# The Happy World of DC Motors



## Kludge (Aug 28, 2008)

As promised, here is the beginning of the thread on DC motors. I have no clue what direction this will go since that will depend an awful lot on questions and problems along the way. There is no possible way I can claim to be any form of expert; I just like them and far prefer them over AC motors. Actually, there is one sort of AC motor I like but it's used and controlled like a DC motor which just serves to confuse issues. I'll get to it another time.

In describing a DC motor, I'm ignoring a few trivial cases which would not be found in a shop other than as toys or experiments. I may hit a few of these later just for fun.

A present day DC motor has two major parts, the field magnet and the armature. Smaller ones use fixed magnets for the field while larger ones use electromagnets. Applying voltage to the armature of either type causes the windings to form mini-electromagnets on the armature pole pieces (those things that stick out that look a little like headsman's axe heads viewed end on) which are either attracted to or repelled by the field magnets. This causes the armature to turn. 

These motors vary in size from the motors that operate the vibrators in pagers on up to locomotive sized and in complexity from a single permanent field magnet & three armature poles (Two in the case of the St. Louis motor which I'll describe at the end of this message) on up to multiple field electromagnet windings and armature poles. Speed and torque vary directly with field strength (ie: the strength of the permanent magnets or the field current) and armature voltage. I won't get into the math (Sorry, Marv), mostly because it's not really needed in everyday use and I've forgotten a lot of it myself through disuse. Actually, the only times I did use it was when I was modifying motors for my own evil purposes.

Among other places, DC motors are found in cordless tools, electric golf carts, wheel chairs, engine starters, pager vibrators and a great number of toys the packages of which are marked "Batteries not included." 

A special case of the DC motor is the universal, AC/DC or series wound motor. They're all just different names for the same thing, a DC motor that has its field and armature wired in series to allow it to run from AC. Sewing machines, most (if not all) corded electric hand tools and virtually all corded kitchen appliances use these motors because they're cheap and easy to manufacture. They are operated from AC normally but they are much happier as DC motors The design dates from when there was still some question as to how the national power grids were going to go, and some areas used AC while others used DC. (Long historical note deleted due to its being boring.) The appliance manufacturers had to find a way to sell to everyone so the universal motor was born. 

One of the features, which also is one of the problems, with the universal motor is the interdependence of the field current and armature voltage. Vary one and the other will go right with it. This is great as an Ohm's Law demonstration but lousy for practical use especially at reduced input. On the other hand, the current increases under load as with most motors which helps bump the field up some for increased strength. 

Speed control for a DC motor can be handled via a simple rheostat or with what's called a Pulse Width Modulator (PWM). A rheostat controls the actual voltage reaching the motor and wastes a lot of energy in the form of heat. A PWM sends varying width full voltage pulses to the motor with a great reduction in wasted energy and, since the pulses are at full voltage, a greater control over the motor, especially at lower speeds. The pulses come at a fixed rate determined mostly by the design of the controller and range from very narrow for slow speeds on up to always on for top speed. 

Even with a PWM, there are several factors that determine the actual motor speed however some of these can be overcome through some rather simple (Okay, maybe not so simple) methods. These will come in a bit.

I mentioned the St. Louis motor above. This type of motor is often used in varying degrees of complexity for experiments in motors and generators due to their almost irreducable minimum count of elements. There are two field poles and two armature poles with a commutator that allows a variety of configurations. The larger versions have field coils instead of field magnets to allow an expansion of the experiments that can be done with them. I ran into and got to play with one of these a great number of years ago which was quite an education. 

Now if only I could remember what I learned ... ???

Best regards,

Kludge


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## pelallito (Sep 3, 2008)

Kludge,
I have read this several times and have been waiting for more write ups. I did not ask questions, since I don't know what to ask. ??? 
Please continue, and hopefully others will chime in.
Thanks for taking the time to give write these articles. I know that I am happy to read them!
Regards,
Fred


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## Kludge (Sep 3, 2008)

I definitely have too many fires heating my iron. Doing the Kludge Has Done It To Himself Again dance is getting old and interfering with progress. So, in a rare moment of clarity, lets talk about motors some more before the reality of too many projects at once catches up with me.

One of the more useful forms of a DC motor is the stepper. This is a rather interesting motor since the permanent magnet rotates and the fixed pole pieces are electromagnets. There are three types of stepper  variable reluctance (an older technology), permanent magnet (cheap and easy to make), and Portescaps new disk stepper which is just finding its way into the real world. Im only going to describe the permanent magnet stepper since its the easiest and its operation is not unlike the other two. There are two broad groups of these devices, bipolar and unipolar. Of the two, the bipolar is the simpler and we all know I like simple things. 

The permanent magnet on the rotor has a number of teeth, the actual number of which relate to how much it will rotate per step; a stepper that rotates 15o/step has significantly fewer teeth than one that rotates .9o/step. The fixed electromagnet pairs are arranged at right angles to each other and each pair is wound in series. That is, the current goes in one end of the first one, out at the end nearest the rotor, across to its partner entering at the end closest to the rotor and back out the end away from it. The end result is that by effectively grounding one end and energizing the other there is a magnetic field across the rotor which will cause it to turn. By reversing the connections, the field is reversed causing it to turn again. 

Ah, but there are two such electromagnets. Continuously energizing one end of each coil (with the other end at an effective ground) around in order will cause the rotor to turn in successive steps. Reverse the direction in which the coils are energized and the motor reverses. Do this six times with a motor that moves 15o/step and youve got one full rotation. (Six rounds of four steps each is 24 steps. 24x15=360. I hope.) 

