Also the field has a different magnetic make up, a solid iron pole .

That surely can't be true if the motor was intended for AC operation.

 

That surely can't be true if the motor was intended for AC operation.

I was referring to a typical shunt field DC motor!
Max.

 

I'm about to embark on this little project. I plan to design a small circuit that will rectify the 220VAC source, and separately control two high-capacity mosfets. The mosfets will be protected by fast diodes at the motor's windings, and a TVS at each of the mosfets themselves.

I plan to experiment first by slowly applying a 220VDC PWM to both rotor and field simultaneously by rising it to 50% duty factor. This should be equivalent to running the motor at 120 VDC. Then I'll start weakening the field by lowering it PWM duty factor, as previously suggested by some of the participants of this thread. And then I'll slowly raise both the field's and rotor's duty factor... see what happens. All this while also measuring the rotor's RPMs.

Questions:

• Can the field's duty factor be lowered so much that either the rotor or the field would burn up? Why?
• Is there a risk of raising the duty factor so much when running the motor at 220 VDC that it would also burn even when no mechanical load is being applied?
• Would it burn be because some sort of saturation point (be it current, or magnetic capacity) would be reached surpassing the motor's original design parameters?
• Is this similar to what would happen to a transformer if it were to be run beyond its design parameters?

 

What ever method you use, weakening the field is going to reduce torque.
It is a trade off, for a given armature voltage, the field current governs max rpm and torque.
I assume you intend controlling the field and armature separately, essentially making it a shunt field motor?
Max.

 

What ever method you use, weakening the field is going to reduce torque.
It is a trade off, for a given armature voltage, the field current governs max rpm and torque.
I assume you intend controlling the field and armature separately, essentially making it a shunt field motor?
Max.

I'm not familiar with the term. But I googled it, and found this:

​

So yes, that's exactly what I'm planning on doing. Except that I'll be controlling both the field and armature independently, as you've just mentioned. So that diagram will end up looking like this:

​

But what do you think about my question regarding saturation by running it at a higher voltage? Is it a strong possibility?

 

And btw, I found this excellent power point presentation explaining several different methods of controlling a DC motor:

https://www.slideshare.net/subhajit798bose/dc-motor-ppt​

 

I think that should work okay but as Max pointed out you will be trading off torque for RPM's (maybe more than you have to work with) which might cause some final speed and workable power output issues.

Then there's the issue of over current on the rotor which being the drill is rated for ~ 6 amps as long as you don't go too far over that on your PWM average current for extended periods it should be fine, assuming the new higher voltage drop across it doesn't make up for some unforeseen added power losses in it.

Last is the issue of whether the rotor itself can handle being spun twice as fast without losing its commutator or fan blades. Best guess is that motor runs at 15K - 20K RPM stock (if not higher given the multistage gearing) which at doubling that might be at or beyond the working limits of their design in longer run time situations.

Don't know really. Might work well and might burndown of blow up after the first 5 minutes!

 

You could fry the brushes.

Good luck.

 

You could fry the brushes.

Good luck.

Is that like, "you'll shoot your eye out kid"?

 

....Or don't run with scissors!
Max.

 

.... Or, never apply 220V to a 120V motor?

 

Or, it sounds like a neat experiment - wear your safety glasses and let me know what happens.

Or, I'm glad he is experimenting on time and equipment that I am not paying for.

 

Or, I'm glad he is experimenting on time and equipment that I am not paying for.

hey... you're the one that taught me that there's no such thing as big mistakes... just expensive lessons...

 

hey... you're the one that taught me that there's no such thing as big mistakes... just expensive lessons...

I said nothing about mistakes - just time and money. And, hopefully, I get to be the smart one instead of the average one here. Smart people learn from other people's mistakes experiences.

 

Smart people learn from other people's mistakes experiences.

@cmartinez is prepared to fall on his sword for the rest of us.


Max.

 

@MaxHeadRoom

I'm not usually the "hey, Jethro, Watch me do this" type of person.
I am more of the "hey, Jethro, do this and I'll watch" type.

Note: 50% of emergency room visits in Louisiana can be traced back to the phrase, "hey, Jethro, watch this!"

 

Well ... I'm back

I was on the verge of starting this little venture, when I was finally able to lay my hands on a drill motor at the factory that is not currently being used. So I disassembled the gear head, and found that the thing has a couple of compound gears that couple the motor's shaft to the output shaft (that is, the drill chuck).

And after counting the teeth in each gear, and a few very simple calculations, I concluded that based on the 600 RPM that the chuck is spinning at (according the the manufacturer's specs) the motor itself must be spinning at close to 23,000 RPM's !!!

So doubling that ratio using electrical means is now out of the question ... 46,000 RPM are not a realistic goal for a tool that is supposed to be robust and reliable.

What I'm going to do now, is make careful measurements of two of the gears, and supplant them with two of my own design (yes, I'm qualified and experienced at that sort of thing ). That will allow me to not just double but triple the chuck's rpm. And then I'm going to use an ordinary triac-based speed motor control to bring the rpm's down to a level that will be determined experimentally.

Many thanks to all who tried to help me in this thread. I'll be back later with my final results... for closure's sake.

 

Keep in mind if running it as the original series motor, the max rpm is generally decided by the motor load, in this case friction,windage, gears, etc, a reduction in any one of these can cause the maximum off load rpm to increase proportionately.
Max.

 

What I'm going to do now, is make careful measurements of two of the gears, and supplant them with two of my own design (yes, I'm qualified and experienced at that sort of thing ). That will allow me to not just double but triple the chuck's rpm. And then I'm going to use an ordinary triac-based speed motor control to bring the rpm's down to a level that will be determined experimentally.

So you're going to try what I hinted at in post #49? The first gear in that train of gears will probably be the biggest problem. I say that because that gear is usually made as the end of the armature shaft it's self. Many of the older drills like that the compound gears were made in two pieces, the smaller one had a pressed on bigger one. And most are helical type gears,to help keep noise down and make the transfer of power smoother.

 

So you're going to try what I hinted at in post #49? The first gear in that train of gears will probably be the biggest problem. I say that because that gear is usually made as the end of the armature shaft it's self. Many of the older drills like that the compound gears were made in two pieces, the smaller one had a pressed on bigger one. And most are helical type gears,to help keep noise down and make the transfer of power smoother.

Yeap, that's what I'm going to do. You know me... I have to prove to myself that I'm on the wrong path before changing course. Both inner compound gears are press assembled from two pieces. Here's a pic, both gears are almost identical:

​

I'll be cutting the gears out of D2 material, hardened to 60 Rc. I'll be drilling on wood and sheet metal, so the work load will be much lighter than the drill's original design. Which is to drill 1/2" diam holes on metal plates of up to 3/4" thick.

Keep in mind if running it as the original series motor, the max rpm is generally decided by the motor load, in this case friction,windage, gears, etc, a reduction in any one of these can cause the maximum off load rpm to increase proportionately.
Max.

If anything, friction will increase, since the output shaft's rpm's are going to increase by a factor of three. In the end, the motor will be spinning a bit slower than the original 23,000 rpm. And a bit more current will be drawn while the motor is spinning unloaded.