So I'm building a DC motor control circuit.. yes, it's another treadmill motor but this time it's on a bandsaw and I need to adjust the speed between slow for metal and fast for wood. For sake of argument, the original controller is not an option.

Insofar as the power circuit.. rectification is already taken care of. The speed control section is where my mind goes offline. The motor is a typical TM, it's DC that maxes at around 120v, "rated" for 2.1hp (we all know that's BS) and it's brushed.


So based on my reading there's a number of ways to control the speed of a DC motor; SCR/Thyristor, H-bridge, BTL amp, PWM driven h-bridge, etc.

What I'm hoping to cultivate from the electro-mechanical collective here is the 'best' or 'most appropriate' control method. Low power consumption, high reliability/resiliency (in terms of it's not going to burn our a component of the motor is forcibly stopped for some reason/a second or two), and not super complex with like a fistful of IC's, a dozen op-amps... you know. Hopefully you know what I mean.

Also this is a learning thing for me, I've always wanted to know how a thing worked.. not just that it did. So knowing why a certain method of control isn't good is just as important as knowing which one is better. Being neuro-diverse probably fuels that, to a degree at least

Thanks!

 

One method to explore, especially if you need fairly tight RPM control, is to use the Motorola IC TDA1085, it was actually intended for the AC/Universal motor type, but with a bridge rectifier after the SCR it can be used for DC motor.
I used one for a Band saw and it controls RPM quite tight as it requires a simple ~6 rpm tach on the motor shaft. which can be easily put together.
Using a SCR?thyrister you do not require the DC power supply when a bridge after it is used.
A lot depends on what accuracy of control you want.
Any reason you are not using a decent designed TM controller?

 

One method to explore, especially if you need fairly tight RPM control, is to use the Motorola IC TDA1085, it was actually intended for the AC/Universal motor type, but with a bridge rectifier after the SCR it can be used for DC motor.
I used one for a Band saw and it controls RPM quite tight as it requires a simple ~6 rpm tach on the motor shaft. which can be easily put together.

I like it.. Im going to look it up.

Using a SCR?thyrister you do not require the DC power supply when a bridge after it is used.
A lot depends on what accuracy of control you want.
Any reason you are not using a decent designed TM controller?

IDK that I need tight RPM control.. I'd prefer to have a set speed not fluctuate more than say 20-30RPMs based on load.

But thats one smaller reason I'm looking to build my own controlling circuit. One to learn, but two because the MC-2100 does not seem to be able to handle loads well. In my experience so far at least. I am making an effort to appropriately size the pulleys so as not to lose too much torque. But also because.. for example, the mc-2100 is kinda fragile. If I'm running the bandsaw trying to cut aluminum and the blade digs in there's a decent chance that something might burnt up because the motor was locked. Granted Im not talking about physically seizing the motor for 30 seconds. But there's reports of having it lock for a second or two being enough because the board was not designed for that.
The tertiary reason is those controllers are big. The ballast resistor gets pretty hot, really quick even with no load, so I'm hoping to improve upon things.

 

Consider that at least some commercial bandsaws just have two speeds Fast and Slow. That is the cheaper ones that I have used in the past 15 years. The very expensive ones with the mechanically variable speed cost lots more..
One simple scheme would be to get a 240 to 120 step down transformer and put the two sections in series, If the phasing is correct you can have the full line voltage, or 2/3 the mains voltage, or just 1/3the voltage. Then rectify that and there are three speeds. Possibly an adequate range, certainly reliable and cheap and about as simple as can be. For more ranges, consider using a surplus 480/240 to 120 volt transformer.

 

All AC-Powered, DC-Motor-Controllers,
are plagued by compromises at every step of the design process.

What is "good-enough", what is "too-big", what is "too-expensive" ?????

Nice features to have ..........
Quiet-Operation, ( no humming, or buzzing, or groaning, sounds ).
Quiet-Electrical-Operation, ( very low levels of "Hash" being broadcasted all over the neighborhood )
Tight RPM Regulation.
Adjustable "Ramp-Up", and Motor-Braking.
Automatic-Current-Limiting.

You can only determine how valuable any of these things are to You by experiencing them yourself,
or, by just starting-out to build it in the "best" possible manner regardless of Time or Cost.

Premium-Performance requires large Filtering-Components, TWICE, AC to DC, and, PWM to DC,
these can be quite expensive if purchased commercially,
or they may be DIY-ed by using MOTs ( Microwave-Oven-Transformers ), for the required Chokes,
but, unfortunately, You can't substitute large, high-Current / Voltage-rated, Filter-Capacitors.

Treadmill-Controllers routinely blow-up because they are built to satisfy a COST target.

Do You have strict Cost constraints ?, ( almost everyone does ),
or, do You want to build a Controller that is a complete pleasure to use, and will last for ever ?
.
.
.