The usual steps are accomplished by energizing both windings in such a way that the armature is attracted to a position half way between each pole. Since both windings are active, this is a far stronger place to be (hence the name sometimes used for it, a strong step) so these are the positions commonly used for stepping. The positions achieved by only energizing one winding at a time is called wave stepping and the two in combination is half-stepping. 

The unipolar steppers are kind of a special case of the bipolar. They have the same two sets of windings at right angles to each other but these are center tapped in two cases and split in the third. The six wire steppers bring out each center tap separately while the five wire has them internally connected. The ones with the completely split windings have eight wires coming out. (There is one other five wire form and thats the combination stepper that has one center tapped winding and one not center tapped. Im not entirely sure why these exist.) In this case, the center tap is connected to the motor power and each lead (or lead pair) is grounded in turn to provide rotation. 

The advantage to the unipolar stepper is that by activating only one side of the windings allows for reduced drive current and simpler control circuits. The disadvantage is reduced power which, in turn, means a larger motor to accomplish the same task as a smaller bipolar one. The six and eight wire unipolar steppers can be wired as bipolar motors, the former by ignoring the center tap and the latter by tying the two windings for the same pole pair together, either in series or parallel. The former configuration requires less current; the latter provides more power. There are a few ways to use a five wire unipolar stepper as a bipolar but they either require more complex drive electronics or surgery to separate the winding center taps. 

Steppers can pretty much stop when you quit pulsing them in a position indicated by what winding is (or windings are) currently active. Theory says they stop instantly but theory also doesnt take into account what the stepper is doing and how much inertia it has to overcome in stopping. As has been discussed in another thread, a stepper has wonderful starting and low speed torque but it falls off at higher step/second counts which, on reflection, works to the advantage of motors with the greater step sizes. The steep part of this fall off starts anywhere from about 200 steps/second to around 1000 steps/second dependent on the motor. The 1000 steps/second rate for a 15o stepper is 41.667 revolutions/second or 2500 rpm. On the other hand, a .72o stepper requires 500 steps for one rotation which means that, at 1000 steps/second, its good for 120 rpm. With 200 step/second being the drop off point, thats 500 and 24 rpm respectively. The motors can, of course, turn faster  up to 8000-11,000 steps/second dependent on the motor  for fast slewing but with significantly reduced torque.

A stepper driver is a fairly easy bit of kit with the new integrated circuits designed for this application. The first IC accepts speed, direction, full step/half step and other information, and decodes it to an A-B-C-D lead output. This is picked up by another IC that has the actual drivers on it, though it may require external power transistors for the higher power motors. From there its off to the motor. Some decoder ICs are made so their internal clocks (not the same as the motor pulse rate) can synch together so that things that are supposed to happen more less at the same time actually do happen at more or less the same time, taking the usual internal delays into consideration. 

All of the decoder and driver ICs Ive looked at (which is by no means an exhaustive list) also had an inhibit and/or select input which, with a little bit of digital magic, would allow one set of control signals operate several steppers in succession. This means that if you have a motor you know you can preposition then ignore during an operation, you can use one set of control signals to do that then use the same signals to control one of the motors that will be actively used during the operation. 

I know this is a bit confusing but explaining things has never really been a long suite. I'll try to answer questions in a more clear fashion.


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## Kludge (Sep 3, 2008)

pelallito  said:
			
		

> I have read this several times and have been waiting for more write ups.



I must apologize for the delay. As a mention in the first paragraph of the new installment, I've got way too many fires heating this poor old iron, one of which is creating a temporary work area so I can produce an (electric) engine or two so people don't think I'm just blowing smoke about making them. I'm also looking for parts I know I have like a bunch of reed relays I can steal the coils from to make the "cylinders". (The wire I'd use to wind my own has disappeared in a sea of chaos yet to be sorted through.) 

Most of the watch parts are sorted out and almost all of the tools have been accounted for (except a few bench blocks I seem to have put away too well) so that much is finished. Mostly. *sigh* ... being an itty bitty thing packrat is such a mess sometimes!



> I did not ask questions, since I don't know what to ask. ???



Whatever you don't understand is a good start. 



> Please continue, and hopefully others will chime in.



I thought someone would already but maybe they're so awestruck that they simply can't. :big:



> Thanks for taking the time to give write these articles. I know that I am happy to read them!



Thanks, Fred. As I said up front, I like DC motors. I like the simplicity, the controlability, the adaptability and the ease (for me, at least) by which they may be customized to some degree. Even brushless motors, which technically are three-phase AC motors but are handled like DC motors, are cool. 

Anyway, ask away about whatever you like. If I don't know the answer (and I'm no expert so that's a very real possibility), I'll find it somewhere. Or make something up. 

BEst regards,

Kludge


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## Mcgyver (Sep 3, 2008)

Kludge, thanks for taking the time to prepare the write up - i need more time to read when my eyes aren't so tired but appreciate the effort!


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## Kludge (Sep 4, 2008)

Mcgyver  said:
			
		

> Kludgee, thanks for taking the time to prepare the write up - i need more time to read when my eyes aren't so tired but appreciate the effort!



No worries. The next one I have to study for since I don't know brushless motors that well - not as well as I'd like, anyway - so there's plenty of time for the rest of the folks to become un-awestruck and chime in. ;D

As I told Fred, if you have any questions, ask away. I'm good with questions. The answers get to be a problem now and then but ... ;D

BEst regards,

Kludge


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