, whatever it is, to smooth that chopped-up sine wave out so that it "imitates" an actual lower voltage. They also make a huge amount of radio interference, and need a noise filter on the input.

Incandescent lights, and heating equipment really are not bothered by that. Motors can be affected. Motors tend to be rougher in operation at low speeds with the light dimmer style than with a variac. The basic "dimmer" circuit can be modified to work better, and any such device advertised for use with motors should be made that way. But they rarely have the smoothness of the same type motors using a variac or a higher frequency PWM setup.

I like the fact that even in the pic for the advert, the heatsink is mounted in a twisted position.... you can see the PCB outline, and the heatsink is clearly askew. Perhaps that is truth in advertising...... ;):rolleyes:

The solder leads of the triac may be the only mechanical connection holding the heatsink to the PC board. If here is any other support, it must not be very steady.

Not sure of your criticisms here. I have no connection with the company but I do know that they work. Measured output is smooth up to 120 vac and, connected to the transformers. output is smooth up to 12 vdc.

Don't get what you mean regarding the heat sink? They are straight on the units that I have, also on some spares that I ordered.

I guess you are a Variac guy. :rolleyes:

MaxHeadRoom
11-27-2017, 12:39 PM
They work nicely with a bridge rectifier and an isolation transformer to provide smooth DC. I built a dual 10 amp, 12 vdc power supply for my G Scale Model Railroad a while back. It uses two of the Yeeco units with bridge rectifiers and 10 amp, center-tapped transformers for isolation and output.

With a proper transformer, you could make very nice DC in any voltage desired. .

If only needing low voltage, I would look at the PWM off ebay, $2.50 for a 12v to 40vdc 10a unit.
If wishing more amps, there is a 60a version for $12.00.
Also if using an isolation transformer, sub it for a 12vac out version and feed the PWM unit with it.
I still have a copy of the VERY early G.E. experimenters SCR/Triac manual and they have a SCR model R.R. supply project in it.
But with the advent of power Mosfets etc, PWM has become the preferred for DC motor control now.
Max.

metalmagpie
11-27-2017, 12:53 PM
In this application, smoothness at low speed is critical.

metalmagpie

J Tiers
11-27-2017, 01:24 PM
Not sure of your criticisms here. .......
I guess you are a Variac guy. :rolleyes:

Not hardly..... More PWM/VFD oriented.

Just pointing out that the light dimmer approach does not always give good results, due to the way it works.

PWM works very well, and is in many ways much more similar to the variac approach.

Paul Alciatore
11-27-2017, 04:01 PM
Yes, I do believe that the triac is the only thing holding that heat sink on. There are no screws or screw holes visible in any of the photos. This is indicative of the cheap nature of that, obviously imported, device. It MAY work just fine, but they did take shortcuts. It wouldn't have taken a lot to add two or even just one screw through the board to attach the heat sink. The holes in the board would have essentially been free as they would have just been added to the board's layout. And tapping one or two holes in the heat sink would be a nominal cost in China. Neither myself nor any US manufacturer that I know of would do it that way.

I am not sure of the point of this thread. The circuit being discussed is an older solution to a common problem; motor speed control. Are we defending it? I don't think it needs defending. Are we trying to understand it? It is not that hard to understand.

Are we debating weather it is better than solid state devices? I guess that depends on what you consider to be better.

Less expensive? NO!

Better regulation? NO!

More resistant to certain types of abuse? YES! But the newer, solid state designs can also be very resistant to those same types of abuse if they are properly designed and constructed. And it does have one big disadvantage in that the variable transformer WILL wear out after a number of adjustment cycles. IT WILL, it is only a matter of time. A properly designed, solid state device WILL outlast it. As I said above, I don't think any decent engineer would use it today. That does not mean it is bad. It means that better, all-around solutions/compromises do exist today. Those better designs can be fowled up, as the Chinese are so willing to prove. But they do exist and even at US-manufacturing prices, they can be less expensive than a device that uses a variac which is the chief expense in it.

Easier to trouble shoot and repair? Depends on many factors. Two important ones would be your skill set and the availability of repair parts. For skill set, solid state devices can be repaired with the try and see approach because everything in them is fairly inexpensive. But purchasing variacs is an expensive proposition when you don't know for sure it is bad.




...<snip>...

I like the fact that even in the pic for the advert, the heatsink is mounted in a twisted position.... you can see the PCB outline, and the heatsink is clearly askew. Perhaps that is truth in advertising...... ;):rolleyes:

The solder leads of the triac may be the only mechanical connection holding the heatsink to the PC board. If here is any other support, it must not be very steady.

J Tiers
11-27-2017, 04:16 PM
Much as I am biased toward electronic solutions, the variac is very probably a more smoothly adjustable device, with a much better repeatability than a cheap "light dimmer" type.

If you set it on "15" it will probably always do what you expect for that setting. The dimmer type tends to be more sensitive to small adjustments, and to voltage variations, which can cause changes in the travel speed. Usually you are not going to want that.

So, between the variac and the simple dimmer type motor control, the variac is probably actually a better solution. A better design of electronic control could give equally good, or better, performance than the variac.

PStechPaul
11-27-2017, 04:20 PM
Some things to consider:

1. An ordinary lamp dimmer TRIAC control may not work well on a motor, because of its inductance and dynamic load properties. The controls designed for a motor should be OK.

2. With a bridge rectifier and capacitor load, any phase delay less than 90 degrees will produce an open circuit voltage of the peak value of the sine wave. Under a sufficiently heavy load, the effective voltage (and motor speed) will be more adjustable. But it might be better to use it without the capacitor. The rapid rise time of the phase-delayed voltage will cause very high peak currents which may cause stress on the TRIAC, rectifiers, and capacitor.

J Tiers
11-27-2017, 04:30 PM
Definitely no cap, no need. Except for one that is part of a "transient snubber".

But this is not really FOR the "cutter cart", seems like it was more of a general question at the moment.

metalmagpie
11-27-2017, 06:52 PM
I am not sure of the point of this thread.

This motor control circuit is worthy of note because of its simplicity. If a guy is thinking of making a track burner, he's probably a welder, not an EE. It has zero active elements. You could even make the rectifiers from a handful of diodes. So, it's simple, easy to make, easy to diagnose if something goes wrong, and is very smoothly adjustable and stable at slow speeds.

Actually, in the beginning I posted this to ask if there were something wrong with this circuit as implemented. Right away I saw that no, there isn't anything really wrong with it, but it did have
some limitations. After that it's mostly informational.

You don't have to make or install a power supply - this simply uses wall power everywhere.

Most people wouldn't have any idea how to specify or buy triacs or SCRs. There are nuances of using such elements that would be easy to miss or get wrong, like heat sinking for example.

metalmagpie

metalmagpie
12-22-2017, 07:21 PM
The old track burners I've seen had a big knob that adjusted a spring on motor shaft driven centrifugal governor. The speed regulation was excellent: you couldn't hurry it or slow it down by hand without skidding the wheels.

Those were Airco Radiagraphs. They were fitted with Lee electric governors. Wonderful to use, very pricey to fix. Also IMO the best looking track burners of all time. Lots of brass bits.

metalmagpie