 

Consider that at least some commercial bandsaws just have two speeds Fast and Slow. That is the cheaper ones that I have used in the past 15 years. The very expensive ones with the mechanically variable speed cost lots more..
One simple scheme would be to get a 240 to 120 step down transformer and put the two sections in series, If the phasing is correct you can have the full line voltage, or 2/3 the mains voltage, or just 1/3the voltage. Then rectify that and there are three speeds. Possibly an adequate range, certainly reliable and cheap and about as simple as can be. For more ranges, consider using a surplus 480/240 to 120 volt transformer.

That idea is not off the table, I’m leaning towards adjustable simply because I already have a PWM generator (off Amazon). But yeah the idea of just having two or three set speeds is on the list.

 

All AC-Powered, DC-Motor-Controllers,
are plagued by compromises at every step of the design process.

What is "good-enough", what is "too-big", what is "too-expensive" ?????

Nice features to have ..........
Quiet-Operation, ( no humming, or buzzing, or groaning, sounds ).
Quiet-Electrical-Operation, ( very low levels of "Hash" being broadcasted all over the neighborhood )
Tight RPM Regulation.
Adjustable "Ramp-Up", and Motor-Braking.
Automatic-Current-Limiting.

You can only determine how valuable any of these things are to You by experiencing them yourself,
or, by just starting-out to build it in the "best" possible manner regardless of Time or Cost.

Premium-Performance requires large Filtering-Components, TWICE, AC to DC, and, PWM to DC,
these can be quite expensive if purchased commercially,
or they may be DIY-ed by using MOTs ( Microwave-Oven-Transformers ), for the required Chokes,
but, unfortunately, You can't substitute large, high-Current / Voltage-rated, Filter-Capacitors.

Treadmill-Controllers routinely blow-up because they are built to satisfy a COST target.

Do You have strict Cost constraints ?, ( almost everyone does ),
or, do You want to build a Controller that is a complete pleasure to use, and will last for ever ?
.
.
.

These are the sorts of questions, and comments you only get from experience. Something I have none of in this arena lol.
Yeah there’s always a cost constraint, and time somewhat.
So can I ask, what would you build if cost wasn’t a huge factor? Then alternatively what would you build if you needed just reliable work cheaply?

 

[QUOTE="MaxHeadRoom, post: 1914454, member: 212212”
I used one for a Band saw and it controls RPM quite tight as it requires a simple ~6 rpm tach on the motor shaft. which can be easily put together.
[/QUOTE]

I had meant to ask, on your bandsaw did you encounter any torque issues at low speeds?

 

Not using the TDA IC, It has a small simple tach on it in order to maintain RPM.
Motorola originally designed it for Washing machine usage.
I picked one up at a local surplus dept. It had been used by a TM manuf.
I would tend to use one of the PWM TM controller such as MC2100 .

 

Not using the TDA IC, It has a small simple tach on it in order to maintain RPM.
Motorola originally designed it for Washing machine usage.
I picked one up at a local surplus dept. It had been used by a TM manuf.
I would tend to use one of the PWM TM controller such as MC2100 .

Pretty sure I saw one of those boards on eBay.

 

These are the sorts of questions, and comments you only get from experience. Something I have none of in this arena lol.
Yeah there’s always a cost constraint, and time somewhat.
So can I ask, what would you build if cost wasn’t a huge factor? Then alternatively what would you build if you needed just reliable work cheaply?

.
For Cheap and reliable, I wouldn't even consider using a DC-Motor, period.
I would use a standard, no frills, AC-Induction-Motor, and
adjust speed with a standard multi-step-Pulley setup.

If I wanted to go wild and crazy, just for the entertainment and "cool-factor",
I would start-out with something like the below block-diagram.
Note that there are no very important details and calculations,
and there are a lot of details that will make or break the project.
This is NOT a working Schematic, only a general representation of the major functions needed.
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.

 

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For Cheap and reliable, I wouldn't even consider using a DC-Motor, period.
I would use a standard, no frills, AC-Induction-Motor, and
adjust speed with a standard multi-step-Pulley setup.

LOL, fair enough, you aren't wrong. It's hard to verbalize and/or relate to another persons definition of adverbs like that.

So in your basic example here, we've got two sets of mains, one thats rectified and the other that's PWM power. Assuming red == + and green == -, but the inductors are throwing me off. Possibly two immediately post-rectifier. Then.. iron core inductors right before the choke is that right? Along with some caps, a zener diode.. the triangle is throwing me off, what does it represent?

 

The "Triangle" is the usual representation of an "Amplifier" of some sort in a Schematic-Diagram.
In this case, it represents a high-Current, very fast-Switching, MOSFET-Gate-Driver.
It insures cool-operation of the main PWM-Switching-FET.

The required Inductance for creating "smooth" DC-Power is split into 2 halves,
this is to help to keep Switching-Noise from getting into the AC-Mains-House-Wiring,
and is an opportunity for adding additional wanted Inductance in a smaller-space.

Brushed-DC-Motors generate Radio-Frequency-Interference, (RFI),
which can, if it happens to get excessive, smoke Electronic-Components instantly.
It isn't "always" excessive, but "IF" it is excessive, things get expensive very fast.
The generated RFI can also be "broadcast" throughout your house and affect other RFI sensitive equipment.

The Inductors alleviate a lot of the stress on the Filter-Capacitors, Bridge-Rectifiers, and other Components,
and reduce audible noises substantially, and, generally increases the Power-Conversion-Efficiency. .
Unfortunately, they're also usually big, heavy, and expensive,
at least when adequately sized for the expected Load.
These reasons are why large-Inductors are avoided by the Tread-Mill manufacturers.

The PWM Circuitry makes possible,
precise Speed-Control, Ramp-Up, and Over-Current-Protection.
( none of these features are represented in the simplified Block-Diagram )
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The inescapable problem with brushed motors is brush wear. When an AC powered induction motor is just ready for the first oil to it's bearings, the DC brush motor is needing a brush replacement. Currently the brushes run a few dollars each. At a specialty supply shop.
The motor bearing oil runs about $4 per quart The induction motor may need a few drops of oil twice a year, under heavy use.

 

The inescapable problem with brushed motors is brush wear. When an AC powered induction motor is just ready for the first oil to it's bearings, the DC brush motor is needing a brush replacement. Currently the brushes run a few dollars each. At a specialty supply shop.
The motor bearing oil runs about $4 per quart The induction motor may need a few drops of oil twice a year, under heavy use.

True, But new or used brushed motor.. I have never worn a set out personally.

 

I have worn some out. Most recently on an inherited Milwaukey angle grinder. Previously on an off-brand electric drill motor.

 

True, But new or used brushed motor.. I have never worn a set out personally.

I have never really seen a great deal of brush wear either in my career, which incidentally spans the early CNC machines which often ran production for 2 or 3 work shifts.
Quite a long period of time passed before brush replacement was required.
Re. the TDA IC, the one I ended up with was surplus from a manuf that used it with a Universal motor on AC for washing machine use.
It worked like a charm for my bandsaw.

 

The required Inductance for creating "smooth" DC-Power is split into 2 halves,
this is to help to keep Switching-Noise from getting into the AC-Mains-House-Wiring,
and is an opportunity for adding additional wanted Inductance in a smaller-space.

This part right here is especially eye-opening with regards to an entirely unrelated issue here at home.

I'm familiar with RFI albeit had no idea it could physically damage components. Also.. inductors, Are the ferrite doughnuts with about 4-7 turns ok for the AC side?
I don't have any hefty DC chokes like, solder/board mount. I do have a failed transformer rewind though. Came out of a typical small 6/12v 2/6 amp battery charger and prolly 50' of the wire that's still good.

 

There are 2 different functions being employed here, RFI-suppression, and Energy-Storage.

The RFI-Suppression is "usually" fairly easy to accomplish, and can be completely accomplished
with a store-bought-Common-Mode-Choke, which are not terribly expensive,
and a small pile of high-Voltage-Ceramic-Capacitors of various capacities, usually less than 100nF.

The Filter-Chokes "can be" a bucket of worms to get "idealized" into the rest of the Circuit.
They can create Voltage "instabilities" and even spikes, if not properly
"tested for values and performance", and "simulated in software", with the proposed Filter-Capacitors,
and all this must be right before being pressed into routine service.

Filtering ~50 or ~60hz AC, into "relatively" low-ripple-DC, at high-Currents,
requires LARGE Value Components,
and they must be matched to each-other to operate correctly and efficiently.

The Core(s) used for the Filter-Choke(s) need to be rated for the same POWER that You expect
to be delivered by your DC-Motor.
In other words, if You expect 1000-Watts out of your Motor,
the Chokes should use a Core rated for roughly 1000-VA.
This is why I mentioned using Microwave-Oven-Transformers, ( MOTs ),
they contain just enough Iron to create some fairly respectable Milli-Henry numbers,
and they are available very cheap, or even free in some cases,
and modification instructions are all over YouTube.
Quite often You can get away with simply removing the very-dangerous Secondary-Winding,
and using the original Primary-Winding "as-is", as a perfectly viable Filter-Choke.
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Well I wasn't concerned earlier.. I am now though. Oh the extra transformer I've got that could be a choke is almost the size of the UPS one. Fluke alligators for scale.


So.. in all of this is there a specific circuit type that you;d recommend or am I speaking out of turn? I just assume that you can categorize most circuits but I could be super wrong as that opinion comes from a position of complete ignorance.